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diff --git a/darwin/ChartesHN_IntentiondeRecherche(2).pdf b/darwin/ChartesHN_IntentiondeRecherche(2).pdf Binary files differ. diff --git a/darwin/biblio/Darwin contre Darwin_ Comment lire 'l'origine des espèces' _ -- Thierry Hoquet -- 2009 -- Editions du Seuil -- 9782021010657 -- 28a1f67e090c37c09155fbfbb73dcedd -- Anna’s Archive.epub b/darwin/biblio/Darwin contre Darwin_ Comment lire 'l'origine des espèces' _ -- Thierry Hoquet -- 2009 -- Editions du Seuil -- 9782021010657 -- 28a1f67e090c37c09155fbfbb73dcedd -- Anna’s Archive.epub Binary files differ. diff --git a/darwin/biblio/Darwin et le darwinisme -- Patrick Tort [Tort, Patrick] -- PUF -- a0d20c8a8a7d7eac4a639ff5e5222c60 -- Anna’s Archive.epub b/darwin/biblio/Darwin et le darwinisme -- Patrick Tort [Tort, Patrick] -- PUF -- a0d20c8a8a7d7eac4a639ff5e5222c60 -- Anna’s Archive.epub Binary files differ. diff --git a/darwin/biblio/Darwin et le darwinisme -- Patrick Tort [Tort, Patrick] -- PUF -- a0d20c8a8a7d7eac4a639ff5e5222c60 -- Anna’s Archive.txt b/darwin/biblio/Darwin et le darwinisme -- Patrick Tort [Tort, Patrick] -- PUF -- a0d20c8a8a7d7eac4a639ff5e5222c60 -- Anna’s Archive.txt @@ -0,0 +1,3223 @@ +QUE SAIS-JE ? + +[1mDarwin et le darwinisme [0m + +[1mPATRICK TORT [0m + +[1mIntroduction [0m + +Si le naturaliste Charles Darwin (1809-1882) a bien été reconnu, à la suite de +Jean-Baptiste Lamarck (1744-1829), mais avec le bénéfice d’une approbation plus +unanime, comme le principal fondateur de la théorie moderne de l’évolution des +organismes, il s’en faut de beaucoup que sa pensée ait été immédiatement agréée +et comprise. + +Jusqu’à une époque assez récente, la théorie de la descendance modifiée par le +moyen de la sélection naturelle exposée par Darwin en 1859 dans De l’origine +des espèces, bien qu’ayant profondément transformé la conception globale de +l’histoire des êtres vivants, n’a été reçue dans ses caractères les plus +profondément originaux ni par les disciplines strictement biologiques ni par +les sciences de l’homme et de la société. + +Depuis son émergence jusqu’à nos jours, elle a fait, de surcroît, +périodiquement l’objet d’appropriations illégitimes et d’applications +dangereusement erronées au sein des théories économiques, sociologiques et +politiques, sans égard pour les développements anthropologiques contenus en +1871 dans un autre grand ouvrage de Darwin plus rarement évoqué, et encore plus +rarement lu, La Filiation de l’homme et la sélection liée au sexe. + +Nos travaux antérieurs et ceux de l’équipe internationale du Dictionnaire du +darwinisme et de l’évolution [1] ont permis d’expliciter les raisons +historiques qui ont fait malheureusement confondre la théorie darwinienne avec +les schèmes d’intervention théorico-politique du darwinisme social (inventé par +Spencer) et de l’eugénisme (inventé par Galton) qui furent développés au cours +de la période – celle des années 1860 – qui s’écoule entre la publication de +L’Origine des espèces et celle de La Filiation de l’homme. + +Or, l’anthropologie darwinienne, telle que nous pensons l’avoir aujourd’hui +restituée dans sa logique et dans ses formulations fondatrices, s’oppose à l’un +et à l’autre – c’est-à-dire s’oppose aux dogmes ordinaires de l’élimination +naturelle ou planifiée des « inaptes » –, comme elle s’oppose en même temps aux +recommandations malthusiennes, aux sociologies biologiques, au racisme « +scientifique », aux exactions coloniales, aux brutalités esclavagistes et à +l’intégrisme libéral qui périodiquement condamne et combat toute législation +protectrice en faveur des pauvres. + +De cela également, le présent ouvrage, sous peine d’acquiescement tacite à des +erreurs courantes dont les conséquences ont été dévastatrices, devra faire +état, en permettant d’accéder, dans la pensée « mûre » de Darwin, à cette +articulation inédite du biologique et du social qui permet de reconnaître en +lui un penseur de la paix et le plus consistant des « généalogistes » de la +morale. + +[1mNotes [0m + +[1] Patrick Tort et al., Dictionnaire du darwinisme et de l’évolution, Paris, +PUF, 1996. + +[1mChapitre I [0m + +[1mLa constitution de la théorie darwinienne [0m + +[1mI. – Les années de formation [0m + +1809 : naissance à Shrewsbury (Shropshire, Angleterre) de Charles Robert +Darwin. Petit-fils d’Erasmus Darwin (1731-1802), médecin, géologue, +naturaliste, inventeur, philosophe et poète, auteur de The Botanic Garden +(1791-1799), de Zoonomia (1794-1796), de Phytologia (1800) et de The Temple of +the Nature, or the Origin of Society (1803), lequel avait déjà profondément +remis en cause le dogme de la fixité des espèces. Fils de Robert Waring Darwin +(1766-1848), médecin établi à Shrewsbury en 1787, futur membre de la Royal +Society, et de Susannah Wedgwood, fille aînée de Josiah Wedgwood (1730-1795), +célèbre céramiste et patron d’industrie. Charles est le cinquième enfant, et le +cadet de deux garçons, venant après Marianne (1798-1858), Caroline (1800-1888), +Susan Elizabeth (1803-1866), Erasmus Alvey (1804-1881) et précédant Emily +Catherine (1810-1866). + +1817 : mort de sa mère. Charles est placé pour un an dans un externat religieux +à Shrewsbury. Début d’une scolarité difficile et apparition de goûts marqués de +naturaliste et de collectionneur. + +1818 : entrée à la « grande école » du docteur Sam Butler dans la même ville. +Il y restera sept ans pensionnaire et portera sur elle un jugement constamment +négatif. Il y apprend le grec et le latin, les classiques, un peu d’histoire et +de géographie anciennes, avec réticence et difficulté. Il se passionne en +revanche pour la pêche et la chasse (qu’il abandonnera plus tard par compassion +envers les animaux), l’observation des plantes, des insectes et des oiseaux. Il +acquiert progressivement le goût des poètes et de la versification, des +paysages, de la géométrie et de la chimie expérimentale. Il éprouve une grande +admiration morale et intellectuelle pour son père, homme d’une corpulence +imposante et dont la mémoire, la générosité, l’intuition psychologique et la « +puissance de sympathie » l’impressionnent. + +1825 : Charles s’inscrit le 22 octobre à l’université d’Édimbourg en vue +d’études médicales, sur les traces de son frère Erasmus qui les y achève, et en +compagnie duquel il s’est déjà livré à des expérimentations chimiques. Il y +passera deux ans, ne s’intéressant qu’aux cours de chimie de Thomas Charles +Hope (1766-1844), éprouvant pour le reste ennui et dégoût, singulièrement +devant la dissection, ce qu’il regrettera vivement plus tard. Il se lie avec +Robert Edmond Grant (1793-1874), de 16 ans son aîné. Brillant naturaliste +passionné par la zoologie marine, ce dernier, adepte de Lamarck, lui expose « +ses vues sur l’évolution », lesquelles, si l’on en croit les souvenirs de +Darwin, laissent le jeune auditeur aussi sceptique que celles de son grand-père +Erasmus. Charles n’a décidément aucun penchant pour la médecine. Seules +l’intéressent les sciences naturelles. Il parvient, en 1826, poussé par Grant, +à faire deux brèves communications devant la Plinian Society de l’université +sur des questions de zoologie marine. Ses visites, agrémentées de parties de +chasse, à Maer, dans la famille de son oncle Josiah Wedgwood fils, sont ses +loisirs préférés. + +1827 : Charles s’inscrit à l’université de Cambridge, décidé par obéissance +filiale, mais sans amertume, à entreprendre une carrière de pasteur de campagne +propre à lui laisser le temps de se livrer à sa passion de naturaliste. Ayant +presque tout oublié de ses études classiques, il travaille chez lui avec un +précepteur et ne rejoint Cambridge qu’au début de 1828. Il y restera trois ans, +résistant par manque d’intérêt à la plupart des enseignements imposés, mais +étudiant avec un grand plaisir la géométrie d’Euclide ainsi que les Principles +of Moral and Political Philosophy, les Evidences of Christianity et la Natural +Theology de William Paley (1743-1805), tout en préparant avec soin son diplôme. +Il s’intéresse vivement, chez Paley, à l’explication des harmonies de la nature +par l’infinie sagesse du Créateur – un providentialisme dont il commencera à +s’éloigner dix ans plus tard. Il s’intéresse encore bien davantage aux +Coléoptères qu’il collectionne avec un soin apprécié des spécialistes. Il suit +les cours de botanique de John Stevens Henslow (1796-1861), dont il devient +l’ami, le compagnon de promenade et l’invité régulier. Il fréquente ses soirées +où il rencontre notamment le philosophe, théologien, mathématicien et +théoricien des sciences William Whewell (1795-1866), ainsi que le naturaliste +Leonard Jenyns (1800-1893). Il lit avec un intérêt profond la relation +historique du Voyage d’Alexander von Humboldt (1769-1859), qui le fera rêver et +l’accompagnera longtemps. + +1831 : au mois de janvier, Charles reçoit son diplôme final de Bachelor of +Arts. Henslow le convainc d’étudier vraiment la géologie, ce qu’il commence à +faire dès son retour dans le Shropshire. Henslow obtient d’Adam Sedgwick +(1785-1873), professeur de géologie à Cambridge, que Charles l’accompagne dans +un voyage d’études au nord du pays de Galles. Revenu de cette excursion, +Charles trouve une lettre de Henslow lui proposant d’embarquer, à titre de +naturaliste sans traitement, sur le HMS Beagle, navire en partance pour un +voyage le long des côtes de l’Amérique du Sud afin d’achever leur relevé +hydrographique. Le navire est commandé par le jeune capitaine Robert FitzRoy +(1805-1865), de 4 ans l’aîné de Charles, tory (conservateur) aux convictions +religieuses radicales et défenseur occasionnel de l’esclavage, à l’opposé de +son futur compagnon de cabine, libéral abolitionniste qui à son retour se +débarrassera de plus en plus nettement des articles de foi du christianisme et +de la croyance en un Dieu créateur. Après de longues discussions avec son père, +et soutenu par son oncle Josiah, Charles s’embarque avec du matériel de +naturaliste et une bibliothèque scientifique choisie – où figure le premier +volume des Principles of Geology de Charles Lyell (1797-1875), promoteur en +géologie de l’uniformitarisme (l’actualisme des géologues continentaux). Cette +nouvelle théorie rejette la doctrine théologique des catastrophes globales et +privilégie le rôle des « causes actuelles », quotidiennement agissantes, +constamment observables et accumulées au cours d’immenses durées – le rôle du +temps se substituant aux cataclysmes et aux miracles – dans le modelage de +l’écorce terrestre. On n’a pas assez insisté jusqu’ici sur l’importance de +cette réforme profonde de la géologie pour la formation de la pensée de Darwin. +La future théorie des petites variations accumulées au cours de périodes +longues (le « gradualisme » darwinien), en tant qu’« actualisme » biologique, +est sans nul doute fortement redevable de ses intuitions premières à +l’uniformitarisme de Lyell. + +Ce voyage, qui durera presque cinq ans, fera du jeune Charles un naturaliste +expérimenté et l’initiateur des théories modernes de la formation et de +l’évolution des espèces vivantes. Il en publiera lui-même en 1839, puis en +1845, le récit détaillé, dont sont extraits les faits et observations suivants. + +[1mII. – Principales étapes et observations du voyage sur le Beagle [0m + +1831, 27 décembre : départ de Devonport (Plymouth). + +1832, 16 janvier : Porto Praia (île de São Tiago, Cap-Vert). Examinant cette +île volcanique, Darwin vérifie la justesse des idées de Lyell sur ce type de +formation : soulèvement (surrection), puis affaissement graduel (subsidence) +autour des cratères. L’idée d’une équilibration compensatoire des mouvements du +sol paraît confirmée. + +4 avril : Rio de Janeiro. Du 8 au 23 avril, voyage terrestre. Réflexions +douloureuses sur l’esclavage des Noirs. + +22 septembre : premières découvertes de fossiles à Punta Alta (province de +Buenos Aires). + +14 novembre : Montevideo, jusqu’au 27 novembre. Darwin y reçoit le deuxième +volume des Principles of Geology de Lyell, comprenant son exposé en forme de +réfutation des idées de Lamarck. + +16 décembre : Terre de Feu. + +1833, 1er mars : îles Falkland, jusqu’au 6 avril. + +3 août : à Punta Alta (Argentine), Darwin découvre, dans du gravier stratifié +et de la boue rougeâtre, une grande quantité de vestiges de Mammifères du +Quaternaire argentin ; il est frappé par la remarquable ressemblance +morphologique entre les grands tatous fossiles et les armadillos contemporains. +La présence conjointe de coquilles très semblables aux coquilles actuelles +confirme la validité des idées de Lyell sur la longévité inférieure des espèces +de Mammifères par rapport à celle des espèces de Mollusques. + +27 septembre : voyage par terre de Buenos Aires à Santa Fe, le long du Paraná, +jusqu’au 4 octobre, et retour à Buenos Aires le 20 octobre. Au cours de ce +voyage, Darwin, qui observe à nouveau des restes de Mammifères terrestres, se +livre à une réflexion fondamentale sur la distribution géographique et les +migrations des animaux, comprenant en particulier le rôle des barrières +physiques dans la délimitation des provinces zoologiques et la diversification +des faunes. + +14 novembre : voyage par terre jusqu’à Mercedes, sur le Río Negro. Observation +des bœufs d’élevage ñatos, handicapés en période sèche par la conformation de +leur museau. Retour le 28 novembre. + +1834, 9 janvier : Puerto San Julián (côte de Patagonie), jusqu’au 19 janvier. +Hypothèses sur les périodes de la formation géologique de la Patagonie. +Découverte, dans la boue rouge recouvrant le gravier de la plaine, à 90 pieds +au-dessus du niveau marin, de la moitié d’un squelette de Macrauchenia +patachonica, quadrupède de la taille du chameau. Coquillages marins récents sur +deux plaines plus élevées. Déduction de l’existence encore plus récente de +Macrauchenia. Réflexion capitale de Darwin, préfigurant les développements +ultérieurs de la théorie de la descendance, sur les relations visibles de +parenté entre les espèces vivantes et fossiles de Mammifères d’Amérique du Sud. +Réflexion très importante également sur les causes de l’extinction relativement +récente des grands animaux retrouvés à l’état fossile. Évocation et rejet de +l’hypothèse d’une catastrophe qui, compte tenu de l’étendue concernée, aurait +dû ébranler le globe d’une façon plus profonde, et qui est contredite en outre +par l’observation du caractère graduel des changements géologiques dans les +régions de La Plata et de la Patagonie. À cet endroit précis du récit de Darwin +(Voyage, 1845, 2e éd.) se trouve la mention sans équivoque de sa lecture de +Malthus, faite en septembre-octobre 1838, et rétroactivement introduite dans ce +passage conjectural sur les causes de l’extinction des espèces. Darwin y évoque +le « frein » nécessaire pour empêcher la multiplication trop rapide de tous les +êtres organisés vivant à l’état naturel : « La provision de nourriture, en +moyenne, demeure constante ; toutefois, chez chaque animal, la tendance à +l’accroissement reproductif est géométrique. » [1] Reste à déterminer, dans la +vie d’une espèce, le moment où un tel frein commence à opérer et sa nature +véritable. « D’où probablement le fait », poursuit Darwin, « que nous +éprouvions si peu de surprise en voyant que, de deux espèces étroitement +proches dans leurs habitudes, l’une est rare et l’autre est abondante au sein +de la même région ». Ce simple énoncé, qui prédit les effets de la concurrence +entre espèces proches vivant sur un même territoire, montre qu’au retour de son +voyage Darwin possédait, compte tenu de sa lecture de Malthus durant l’automne +de 1838, les clés de la future théorie de la sélection naturelle. Quant à +l’extinction, elle est toujours précédée selon lui d’une raréfaction, indice de +conditions d’existence moins favorables pour l’espèce, ce qui plaide en faveur +des processus graduels. C’est également en Patagonie que Darwin observera le +cas de deux espèces de nandous (Rhea americana et Rhea darwinii, plus petit et +qui sera spécifiquement distingué sous ce nom par l’ornithologue John Gould) +habitant respectivement les parties septentrionale et méridionale du +territoire. + +26 janvier : détroit de Magellan. + +17 février : Terre de Feu. + +10 mars : îles Falkland. + +18 avril-8 mai : remontée du Santa Cruz et retour. + +11 juin : entrée dans le Pacifique. + +28 juin : île Chiloé, jusqu’au 14 juillet. + +23 juillet : Valparaiso (Chili), jusqu’au 10 novembre. Voyage au pied des Andes +(14 août-27 septembre). + +21 novembre : Chiloé. Exploration de l’archipel. + +1835, 8février : Valdivia. Excursion jusqu’au 14. Tremblement de terre le 20. +Brutale surélévation du sol sur une vaste distance. Confirmation de l’idée de +la surrection continentale et hypothèse sur la formation des chaînes de +montagnes. + +4 mars : île de Concepción. + +11 mars : Valparaiso. + +13 mars : Darwin se rend à Santiago, d’où il part le 18 pour une traversée des +Andes jusqu’à Mendoza. La vue des terrasses de galets qui s’étendent de part et +d’autre des grandes vallées de la Cordillère confirme sa théorie de l’élévation +graduelle du sol. Retour à Santiago le 10 avril, puis à Valparaiso. Darwin +constate une concordance entre les différenciations spécifiques présentées par +les animaux (notamment les quadrupèdes, et en particulier les souris) et l’âge +de la barrière des Andes. + +27 avril : voyage par terre à Coquimbo et Copiapó. Darwin observe les terrasses +de galets (17 mai). La haute plaine de Coquimbo est emplie de coquilles +fossiles très proches de celles des plages. Excursion dans la Cordillère. Le 4 +juillet, le Beagle rejoint Darwin sur la côte. + +12 juillet : Pérou. + +16 septembre : archipel des Galápagos, jusqu’au 20 octobre. Darwin rapporte de +cet archipel océanique des observations botaniques et zoologiques capitales +pour l’élaboration de sa future théorie transformiste – notamment sur +différentes espèces, qu’il prend alors pour de simples variétés, de « pinsons » +insulaires, sur les iguanes marins et terrestres, et sur les tortues trop +rapidement vues –, observations qu’il n’interprétera que plus tard. + +9 novembre : Archipel Dangereux (Low Islands). Il voit pour la première fois un +récif de corail. Il en étudiera la structure l’année suivante à l’île Keeling. + +15 novembre : Tahiti. + +21 décembre : Nouvelle-Zélande. + +1836, 12 janvier : Sydney (Australie). + +5 février : Hobart Town (Tasmanie). + +1er avril : île Keeling (ou île des Cocos). Il se livre enfin à l’étude +approfondie d’un atoll. + +29 avril : Port-Louis (île Maurice). + +31 mai : cap de Bonne-Espérance. + +8 juillet : île de Sainte-Hélène. + +19 juillet : île de l’Ascension. + +1er août : Bahia (Brésil). + +12-19 août : Pernambouc (Brésil). + +31 août : Porto Praia (archipel du Cap-Vert). + +19-24 septembre : six jours aux Açores. + +2 octobre : arrivée à Falmouth, Angleterre. + +[1mIII. – Conséquences scientifiques du voyage [0m + +Lors de son retour en Angleterre, Darwin, qui n’a cessé de correspondre avec +ses aînés (en particulier Henslow, relais de ses observations auprès des +sociétés savantes, et Sedgwick, heureux de lui prédire un brillant avenir +d’homme de science), possède l’essentiel des éléments qui, une fois +réinterprétés, reliés et mis en ordre, constitueront les points d’ancrage de sa +théorie. Il a confirmé la validité des thèses uniformitaristes en géologie, et +l’a illustrée par l’étude des îles volcaniques, par une intelligence nouvelle +du mécanisme de formation des récifs coralliens, ainsi que par la vérification +de l’alternance compensatoire entre élévation et affaissement du sol. Il a +perçu l’influence universelle des facteurs climatiques. Il a observé et reconnu +les phénomènes de distribution géographique des organismes et noté la fonction +probable des barrières d’isolement extrinsèque. Il a souligné la ressemblance, +sur un même territoire, entre espèces vivantes d’un même genre, et entre +espèces vivantes et fossiles. Il a noté la variation des animaux domestiques +sous l’action de la domestication. Il a étudié l’acclimatation des espèces de +végétaux cultivés. Il a constaté l’action positive et négative, voire +destructrice, de l’Homme sur la nature et ses équilibres initiaux. Il a pris +définitivement conscience des interactions entre les êtres vivants, et des +chaînes alimentaires. Il a réfléchi sur les migrations des organismes et sur +les modes de transport des individus ou des semences. Il a effleuré aux +Galápagos la question des spéciations insulaires et des adaptations aux +conditions locales. Il a éprouvé à la fin de 1838 la nécessité logique de la +concurrence vitale intra- et interspécifique comme mécanisme régulateur +assurant les équilibres populationnels sur des territoires aux dimensions et +aux ressources limitées. Il a observé enfin la diversité des coutumes, des +croyances et des comportements humains (ce qui installera chez lui un puissant +relativisme), il a constaté que les sauvages sont hautement civilisables +(appréciant l’écart vertigineux séparant les Fuégiens gesticulant sur la plage +de ceux, acculturés, que le Beagle raccompagnait sur leur terre natale), et il +a ressenti avec horreur l’esclavage, institutionnalisé, en particulier au +Brésil. + +La publication des résultats de son voyage, qui mobilise plusieurs spécialistes +– Richard Owen (1804-1892) pour les Mammifères fossiles, George Robert +Waterhouse (1810-1888) pour les Mammifères, John Gould (1804-1881) pour les +oiseaux, Leonard Jenyns pour les poissons, Thomas Bell (1792-1880) pour les +reptiles –, ainsi que celle de ses monographies personnelles et de son propre +journal vont désormais occuper Darwin. + +1837 : Darwin, nouveau membre de la Geological Society of London (où il fait +plusieurs communications, ainsi qu’à la Zoological Society), s’installe à +Londres et, tandis que la reine Victoria monte sur le trône d’Angleterre, +prépare la publication de son journal de voyage. L’expertise ornithologique de +Gould a montré que les « pinsons » des Galápagos (Geospizinae) rapportés sur le +Beagle sont en réalité de véritables espèces et non de simples variétés, ce qui +stimule l’interprétation transformiste du jeune naturaliste, lequel commence en +juillet à remplir son premier carnet de notes sur la « transmutation » des +espèces et entame un programme étendu et diversifié de lectures dans l’ensemble +des sciences naturelles et humaines. + +1838 : Darwin travaille sur la zoologie et la géologie du voyage. Élu le 16 +février secrétaire de la Geological Society, il étudie les « routes parallèles +» de Glen Roy (Écosse) et publie une interprétation malheureuse de leur +origine. Il s’attaque à la mise en forme de son travail sur les récifs +coralliens, s’intéresse à la psychologie, aux instincts, à l’expression, au +comportement animal, à la métaphysique et à la morale. + +[1mIV. – L’épisode Malthus [0m + +C’est à cette époque que prend place un événement souvent commenté du +cheminement intellectuel de Darwin : sa lecture de Malthus. Il en rend compte +ainsi dans son Autobiographie : + +« Après mon retour en Angleterre, il m’apparut qu’en suivant l’exemple de Lyell +en géologie et en recueillant tous les faits qui, d’une quelconque manière, +avaient trait à la variation des animaux et des plantes à l’état domestique et +à l’état naturel, on pourrait peut-être jeter quelque lumière sur l’ensemble du +sujet. J’ouvris mon premier carnet de notes en juillet 1837. Je travaillais +suivant les véritables principes baconiens, et, sans aucune théorie, je fis une +récolte massive de faits, plus spécialement tournée vers les productions +domestiques, par le biais de questionnaires imprimés, de conversations avec des +éleveurs et des jardiniers habiles, et de lectures étendues. Quand je vois la +liste des livres de toute sorte que j’ai lus et résumés, y compris la série +entière des journaux et des comptes rendus de sociétés savantes, je suis +surpris de mon industrie. J’aperçus bientôt que la sélection était la clé de +voûte de la réussite de l’homme en matière de production de races utiles +d’animaux et de plantes. Mais la manière dont la sélection pouvait s’appliquer +à des organismes vivant à l’état de nature demeura pendant un bon moment un +mystère pour moi. + +« [3mEn octobre 1838, c’est-à-dire quinze mois après que j’eus commencé mon +enquête systématique, il m’arriva de lire, pour me distraire, l’essai de +Malthus sur la Population ; et comme j’étais bien préparé, du [0m [3mfait +de mes observations prolongées sur les habitudes des animaux et des plantes, à +apprécier la présence universelle de la lutte pour l’existence, je fus soudain +frappé par l’idée que dans ces circonstances, [0m [3mles variations +favorables auraient tendance à être préservées, et les défavorables à être +anéanties. Le résultat de cela serait la formation de nouvelles espèces. +J’avais donc trouvé là, enfin, une théorie [0m [3mpour travailler ; mais +j’étais si anxieux d’éviter les idées préconçues que je décidai de n’en pas +écrire la plus courte esquisse avant un bon moment. [0m» + +La lecture de l’Essay on the Principle of Population (dont la première +publication remonte à 1798) provoque chez Darwin, pour emprunter une image à la +chimie, une sorte de « précipité » théorique. Il n’y découvre pas, absolument, +l’idée de concurrence vitale, déjà approchée par Augustin-Pyramus de Candolle +(1778-1841), qu’il a lu. A fortiori, il n’y découvre pas non plus l’idée de +variation interindividuelle, qui lui est depuis longtemps familière. Il y +découvre davantage peut-être une formalisation des conséquences éliminatoires +de la compétition, bien que, avant Malthus, le RP Joseph Townsend (1739-1816) +en eut formulé une version clairement anticipative, résumée par Nora Barlow – +la petite-fille de Darwin – dans la première Annexe de l’Autobiographie du +naturaliste : + +« [3mDans la sphère de l’histoire sociale, avant que Malthus n’eût acquis +pour ses vues la reconnaissance du public, d’autres que lui observaient comment +la lutte pour l’existence affectait réellement les [0m [3mpopulations. Dans +son Histoire du peuple anglais, Halévy renvoie à un obscur pamphlet au sujet +des lois sur les pauvres, écrit en 1786 par un “bienveillant ami de +l’humanité”. L’écrivain, le RP M. Townsend, [0m [3mblâme les lois sur les +pauvres, car elles préservent le faible aux dépens du fort, avec toutes les +implications qui tiennent au travail de la sélection naturelle. Il utilise +l’analogie des populations de [0m [3mchèvres et de lévriers sur l’île de +Juan Fernández, dont parle Dampier. Les chèvres au début étaient les seules +occupantes et atteignaient un niveau de subsistance en dépit de quelques +maladies et des [0m [3mincursions des corsaires anglais. Puis les Espagnols +mirent sur l’île un couple de lévriers pour exterminer les chèvres et ennuyer +les Anglais. Ces lévriers “augmentèrent à proportion de la quantité de [0m +[3mnourriture qu’ils trouvaient”. » Les chèvres diminuèrent en nombre et se +retirèrent sur les rochers, et un nouvel équilibre s’installa : « Les plus +faibles des deux espèces furent parmi les premiers à payer [0m [3mle tribut +de la nature, les plus actifs et les plus vigoureux préservèrent leur vie. +C’est la quantité de nourriture qui régule l’effectif de l’espèce humaine… le +faible doit dépendre de la libéralité [0m [3mprécaire du fort… [0m» + +Ce que rapporte Nora Barlow, c’est, presque un siècle avant la lettre, la +naissance du « darwinisme social » tel qu’il sera impitoyablement développé par +Herbert Spencer (1820-1903), lui aussi lecteur du pasteur Malthus. La vision +libérale extrême de l’économie et de la société puise un modèle dans la nature +(en l’occurrence dans le règne animal, et plus précisément ici chez des animaux +domestiques retournés à l’état de nature) pour l’exporter dans l’analyse de la +société des hommes afin d’y argumenter en faveur de la naturalité des rapports +de compétition-élimination. + +Or, Darwin fera la démarche inverse : partant de la modélisation malthusienne, +qui concerne les sociétés humaines (distorsion entre croissance géométrique de +la population et croissance seulement arithmétique des ressources), il en +appliquera d’abord les conséquences dynamiques aux règnes végétal et animal, +mais en refusera ensuite l’application aux sociétés humaines civilisées en +retournant contre Malthus, dans La Filiation de l’homme (1871), le principe +même dont ce dernier lui a fait apercevoir l’importance dans la nature. + +Il serait vain de chercher à minimiser l’importance déterminante de la lecture +de Malthus par Darwin. Outre que l’on n’est jamais fondé à mettre en doute +l’aveu spontané d’un emprunt théorique ou conceptuel, un élément factuel vient +confirmer l’importance des structures logiques du malthusianisme dans la +formation de la théorie sélective. Dans son autobiographie publiée en 1905, +l’autre inventeur de la théorie de la descendance modifiée par le moyen de la +sélection naturelle, Alfred Russel Wallace (1823-1913), raconte que souffrant +d’un accès de fièvre au cours d’une expédition en Asie tropicale, il s’est +ressouvenu des Principes de Malthus (il semble confondre les Principes et +l’Essai) et s’est mis à réfléchir sur son analyse si claire des freins positifs +à la croissance : + +« La plupart des animaux se reproduisant bien plus rapidement que l’homme, ces +freins doivent provoquer chaque année des hécatombes animales énormes afin de +contenir dans les limites appropriées le nombre des individus de chaque espèce. +Et la réponse était manifestement : ce sont dans l’ensemble les plus aptes qui +vivent […]. Il m’apparut soudain comme un éclair que ce processus automatique +aurait nécessairement pour effet d’améliorer la race, puisque chaque génération +verrait inévitablement la mort des inférieurs et la survie des supérieurs. » + [2] + +Il est vrai également que Darwin a reconnu par ailleurs la présence de l’idée +de la lutte pour la vie (struggle for life) chez Augustin-Pyramus de Candolle, +Charles Lyell et William Herbert (1778-1847), comme l’indiquent expressément +une lettre à Asa Gray datée du 5 septembre 1857 et le deuxième paragraphe du +troisième chapitre de L’origine. + +Darwin, bien avant Wallace, a ressenti cet « éclair » théorique émanant d’un +auteur qui s’énonçait encore au sein du cadre providentialiste d’une théologie +naturelle construisant une théologie sociale – ces mécanismes de régulation +étant pour Malthus les moyens dont se servirait Dieu pour inciter les hommes à +peupler et à cultiver la Terre. Ayant appliqué ce modèle, dégagé de toute +théologie, à la nature, Darwin le rejettera comme inapplicable dans l’état de +civilisation, pour des raisons à la fois (et indissociablement) évolutives et +éthiques, comme nous le verrons plus loin (chap. VI). Il est radicalement faux +que Darwin ait emprunté à Malthus d’autres éléments (qui seraient de l’ordre +d’une « philosophie » générale de la vie et de la société) que celui de cette +modélisation dont il trouve lui-même immédiatement le véritable champ +d’application : la « nature », et non la société. Il y aura gagné un outil de +travail, ainsi qu’une conscience claire de l’importance des variations +individuelles susceptibles de favoriser certains organismes au sein de +circonstances et d’un milieu donnés. + +[1mV. – Les années d’élaboration [0m + +En 1839, Darwin, qui a été élu le 24 janvier membre de la Royal Society of +London, épouse cinq jours plus tard sa cousine germaine Emma Wedgwood, fille de +son oncle Josiah, laquelle met au monde leur premier enfant (William Erasmus) +le 27 décembre. C’est à cette époque que Darwin se met à observer le +comportement infantile. S’il soutient en février une mauvaise interprétation +(celle de l’origine marine) des terrasses de Glen Roy, qu’il abandonnera plus +tard en faveur de la thèse glaciaire de Louis Agassiz (1807-1873), Darwin +obtient en revanche un grand succès avec la publication, le 15 août, de son +Journal of Researches. Il rencontre le jeune botaniste Joseph Dalton Hooker +(1817-1911), qui sera par la suite son ami et son interlocuteur le plus proche, +avant le départ de ce dernier pour l’Antarctique. Il veille à l’édition des +fascicules de la Zoology, lit des ouvrages sur la génération (Johann Friedrich +Blumenbach [1752-1840]), sur la reproduction animale (Lazzaro Spallanzani +[1729-1799]), sur l’instinct (Algernon Wells), sur l’éthique (James Mackintosh +[1765-1832]), ainsi que le premier volume (1794) de la Zoonomia de son +grand-père Erasmus et le deuxième volume de la Philosophie zoologique de +Lamarck. Entre janvier et mai, il diffuse un questionnaire sur l’élevage des +animaux qui élargit une enquête commencée un an plus tôt. Sa mauvaise santé (il +a contracté la maladie de Chagas en Amérique du Sud) rend son travail lent et +pénible. L’année 1840 sera consacrée malgré cela à poursuivre le travail en +cours sur les espèces et à lire Johannes Müller (1801-1858) sur la physiologie, +Henry Holland (1788-1873) sur la médecine, Erasmus Darwin sur la phytologie, +Spallanzani sur la génération, Charles Bell (1774-1842) sur l’expression, +William Buckland (1784-1856) sur la géologie et John Fleming (1785-1857) sur la +philosophie de la zoologie, ainsi que de nombreux récits de voyages et ouvrages +d’histoire et de littérature. L’année suivante, il s’intéresse aux +développements du Suédois Carl von Linné (1707-1778) sur la philosophie +botanique, tandis qu’il poursuit ses travaux géologiques et commence à publier +sur la pollinisation. + +En 1842, après avoir lu The Botanic Garden et The Temple of Nature de son +grand-père Erasmus, il achève la première esquisse (sketch) de son travail sur +les espèces [3]. Il publie son ouvrage sur les récifs coralliens, The +Structure and Distribution of Coral Reefs, premier volume de la Geology du +Voyage, expliquant la formation des atolls : « Dans un premier temps, écrit à +ce propos Charles Devillers [4], un volcan émergé s’entoure d’un récif +frangeant ; si le fond marin s’abaisse progressivement, la base du récif meurt +peu à peu (les coraux ne pouvant se développer au-dessous de 80 m en raison du +manque de lumière), tandis que sur le sommet s’installent de nouveaux coraux +qui d’une part compensent l’enfoncement et d’autre part élargissent le diamètre +du récif. Si un équilibre se maintient entre enfoncement et exhaussement, le +récif devient peu à peu une barrière séparée de la pointe du volcan par un +chenal. Lorsque, finalement, la montagne disparaîtra sous les eaux, sur son +emplacement subsistera un atoll avec son lagon. En conclusion, barrières et +atolls se développent dans les zones d’affaissement de fonds marins, tandis que +les frangeants occupent des zones stables ou en soulèvement. » + +Le 17 septembre 1842, la famille Darwin, qui verra une petite fille, Mary +Eleanor, naître et mourir entre le 23 de ce même mois et le 16 octobre, +s’établit à Downe, petit village au Sud-Est de Londres. Une autre fille, +Henrietta Emma, naîtra le 25 septembre 1843, année de la mort de Josiah +Wedgwood et du début de l’amitié de Darwin avec Joseph Dalton Hooker. Darwin +relit Paley, réfléchit sur la variation, publie sur l’origine des fleurs +doubles. En 1844, il achève son Essay sur la théorie des espèces (seconde +esquisse), qu’il recommande aux soins de son ami Hooker, craignant que sa +mauvaise santé ne lui interdise de terminer son œuvre. Il publie en mars un +volume sur les îles volcaniques, poursuit et achève son travail sur la géologie +de l’Amérique du Sud, qui paraît fin 1846, plus d’un an après la parution de la +deuxième édition du Voyage. Il s’attaque ensuite à une monographie sur les +Cirripèdes, qui l’occupera jusqu’à sa publication, en 1851 (année de la mort de +sa fille Anne Elizabeth, dite Annie, âgée de 10 ans) et 1854. En 1855 et 1856, +il travaille sur la distribution géographique des organismes, parallèlement aux +recherches de Wallace (qui publie en septembre 1855 dans Annals and Magazine of +Natural History son « Essay on the Law which Has Regulated the Introduction of +New Species ») sur le même sujet. Lyell, au début de l’année 1856, conscient du +risque couru par Darwin de se voir ravir une légitime priorité, l’incite à +publier sa théorie. Darwin prépare alors le texte qui va aboutir, une fois +allégé, à L’Origine des espèces. Après la réception, le 18 juin 1858, d’un +manuscrit de Wallace intitulé On the Tendency of Varieties to Depart +Indefinitely from the Original Type, où l’évolution par sélection naturelle se +trouve nettement thématisée, Darwin, fort de sa réelle antériorité et soutenu +par l’amitié de Charles Lyell, de Joseph Dalton Hooker et de Thomas Henry +Huxley, laisse les deux premiers organiser une communication de textes de +lui-même et de Wallace devant la Linnean Society of London, le 1er juillet +1858. La très grande estime réciproque des deux hommes et leur rigueur morale +exemplaire relativement au partage de la découverte l’ont emporté sur toute +tentation de querelle. Wallace sera désormais un allié solide dans la défense +de la théorie commune. Darwin prépare alors le résumé de son énorme manuscrit +inachevé sur les espèces et le publie le 24 novembre 1859 sous le titre On the +Origin of Species by Means of Natural Selection, or The Preservation of +Favoured Races in the Struggle for Life. Plus de vingt années ont passé depuis +ses premières intuitions. La première édition est épuisée dès sa mise en vente. +Prudent, Darwin accentue, dans la deuxième édition, sa mention du Créateur, +bien que sa pensée ait déjà rompu avec les convictions religieuses et la +théologie naturelle providentialiste qui ont dominé ses années d’apprentissage. +En outre, la publication anonyme du géologue amateur Robert Chambers +(1802-1871), Vestiges of the Natural History of Creation (1844), revendiquant +un évolutionnisme cosmique progressionniste heurtant le dogme, et de type +partiellement lamarckien, avait suscité des réactions parfois violentes dans un +milieu scientifique qui s’en tenait soit au dogme de la Création, soit à sa +réinterprétation en termes d’archétypes par Richard Owen. + +Au cours de cette période qui correspond à la mise au point de la théorie +sélective proprement dite, Darwin et son épouse ont achevé de fonder leur +famille : George Howard, qui s’occupera d’astronomie et de mathématiques (et +prêtera son aide à son père dans ses recherches sur la consanguinité), est né +en 1845 ; Elizabeth en 1847 ; Francis, futur botaniste, qui sera l’élève de +Julius von Sachs (1832-1897) en physiologie des plantes, ainsi que le +collaborateur et biographe de son père, en 1848 ; Leonard, futur militaire un +peu économiste qui se reconvertira dans l’activisme eugéniste galtonien, en +1850 ; Horace, futur ingénieur et constructeur d’instruments scientifiques, en +1851, et Charles Waring, très probablement atteint du « syndrome de Down » +(trisomie 21), qui ne vivra que dix-huit mois, en 1856. Depuis 1839, dix +enfants sont nés et trois sont morts. La pensée et l’œuvre de Darwin ont grandi +dans la douleur, celle de la maladie et celle du deuil, et l’on ne peut +s’empêcher de penser que l’attention extrême qu’il témoignera constamment au +problème des unions consanguines et de leurs conséquences souvent néfastes a +été grandement favorisée par des épreuves personnelles qu’il pouvait attribuer +au fait d’avoir fait de sa cousine germaine, pour laquelle il éprouva toujours +tendresse, reconnaissance et respect, la mère de ses enfants. + +[1mNotes [0m + +[1] Charles Darwin, Voyage d’un naturaliste, 1845, 2e éd. + +[2] Cité par J. Dupâquier dans l’article « Malthus » du Dictionnaire du +darwinisme et de l’évolution, Paris, PUF, 1996. + +[3] Charles Darwin, Esquisse au crayon de ma théorie des espèces (Essai de +1842), vol. 10 des Œuvres complètes de Darwin, Tort (éd.), Travaux de +l’institut Charles-Darwin international, Genève, Slatkine, 2007. + +[4] Charles Devillers, art. « Récifs coralliens » du Dictionnaire du darwinisme +et de l’évolution, Paris, PUF, 1996. + +[1mChapitre II [0m + +[1mL’origine des espèces et la sélection naturelle [0m + +L’idée que les espèces vivantes dérivent les unes des autres à travers des +variations transmises par voie de génération a mis exactement un demi-siècle – +entre la publication de la Philosophie zoologique de Lamarck (1809), ouvrage +qui n’est pas le premier, mais seulement le plus célèbre des écrits +transformistes du naturaliste français, et celle de L’Origine des espèces +(1859) de Darwin – à élaborer les fondements du transformisme moderne. + +[1mI. – Variation domestique et sélection artificielle [0m + +L’observation de la variabilité naturelle des organismes – matérialisée au +niveau le plus simple par les différences interindividuelles – et de la +transmission de variations organiques d’une plus grande amplitude chez des +animaux et des plantes vivant à l’état domestique semble avoir été le point de +départ de la construction par Darwin de sa théorie de la filiation des espèces. + +En effet, si l’on s’en remet, sans entrer ici dans des discussions de détail, +aux indications fournies par Darwin lui-même, la genèse intellectuelle du +concept de sélection naturelle ne peut, pour une part significative, être +véritablement comprise qu’en tenant compte du point d’appui que constituait +l’existence, attestée par ses produits, de la sélection artificielle pratiquée +par les éleveurs et les horticulteurs, capables depuis longtemps, suivant des +recettes empiriques de contrôle de la reproduction, de « créer » de nouvelles « +races » par tri méthodique et consolidation héréditaire de variations +fortuitement apparues au sein de groupes d’individus conspécifiques vivant et +se multipliant dans des conditions domestiques. + +Au début du chapitre IV de L’Origine des espèces, Darwin formule en termes +clairs la question à laquelle ce chapitre clé a pour mission de répondre : « +Est-ce que le principe de la sélection, dont nous avons vu qu’il est si +puissant entre les mains de l’homme, peut s’appliquer au sein de la nature ? » + +Tout commence par la variation, qui affecte l’individu organique vivant dans la +nature ou à l’état domestique. Darwin, après d’autres observateurs, fait +remarquer que dans ce dernier cas la variabilité est plus grande et comme +activée par la domestication elle-même. Un tel fait, régulièrement observable, +est sans nul doute à rapporter au type d’attention ou d’intention – plus ou +moins consciente – qui anime le comportement de l’éleveur ou de l’horticulteur +à l’égard des êtres dont il surveille et oriente la reproduction. En +particulier, le changement intentionnel et répété des conditions de la +captivité, de l’entretien ou de la culture accroît cette variabilité. Mais, +ajoute Darwin, « de pareils changements de conditions ont pu survenir, et +surviennent, dans la nature ». + +La question des causes des variations, des déterminants biologiques intimes de +leur surgissement apparemment fortuit, demeure pour Darwin une énigme. Mais +c’est une énigme repérée, circonscrite à la manière d’une zone de non-savoir +temporaire destinée à être occupée dans l’avenir par la connaissance, jugée +indispensable, des lois de l’hérédité. + +Cette ignorance provisoire du déterminisme secret des variations n’est +d’ailleurs nullement paralysante pour la construction d’une théorie qui +s’appuie essentiellement sur le fait global de la variation, fait d’observation +courante dans l’univers de la domestication et fait d’observation – moins aisée +sans doute, mais tout aussi réelle et, en tout état de cause, certitude +inductive – en milieu naturel. Car, comme l’écrira Darwin en 1868 dans +l’introduction de La Variation des animaux et des plantes à l’état domestique : +« Si les êtres organiques n’avaient pas possédé une tendance inhérente à la +variation, l’homme n’aurait rien pu faire. » + +Par ailleurs, le surgissement d’une variation pose la question de sa +compatibilité avec cet ensemble, à régulation complexe, de composantes +organiques et inorganiques constamment interagissantes que l’on nomme le +milieu, idée enracinée chez Darwin depuis l’époque du voyage. + +[1mII. – La théorie de la variation en 1859 [0m + +Le premier chapitre de L’Origine s’intitule « De la variation des espèces à +l’état domestique » et commence par interroger, dès son premier paragraphe, les +causes probables de la variabilité. Il y est rappelé que la variation est plus +forte, à l’intérieur d’une même variété ou sous-variété, entre les individus de +domestication ancienne qu’entre les individus appartenant à une espèce ou à une +variété naturelle. Quant à cela, Darwin épouse les conclusions d’une +observation depuis longtemps reçue par les naturalistes comme par la conscience +commune, et il est frappant de relever à la fois la simplicité du propos et le +caractère « buffonien » de la première page de l’ouvrage de 1859. Darwin cite +l’opinion d’Andrew Knight (1759-1838) suivant laquelle la variabilité +domestique serait peut-être due à un excès de nourriture, et l’on se souvient +de l’importance du facteur nutritionnel comme source de modifications chez +Buffon [1]. Sans doute est-ce là une approche tactique – à la fois prudente et +rassurante – d’un problème à propos duquel les hypothèses théoriques de Darwin +vont diverger progressivement des idées couramment admises à la faveur des +faits livrés par l’élevage et par l’horticulture. L’affirmation buffonienne et +lamarckienne d’une variation de fait sous l’influence d’un changement de +conditions est toutefois un élément clé de la construction et de l’explication +darwiniennes – un point de départ que nul ne songe sérieusement à remettre en +cause, car reposant sur une réalité constamment accessible à l’observation +directe et sur une longue tradition empirique. + +Outre l’affirmation générale d’un plus fort coefficient de variation (donc +d’une plus forte variabilité) au sein des espèces vivant à l’état domestique, +le premier chapitre de L’Origine développe, sur un mode qui oscille entre +l’assertion et l’hypothèse probable, les propositions suivantes : + +– une longue durée d’exposition (plusieurs générations) à de nouvelles +conditions est généralement nécessaire à la production d’une somme (amount) +appréciable de variations chez les organismes ; + +– une fois ce stade atteint, la variabilité perdure à travers de nombreuses +générations ; + +– la cause la plus fréquente de variabilité réside dans le fait que les +éléments reproductifs – et non les « organes reproducteurs », ainsi que le +voulait en 1862 Clémence Royer (1830-1902), la première traductrice de +L’Origine en français – mâles et femelles (male and female reproductive +elements) ont été affectés avant la conception. Les produits, animaux ou +végétaux, de la domestication, bien que se signalant parfois par une vigueur +physique apparente, souffrent souvent des effets de la réclusion ou de la +culture au niveau de leur aptitude à se reproduire. Le déterminisme de cette +affection du système reproducteur (reproductive system) est complexe, en grande +partie inconnu et variable suivant les organismes ; + +– la même cause détermine la variabilité et la stérilité, « et la variabilité +est la source de tous les plus beaux produits de nos jardins », ajoute Darwin. +Cet énoncé, qui indique une relation éventuellement régulière entre choix +humain d’une valeur culturelle (ici la « beauté », mais ce peut être aussi le « +bien » ou la « civilisation », ainsi qu’on le verra plus loin) et la perte de +certains avantages naturels, est en quelque sorte le modèle du rapport +nature/culture chez Darwin, à l’intérieur duquel l’Homme, suivant une modalité +singulière souvent mésinterprétée, s’inscrit comme espèce à la fois +domesticatrice et domestiquée ; + +– l’effet direct des conditions de vie (conditions of life) est moins important +dans la production des variations que les « lois de reproduction, de croissance +et d’hérédité ». Cela est confirmé par le fait que, bien souvent, des +conditions opposées voient néanmoins se produire des changements identiques +dans les organismes ; + +– l’action directe des conditions ne concerne que quelques variations +(accroissement de taille dû à une augmentation de la nourriture, épaississement +de la fourrure dû au froid, etc.) ; + +– le non-usage habituel d’un organe chez un animal en captivité est générateur +d’un affaiblissement par rapport à l’état naturel de cet organe (oreilles +pendantes chez de nombreux animaux domestiques, poids diminué des os de l’aile +et de la cuisse chez le canard domestique comparé au canard sauvage) ; + +– bon nombre de lois gouvernent la variation. L’une d’entre elles est la loi +dite de la « corrélation de croissance » : la modification d’un caractère d’un +organe au cours de son développement entraîne des conséquences régulières sur +des caractères d’organes différents. Le corollaire en sera que toute « +sélection » par l’Homme devra compter avec « les lois mystérieuses de la +corrélation de croissance » ; + +– seules importent, du point de vue de Darwin, les variations transmises +héréditairement. Au sujet de leur héritabilité, Darwin développe un +raisonnement a fortiori à partir de la transmissibilité des déviations « +monstrueuses » (albinisme, peau épineuse, pilosité. Le grand exemple du +sexdigitisme, développé au siècle précédent par le Français Maupertuis dans sa +Vénus physique, n’est pas évoqué [2]). Le caractère tératologique transmis +sert ici d’indice visible d’une transmission qui s’effectue couramment, sans +être aussi clairement repérable, dans des conditions normales. Darwin insiste +par ailleurs sur l’hérédité sexuelle (hérédité « limitée à un seul sexe ») et +sur l’« hérédité aux âges correspondants de la vie », marquant, comme un vide à +combler, la place d’une science des « lois de l’hérédité » non encore advenue, +mais nécessaire ; + +– en retournant à l’état sauvage, les espèces domestiques reprennent +graduellement, si elles survivent, les caractères de leur type originel. +Maintenues dans les mêmes conditions, elles demeurent semblables à elles-mêmes +et perpétuent leurs caractères acquis qu’elles modifient dans le cas d’un +changement de conditions. On peut en induire l’existence de phénomènes de +variation et de retour (reversion) au type ancestral dans des circonstances +analogues (changement de conditions) au sein de l’état naturel ; + +– les races domestiques appartenant à la même espèce entretiennent entre elles +des différences du même ordre, mais généralement moindres, que celles +présentées par les espèces voisines ou proches alliées du même genre à l’état +de nature ; + +– la variabilité due à la domestication s’étend potentiellement à un très grand +nombre d’espèces sauvages ; + +– la question de l’origine unique ou plurielle de la plupart des espèces +domestiques restera peut-être à jamais indécidable. Toutefois, la multiplicité +extrême des races domestiques, comparée au petit nombre d’espèces sauvages, +tend à favoriser le plus souvent l’hypothèse d’une communauté de souche, à +l’origine de descendances diversement modifiées en fonction des lieux et des +conditions. S’il n’en est pas ainsi, selon Darwin, pour les différentes races +de chiens domestiques, qui descendent probablement de plusieurs espèces +distinctes de Canidés domestiqués ayant produit des variations héréditaires +sous des influences diverses, c’est en revanche le cas des pigeons, qui +proviennent sans doute d’une seule espèce souche, le pigeon de Roche ou Biset +(Columba livia), par le même processus ; + +– la variation à l’état domestique, si elle est bien l’indéniable indice de la +variabilité naturelle des organismes, a pour caractéristique de n’en être pas +toutefois la simple traduction : en effet, la domestication favorise des +adaptations qui ne représentent, pour l’organisme concerné, aucun avantage +propre, mais qui correspondent en fait le plus souvent à un avantage pour +l’Homme domesticateur. La première conclusion claire à laquelle aboutit celui +qui considère le travail des éleveurs, c’est que les variations fournies par la +nature – variations premières dont l’Homme ne décide pas, mais qui, précise +Darwin, peuvent se produire à l’état naturel comme des modifications +avantageuses pour les organismes eux-mêmes – ont été sélectionnées par l’Homme +pour son propre avantage. Cela permet deux hypothèses : + +– soit les variations utiles à l’Homme se sont produites soudainement, en une +seule fois, comme dans les cas du chardon à foulon (issu du Dipsacus sauvage) +et de ses aiguilles, du chien tourne-broche et du mouton ancon, + +– soit les variations premières se sont produites lentement à l’état naturel +comme des modifications utiles aux organismes eux-mêmes, et l’Homme s’est +accoutumé à tirer de ces avantages naturels une utilité propre quelconque qu’il +a ensuite cherché à accroître. + +Il est intéressant de constater ici que cette seconde possibilité, à l’origine +de la variation domestique orientée, implique l’opération, à l’intérieur d’un +exposé didactique qui n’en a pas encore explicité le concept, de la théorie de +la sélection elle-même : la théorie de l’avantage vital, explicative de la +conservation d’une modification utile à l’organisme naturel, appartient au +système logique de la théorie sélective et apparaît ici avant même que ce +dernier ne soit développé dans le corps même de l’exposé. On pourrait ici +conclure, à propos de cette anticipation, à une faute logique ou, tout au +moins, didactique de Darwin ayant besoin, pour expliquer l’origine de la +variation domestique, d’éléments théoriques appartenant à la doctrine sélective +constituée. En fait, cette apparente difficulté expositionnelle est révélatrice +du caractère direct de l’intuition qui associe sur le mode de l’induction +analogique sélection artificielle sélectionnant des variations pour l’avantage +de l’Homme (vérité première d’observation et d’expérience) et sélection +naturelle sélectionnant des variations pour l’avantage des organismes +eux-mêmes. Dans la nature domestiquée, Darwin voit immédiatement et d’abord le +résultat de la sélection ; et comme la nature domestiquée n’a pas pour autant +cessé d’être naturelle, la sélection apparaît immédiatement comme une capacité +de la nature. De même que la variation prouve la variabilité, la sélection +prouve la sélectionnabilité (voir le paragraphe suivant et le diagramme +correspondant à la genèse complexe de la théorie sélective). Ce schème de +pensée, aussi simple soit-il, résout bien des questions inutiles sur +l’avènement réel de l’intuition sélective chez Darwin. La sélection +artificielle est bien le seul modèle initial qui, dans le cadre de l’analyse +des phénomènes de domestication et de la comparaison entre le domestiqué et le +sauvage, peut permettre d’anticiper d’une manière hypothétiquement cohérente +les conclusions de la théorie de la sélection naturelle, dont elle sert ainsi +très légitimement à construire l’exposé. + +La démarche de Darwin dans l’exposition de sa théorie de la variation inclut +donc le fait et les inductions de la sélection artificielle : + +– les organismes varient à l’état domestique sous l’influence d’un changement +de conditions et de l’action canalisante (choix continué) des éleveurs à partir +d’une variation initiale apparue dans la nature ; + +– l’action sélective des éleveurs (sélection artificielle) fait elle-même +partie d’un changement de conditions ; + +– cette action sélective s’exerce sur des organismes (donc sur une nature) +susceptibles de varier et variant effectivement dans le monde naturel ; + +– cette action révèle que la variation peut être avantageuse à ceux qui la +mettent en œuvre ; + +– la variation avantageuse à l’état domestique (avantage pour l’Homme) induit +l’hypothèse d’une variabilité avantageuse à l’état naturel (avantage pour +l’organisme). + +La variation sélectionnée par les éleveurs est le plus souvent presque +imperceptible et doit être « accumulée » dans le même sens au cours d’une +longue durée pour produire les améliorations souhaitées. En horticulture, elle +est plus fréquemment susceptible d’être soudaine, et fixée immédiatement, mais +elle requiert dans tous les autres cas un travail de sélection long et +attentif. La variation enfin, du fait de la sélection artificielle, peut porter +sur un seul organe (fleurs chez les plantes d’ornement, feuilles ou tubercules +chez les plantes potagères). + +Dès le premier chapitre de L’Origine, la variation à l’état de nature est +induite. Le deuxième chapitre, qui en traite, n’aura qu’à l’illustrer. + +[1mIII. – Taux reproductifs et équilibres du vivant : nécessité d’une +régulation par la lutte [0m + +La deuxième idée fondamentale de Darwin est celle de la « lutte pour +l’existence » (struggle for life), c’est-à-dire du combat que doit mener pour +sa survie chaque être vivant contre l’ensemble des obstacles que lui oppose son +milieu naturel. Par une sorte d’habitude simplificatrice, on tend souvent à +identifier la lutte pour l’existence, qui concerne d’une façon globale +l’exercice de survie de chaque individu ou groupe face aux pressions de +l’environnement naturel en général (un organisme ou un groupe peut lutter +contre la sécheresse – facteur inorganique – ou contre un prédateur – facteur +organique –, etc.), et la concurrence vitale, qui concerne plus spécialement la +lutte que se livrent, à l’intérieur d’un milieu donné, les organismes ou +groupes d’organismes entre eux. L’idée de lutte pour l’existence, qui comprend +donc celle de concurrence vitale, serait due, suivant ce qu’en dit Darwin +lui-même [3], à Malthus, ce que tendrait à confirmer l’emprunt qu’il fait, +dans son exposé, du concept d’accroissement géométrique qui était à l’œuvre +dans l’Essai sur le principe de population de 1798 : « Une lutte pour +l’existence, écrit Darwin, est la conséquence inévitable du taux élevé suivant +lequel tous les êtres organiques tendent à s’accroître. Chaque être, produisant +au cours de sa durée de vie naturelle plusieurs œufs ou graines, doit, durant +une certaine période de sa vie, et durant une certaine saison ou, +occasionnellement, une année, être soumis à la destruction, car autrement, en +vertu du principe de l’accroissement géométrique, ses effectifs grossiraient +bientôt si extraordinairement qu’aucun territoire n’en pourrait entretenir la +production. Dès lors, puisqu’il se produit plus d’individus qu’il n’en peut +survivre, il faut, dans tous les cas, qu’il y ait une lutte pour l’existence +soit entre individus de la même espèce, soit entre individus d’espèces +distinctes, ou encore avec les conditions de vie physiques. C’est la doctrine +de Malthus appliquée avec une force variée à l’ensemble des règnes animal et +végétal… » (chap. III). + +On se souvient que Joseph Townsend était parvenu dès 1786, dans une +dissertation à propos de la loi sur les pauvres, à des conclusions très +voisines de celles de Malthus et intéressantes pour Darwin (qui semble +toutefois n’avoir pas eu directement connaissance de ce texte) à cause de +l’exemple analogique didactiquement emprunté au cas des chèvres et des chiens +(leurs prédateurs) vivant sur l’île de Juan Fernández, dont les équilibres +démographiques se sont régulés suivant la loi du triomphe des mieux armés dans +la lutte pour l’appropriation de la nourriture. + +Quoi qu’il en soit de l’occurrence simplement événementielle, dans la pensée de +Darwin, de cette inspiration théorique par ailleurs indéniable, le socle de +l’édifice doctrinal darwinien est posé, sans que Darwin ait jamais approuvé par +ailleurs le courant idéologique dont participaient ensemble Townsend et +Malthus, non plus que les attitudes politico-sociales qui trouvaient en lui le +dogme d’une nouvelle religion. + +Au départ, donc, deux faits d’observation : la variation des organismes et leur +capacité reproductive, et deux certitudes, l’une inductive, l’autre déductive : +la capacité indéfinie de variation des organismes et la capacité de +surpeuplement, tendant naturellement à la saturation de tout espace de vie par +n’importe quelle catégorie d’organismes se reproduisant sans entrave et +entraînant de ce fait la nécessité d’une lutte éliminatoire. À quoi il faut +ajouter – rappelons-le –, dès les commencements de la réflexion de Darwin, le +fait empirique de la sélection artificielle et l’aptitude, qui s’en induit, des +organismes à être sélectionnés par tri cumulatif de variations et fixation +héréditaire. À chaque étape de l’avancée de Darwin, une capacité est inférée +d’un fait d’observation universellement constaté et admis. Pour que le tableau +soit complet, il faut également rappeler la présence, à l’autre pôle de ce +réseau de données et assurant sa cohérence, d’un autre fait d’observation +courante, presque trivial celui-là : c’est l’existence même des équilibres +naturels. En effet, on observe dans la plupart des lieux de la nature la +coexistence de représentants de plusieurs espèces vivantes sur un même +territoire. Ce simple fait, qui contredit d’une manière universelle la tendance +intrinsèque de chaque population d’organismes conspécifiques à se reproduire et +à s’étendre sans limites, est lui-même l’élément qui requiert, pour sa propre +explication, l’idée de la nécessité d’un mécanisme régulateur immanent, +responsable des limitations et des stabilisations relatives dans les +proportions numériques des représentants des diverses espèces, c’est-à-dire +responsable des éliminations nécessaires au maintien d’un certain pluralisme de +vie, indispensable lui-même à la survie de chaque espèce. C’est alors que +s’impose véritablement l’idée de la lutte pour l’existence, idée qui, +confrontée au fait fondamental de la variation et à l’induction de la +variabilité indéfinie, entraîne l’hypothèse d’une sélection de variations – +cette hypothèse répondant elle-même à la question qui porte inévitablement sur +les raisons du triomphe de certaines espèces comme de certains individus dans +cette lutte –, et cette réponse étant à son tour très probablement guidée par +l’incontestable pression analogique qu’exerçait alors la connaissance des +procédés empiriques de la sélection artificielle. + +Cette démarche peut se résumer en dix points : + +On observe des variations individuelles chez les êtres soumis à la +domestication ou vivant à l’état naturel. + +On en induit l’existence d’une capacité naturelle indéfinie de variation des +organismes (variabilité). + +On observe qu’une reproduction orientée peut fixer héréditairement certaines de +ces variations (avantageuses pour l’Homme) par accumulation dans un sens +déterminé, avec ou sans projet raisonné ou méthodique (sélection artificielle, +sélection inconsciente). + +On en induit l’hypothèse d’une aptitude des organismes à être sélectionnés +d’une manière analogue au sein de la nature (« sélectionnabilité »). Question : +quel peut être l’agent de la « sélection naturelle » ainsi inférée de cette « +sélectionnabilité » avérée (par ses actualisations domestiques) des variations +organiques ? + +On évalue le taux de reproduction des diverses espèces et leur capacité de +peuplement. + +On en déduit l’existence d’une capacité naturelle d’occupation totale rapide de +tout territoire par les représentants d’une seule espèce, animale ou végétale, +se reproduisant sans obstacle. + +On observe cependant à peu près universellement, au lieu de cette saturation, +l’existence d’équilibres naturels constitués par la coexistence, sur un même +territoire, de représentants de multiples espèces. + +On déduit de l’opposition entre les points 6 et 7 la nécessité d’un mécanisme +régulateur opérant au sein de la nature et réduisant l’extension numérique de +chaque population. Un tel mécanisme est nécessairement éliminatoire et s’oppose +par la destruction à la tendance naturelle de chaque groupe d’organismes à la +prolifération illimitée. C’est la lutte pour la vie (struggle for life) qui +effectue une sélection naturelle dont le principal effet est la survie des plus +aptes (par le jeu de l’élimination des moins aptes). Question : qu’est-ce qui +détermine une meilleure adaptation ? + +On observe la lutte pour l’existence au sein de la nature (dans le cas de l’île +de Juan Fernández, où il s’agit d’un retour organisé d’animaux domestiques à +l’état naturel, l’intervention humaine peut faire considérer l’anecdote comme +une confirmation expérimentale, avant la lettre, de la théorie darwinienne en +tant qu’application à la nature de la doctrine de Malthus). + +Pour répondre à la question des facteurs d’une meilleure adaptation, on fait +retour à la variabilité, et, sous la pression analogique du modèle de la +sélection artificielle, on forge l’hypothèse d’une sélection naturelle qui, à +travers la lutte (interindividuelle, interspécifique et avec le milieu), +effectuerait le tri et la préservation des variations avantageuses dans un +contexte donné et assurerait ainsi le triomphe vital, transmissible +héréditairement, des individus qui en seraient porteurs. Ces derniers seraient +par là même sur la voie d’une amélioration constante de leur adaptation à leurs +conditions de vie et à celles de la lutte : « Ce principe de préservation, ou +Survie des plus aptes, écrit Darwin, je l’ai nommé Sélection naturelle » +(L’Origine des espèces, chap. IV). + +Tels sont les principaux moments logiques, depuis le fait empiriquement observé +et orienté de la variation des organismes (phénomène essentiellement +individuel) jusqu’à la formulation de la théorie de la sélection naturelle (qui +étend l’effectif des organismes porteurs de la variation avantageuse), de la +réflexion transformiste de Darwin. Cette logique ici reconstituée reproduit +l’ordre suivi par les premiers chapitres de L’Origine des espèces (I. « +Variation sous l’influence de la domestication » ; II. « Variation dans la +nature » ; III. « La lutte pour l’existence » ; IV. « Sélection naturelle ou +Survie des plus aptes »), tel qu’il se trouve exposé dans l’introduction de +l’ouvrage : + +« Nous verrons ainsi [4] qu’une grande accumulation de modifications +héréditaires est au moins possible ; et nous verrons, ce qui est tout aussi +important, voire davantage, combien est grand le pouvoir qu’a l’homme +d’accumuler par sa sélection de légères variations successives. Je passerai +ensuite à la variabilité des espèces à l’état de nature […] Dans le chapitre +suivant, on examinera la lutte pour l’existence entre tous les êtres organiques +à travers le monde, conséquence inévitable de la forte raison géométrique de +leur accroissement. C’est la doctrine de Malthus appliquée à l’ensemble des +règnes animal et végétal. Comme il naît beaucoup plus d’individus de chaque +espèce qu’il n’en peut survivre ; et comme, en conséquence, il y a une action +périodique fréquente de la Lutte pour l’Existence, il s’ensuit que tout être +qui, soumis aux conditions de vie complexes et parfois variables, aura varié +légèrement d’une quelconque manière qui lui soit profitable, aura une meilleure +chance de survivre et d’être ainsi naturellement sélectionné. En vertu du +puissant principe de l’hérédité, toute variété sélectionnée tendra à propager +sa forme modifiée et nouvelle. Ce point fondamental de la sélection naturelle +sera traité avec quelque étendue dans le quatrième chapitre ; et nous verrons +alors comment la sélection naturelle cause presque inévitablement beaucoup +d’extinctions dans les formes de vie les moins perfectionnées et conduit à ce +que j’ai nommé Divergence de Caractère. » [5] + +[1mIV. – Succès, polémiques et nouveaux développements [0m + +La première édition de L’Origine des espèces (1 250 exemplaires) est épuisée le +jour même de sa parution, le 24 novembre 1859. Huxley en fait, le mois suivant, +une recension enthousiaste dans le Times. + +La deuxième édition (3 000 exemplaires) sort des presses six semaines après la +première, le 7 janvier 1860, porteuse de quelques modifications légères et de +l’adjonction d’une mention finale concernant le « Créateur » destinée à calmer +les inquiétudes des esprits religieux. Malgré cela, le débat qui s’ouvre prend +très vite une ampleur exceptionnelle : bataille de comptes rendus, +revendication d’antériorité, discussions théologiques, réunion houleuse, enfin, +de la British Association for the Advancement of Science à Oxford (27 juin-4 +juillet 1860) qui voit s’affronter T. H. Huxley (que l’on appellera par la +suite le « bulldog de Darwin ») et le puissant Richard Owen, puis le même +Huxley et l’évêque Samuel Wilberforce (1805-1873), coupable d’une apostrophe +célèbre dans laquelle il demande à son interlocuteur s’il s’apparente au singe +par sa grand-mère ou par son grand-père. La question de l’évêque et la réponse +à la fois sérieuse et méprisante de Huxley – concluant qu’il préférait avoir un +singe pour ancêtre qu’un homme capable, en cette matière, de telles +plaisanteries – donneront lieu à autant de versions variées qu’il y aura de +narrateurs témoignant de l’âpreté des débats entre conservatisme et +progressisme théologiques, et de la lutte entre deux générations d’acteurs +scientifiques intéressés les uns à maintenir, les autres à bouleverser la « +politique » de la science. + +Le 3 avril 1861 paraît la troisième édition de L’Origine (2 000 exemplaires), +modifiée en fonction des critiques et augmentée d’une notice historique +(Historical Sketch) concernant les auteurs que Darwin accepte de reconnaître +comme étant, à certains égards, ses prédécesseurs. Leur nombre, leur célébrité +et leur diversité (de Buffon, Lamarck, Goethe et Étienne Geoffroy Saint-Hilaire +pour les prédécesseurs « réels » à A. R. Wallace, T. H. Huxley et J. D. Hooker +pour les contemporains « sérieux ») ont pour effet principal de rendre +ridicules les prétentions d’un Patrick Matthew (1790-1874), obscur auteur en +1831 d’un ouvrage sur les bois de marine, auxquelles Darwin a cependant résolu, +formellement, de faire droit. Il usera d’une ironie plus directe dans le Sketch +de la quatrième édition (1866) en mentionnant Richard Owen, dont l’irrite +l’étonnant et rapide revirement à l’égard de sa théorie. Owen était en effet +passé en peu de temps d’une opposition idéologiquement compréhensible à une +revendication de précursion peu convaincante, si ce n’est par le biais tortueux +de ses rapports avec la « philosophie de la nature » allemande, dont Darwin, +intentionnellement prodigue, cite également dans sa liste l’un des +représentants majeurs, Lorenz Oken (1779-1851). + +Dès le 9 janvier 1861, Darwin a entamé la réalisation de ce qui deviendra son +deuxième grand ouvrage de synthèse, The Variation of Animals and Plants under +Domestication. Il intensifie ses recherches dans le domaine botanique. Son +ouvrage sur la fécondation des orchidées par les insectes paraît le 15 mai de +l’année suivante et porte un coup sérieux au finalisme des explications +physico-théologiques des phénomènes adaptatifs et coadaptatifs. Il poursuit la +rédaction de La Variation, dont il achève en 1863 plusieurs chapitres +importants sur l’hérédité, les croisements, la sélection. En 1864, il travaille +sur les plantes grimpantes et reçoit le 30 novembre la médaille Copley de la +Royal Society of London. L’année 1865 voit la publication de son travail sur +les plantes grimpantes, la rédaction du chapitre de La Variation sur la +pangenèse (théorie conjecturale sur les mécanismes de l’hérédité) et le début +de sa correspondance avec l’un de ses plus brillants admirateurs, Fritz Müller +(1822-1897), émigré au Brésil, et dont il lit avec une très grande satisfaction +l’ouvrage Für Darwin (« Pour Darwin »), paru à Leipzig en 1864. + +En décembre 1866 sort des presses la quatrième édition de L’Origine (1 500 +exemplaires), remaniée en fonction de critiques récentes, de faits nouveaux et +de développements théoriques et naturalistes inédits. En 1867, Darwin +s’oriente vers l’étude de l’Homme et diffuse un questionnaire sur l’expression +qui servira de base documentaire à l’ouvrage de 1872, L’Expression des émotions +chez l’homme et les animaux. + +Le 30 janvier 1868 paraît The Variation of Animals and Plants under +Domestication. + +[1mNotes [0m + +[1] Buffon, Histoire des animaux, t. III, 2e partie, chap. III : « De la +nutrition et du développement ». + +[2] Pierre-Louis Moreau de Maupertuis (1698-1759), savant et philosophe +français, fut en 1732 l’introducteur en France de la cosmologie de Newton et +des « lois de l’attraction », dont il s’inspira pour imaginer une nouvelle +théorie de la génération des êtres vivants fondée sur l’attraction réciproque +des particules issues du père et de la mère, qu’il exposa en 1745 dans sa Vénus +physique (Tort(éd.), Aubier, 1980). En ce qui concerne la génération, cette +inspiration newtonienne est aussi, à la même époque, celle de Georges-Louis +Leclerc de Buffon (1707-1788). On la retrouvera en 1868 chez Darwin, au chap. +XXVII de La Variation. + +[3] 3L’Origine des espèces, introd. et chap. III. + +[4] Dans le premier chapitre. + +[5] Ibid., introd. + +[1mChapitre III [0m + +[1mIllustrations et extensions naturalistes de la théorie [0m + +[1mI. – La variation des animaux et des plantes à l’état domestique (1868) +[0m + +Dans ce deuxième grand ouvrage de synthèse, étroitement lié dans sa genèse au « +grand livre sur les espèces » dont L’Origine n’était elle-même qu’un résumé, +Darwin réaffirme et illustre avec netteté les principes exposés dans les deux +premiers chapitres de l’ouvrage de 1859. + +L’idée principale reste que la domestication humaine stimule chez les êtres +vivants, du fait de leur transplantation et des changements introduits dans +leur alimentation, des variations qui prouvent l’existence d’une aptitude +naturelle à varier, d’une variabilité inhérente à la vie même des organismes. + +L’Homme, écrit Darwin, a certes le pouvoir de jeter un morceau de fer dans de +l’acide sulfurique, mais nul n’est en droit d’en conclure qu’il fait du sulfate +de fer. + +Son rôle se borne à mettre en présence certaines affinités qui se manifestent +dans une réaction dont il n’est pas le maître. Il en va de même pour la +domestication qui ne fait que révéler une tendance inhérente des organismes à +varier suivant leurs conditions d’existence, ce qui n’a pu manquer de se +produire également dans l’état naturel et d’une manière tout aussi +transmissible. De fait, l’introduction de La Variation est un simple résumé des +conclusions des deux premiers chapitres de L’Origine, résumé annonçant leur +illustration par des séries de cas finement analysés et décrits : rôle de +l’action directe du climat et de la nourriture, effets de l’usage et du +non-usage, lois de l’hérédité, effets du croisement sur la stérilité en +condition domestique, effets de la répétition des croisements consanguins, +importance du principe de sélection, sélection méthodique et sélection +inconsciente, adaptation des races domestiques aux besoins humains (justifiant +leurs fréquentes anomalies). Darwin renvoie à un « autre ouvrage » [1] le +complément du programme extrait de L’Origine : variabilité à l’état naturel, +différences individuelles, différences intervariétales (les « variétés » étant +des « races géographiques ») ; difficulté de distinguer entre races et +sous-espèces, entre sous-espèces et espèces ; variation plus accentuée chez les +espèces dominantes, nombre plus grand d’espèces variables dans les grands +genres ; variétés, enfin, considérées comme « espèces naissantes ». + +Au cours de cette même Introduction, qui présente une homogénéité frappante +avec L’Origine, Darwin réévoque le voyage sur le Beagle d’une façon qui ne peut +plus laisser de doute sur l’importance respective et partagée des facteurs +qu’il désigne comme responsables de son intuition théorique majeure. Dans +l’archipel des Galápagos, à environ 500 milles des côtes de l’Amérique du Sud, +Darwin s’est vu environné d’espèces d’oiseaux, de reptiles et de plantes +particulières, inconnues ailleurs, et offrant cependant un faciès américain. +Les habitants des multiples îles, quoique visiblement apparentés, présentaient +entre eux des différences dont beaucoup, à l’examen, se révélèrent spécifiques. +L’observation répétée de ce phénomène suggérait l’idée d’une descendance +commune avec modifications, à partir d’une origine continentale. Revenant sur +les résultats expertisés du voyage, Darwin s’était interrogé sur le processus +de ces différentes spéciations. Comment ces modifications avaient-elles pu +avoir lieu ? C’est en ce point précis de sa remémoration que Darwin avoue que +ce phénomène fut pour lui pendant longtemps inexplicable et le fut resté s’il +n’avait étudié les animaux domestiques et acquis de cette manière une idée +nette de la sélection. Ce n’est que plus tard – toujours d’après ce récit – que +la lecture de Malthus (septembre 1838) allait lui permettre de lier +augmentation du nombre des êtres organisés, lutte pour l’existence et sélection +naturelle. + +Ainsi, dans l’œuvre darwinienne, La Variation apparaît d’une manière générale +comme une immense corroboration des thèses de L’Origine au moyen d’une +compilation systématique et ordonnée de faits d’observation et d’informations +lues ou recueillies auprès d’une multitude de correspondants. C’est dans ce +texte que Darwin développe l’exemple des pigeons qu’il connaissait par le +témoignage d’un grand nombre de spécialistes anglais et étrangers, ainsi que +par son expérience personnelle d’éleveur. + +[1mII. – La théorie de la pangenèse [0m + +C’est dans la dernière partie de La Variation que Darwin, tentant de substituer +à l’ignorance générale en ce domaine un essai conjectural d’explication des +mécanismes de la génération et de la transmission héréditaire, formule son « +hypothèse provisoire [provisional hypothesis] de la pangenèse ». De l’ensemble +des parties du corps proviendraient de très fines particules (les « gemmules ») +représentatives de chacune d’elles et qui seraient adressées aux organes de la +reproduction, pour s’unir enfin, lors de la fécondation aux gemmules issues du +partenaire. On reconnaît évidemment là l’influence des premières hypothèses +formulées par les naturalistes (en particulier Buffon et Maupertuis) concernant +une génération de type particulaire, dans le sillage newtonien (car requérant +une force d’attraction ou d’affinité entre les particules s’assemblant pour la +formation du nouvel être). + +Cette hypothèse d’attente, qui permettait notamment une explication théorique +de la transmission de caractères acquis, ne connaîtra naturellement aucune +validation expérimentale (Galton en particulier échouera dans cette tentative), +mais permettra de construire d’autres hypothèses (notamment celle de la « +pangenèse intracellulaire » de Hugo De Vries) qui pourront, à l’extrême fin du +xixe siècle, favoriser la redécouverte des lois de Mendel. + +Parallèlement, Darwin a entamé en février la rédaction de son travail sur la +théorie de la descendance et l’humanité. L’année 1869 voit paraître la +cinquième édition de L’Origine (2 000 exemplaires), à nouveau modifiée en +fonction des critiques. + +Le 24 février 1871, il publie enfin l’ouvrage, depuis longtemps sollicité par +ses partisans, qui devait étendre à l’Homme la théorie de la descendance et qui +sera sans nul doute, entre tous, son ouvrage le plus gravement mésinterprété : +The Descent of Man. + +[1mNotes [0m + +[1] Il s’agit du « grand livre sur les espèces », commencé en 1856 et qui ne +fut jamais achevé, mais qui fournit beaucoup de ses thèmes et de sa +documentation aux principaux ouvrages publiés par Darwin. + +[1mChapitre IV [0m + +[1mLa filiation de l’homme et la sélection sexuelle [0m + +The Descent of Man and Selection in Relation to Sex, troisième grand ouvrage de +synthèse de Darwin, a été introduit en France à travers une première traduction +de Jean-Jacques Moulinié (1872), où Descent – qui signifie le fait de « +descendre de », d’être issu d’une souche ou d’une lignée, de provenir d’une +origine, de procéder d’une série d’ancêtres, de représenter le point +d’aboutissement actuel d’une généalogie, bref, d’avoir une ascendance – est +rendu par « descendance », dont l’usage en français, dans un tel registre, est +rare et contesté. Nous donnons dans l’article « Descendance » du Dictionnaire +du darwinisme les raisons sémantiques qui nous font préférer pour cette +traduction le terme de « filiation » entendu dans son acception juridique – +établir la filiation de quelqu’un consistant à authentifier son ascendance en +remontant le long du lien (de « descendance ») qui unit jusqu’à lui des +individus directement issus les uns des autres par un acte de génération. +L’usage s’étant toutefois longtemps imposé, dans le contexte francophone, du +terme malaisé de « descendance », on le trouve encore, mais de plus en plus +rarement, sous la plume des commentateurs de Darwin qui se sont nourris des +anciennes traductions. + +Si l’on mesure dans toute son ampleur le choc produit par L’Origine des +espèces, déjà largement diffusée à ce moment aux États-Unis et sur le continent +européen, on pourra évaluer l’intérêt que pouvait susciter, en 1871, un ouvrage +attendu et présenté comme l’extension à l’Homme de la théorie de la descendance +avec modifications, et donc comme l’émancipation définitive du discours +naturaliste par rapport au plus résistant des interdits théologiques : celui +qui tendait à préserver ultimement l’humanité de son inscription au sein de la +série animale. Mais l’ouvrage était en même temps ressenti comme devant être, +très logiquement, le lieu de l’extension de la théorie sélective à l’homme et +aux sociétés humaines. L’enjeu scientifique d’un tel livre apparaissait dès +lors comme indissociable d’enjeux philosophiques et politiques déterminants au +cœur d’une époque d’expansion et de consolidation des emprises coloniales, et +dans une société en restructuration traversée par les luttes sociales – théâtre +d’un conflit non seulement entre conservatisme et libéralisme, mais aussi bien +entre des versions différentes du libéralisme conquérant. + +[1mI. – Le transformisme darwinien étendu à l’Homme [0m + +« L’unique objet de cet ouvrage, écrit Darwin, est de considérer, premièrement, +si l’homme, comme toute autre espèce, est issu par filiation de quelque forme +préexistante ; deuxièmement, le mode de son développement ; et, troisièmement, +la valeur des différences entre ce que l’on appelle les races de l’Homme. » + +Le premier acte de la démonstration de Darwin consiste à dresser la liste des +phénomènes de ressemblance qui, selon lui, rendent indiscutable le lien qu’il +veut établir entre la constitution anatomo-physiologique de l’Homme et celle +des autres membres du groupe des Vertébrés. Ses arguments, empruntés d’abord à +l’anatomie comparée, et particulièrement à Huxley, sont déjà classiques : +identité de conformation du squelette, des muscles, des nerfs, des vaisseaux, +des viscères et même de l’encéphale lorsqu’il s’agit des signes supérieurs ; +communicabilité réciproque de certaines maladies entre les animaux – les singes +surtout – et l’Homme ; parenté entre les parasites qui affectent les Hommes et +les animaux ; analogie également entre les processus qui, chez les uns et les +autres, suivent les phases de la lune, entre les phénomènes cicatriciels, entre +les comportements reproducteurs, entre les différences qui séparent les +générations et les sexes, entre les stades et les mécanismes du développement +embryonnaire, singulièrement lorsque l’on observe la parturition des singes ; +communauté de la détention d’organes rudimentaires ; existence d’un revêtement +laineux (lanugo) chez le fœtus humain au sixième mois ; traces persistantes, +chez l’Homme, à l’extrémité inférieure de l’humérus, du foramen +supracondyloïde, ouverture par laquelle passent, chez « quelques quadrumanes, +les Lémuridés et surtout les Carnivores aussi bien que beaucoup de Marsupiaux +», le « grand nerf de l’avant-bras et souvent son artère principale », etc. + +Mais les données mises en œuvre par la grande somme compilatoire et +illustrative que constitue The Descent excèdent considérablement les seuls +domaines de l’anatomie et de la physiologie comparées. Celles que Darwin +emprunte également à l’anthropologie physique, à l’anthropométrie, à +l’observation du comportement humain et à l’étude des sociétés « civilisées » +et des cultures exotiques (dont certaines remontent à sa propre expérience de +voyageur) lui fournissent les éléments qui lui permettent de mettre en évidence +le fait que la variabilité, prouvée chez l’Homme sur le terrain de l’anatomie, +l’est également sur les plans raciologique et sociologique, et que, sous des +modalités qui n’ont été, hélas, convenablement analysées que bien tard, la +sélection se poursuit au sein de l’humanité. + +[1mII. – L’effet réversif de l’évolution [0m + +Darwin se livre donc, dans La Filiation, à un essai – inévitable du point de +vue de la cohérence et de la portée de sa théorie – d’unification de l’ensemble +des phénomènes biologiques et humains sous l’opération d’un seul principe +d’explication du devenir : ce dernier dérive très normalement des sciences qui +viennent d’être énumérées, Darwin parcourant leurs différents domaines pour +aboutir sans heurt au champ de ce que l’on nommerait aujourd’hui +l’anthropologie sociale, ainsi qu’à des observations psychosociologiques et +éthiques qui, pour être spécifiquement humaines, n’en sont pas moins +évolutivement liées à des données et à des conduites dont l’analyse tend à +faire apparaître l’origine au sein des groupes animaux. + +Or, contrairement aux interprétations qui ont dominé pendant plus d’un siècle +la lecture (en réalité, dans la plupart des cas, la non-lecture) du texte de La +Filiation, ce continuisme ne fonde ni ce que l’on a appelé d’une manière +expéditive le « darwinisme social », présent au contraire chez Herbert Spencer +et Ernst Haeckel ni, sous le motif de la « poursuite de la sélection », aucune +forme ultérieure d’inégalitarisme social ou racial. + +En effet, La Filiation établit qu’un renversement s’est opéré, chez l’Homme, à +mesure que s’avançait le processus civilisationnel. La marche conjointe du +progrès (sélectionné) de la rationalité et du développement (également +sélectionné) des instincts sociaux, l’accroissement corrélatif du sentiment de +sympathie, l’essor des sentiments moraux en général et de l’ensemble des +conduites et des institutions qui caractérisent la vie individuelle et +l’organisation communautaire dans une nation civilisée permettent à Darwin de +constater que la sélection naturelle n’est plus, à ce stade de l’évolution, la +force principale qui gouverne le devenir des groupes humains, mais qu’elle a +laissé place dans ce rôle à l’éducation. Or, cette dernière dote les individus +et la nation de principes et de comportements qui s’opposent, précisément, aux +effets anciennement éliminatoires de la sélection naturelle et qui orientent à +l’inverse une partie de l’activité sociale vers la protection et la sauvegarde +des faibles de corps et d’esprit, aussi bien que vers l’assistance aux +déshérités. La sélection naturelle a ainsi sélectionné les instincts sociaux +qui à leur tour ont développé des comportements et favorisé des dispositions +éthiques ainsi que des dispositifs institutionnels et légaux antisélectifs et +antiéliminatoires. Ce faisant, la sélection naturelle a travaillé à son propre +déclin (sous la forme éliminatoire qu’elle revêtait dans la sphère +infracivilisationnelle), en suivant le modèle même de l’évolution sélective – +le dépérissement de l’ancienne forme et le développement substitué d’une forme +nouvelle : en l’occurrence, une compétition dont les fins sont de plus en plus +la moralité, l’altruisme et les valeurs de l’intelligence et de l’éducation. +Sans rupture, Darwin, à travers cette dialectique évolutive qui passe par un +renversement progressif que nous avons nommé l’effet réversif de l’évolution, +installe toutefois dans le devenir, entre biologie et civilisation, un effet de +rupture qui interdit que l’on puisse rendre son anthropologie responsable d’une +quelconque dérive en direction des désastreuses « sociologies biologiques ». +Cette remarquable dialectique du biologique et du social, qui se construit pour +l’essentiel entre les chapitres III, IV, V et XXI de La Filiation et qui, en +plus de s’opposer à toutes les conduites oppressives, préserve l’indépendance +des sciences sociales en même temps qu’elle autorise et même requiert le +matérialisme éthique déductible d’une généalogie scientifique de la morale, n’a +été reconnue dans toute sa force logique qu’à partir du début des années 1980. +Le continuum biologico- social darwinien, dont une bonne métaphore didactique +est l’image topologique de la torsion du ruban de Möbius [1], est un continuum +réversif, impliquant donc un passage progressif au revers de la loi évolutive +initiale – la sélection naturelle, en tant que mécanisme en évolution, se +soumettant elle-même, de ce fait, à sa propre loi. Il faudra revenir plus loin +sur ce concept qui rend caduque la prétention ordinaire de la plupart des +philosophies à déclarer inconcevable la possibilité même d’un matérialisme +intégral englobant l’éthique. + +[1mIII. – La sélection sexuelle [0m + +Le traitement de la sélection sexuelle dans La Filiation est extrêmement +documenté et parcourt un domaine zoologique très vaste avant de revenir à +l’Homme après un long détour passant par l’interrogation de la proportion +numérique des sexes (sex-ratio) et des différences entre les sexes dans les +espèces animales. + +La sélection sexuelle « dépend de l’avantage que possèdent certains individus +sur d’autres de même sexe et de même espèce, uniquement en ce qui concerne la +reproduction » (chap. VIII). + +En d’autres termes, la sélection sexuelle ne repose pas directement sur la +lutte pour l’existence, mais essentiellement sur une rivalité des mâles dans la +compétition pour la possession des femelles, compétition dont les effets, moins +rigoureux en règle générale que ceux de la sélection naturelle, sont +momentanément disqualifiants pour les vaincus ou les évincés sans être en +principe définitivement éliminatoires. La sélection sexuelle, qui sélectionne +des caractères sexuels secondaires et repose en grande partie sur l’hérédité « +liée à un seul sexe », assure généralement le triomphe des mâles les plus +vigoureux et les plus combatifs, ou de ceux qui possèdent une particularité +morphologique favorisant leur suprématie au sein de cette compétition (cornes +et ergots plus développés respectivement chez le cerf et le coq, crinière plus +épaisse chez le lion, plumage plus éclatant et chant plus mélodieux chez les +oiseaux). La préférence et le choix exercés par les femelles jouent dans ce +processus un rôle déterminant. Darwin retrouve, au sein de l’espèce humaine, +des traits de comportement qui manifestent la persistance d’une sélection +sexuelle sous les critères (variables suivant les cultures) de la beauté +masculine et féminine, et reconnaît le rôle qu’ils jouent lors des choix +nuptiaux. + +La sélection sexuelle, complément de la sélection naturelle, peut cependant +avoir des effets anti-adaptatifs : par exemple, la lourde parure de noce de tel +oiseau mâle pendant la période des parades nuptiales, en l’empêchant presque de +voler et en l’exposant ainsi davantage à la prédation, constitue +potentiellement un obstacle à sa survie. Que la tension vers l’union sexuelle +reproductive – qui possède à l’évidence un lien d’origine avec ce que l’on +appelle l’amour – puisse comporter d’une manière intime et permanente un risque +de mort est une observation darwinienne qui ne devrait pas échapper à la +perspicacité de la psychanalyse. + +[1mIV. – Sélection sexuelle et sélection naturelle [0m + +La sélection sexuelle, on l’a dit, sélectionne des caractères sexuels +secondaires, c’est-à-dire des organes ou des traits morpho-anatomiques +appartenant en propre à un seul sexe (le sexe mâle en l’occurrence), lesquels, +sans avoir de lien direct avec la génération, en favorisent cependant +l’accomplissement : c’est le cas par exemple des organes de préhension +développés chez les seuls mâles de nombreuses espèces (certains Crustacés +notamment), et qui leur servent occasionnellement à saisir et à maintenir la +femelle lors de l’accouplement. + +L’hérédité liée à un seul sexe est donc nécessaire pour penser la transmission +des caractères sexuels secondaires. Lorsque ces derniers sont l’occasion d’une +supériorité dans la lutte, les individus qui en sont porteurs, et qui sont de +ce fait capables d’engendrer un plus grand nombre de descendants et d’en +assurer la protection, transmettent à ceux-ci cet avantage. Certes, la +sélection naturelle suffit à expliquer chez le mâle l’existence d’organes tels +que les organes des sens et de la locomotion, qui servent à trouver la femelle +en même temps qu’à de nombreux autres usages. Cependant, la sélection sexuelle +a dû jouer un rôle non négligeable dans la formation et le perfectionnement de +ces organes, dans la mesure où c’est ce perfectionnement même qui assure à +certains mâles leur domination sur d’autres mâles et confère aux mieux armés la +faculté de transmettre cet avantage à leurs descendants du même sexe. Il faut +également noter que les mâles avantagés ayant la possibilité de conquérir les +femelles les plus saines et les plus vigoureuses, qui sont également les plus +précoces sous le rapport de la fécondité, l’avantage se répartit entre les +descendants des deux sexes sous la forme commune d’une santé et d’une vigueur +physique augmentées. + +« [3mIl y a beaucoup d’autres structures, écrit Darwin, et beaucoup d’autres +instincts qui ont dû se développer sous l’effet de la sélection sexuelle, comme +les armes offensives et les moyens de défense des [0m [3mmâles pour +combattre et chasser leurs rivaux ; leur courage et leur pugnacité ; leurs +ornements variés ; leurs dispositifs pour produire de la musique vocale ou +instrumentale et leurs glandes pour émettre des [0m [3modeurs, la plupart +de ces structures ne servant qu’à attirer et exciter la femelle. Il est clair +que ces caractères sont le résultat de la sélection sexuelle et non de la +sélection ordinaire, puisque des [0m [3mmâles sans armes, sans ornements et +sans attraits réussiraient tout aussi bien dans la bataille pour la vie et +parviendraient à laisser une nombreuse progéniture, s’ils ne se trouvaient en +présence de mâles [0m [3mmieux doués. Nous pouvons conclure qu’il en serait +ainsi, parce que les femelles, qui n’ont ni armes ni ornements, sont capables +de survivre et de propager leur espèce [0m» + +(chap. VIII). + +Ainsi, la sélection sexuelle se superpose à la sélection naturelle, travaillant +elle aussi à une amélioration qui, pour être de l’ordre de l’aptitude +reproductive et de la transmission en ligne mâle de caractères sexuels +secondaires avantageux, n’en atteint pas moins bénéfiquement l’ensemble de la +conformation et de la santé foncière des individus des deux sexes, par le +double mouvement qui pousse les mâles les mieux doués à s’emparer des femelles +les plus saines et les plus tôt prêtes à la fécondation, et les femelles à +préférer les mâles les plus attrayants, ce qui a pour conséquence une +amélioration globale du niveau physique de la descendance : il devient dès lors +difficile de démêler ce qui est dû à la sélection sexuelle et ce qui est +l’effet ordinaire de la sélection naturelle. + +Il est intéressant de noter que le raisonnement qui, chez Darwin, sert à +établir la naturalité de la sélection sexuelle est le même qui a servi à +établir celle de la sélection naturelle : si en effet l’Homme pratique une +sélection sexuelle artificielle sur ses animaux domestiques – améliorant dans +le sens de ses goûts ou de ses besoins telle ou telle race de coqs par exemple +–, il s’en induit nécessairement que la nature détient la capacité de +sélectionner les caractères sexuels secondaires (dont la variabilité est +nettement accusée), dans le sens d’un avantage reproductif, et d’améliorer +ainsi l’aspect physique des mâles de telle ou telle espèce. La démarche +explicative de Darwin à propos de la sélection sexuelle dans La Filiation de +l’homme est en cela exactement parallèle à celle qui a été mise en œuvre en +1859 dans L’Origine des espèces pour faire comprendre, à travers l’existence +avérée de la sélection artificielle, l’existence probable d’une sélection +opérant librement au sein de la nature : + +« [3mDe même que l’homme peut améliorer la race de ses coqs de combat en +sélectionnant les volatiles qui sont victorieux dans l’arène, de même il +apparaît que les mâles les plus forts et les plus vigoureux, ou [0m [3mceux +qui sont pourvus des meilleures armes, l’ont emporté à l’état de nature et ont +conduit à l’amélioration de la race naturelle ou de l’espèce. Un léger degré de +variabilité conduisant à quelque avantage, [0m [3mmême léger, au cours de +combats à mort réitérés suffirait à mettre en œuvre la sélection sexuelle ; or, +il est certain que les caractères sexuels secondaires sont éminemment +variables. De même que l’homme [0m [3mpeut rendre plus beaux, selon ses +critères de goût, ses coqs de basse-cour, ou plus précisément peut modifier la +beauté acquise à l’origine par l’espèce parente, de même qu’il peut donner au +Bantam Sebright [0m [3mun plumage nouveau et élégant, un port dressé +particulier, de même il apparaît que les oiseaux femelles, à l’état de nature, +par une longue sélection des mâles les plus attrayants, ont ajouté à leur +beauté ou [0m [3mà d’autres qualités qui les rendent attrayants [0m» + +(ibid.). + +Il semble donc d’une manière générale que chez presque tous les animaux à sexes +séparés, il doive y avoir une compétition « périodique et constante » entre les +mâles pour la possession des femelles, compétition au sein de laquelle la +force, les armes et la beauté physiques des mâles d’une part, le choix exercé +par les femelles d’autre part jouent un rôle déterminant. + +Au terme d’un long recensement, Darwin aboutit à la conclusion suivant laquelle +les caractères sexuels secondaires sont généralement plus accentués chez les +mâles des espèces polygames. En voici la raison : on admet au départ qu’une +prépondérance numérique des mâles sur les femelles constitue une condition +favorable à la rivalité des mâles, donc au développement chez ces derniers de +caractères sexuels secondaires plus ou moins marqués selon les individus, d’où +il suit que les mieux armés l’emporteront dans la compétition reproductive. Or, +la polygamie, qui est la situation où un seul mâle, en raison de sa force, de +sa combativité ou de sa séduction, gouverne un harem de femelles, produit les +mêmes effets que l’inégalité numérique des sexes : de nombreux mâles – « et ce +sont certainement, écrit Darwin, les plus faibles et les moins attrayants » +(ibid.) – ne pourront pas s’accoupler. On peut penser également qu’étant donné +cette situation, il faudra d’autant plus de qualités à un mâle non seulement +pour conquérir, mais pour conserver ses femelles et protéger ses petits. Les +mâles écartés de l’accouplement ne le sont pas toutefois d’une manière +définitive, mais ne peuvent la plupart du temps s’unir qu’à des femelles moins +vives, ce qui rejaillit négativement sur la qualité de leur descendance des +deux sexes. + +Les modifications qui déterminent les différences intersexuelles de l’apparence +extérieure chez de nombreuses espèces sont généralement plus accusées chez le +mâle que chez la femelle. Le fait que les mâles soient plus ardents, plus +combatifs et qu’ils aient presque toujours l’initiative de la poursuite +amoureuse entraîne indirectement chez eux un développement plus fréquemment +remarquable des caractères sexuels secondaires. Il faut cependant se souvenir +que l’apparente passivité des femelles n’exclut pas cependant, de leur part, un +choix déterminant lors de l’acceptation du mâle. + +[1mNotes [0m + +[1] Soit une bande de papier de faible largeur, comportant évidemment deux +faces. On en rapproche les deux extrémités, mais, avant de la refermer en +anneau, on fait subir à l’une de ses extrémités une torsion de 180o. On referme +par collage et l’on obtient un objet qui ne possède plus qu’une seule face et +un seul bord, permettant donc, en suivant ce qui était au départ l’une de ses +faces, de rejoindre d’une manière continue ce qui était initialement son +revers. Son nom est celui de son inventeur, August Ferdinand Möbius +(1790-1868), astronome et mathématicien allemand. + +[1mChapitre V [0m + +[1mL’expression des émotions [0m + +L’année qui suit la publication de La Filiation de l’homme voit paraître en +février la sixième et dernière édition de L’Origine des espèces (3 000 +exemplaires), qui comporte la mise au point de nouvelles réponses aux +objections adressées à la théorie, et demeurera l’édition standard. Cette même +année 1872 est également (26 novembre) celle de la parution d’un ouvrage qui +pose les fondements de la psychologie animale et de l’éthologie comparative +évolutionnistes, tout en étant lui aussi un effort d’illustration de la théorie +de la descendance étendue à l’Homme : The Expression of the Emotions in Man and +Animals. + +Cet ouvrage, dont une grande partie de la documentation a été recueillie par +voie d’enquêtes et de questionnaires, constitue en fait une sorte de chapitre +détaché de The Descent of Man. Il s’agit pour Darwin de se livrer à une étude +comparative des manifestations émotionnelles chez l’Homme et chez les animaux, +afin de mettre en évidence entre eux, par le biais du rapport de la +psychologie, de la physionomie et des attitudes corporelles, une continuité des +comportements réactionnels. Cette démonstration est destinée, d’une façon +indissociable, à argumenter contre l’interprétation créationniste et finaliste +de l’expression défendue à partir de 1806 par l’anatomiste Charles Bell dans un +travail, par ailleurs techniquement remarquable, qui sera plusieurs fois +réédité sous le titre de The Anatomy and Philosophy of Expression, et qui +professe que les muscles de la face sont « uniquement des instruments de +l’expression » ou sont « spécialement disposés à cet effet ». + +Darwin distingue d’emblée trois règles structurantes permettant de comprendre +le mécanisme effecteur de la plupart des mouvements expressifs : + +principe d’association des habitudes utiles. – À certains états tensionnels de +l’esprit sont habituellement associés certains actes complexes directement ou +indirectement utiles, et qui tendent à se déclencher, par une association +involontaire, chaque fois que ces états se reproduisent, et lors même qu’ils +n’offrent aucune utilité réelle. Voici un exemple propre à Darwin : son +mouvement de recul incontrôlé lors de l’assaut d’un serpent dont il était +protégé par une vitre. De tels actes peuvent alors faire l’objet d’une +inhibition volontaire qui s’accompagne souvent de mouvements de contraction +traduisant l’action répressive, et qui sont en eux-mêmes également expressifs ; + +principe de l’antithèse. – Le passage de l’un de ces états d’esprit induisant +de tels actes « utiles » à un état exactement inverse s’accompagne de +mouvements expressifs involontaires directement opposés, en dépit de +l’inutilité de ces derniers. C’est, par exemple, le comportement du chien +passant d’une attitude agressive à un transport de joie affectueuse en +reconnaissant son maître ; + +principe des actes dus à la constitution du système nerveux, complètement +indépendants de la volonté et jusqu’à un certain point de l’habitude (ou plus +brièvement principe de l’action directe du système nerveux). – L’excès de force +nerveuse engendré par une violente excitation est transmis par la connexion des +cellules nerveuses et par l’habitude, ou encore brutalement interrompu, +produisant dans les deux cas des actes expressifs. Exemples des tremblements +nerveux et de l’accélération du rythme cardiaque lors d’une vive émotion. + +Les trois premiers chapitres sont consacrés à illustrer ces trois principes par +des faits empruntés à l’observation de l’Homme et des animaux. Les deux +suivants décrivent les « expressions spéciales de quelques animaux ». Les +chapitres VI à XIII sont employés à l’étude systématique de l’expression des +sentiments humains : souffrance et pleurs ; abattement, anxiété, chagrin, +découragement, désespoir ; joie, gaieté, amour, sentiments tendres, piété ; +dédain, mépris, dégoût, culpabilité, orgueil, impuissance, patience, +affirmation et négation ; surprise, étonnement, crainte, horreur ; attention +portée sur soi-même, honte, timidité, modestie (rougissement). + +La plupart des actes expressifs des émotions sont héréditaires et innés. +Certains autres, en petit nombre, paraissent avoir été appris « comme les mots +du langage ». Toutefois, on peut supposer, à l’origine des actes expressifs +involontaires, une volonté utilitaire relayée par l’habitude transmise. D’autre +part, un mouvement involontaire peut être ensuite utilisé volontairement (on +retrouve ici, sans doute à l’insu de Darwin, le thème condillacien si +remarquable de l’apprentissage des signes naturels). + +L’hérédité des habitudes acquises est le principe sur lequel se fonde la +théorie darwinienne de l’expression des émotions. Certains actes d’abord +volontaires deviennent bientôt habituels, finissent par devenir héréditaires, +et peuvent même, alors, se produire malgré l’opposition de la volonté. Ces +conduites, non consciemment finalisées à l’origine en tant qu’actes expressifs, +étaient au départ des actes directement utiles ou des résultats indirects de +l’excitation du sensorium. Dans le commerce entre les individus, la question +demeure de savoir si la compréhension des actes expressifs d’autrui est innée +ou bien dérive de l’expérience et de l’apprentissage. Darwin penche pour la +thèse d’une sympathie instinctive (ce qui est cohérent avec le développement de +la théorie des instincts sociaux dans La Filiation). L’identité remarquable +des principales expressions humaines dans le monde tend à confirmer l’idée +d’une origine unique des différentes races de l’humanité à partir d’une souche +commune (monogénisme). + +Le travail de Darwin influencera l’histoire ultérieure de la réflexion +éthologique et de la psychologie animale et humaine, par l’intermédiaire +notamment de George John Romanes (1848-1894) – qui publiera en 1883 Mental +Evolution in Animals, ouvrage dans lequel il recherche systématiquement les +indices des manifestations antéhumaines de certaines émotions – et de William +James (1842-1910), qui entame en 1875 un vaste programme d’étude des relations +entre psychologie et physiologie. + +[1mChapitre VI [0m + +[1mLe darwinisme dénaturé : darwinisme social, sociobiologie, eugénisme [0m + +[1mI. – Erreurs premières [0m + +L’idée que la sélection naturelle, en tant que loi universelle de l’évolution, +doit nécessairement, de ce fait, s’appliquer aussi, avec toutes ses +conséquences cruellement éliminatoires, au fonctionnement et au devenir +historique des sociétés humaines imprègne l’Occident depuis les années qui +suivirent la parution de L’Origine des espèces. + +Or, Darwin, dans le premier ouvrage zoologique – le Voyage, étant un récit, +possède un statut différent – où il traite expressément de l’Homme et de la +civilisation (La Filiation de l’homme de 1871), a argumenté avec une +remarquable cohérence théorique son opposition à cette idée triviale dont +l’hyperlibéralisme sélectionniste a fait, de Spencer à Friedrich von Hayek +(1899-1992), l’axe de son idéologie. + +Cette invraisemblable confusion, qui a dénaturé pendant plus d’un siècle +l’interprétation complète et rigoureuse de la pensée de Darwin par le biais de +l’ignorance ou du travestissement résolu de son anthropologie, est celle-là +même que l’on voit se perpétuer dans la conscience commune et que réactivent +encore trop de commentaires superficiels et d’introductions hâtives à une œuvre +non lue Darwin serait à la fois le fondateur du « darwinisme social » propre au +libéralisme extrême en même temps que le père d’un eugénisme +ultra-interventionniste – ce qui est pour le moins contradictoire –, le garant +de l’expansion coloniale impérialiste – combattue en principe par les libéraux +anglais, dont il était proche –, le propagandiste de la concurrence économique +dure, un partisan du malthusianisme (alors qu’il en rejetait la principale +recommandation : la limitation des naissances dans les classes pauvres) et le +théoricien du « racisme scientifique » (qu’il haïssait). + +Cette confusion a une histoire, qui commence aujourd’hui à être connue. Elle +est due pour l’essentiel à trois causes parfaitement établies : l’hégémonie de +la philosophie évolutionniste de Spencer pendant la période de l’ascension +scientifique de la théorie darwinienne ; le développement coextensif de +l’eugénisme de Galton en référence à la théorie sélective ; la conviction des « +darwiniens » considérant La Filiation comme l’extension homogène de L’Origine. + +[1mII. – Le « darwinisme social » de Spencer et l’anthropologie de Darwin +[0m + +L’ingénieur philosophe Herbert Spencer (1823-1903) expose sa propre « loi +d’évolution » dans le « Plan général de la philosophie synthétique » du 6 +janvier 1858, publié sous forme de « Programme » en 1860, deux ans avant la +publication des Premiers principes, qui paraissent en 1862. La « loi +d’évolution » décrit le passage des agrégats, par un processus d’intégration et +de différenciation, d’un état primitif indéfini, incohérent et homogène vers un +état défini, cohérent et hétérogène (processus correspondant à un accroissement +de complexité conduisant jusqu’aux extrêmes raffinements organisationnels des +corps vivants, de l’individualité humaine et des sociétés). + +La « loi » ainsi énoncée sera appliquée à toutes les catégories de phénomènes +et à tous les domaines du savoir, ainsi qu’à la théorie de la connaissance +elle-même. Le versant sociologique de la pensée spencérienne est +particulièrement représentatif des aspirations de la bourgeoisie industrielle +anglaise : la société est un organisme et évolue comme un organisme. +L’adaptation (pensée par Spencer en des termes fondamentalement lamarckiens qui +n’intégreront le darwinisme que pour le trahir) est la règle de survie au sein +d’une concurrence interindividuelle généralisée : les moins adaptés doivent +être éliminés sans recours et sans égard. Spencer s’opposera ainsi à toute +mesure visant à venir en aide aux défavorisés et à toute forme de loi +d’assistance. Ce qu’il emprunte à Darwin (mais à ce niveau, ce pourrait être +aussi bien à Malthus) est donc le « noyau dur » de la théorie sélective telle +qu’il le découvre au mois d’octobre 1858 lorsqu’il prend connaissance de la +présentation commune des textes de Darwin et de Wallace devant la Linnean +Society de Londres. Dès lors, son souci sera de l’appliquer non pas tant au +domaine où son usage serait légitime (l’évolution des organismes), qu’à un +univers au sein duquel Darwin en refuse précisément l’application mécanique : +les sociétés humaines. C’est ainsi qu’il emprunte à Darwin, en dépit d’un +lamarckisme revendiqué, la théorie de la sélection naturelle, rebaptisée « +survie des plus aptes » afin d’en évacuer les éventuelles connotations +anthropomorphiques. + +À la faveur du concurrentialisme qui règne sur les mentalités des grands +acteurs et soutiens de l’industrialisme victorien, la confusion s’établira +entre les concepts et les théories. Darwin sera lu à travers la lunette de +Spencer et doté d’appendices sociologiques spencériens ajustés aux réquisits +idéologiques du nouvel état économique et social de la nation anglaise. La « +sociobiologie » américaine, popularisée par Edward O. Wilson dans le dernier +quart du xxe siècle, ultime remaniement systématique du versant « +social-darwiniste » du spencérisme, sera, dans son pansélectionnisme et ses +tentatives d’intégration des sciences sociales sous l’autorité des sciences +biologiques, l’héritière de cette erreur de lecture. + +Il importe donc d’identifier, dans la logique de l’anthropologie de Darwin +telle qu’elle s’expose intelligiblement, pour qui sait lire, au sein de La +Filiation de l’homme et la sélection sexuelle (1871), ce qui l’oppose à +l’hypersélectionnisme biologico-social de Spencer, véritable inventeur de +l’improprement nommé « darwinisme social » et créateur de tous les paradigmes +communs aux « sociobiologies » qui dans le monde entier accompagneront +périodiquement l’essor tumultueux du système libéral. + +[1mIII. – La civilisation, le matérialisme et la morale [0m + +Il faut ici revenir d’une manière plus approfondie, comme nous l’annoncions au +chapitre IV, sur l’analyse de cette clé du passage à la culture que constitue +le concept d’effet réversif de l’évolution. + +Figure logique centrale de l’anthropologie darwinienne (à distinguer de +l’anthropologie évolutionniste), l’effet réversif de l’évolution [1] est ce +qui permet de penser chez Darwin la transition progressive entre ce que l’on +nommera par commodité la sphère de la nature, régie par la stricte loi de la +sélection, et l’état d’une société civilisée, à l’intérieur de laquelle +s’institutionnalisent des conduites qui s’opposent au libre jeu de cette loi. + +Si ce concept n’est nulle part nommé dans l’œuvre de Darwin, il y est cependant +décrit et opère dans certains développements importants (notamment les chap. +IV, V et XXI) de La Filiation de l’homme de 1871, ouvrage dont nous avons vu en +quel sens il constitue bien la poursuite cohérente, dans le registre de +l’histoire évolutive de l’Homme naturel et social, de la théorie sélective +exposée dans L’Origine des espèces de 1859. Il résulte d’un paradoxe identifié +par Darwin au cours de son effort pour penser le devenir social et moral de +l’humanité comme une conséquence particulière de l’application antérieure et +universelle de la loi sélective à l’ensemble du monde animal. + +Ce paradoxe peut se formuler ainsi : la sélection naturelle, principe directeur +de l’évolution impliquant l’élimination des moins aptes dans la lutte pour +l’existence, sélectionne dans l’humanité une forme de vie sociale dont la +marche vers la « civilisation » tend à exclure de plus en plus, à travers le +jeu lié de la morale et des institutions, les comportements éliminatoires. En +termes simplifiés, la sélection naturelle sélectionne la civilisation qui +s’oppose à la sélection naturelle. Comment résoudre un tel paradoxe ? + +On doit avoir présent à l’esprit le fait que la sélection naturelle – il s’agit +chez Darwin d’un point fondamental – sélectionne non seulement des variations +organiques présentant un avantage adaptatif, mais aussi des instincts. + +Parmi ces instincts avantageux, ceux que Darwin nomme les instincts sociaux ont +été tout particulièrement retenus et développés, ainsi que le prouvent le +triomphe universel du mode de vie sociale au sein de l’humanité et la +tendancielle hégémonie des peuples « civilisés ». + +Or, dans l’état de « civilisation », résultat complexe d’un accroissement de la +rationalité, de l’emprise grandissante du sentiment de « sympathie » et des +différentes formes morales et institutionnelles de l’altruisme, on assiste à un +renversement de plus en plus accentué des conduites individuelles et sociales +par rapport à ce que serait la poursuite pure et simple du fonctionnement +sélectif antérieur : au lieu de l’élimination des moins aptes apparaît, avec la +civilisation, le devoir d’assistance qui met en œuvre à leur endroit de +multiples démarches de secours et de réhabilitation ; au lieu de l’extinction +naturelle des malades et des infirmes, leur sauvegarde par la mobilisation de +technologies et de savoirs (hygiène, médecine, exercice corporel) visant à la +réduction et à la compensation des déficits organiques ; au lieu de +l’acceptation des conséquences destructrices des hiérarchies naturelles de la +force, du nombre et de l’aptitude vitale, un interventionnisme rééquilibrateur +qui s’oppose à la disqualification sociale. + +Par le biais des instincts sociaux, la sélection naturelle, sans « saut » ni +rupture, a ainsi sélectionné son contraire, soit : un ensemble normé, et en +extension, de comportements sociaux antiéliminatoires – donc anti-sélectifs au +sens que revêt le terme de sélection dans la théorie développée par L’Origine +des espèces –, ainsi, corrélativement, qu’une éthique de la protection des +faibles traduite en principes, en règles de conduite et en lois. + +L’émergence progressive de la morale apparaît donc comme un phénomène +indissociable de l’évolution, et c’est là une suite normale du matérialisme de +Darwin et de l’inévitable extension de la théorie de la sélection naturelle à +l’explication du devenir des sociétés humaines. Mais cette extension, que trop +de théoriciens, abusés par l’écran tissé autour de Darwin par la philosophie +évolutionniste de Spencer, ont interprétée hâtivement sur le modèle simpliste +et faux du darwinisme social libéral (application aux sociétés humaines du +principe de l’élimination des moins aptes au sein d’une concurrence vitale +généralisée), ne peut en toute rigueur s’effectuer que sous la modalité de +l’effet réversif, qui oblige à concevoir le renversement même de l’opération +sélective comme base et condition évolutive de l’accession à la « civilisation +». C’est ce qui interdit définitivement que la « sociobiologie », qui défend au +contraire, à l’opposé de toute la logique anthropologique de Darwin, l’idée +d’une continuité simple (sans renversement) entre nature et société, puisse à +bon droit se réclamer du darwinisme. + +L’opération réversive est ainsi ce qui fonde la justesse finale de l’opposition +nature/culture, en évitant le piège d’une « rupture » magiquement installée +entre ses deux termes : la continuité évolutive, à travers cette opération de +renversement progressif liée au développement (lui-même sélectionné) des +instincts sociaux, produit de cette manière non pas une rupture effective, mais +un effet de rupture qui provient de ce que la sélection naturelle s’est +trouvée, dans le cours de sa propre évolution, soumise elle-même à sa propre +loi – sa forme nouvellement sélectionnée, qui favorise la protection des +faibles, l’emportant, parce que avantageuse, sur sa forme ancienne, qui +privilégiait leur élimination. L’avantage nouveau n’est plus alors d’ordre +biologique : il est devenu social. + +Darwin permet ainsi de penser le rapport nature/civilisation en échappant au +double dogmatisme de la continuité (discours de type « sociobiologique ») et de +la rupture (discours de type lévi-straussien), évitant aussi bien la réciproque +extériorité du biologique et du social (un sociologisme qui exclurait +méthodologiquement la prise en compte de tout facteur naturaliste) que le +réductionnisme ordinaire, pour lequel tout le social n’est que la traduction +d’impulsions issues d’un niveau quelconque (variable suivant l’état historique +des investigations sur le vivant) de la biologie. En bref, Darwin rend +possible, dans la pensée de ce rapport complexe, un continuisme matérialiste +imposant la représentation d’un renversement progressif (pensable en termes de +divergence sélectionnée à l’intérieur de la sélection naturelle, elle-même en +évolution et se soumettant de ce fait à sa propre règle avant d’entrer en +régression sous sa forme ancienne) qui s’écarte des artefacts théoriques tels +que le « bond qualitatif » tout en sauvant évolutivement l’indépendance finale +des sciences de l’homme et de la société. + +Corrélativement, Darwin produit, à travers le motif dialectique de la sélection +des conduites antisélectives et du sentiment de sympathie, couplé avec celui de +l’accroissement de la rationalité et de l’importance grandissante accordée par +chaque sujet à l’opinion publique, une théorie matérialiste des bases de la +morale qui préserve de même l’indépendance conquise par les décisions et la +réflexion éthiques (grâce à l’effet de rupture produit par le renversement), +tout en permettant de soustraire celles-ci à l’emprise dogmatique des morales +théologiques de l’obligation transcendante. + +[1mIV. – Galton et l’eugénisme [0m + +La deuxième cause de méprise et de confusion fut la naissance de l’eugénisme. +Son premier et principal théoricien fut un cousin de Darwin, Francis Galton +(1822-1911), statisticien passionné en particulier par l’étude des phénomènes +héréditaires. Il fit quelques études médicales, fut profondément marqué en 1859 +par la lecture de L’Origine des espèces, et dès 1865 commença à produire les +thèses fondamentales de la doctrine qui – combinant une forte conviction +héréditariste, la crainte de la dégénérescence et le vœu compensatoire d’une +sélection artificielle appliquée à l’humanité – allait prendre avec lui le nom +d’eugénisme. La proposition de base en est assez simple : la sélection +naturelle assurant dans l’ensemble du monde vivant la diversité des espèces et +la promotion des plus aptes à partir du tri des variations avantageuses, la +même chose devrait se produire dans la société humaine, eu égard en particulier +aux caractères intellectuels. Or, la civilisation développée entrave le libre +jeu de la sélection naturelle en favorisant la protection et la reproduction +des existences médiocres. Il faut donc engager une action de sélection +artificielle institutionnalisée afin de compenser ce déficit et d’alléger ce +fardeau nuisible à la qualité biologique et psychique du groupe social. À la +lumière de La Filiation de 1871, on comprend que cette attitude était +inconciliable avec ce qu’allait énoncer l’anthropologie de Darwin et +contradictoire avec le darwinisme strictement entendu (celui de Darwin), pour +lequel la sélection artificielle ne peut être appliquée qu’aux plantes +cultivées et aux animaux d’élevage. Pour Darwin, en effet, celui qui traiterait +un autre être humain, quels que soient son degré d’éloignement racio-culturel +et ses caractéristiques physiques ou psychiques, comme autre chose que son « +semblable » contreviendrait à la loi proprement civilisationnelle de +l’extension progressive de la sympathie et régresserait sur l’échelle de +l’évolution humaine jusqu’à l’état de sauvagerie ancestrale : + +« L’aide, écrit Darwin, que nous nous sentons poussés à apporter à ceux qui +sont privés de secours est pour l’essentiel une conséquence inhérente de +l’instinct de sympathie, qui fut acquis originellement comme une partie des +instincts sociaux, mais a été ensuite, de la manière dont nous l’avons +antérieurement indiqué, rendu plus délicat et étendu plus largement. Nous ne +saurions réfréner notre sympathie, même sous la pression d’une raison +implacable, sans porter atteinte à la partie la plus noble de notre nature. Le +chirurgien peut se durcir en pratiquant une opération, car il sait qu’il est en +train d’agir pour le bien de son patient ; mais si nous devions +intentionnellement négliger ceux qui sont faibles et sans secours, ce ne +pourrait être qu’en vue d’un bénéfice imprévisible, lié à un mal présent qui +nous submerge. Nous devons par conséquent supporter les effets indubitablement +mauvais de la survie des faibles et de la propagation de leur nature » (La +Filiation de l’homme, chap. V). + +Galton, qui non plus que d’autres ne pressent ce que Darwin écrira en 1871, et +qui ne peut, en tout état de cause, l’entrevoir dans les années 1860, poursuit +donc sa tâche d’explication militante de l’urgence d’une politique eugéniste +scientifiquement conduite pour freiner la dégénérescence probable des civilisés +: il tente de démontrer le caractère héréditaire des qualités intellectuelles +et d’établir statistiquement la stricte hérédité du génie (Hereditary Genius, +1869) en faisant totalement abstraction des facteurs éducatifs. L’eugénisme de +Galton, ainsi que le rapporte Darwin lui-même dans La Filiation (chap. V), +était hostile à la « reproduction des pauvres et des insouciants », pensée +comme un obstacle à l’augmentation numérique des hommes « supérieurs ». Darwin +conclura quant à lui, à l’inverse, en défendant le principe d’une compétition +ouverte à tous, répondant ainsi, à partir de l’idée d’une sélection encore +nécessaire pour améliorer l’homme en tant que sujet des valeurs de +civilisation, à la fois au malthusianisme et à l’eugénisme galtonien. À la fin +de La Filiation de l’homme, en effet (chap. XXI : « Conclusion principale »), +Darwin, défendant ses propres conclusions contre toutes formes de sélection +artificielle appliquée aux sociétés humaines, écrit : + +« Par conséquent, notre taux naturel de croissance, même s’il conduit à de +nombreux et évidents malheurs, ne doit d’aucune manière être grandement +diminué. Il devrait y avoir compétition ouverte pour tous les hommes ; et l’on +ne devrait pas empêcher, par des lois ou des coutumes, les plus capables de +réussir le mieux et d’élever le plus grand nombre de descendants. » + +Il s’agit de n’entraver par aucun artifice coercitif ou limitatif le libre jeu +d’une compétition dans laquelle les individus, quelle que soit leur origine +sociale,doivent avoir la possibilité de prouver leur valeur. Darwin défend en +toutes lettres le droit des plus pauvres à la procréation, refusant donc +implicitement de considérer la pauvreté comme l’indice d’une infériorité +héréditaire. Le malthusianisme est d’abord rejeté parce qu’il heurte le +principe même de la poursuite du perfectionnement sélectif au sein des sociétés +humaines. Mais il ne faut pas oublier que les modalités de cette « poursuite » +sont éthiques, donc opposées à la version éliminatoire, supplantée dans +l’évolution, de la sélection naturelle : + +« Si importante qu’ait été, et soit encore, la lutte pour l’existence, +cependant, en ce qui concerne la partie la plus élevée de la nature de l’homme, +il y a d’autres facteurs plus importants. Car les qualités morales progressent, +directement ou indirectement, beaucoup plus grâce aux effets de l’habitude, aux +capacités de raisonnement, à l’instruction, à la religion, etc. que grâce à la +sélection naturelle ; et ce, bien que l’on puisse attribuer en toute assurance +à ce dernier facteur les instincts sociaux qui ont fourni la base du +développement du sens moral. » + +L’interventionnisme social de Darwin est donc rééquilibrateur : il s’agit de +maintenir ouverte la voie de la civilisation qui impose de produire « un grand +nombre d’hommes bien doués », c’est-à-dire faisant le plus grand cas de +l’altruisme et de la solidarité. + +Tout en affinant l’application des méthodes statistiques à la biologie, les +continuateurs de Galton – les « biométriciens » ralliés au principe de la +sélection darwinienne, notamment Karl Pearson (1857-1936) – partageront avec +lui la responsabilité historique de l’élaboration première de la doctrine +eugéniste. On pourrait noter ici que l’infléchissement propre au travail de +Galton, qui substitue à l’importance chez Darwin de l’individualité biologique +et de ses avatars évolutifs celle d’un ensemble populationnel soumis en tant +que tel à la sélection, favorise d’emblée tous les discours et entreprises à +venir qui vont, au nom de l’amélioration de la qualité biologique du groupe +social en tant que tel, recommander comme nécessaire l’élimination de certaines +catégories d’individus porteurs de « mauvaises » variations. L’opposition +entre la conception galtonienne et la conception darwinienne de la variation +éclate d’ailleurs vers la fin des années 1870, au sein d’une histoire complexe +qui est à la fois celle de l’émergence de la biologie mathématique et celle des +idéologies de l’optimisation normalisante du niveau biologique des populations. +On retiendra ici, seulement, l’idée d’un risque constant d’enfermement de la +rationalité mathématique en elle-même, d’oubli des réalités biologiques de +l’organisme et de disparition de l’individu derrière l’écran des mesures, des +caractères quantifiables et des abstractions statistiques. Comme à un autre +niveau l’anthropologie physique, la biométrie, comportant le trait +constitutionnel d’une déshumanisation méthodologique relative de son objet +était parfaitement susceptible, sous l’action de certaines forces +politico-idéologiques, de servir d’instrument à des prescriptions et à des +pratiques interventionnistes sur la vie et la reproduction des individus, au +nom de la qualité biologique de la communauté, et ce, d’autant plus que chez +son principal inspirateur, Galton, le penchant élitiste était initial, et que +son premier continuateur, Pearson, optait clairement pour une « modification de +la fertilité relative des bonnes et des mauvaises souches » au sein du groupe +social (formule qui, au passage, rend problématique et toujours instable la +distinction entre un eugénisme « positif » et un eugénisme « négatif »). +Corrélativement, la science quantitative naissante nourrissait déjà le projet +d’une annexion des sciences sociales. La génétique mendélienne, après un débat +houleux avec la biométrie, l’intégrera progressivement à son versant +quantitatif, et des généticiens prendront alors le relais de l’eugénisme, +soutenu également par de nombreux médecins, naturalistes et sociologues +biologistes au cours des premières décennies du xxe siècle. + +Une chose est à retenir : l’eugénisme, dans son acte de fondation moderne +(galtonien), est profondément pénétré de l’idée, qui sera universellement +reprise, que dans les sociétés civilisées, la sélection naturelle, du fait des +diverses mesures de protection sociale et sanitaire, ainsi que des conditions +générales de confort qui maintiennent les existences individuelles à l’écart de +tout risque majeur, ne joue plus le rôle discriminant et éliminatoire qu’elle +assurait dans la « nature », et dont l’effet était de privilégier les +meilleures souches sur le plan de la survie différentielle et de la +reproduction. D’où la crainte, étayée de mille exemples, d’une « dégénérescence +» globale (thème déjà acclimaté par la psychiatrie héréditariste) des +populations humaines au niveau de leurs caractéristiques biologiques. D’où +enfin la recommandation de mesures institutionnelles d’intervention correctrice +et compensatoire visant à restaurer la qualité biologique du groupe par +l’introduction d’une sélection artificielle appliquée à ses membres. Là se +trouve le noyau théorique de l’eugénisme moderne, et l’on a vu à quel point +Darwin y était opposé. L’idée d’un Darwin eugéniste est l’une des +mystifications les plus graves qui aient pu émaner des eugénistes eux-mêmes et, +simultanément, de l’ignorance de la plupart de ceux qui ont fait mine de les +combattre. + +La complexité extraordinaire des rapports entre eugénisme et darwinisme social +dans les différents pays qui ont été le théâtre de la diffusion des idées nées +de la biologie moderne est telle qu’aucune règle absolument constante ne +saurait être formulée pour définir une homogénéité doctrinale réellement +stable, à l’exception peut-être de celle attachée au schéma de base qui vient +d’être décrit (défaut de sélection naturelle → dégénérescence → sélection +artificielle). Aux États-Unis, qui sont à la fois le territoire de +l’exportation massive du « darwinisme social » hyperlibéral de Spencer (lequel +ne comportait pas, tout au moins chez son fondateur, la prescription de mesures +eugénistes ou racistes) et une terre d’immigration multiraciale, d’esclavage et +de ségrégation, l’eugénisme stérilisateur d’activistes institutionnels tels que +Charles B. Davenport (1866-1944) et Henry H. Laughlin (1880-1943) sévit +cruellement pendant une longue période qui commence vers 1904. Les « faibles +d’esprit », les porteurs de maladies déclarées « héréditaires » et les pauvres +sont les cibles de ce terrible mouvement. Des pratiques analogues se +développent dans les pays scandinaves. En Allemagne, la grande figure d’Ernst +Haeckel (1834-1919), naturaliste lamarckien fondateur du Sozial-Darwinismus +national, se mêle à l’eugénisme et au « combat pour la civilisation » +(Kulturkampf) engagé par Bismarck, développant dans ses ouvrages de +vulgarisation les thèmes de l’euthanasie et de la « sélection spartiate », qui +se retrouveront au cœur des motifs clés du nazisme, que développeront dans +leurs laboratoires les « hygiénistes raciaux » Alfred Ploetz (1860-1940), Ernst +Rüdin (1874-1952), Eugen Fischer (1874-1967) et bien d’autres. Les mesures +nazies de stérilisation humaine se déploieront entre 1933 et 1940, et le +programme d’élimination des Juifs, considérés comme dysgéniques, sera appliqué +à leur suite. + +En France, l’eugénisme d’un Georges Vacher de Lapouge (1854-1936) demeure +étroitement lié aux slogans d’un « darwinisme social » peu nuancé, et les +déclarations eugénistes demeureront essentiellement des exhortations, bien que +la proximité de l’Allemagne et l’épuration nazie aient fait rêver plus tard +quelques médecins racistes tels que le gobinien René Martial. L’eugéniste le +plus connu fut sans doute alors le médecin (émigré aux États-Unis) Alexis +Carrel (1873-1944), prix Nobel en 1912, auteur du célèbre ouvrage L’Homme, cet +inconnu (1935), où il se montre le répétiteur médiocre du discours ordinaire de +l’hygiène raciale allemande et de l’eugénisme stérilisateur américain. Carrel, +partisan déclaré, en 1936, des mesures nazies d’épuration biologique de la race +et promoteur de l’usage des chambres à gaz pour le traitement « humain et +économique » du problème posé à la société par certains délinquants et malades +mentaux. Carrel, créant et dirigeant sous l’autorité du maréchal Pétain une « +Fondation française pour l’étude des problèmes humains » à vocation eugéniste +fort proche de celle dont le Norvégien Jon Alfred Hansen Mjøen (1860-1939) +avait fait le projet en 1915. Carrel, envoyant ses équipes enquêter sur la « +qualité biologique » des familles immigrées pendant l’Occupation de la France +par les nazis. Carrel, adhérant au Parti populaire français, +collaborationniste, de Jacques Doriot. Carrel, rêvant d’une « aristocratie +biologique héréditaire » et de la fin de la démocratie. Carrel, dont la France +efface aujourd’hui le nom, malgré les efforts de certains nostalgiques, sur les +plaques de ses rues et de ses établissements publics. + +De Darwin au nazisme, la voie est sinueuse et traversée de courants dont la +caractéristique commune est la trahison de la pensée intégralement développée +de Darwin. La vérité sur ce que Darwin a écrit sur l’Homme doit être cherchée +non pas dans l’ouvrage où il n’en dit rien (L’Origine des espèces), mais dans +celui où il en parle (La Filiation de l’homme) – et où il explique que +l’émergence de la morale et de la civilisation, fruit de l’évolution conjointe +des instincts sociaux et de la rationalité, est indissociable du mouvement +d’extension indéfini de la sympathie. Le transformisme darwinien en +anthropologie ouvre ainsi la voie d’une compréhension scientifique des +conditions de possibilité d’une éthique débarrassée de tout présupposé +religieux. D’une éthique qui explique les valeurs dans leur genèse évolutive, +comme tous les autres faits du devenir humain, en les rendant à la +responsabilité purement humaine de ceux qui s’en reconnaissent les sujets. + +[1mV. – L’antiracisme de Darwin [0m + +L’engagement antiraciste de Darwin est d’abord une question de fait. On ne +saurait trop rappeler sa détestation violente de l’esclavage, inscrite dans le +récit du Voyage, dans sa correspondance de jeunesse avec Henslow ou John +Maurice Herbert (1808-1882), ainsi que dans les lettres écrites beaucoup plus +tard à son ami le botaniste américain Asa Gray (1810-1888) durant la guerre de +Sécession. On connaît par ailleurs sa sympathie pour la théorie monogéniste +(défendant la thèse d’une origine unique pour l’humanité) et son opposition – à +travers ses partisans du X-Club et de l’Ethnological Society – au polygénisme +raciste et antiabolitionniste des membres de l’Anthropological Society of +London, dont l’animateur, James Hunt (1833-1869), ennemi des darwiniens, +tournait en dérision les traits physiques et comportementaux des Noirs, et +faisait du thème de l’inégalité irréductible des races le motif central de son +plaidoyer en faveur d’une politique coloniale dure. + +Toujours aux fins d’accréditer une thèse illogique, on s’est longtemps complu à +évoquer les réactions du jeune Darwin voyant pour la première fois des Fuégiens +dans leur milieu naturel, mais on a toujours oublié de citer le texte du Voyage +où il souligne les effets rapides de la civilisation sur les Fuégiens +introduits à la cour d’Angleterre et ramenés par le Beagle dans leur pays +natal, ainsi que leur grande similitude psychologique et affective avec ceux +qui les raccompagnaient. + +L’antiracisme de Darwin est ensuite un choix éthique enraciné dans sa théorie, +et la conséquence stricte de son anthropologie évolutive ; le sentiment de +sympathie, produit des instincts sociaux sélectionnés, tend naturellement à +s’étendre à l’humanité entière : + +« [3mÀ mesure que l’homme avance en civilisation, et que les petites tribus +se réunissent en communautés plus larges, la plus simple raison devrait aviser +chaque individu qu’il doit étendre ses instincts sociaux [0m [3met sa +sympathie à tous les membres de la même nation, même s’ils lui sont +personnellement inconnus. Une fois ce point atteint, seule une barrière +artificielle peut empêcher ses sympathies de s’étendre aux [0m [3mhommes de +toutes les nations et de toutes les races. Il est vrai que si ces hommes sont +séparés de lui par de grandes différences d’apparence ou d’habitudes, +l’expérience malheureusement nous montre combien [0m [3mle temps est long +avant que nous les regardions comme nos semblables [0m» + +(La Filiation de l’homme, chap. IV). + +Il faudra répéter longtemps encore, faute d’obtenir que l’on lise La Filiation +de l’homme avec l’intelligence requise par son articulation au sein de la +cohérence globale de la théorie sélective, que Darwin n’était ni « darwiniste +social », ni eugéniste, ni raciste, ni néo-malthusien, ni impérialiste, ni +proesclavagiste, mais très exactement l’ennemi de tous ces dispositifs de +forces idéologiques qui ont tenté récursivement d’utiliser son rayonnement +scientifique pour se donner l’ancrage naturaliste dont ils avaient besoin lors +de chacune de leurs résurgences. + +Darwin a pris position clairement, on l’a vu, contre l’eugénique proposée par +Galton, ce qui ne l’empêchait nullement de prendre en considération point par +point les arguments galtoniens et la statistique biométrique ; malgré le calme +de son caractère et sa prudence d’innovateur scientifique toujours soucieux +d’éviter les éclats publics, il s’est toutefois engagé personnellement contre +le racisme, et il a argumenté cette position éthique dans La Filiation ; par +ailleurs, tout en ayant emprunté à Malthus un élément de modélisation +mathématique qu’il a clairement appliqué aux dynamiques d’accroissement des +populations végétales et animales sur des territoires aux dimensions et aux +ressources limitées, il a refusé l’application des recommandations +malthusiennes aux sociétés humaines ; il a combattu de même les effets +d’exténuation physique et morale introduits outre-mer dans les populations +indigènes par l’effraction mortifère de la domination coloniale ; il éprouva +enfin durant sa vie entière une aversion sans limites pour l’esclavage et pour +chaque forme rencontrée d’humiliation ou de domination brutale de l’homme par +l’homme. Tout cela est aujourd’hui – au prix d’un effort explicatif qui est +encore loin d’être achevé – connu et vérifié. + +La mise en accusation, contre l’évidence logique, historique et textuelle, d’un +Darwin rendu responsable, suivant l’opportunité, de tous les fléaux +inégalitaires et suprématistes qui ont défiguré le xxe siècle, ne saurait être +totalement innocente chez ceux qui détiennent en principe la possibilité +d’accéder aux sources les plus propres à l’invalider. L’un des programmes +d’étude de l’épistémologie historique des discours circum-darwiniens pourrait +être, dans la période contemporaine, l’analyse des stratégies sous-jacentes à +ce genre d’obstination. + +[1mNotes [0m + +[1] Patrick Tort, La Pensée hiérarchique et l’évolution, Paris, Aubier, 1983. + +[1mChapitre VII [0m + +[1mObjections et réponses [0m + +Darwin a mis plus de onze ans – de la troisième édition de L’Origine en 1861 +jusqu’à la sixième et dernière en 1872 – à affiner ses réponses aux critiques +dont la théorie de la descendance modifiée par sélection naturelle a été la +cible. S’il s’est peu préoccupé des objections de Wilberforce et d’Owen, le +premier tentant de réintégrer la sélection dans le sein de la théologie +naturelle en lui déniant son rôle créateur, et le second de lui substituer une +théorie de la dérivation des espèces à partir de types fixes inspirée de la +morphologie idéaliste, de la philosophie de la nature allemande et d’un +providentialisme des causes secondes, il s’est en revanche intensément attaché +à répondre à celles qui lui paraissaient poser de véritables problèmes au +niveau de l’application universelle du principe sélectif à l’interprétation des +faits d’évolution. + +Au chapitre VI de L’Origine (intitulé « Difficultés de la théorie »), Darwin +répond à deux objections qui demeureront classiques : celle de la rareté des +formes de transition entre des espèces censées descendre les unes des autres +par gradations insensibles et celle de la production, par le même mécanisme +sélectif, d’organes très peu importants comme la queue de la girafe – simple +chasse-mouches – et d’organes d’une haute complexité et d’une grande perfection +fonctionnelle tels que l’œil. + +L’objection de la rareté des formes de transition actuelles entre espèces +distinctes voisinant sur un territoire continu suscite la réponse suivante : la +sélection naturelle est un processus lent n’agissant que sur un petit nombre de +formes et impliquant le remplacement continuel et l’extinction des formes +intermédiaires antérieures. En outre, les mouvements géologiques tendent à +faire penser que des spéciations peuvent avoir eu lieu sur des territoires +anciennement discontinus (comme des archipels) et climatiquement différents, +aujourd’hui réunis. Par ailleurs, des variétés surgissant dans deux régions +d’un territoire continu peuvent voir naître dans une étroite zone limitrophe +une variété intermédiaire nécessairement moins nombreuse qu’elles, ayant donc +beaucoup moins de chances de présenter des variations favorables, et donc +aisément supplantée. Dans tous les cas de figure, la tendance est à la +production de différences spécifiques marquées et à la rareté de la survie des +formes intermédiaires. Cela n’élimine pas toutefois l’objection qui concerne la +rareté des formes de transition fossiles, qui ont dû être dans le temps, ainsi +que Darwin le reconnaît au chapitre X de l’édition définitive, extrêmement +nombreuses. C’est à elle que répondent dans ce même chapitre et dans le +suivant les longs développements sur l’imperfection des archives géologiques : + +« [3mJ’ai tenté de montrer que l’archive géologique est extrêmement +imparfaite ; qu’une petite portion du globe seulement a été explorée avec soin +sur le plan de la géologie ; que certaines classes seulement [0m [3md’êtres +organiques ont été conservées en abondance à l’état fossile ; que le nombre +d’espèces et de spécimens conservés dans nos musées n’est absolument rien +comparé au nombre des générations qui ont dû se [0m [3msuccéder, ne +serait-ce qu’au cours d’une seule formation ; que, en raison du fait qu’un +affaissement du sol est presque nécessaire pour permettre l’accumulation de +dépôts riches en espèces fossiles de toutes [0m [3msortes et assez épais +pour résister aux dégradations futures, d’immenses intervalles de temps ont dû +s’écouler entre la plupart de nos formations successives ; qu’il y a +probablement eu plus d’extinction [0m [3mpendant les périodes +d’affaissement et plus de variation pendant les périodes d’exhaussement, +l’enregistrement ayant été moins parfait au cours de ces dernières ; que chaque +formation particulière n’a pas été [0m [3mdéposée d’une manière continue ; +que la durée de chaque formation est, probablement, courte en comparaison de la +durée moyenne des formes spécifiques ; que la migration a joué un rôle +important dans la [0m [3mpremière apparition de formes nouvelles dans toute +zone et dans toute formation ; que les espèces largement répandues sont celles +qui ont varié le plus fréquemment et ont le plus souvent donné naissance à de +[0m [3mnouvelles espèces ; que les variétés ont d’abord été locales ; et +enfin que, bien que chaque espèce ait dû parcourir de nombreux stades de +transition, il est probable que les périodes pendant lesquelles elle [0m +[3ma subi des modifications, bien que nombreuses et longues, si on les mesure +en années, ont dû être courtes, en comparaison des périodes pendant lesquelles +elle est demeurée sans modification [1]. Ces causes, [0m [3mprises +conjointement, expliquent dans une large mesure pourquoi – bien que nous +trouvions de nombreux maillons – nous ne trouvons pas d’interminables variétés, +reliant entre elles toutes les formes éteintes [0m [3met vivantes par les +étapes les plus finement graduées. Il faudrait aussi avoir constamment à +l’esprit que toute variété qui se trouverait faire le lien entre deux ou +plusieurs forme(s) serait infailliblement [0m [3mconsidérée, à moins que +l’on ne pût reconstituer parfaitement la chaîne entière, comme une espèce +nouvelle et distincte ; car on ne prétend point détenir de critère certain +permettant d’effectuer la [0m [3mséparation entre espèces et variétés +[0m» + +(chap. XI, résumé des chap. X et XI). + +Une erreur courante des adversaires – anciens et modernes – de Darwin consiste +à se tromper sur la notion même de forme intermédiaire. Le chapitre X contient +à ce sujet un second paragraphe lumineux : + +« [3mEn premier lieu, il faudrait toujours avoir à l’esprit quelle sorte de +formes intermédiaires a dû, d’après la théorie, exister autrefois. J’ai éprouvé +la difficulté qu’il y a, lorsque l’on regarde deux [0m [3mespèces +quelconques, à éviter de se représenter des formes directement intermédiaires +entre elles. Mais c’est là une vision totalement fausse ; ce que nous devrions +toujours rechercher, ce sont des formes [0m [3mintermédiaires entre chaque +espèce et un ancêtre commun et inconnu ; et cet ancêtre aura généralement +différé à certains égards de tous ses descendants modifiés. Pour en donner une +simple illustration : les [0m [3mpigeons paon et grosse-gorge descendent +tous deux du pigeon de roche ; si nous possédions toutes les variétés +intermédiaires qui ont existé, nous aurions une série extrêmement précise entre +chacune de ces [0m [3mdeux variétés et le pigeon de roche ; mais nous +n’aurions pas de variétés directement intermédiaires entre le paon et le +grosse-gorge ; aucune qui, par exemple, réunît à la fois une queue quelque peu +étalée [0m [3met un jabot quelque peu dilaté, traits caractéristiques de +ces deux races. En outre, ces deux races se sont tellement modifiées que, si +nous n’avions de témoignage historique ou indirect concernant leur [0m +[3morigine, il aurait été impossible de déterminer, par une simple +comparaison de leur structure avec celle du pigeon de roche, C. livia, si elles +descendent de cette espèce, ou de quelque autre espèce voisine [0m [3mtelle +que C. œnas. [0m» + +On ne saurait surévaluer chez Darwin l’importance capitale de la longue durée +des temps géologiques. À ce réquisit foncier de sa théorie s’adresse en +particulier une objection qui le tourmentera fortement, sans toutefois qu’il en +ait laissé s’inscrire une trace proportionnée dans L’Origine des espèces : +celle du célèbre thermodynamicien William Thomson (1824-1907), le futur lord +Kelvin. En voici le contenu. + +L’uniformitarisme de Lyell était, on l’a vu, une composante fondamentale de la +géologie darwinienne, qui tendait, en accord avec la théorie de la descendance +modifiée, à attribuer à la Terre la grande ancienneté requise par l’extrême +lenteur des changements évolutifs. Or, Thomson pensa tirer des données de la +thermodynamique la preuve de l’impossibilité pour la Terre, en raison de son +degré insuffisant de refroidissement, d’être aussi ancienne. Suivant les +conclusions de ses travaux d’alors en ce domaine (1862-1869), l’âge de la Terre +pouvait être compris entre 20 et 200 millions d’années, durée trop courte pour +le gradualisme darwinien. C’est Huxley qui lui répondit en février 1869, et +Darwin ne fit que souligner dans la dernière édition de L’Origine +l’insuffisance des connaissances relatives à la constitution physique de +l’intérieur du globe pour raisonner avec sûreté sur son âge. Il faudra attendre +Pierre Curie (1859-1906), Ernest Rutherford (1871-1937) et le développement des +études sur la radioactivité (dont la perte fournit une énergie qui explique le +« retardement » du refroidissement) pour que raison soit donnée aux biologistes +darwiniens. + +Comme William Thomson, dont il était un proche, l’ingénieur écossais Fleeming +Jenkin (1833-1885) a combattu Darwin. Sa critique, parue dans la North British +Review au mois de juin 1867, et portant elle aussi le sceau de la physique, a +été écoutée. Elle comportait cinq arguments principaux : + +les limites de la variation, liées à la force de la tendance au retour, qui +s’oppose à ce qu’un variant échappe à l’attraction du type, représenté comme le +centre d’une sphère dont aucune forme variante ne saurait franchir l’enveloppe +; + +le pouvoir réel de la sélection naturelle, limité selon lui à l’amélioration +d’organes déjà existants, mais incapable d’en former de nouveaux. Il distingue +la variation ordinaire et les sauts brusques (« sports »), dont le calcul +montre que leur fixation dans une espèce est très peu probable. La sélection et +le gradualisme ne sauraient combattre la dilution due aux croisements (cela, +bien sûr, dans le cadre d’une théorie de l’hérédité mélangeante) ; + +le temps géologique trop court (reprise commentée de l’argument de Thomson) ; + +les difficultés de la classification, qui sont un phénomène commun à la plupart +des systèmes classificatoires (problème de la délimitation des espèces) et qui +ne militent pas nécessairement en faveur de la transmutation ; + +diverses observations, liées notamment aux données biogéographiques, paraissent +donner raison à Darwin, mais n’entraînent pas que sa théorie soit la seule à +pouvoir les expliquer. + +Afin de répondre à ce qu’il pensait être la plus sérieuse de ces critiques +(celle fondée sur la dilution nécessaire de toute variation individuelle), +Darwin émit l’hypothèse qu’une variation pouvait affecter un certain nombre +d’organismes, sauvant ainsi la possibilité de la sélection et de l’évolution +graduelle. Il faudra attendre la redécouverte des lois de Mendel (loi de +ségrégation des caractères) et l’avènement de la génétique pour que les +objections telles que celle de Jenkin perdent leur pertinence. + +Le plus important des adversaires de la théorie sélective, le juriste et +zoologiste converti au catholicisme saint George Jackson Mivart (1827-1900), +proche d’Owen, partage avec ce dernier le privilège d’être plusieurs fois cité +dans L’Origine. En 1871, il publie On the Genesis of Species, ouvrage dans +lequel il expose sa croyance en un pouvoir divin moteur et guide de +l’évolution, et adresse à la théorie de la sélection naturelle les critiques +suivantes – résumées ici, de même que celles de Jenkin, d’après une étude de +J.-M. Goux destinée au congrès international Pour Darwin [2] : + +La sélection naturelle ne saurait favoriser les variations (non encore +adaptées) figurant les stades initiaux des structures utiles. + +Elle ne s’harmonise pas avec la coexistence de structures étroitement +similaires dans des groupes éloignés. + +Les différences spécifiques peuvent s’être développées soudainement, et non +graduellement. + +L’opinion suivant laquelle les espèces ont des limites définies, quoique très +différentes, est toujours recevable. Mivart cite, à ce propos, le modèle de la +sphère de Jenkin. + +Mivart interprète la question du temps disponible d’après W. Thomson tout en +défendant le point de vue d’une évolution dirigée. + +Certaines formes intermédiaires fossiles sont absentes, alors qu’on aurait pu +les attendre. + +Certains faits de distribution géographique renforcent d’autres difficultés de +la théorie. + +L’objection de la différence physiologique entre « espèce » et « race » +(variété) n’est pas réfutée. + +Il existe de nombreux phénomènes remarquables sur lesquels la sélection +naturelle ne jette aucune lumière, en particulier les homologies (latérales ou +sérielles). + +Il n’est pas possible d’expliquer la « moralité », l’existence du bien, +l’esprit de sacrifice, la dévotion à Dieu en termes darwiniens. Cette objection +ouvre sur les nombreux développements que Darwin consacre au cours de la même +année à la question de l’évolution mentale et morale de l’Homme et des sociétés +humaines. + +La pangenèse est pour Mivart une explication du type obscurum per obscurius +(c’est-à-dire consistant à « éclairer » une chose obscure par une explication +plus obscure encore). + +De ces critiques, seule la première inquiétera vraiment Darwin : une variation +ne peut être avantageuse que lorsqu’elle aboutit à une modification organique +achevée, adaptée et fonctionnelle. Or, les stades commençants d’une variation +ne pouvant d’emblée être détenteurs de ce degré d’utilité, ils ne peuvent donc +être sélectionnés. + +Darwin consacrera une partie du chapitre additionnel (VII) de la 6e édition de +L’Origine à y répondre dans le plus grand détail. Mais il y a déjà globalement +répondu dans le chapitre VI à travers ce qui deviendra en 1875, chez son +disciple Anton Dohrn (1840-1909), le principe du changement ou de la succession +de fonctions. + +Un organe possède en effet plusieurs fonctions. Si l’on prend le cas d’un +organe jouissant d’une fonction principale et d’une fonction secondaire, il se +peut que progressivement, sous la pression des conditions, l’ordre de +l’importance fonctionnelle s’inverse, et que la fonction secondaire devienne +prépondérante, aboutissant à une transformation de l’organe au cours de +laquelle chaque stade de modification voit croître ses avantages adaptatifs +sans interruption de sa fonctionnalité générale. De même, l’argument consistant +à estimer impossible la constitution, par accumulation de petites variations +sélectionnées, de la perfection structuro-fonctionnelle de l’œil d’un Vertébré +disparaît si l’on admet que chaque stade de complexification de cet organe a +été effectivement retenu comme présentant un degré d’utilité supérieur dans +l’adaptation à leurs milieux respectifs des divers organismes composant +l’échelle animale. La formation des instincts spéciaux de certaines espèces +(coucous, fourmis esclavagistes, abeilles) s’explique de même non par une +tendance « merveilleuse » déchiffrée à l’aide d’une grille anthropomorphique ou +théologique, mais, utilitairement, par l’avantage vital, en l’occurrence +reproductif, lequel est susceptible de s’investir dans des formes originales, +voire apparemment paradoxales, d’organisation communautaire. + +Des principales objections adressées à la théorie darwinienne, on retiendra ici +deux constantes : + +elles se recoupent mutuellement sur le mode de la reprise ou du remaniement. +Mivart, par exemple, reprend Jenkin, qui reprend Thomson, et ses objections +seront elles-mêmes reprises plus tard par le zoologiste français Louis +Vialleton (1859-1929), et ainsi de suite jusqu’aux critiques réitérées dans la +période contemporaine, qui ont toutes, de Michaël Denton aux antidarwiniens +marginaux actuels, répété d’une façon consciente ou inconsciente les mêmes +objections, évidemment de moins en moins recevables et de plus en plus +anachroniques en dépit de leurs efforts d’actualisation ; + +elles ont toutes, d’une manière plus ou moins apparente ou voilée, un +arrière-fond théologique. La reconnaissance tardive par le pape Jean-Paul II de +la consistance du darwinisme, si elle a manifesté un recul forcé de la +théologie dogmatique et des prétentions de l’Église, a toutefois évidemment +préservé – en soustrayant du processus naturel la conscience humaine, qu’il +déclare issue d’un don transcendant – un dualisme incompatible avec toute la +théorie biologique de l’évolution des facultés. Car il existe une histoire +naturelle de la conscience, de l’intelligence et même des capacités morales, ce +que Darwin, lui, avait depuis longtemps compris, expliqué et illustré. + +[1mNotes [0m + +[1] Cette dernière phrase de Darwin paraît relativiser beaucoup la nouveauté +revendiquée par le modèle des « équilibres ponctués » de N. Eldredge et S. J. +Gould. + +[2] P. Tort (dir.), Pour Darwin, Paris, PUF, 1997. + +[1mChapitre VIII [0m + +[1mLes dix dernières années [0m + +Après 1872, Darwin travaillera avec constance à la réédition de plusieurs +ouvrages antérieurs (Récifs de corail, Filiation de l’homme, Variation, +Fécondation des orchidées). En même temps, soucieux d’approfondir certaines +réflexions propres à apporter des compléments d’illustration à sa théorie, il +renforcera son travail dans quelques voies de recherche déjà frayées : facultés +psychiques et instincts des animaux inférieurs, fécondation végétale, +opportunité des croisements, mouvement des plantes, résistance des végétaux et +des semences, transport des éléments reproductifs. + +Son ouvrage sur Les Plantes insectivores paraît en 1875 (2 juillet), suivi en +novembre de l’édition en volume de son essai sur Les Mouvements et les +habitudes des plantes grimpantes, et en 1876 (10 novembre) d’un travail +important sur Les Effets de la fécondation croisée et de l’autofécondation dans +le règne végétal. Au cours de cette dernière année, il rédige à l’intention de +sa famille son Autobiographie, dont son fils Francis, obéissant aux souhaits +puritains d’Emma Darwin, veuve soucieuse de ne pas choquer les sentiments +religieux et les personnes, inclura une version expurgée en tête du premier +volume de son édition de Life and Letters of Charles Darwin (1887). En 1877 (6 +juillet), Darwin publie son livre sur Les Différentes Formes de fleurs dans les +plantes de la même espèce qui approfondit la question des croisements, comme le +fait également la deuxième édition de son ouvrage sur La Fécondation des +orchidées par les insectes. + +En 1879, Darwin rédige un long essai préfaçant la traduction anglaise de la +biographie de son grand-père Erasmus par Ernst Krause (1839-1903), ouvrage dont +la publication en novembre déclenchera d’âpres attaques de la part d’un ancien +admirateur, l’écrivain évolutionniste finaliste Samuel Butler (1835-1902), +évoqué par Krause comme un esprit attardé désireux de redonner une actualité +impossible aux vieilles doctrines d’Erasmus. En 1880 paraît le travail, préparé +avec la collaboration de Francis, sur La Faculté motrice des plantes. L’année +suivante, Darwin doit s’engager dans la querelle autour de la vivisection qu’il +défend au nom des avancées de la physiologie et de la sauvegarde des vies +humaines, tout en recommandant la plus grande « humanité » envers les animaux – +sentiment dans lequel La Filiation de l’homme reconnaissait déjà « l’une des +dernières acquisitions morales ». Le 10 octobre 1881, un mois et demi après la +mort de son frère Erasmus Alvey (26 août), paraît son dernier livre, La +Formation de la terre végétale par l’action des vers, avec des observations sur +leurs habitudes, qui tente de mettre en lumière l’action géologique +transformatrice que ces animaux exercent au cours de longues périodes ainsi que +l’existence chez eux, au-delà de l’instinct, d’une forme embryonnaire +d’intelligence acquisitive révélée par leur comportement. + +L’année 1882 laissera à Darwin le temps de présenter ses travaux sur l’action +de certaines substances chimiques sur les tissus végétaux, sur la dispersion +géographique des Bivalves d’eau douce, sur le comportement animal ; de préfacer +en outre une édition anglaise de l’ouvrage d’August Weismann Studies in the +Theory of Descent et une autre d’un livre de Hermann Müller (1829-1883) – le +frère de Fritz –, The Fertilisation of Flowers, qui paraîtra l’année suivante. +Le 19 avril, il s’éteint dans sa demeure de Down House. Sans doute aurait-il +vécu plus longtemps sans l’épuisement périodique que lui avait procuré pendant +quatre décennies la maladie de Chagas (trypanosomiase transmise par la morsure +toxique d’une punaise du genre Reduvius), contractée au cours de son voyage en +Amérique du Sud. Une semaine plus tard, le 26, accompagné par sa famille, par +ses plus proches amis scientifiques, mais aussi par des représentants de +l’aristocratie victorienne et par une foule de personnalités très diverses +réunies pour honorer en lui à la fois un savant éminent, un homme de progrès et +un citoyen demeuré toujours formellement respectueux des valeurs +traditionnelles, il est glorieusement inhumé sous les voûtes de l’abbaye de +Westminster. + +[1mChapitre IX [0m + +[1mLe darwinisme et la biologie moderne [0m + +Le siècle qui a suivi la mort de Darwin a produit les avancées les plus +remarquables de l’expérimentation et de la théorie biologiques. Sans cesser +d’être la cible de la lutte idéologique défensive des théologiens et des +tentatives concordistes du spiritualisme philosophique et de la théologie +naturelle (Henri Bergson et Pierre Teilhard de Chardin peuvent être +respectivement considérés comme des représentants « modernes » de ces dernières +tendances), le transformisme – c’est-à-dire la théorie de la dérivation des +espèces à partir d’ancêtres communs –, dans sa version darwinienne ou +néolamarckienne, a fini par s’imposer comme la seule interprétation +scientifiquement acceptable des données complexes offertes à l’observation des +naturalistes. + +Les dernières décennies du xixe siècle avaient à résoudre cependant d’immenses +questions, dont celle de la nature des variations – laissée en suspens par +Darwin qui en avait toutefois indiqué l’importance cruciale – et celle, liée, +des lois de la transmission héréditaire. + +[1mI. – Weismann et le néodarwinisme [0m + +À la suite de recherches embryologiques et d’une réflexion sur la durée de vie +naturelle des individus biologiques (caractère ne pouvant dépendre d’aucun +déterminisme autre que l’hérédité), le biologiste allemand August Weismann +(1834-1914), dans un mémoire sur l’hérédité (Über Vererbung, 1883), développant +les thèses de Gustav Jaeger (1832-1917), produit sa théorie de la continuité du +plasma germinatif. Comme l’avait postulé Jaeger en 1878, chaque être vivant se +compose d’un soma, corps composé de cellules périssables et d’un germen, qui +est l’ensemble de ses cellules « germinatives » (dites encore « germinales », +ou sexuelles), et qui détient une immortalité virtuelle liée à la reproduction, +dont il est l’agent. Cette distinction fondant en nature une incommunicabilité +entre les cellules du corps et les cellules spécialisées qui constituent la +lignée germinale, la théorie de Weismann, pour qui le « plasma germinatif », +contenu dans le noyau de la cellule sexuelle, descend en ligne directe d’une +cellule germinale primordiale, évacue la possibilité de la transmission +héréditaire des modifications acquises par le soma au cours de son existence, +ce qui rompt à l’évidence tout compromis possible avec le lamarckisme (et +l’éloigne de ce fait des premiers travaux qu’il avait réalisés sous l’influence +de Haeckel, mais aussi des concessions de Darwin à l’hérédité de l’acquis, en +particulier celles que tendait à légitimer l’hypothèse provisoire de la +pangenèse). Weismann demeura cependant un partisan extrême du sélectionnisme +darwinien, situant dans le processus recombinant de l’union reproductive la +source de variation nécessaire pour nourrir la dynamique sélective. Les +validations expérimentales de sa théorie paraissent étranges, si l’on en croit +la tradition qui veut qu’il ait coupé la queue à nombre de générations +successives de rats pour vérifier une non-transmission dont la pratique humaine +séculaire de la circoncision, par exemple, avait déjà amplement démontré le +fait. Bien que Weismann ait nuancé sensiblement ses positions dans la dernière +partie de sa carrière, la notion moderne de « cellule totipotente » est un +argument de plus en faveur de la séparation soma/germen. Le travail de Weismann +a permis de dépasser, sur la voie qui conduit aux chromosomes de Waldeyer et à +la génétique future, des objections longtemps maintenues (de Herbert Spencer à +William McBride, d’Edmond Perrier à Alfred Giard, Félix Le Dantec, Yves Delage +et Étienne Rabaud, d’Ernst Haeckel à Paul Kammerer, d’Edward Drinker Cope, +Alpheus Spring Packard et Alpheus Hyatt à Henry Fairfield Osborn et William +McDougall, mais aussi à Staline, Mitchourine et Lyssenko) par les +néolamarckismes européen, américain et soviétique. + +[1mII. – Mutationnisme, hérédité mendélienne et darwinisme [0m + +Si le travail de Weismann, à la différence de celui du botaniste Carl von +Nägeli (1817-1891) – qui, tout en partageant avec lui l’héritage de la théorie +cellulaire issue de Matthias Schleiden et de Theodor Schwann, ainsi que la +distinction entre deux formes de lignées cellulaires dans la constitution des +êtres vivants, était demeuré assez imprégné de lamarckisme et de recherche des +causes mécaniques pour négliger l’importance des résultats formels que Mendel +lui avait transmis –, a permis un affranchissement réel de la théorie de +l’hérédité, d’autres biologistes, tel le botaniste Hugo De Vries (1848-1935), +ont participé d’une façon encore plus directe à l’introduction de la nouvelle +science, ainsi qu’au remodelage subséquent de la théorie de l’évolution. + +De Vries, spécialiste de cytologie végétale, s’était déjà intéressé aux +mécanismes cellulaires de la croissance des plantes et à des expériences +d’hybridation lorsqu’il découvrit en 1886 les « mutations » (en réalité des +effets de recombinaison ou de polyploïdie – multiplication du nombre des +chromosomes, très fréquente chez les végétaux –) de la plante Œnothera +lamarckiana. Son souci de produire une explication de l’indépendance des +caractères constatée chez les hybrides aboutit trois ans plus tard à la +publication de sa Pangenèse intracellulaire, qui propose, sous un titre +clairement inspiré de Darwin, une théorie particulaire de l’hérédité où les +gemmules sont remplacées par les pangènes, supports et véhicules des caractères +héréditaires, reproductibles, situés dans le noyau cellulaire et composés de +molécules chimiques. Lors de la fécondation (fusion des noyaux cellulaires), +chaque participant transmet ses pangènes, à chacun desquels correspond un +caractère. L’expression d’un caractère est liée au passage d’un pangène vers le +cytoplasme. Ainsi, tout le protoplasme vivant – même le plus primitif – est +constitué de pangènes, actifs ou inactifs. Il est le lieu du développement des +caractères, tandis que le noyau – où sont représentés à l’état latent tous les +types de pangènes – est celui de leur transmission. + +Rompant comme Weismann avec l’idée lamarckienne d’une transmission de +caractères acquis, De Vries rejette cependant de facto l’idée d’une séparation +radicale du germinal et du somatique. Chez Darwin, le cadre du transfert des +gemmules était l’organisme entier. Chez De Vries, ce transfert a lieu dans le +cadre étroit de la cellule. À côté de la simple variation fluctuante dépendant +de la proportion variable des différents types de pangènes, la « mutation », +créatrice de nouvelles espèces, provient de la dissemblance accidentelle des +deux nouveaux pangènes issus par division d’un pangène père – et de ses +conséquences transformatrices sur les caractères visibles de l’organisme. La +variation brusque (saltation) se substitue, pour De Vries, à l’accumulation +graduelle des petites variations, qui était pour Darwin le procédé majeur des +transformations évolutives. + +Cette théorie devait préparer De Vries à la fameuse « redécouverte » des lois +de Mendel (publiées par le moine morave en 1866, mais passées inaperçues ou +négligées pendant un tiers de siècle) – redécouverte dont on le gratifie +ordinairement, en soulignant qu’elle fut indépendante de celles qu’effectuèrent +à la même époque, isolément, le botaniste allemand Carl Correns (1864-1933) et +son homologue autrichien Erich von Tschermak (1871-1962). Elle se produit en +1900, avec, pour ce qui concerne De Vries, la publication, en France et en +Allemagne, de courts articles tels que celui intitulé « Sur la loi de +disjonction des hybrides » [1], où il expose l’équivalent de la loi +mendélienne de ségrégation des caractères et inaugure la génétique moderne. +Celle-ci réinstalle une vision préformationniste des transformations organiques +qui va s’opposer tendanciellement à l’épigenèse mécaniste dont s’inspirait +jusqu’alors, de Lamarck à Haeckel, l’ensemble du grand discours évolutionniste. + +La redécouverte des lois de Mendel va être suivie d’une accélération de la +recherche expérimentale et théorique sur les mécanismes de l’hérédité, avec les +travaux du Danois Wilhelm Johannsen (1857-1927), inventeur du terme de « gène » +et de la distinction entre génotype et phénotype, et de l’Américain Thomas Hunt +Morgan (1866-1945), qui commence en 1910 ses expériences sur les mutations des +drosophiles, lesquelles vérifient les lois mendéliennes et confirment le +mutationnisme, en concluant vers 1917 à la négation de la fonction créatrice de +la sélection naturelle, cantonnée dans un rôle secondaire d’élimination ou de +conservation des variations. S’il est clair que le génotype ne peut être +affecté par les influences reçues de l’extérieur, la valeur sélective d’une +mutation ne saurait toutefois être indépendante d’un milieu au sein duquel +cette dernière se révèle avantageuse ou désavantageuse. + +Dès 1902, William Bateson (1861-1926) avait publié une défense des principes de +l’hérédité de Mendel et entamé une croisade en faveur de la science nouvelle, +en particulier contre le gradualisme des biométriciens darwiniens de l’École de +Pearson, lui-même héritier de Galton et d’un darwinisme réduit et réinterprété, +qui défendaient quant à eux la théorie de l’hérédité mélangeante (blending +inheritance), compatible avec la théorie de l’accumulation des petites +variations. Cette bataille dura jusqu’à ce que fussent mis en lumière +expérimentalement, par Hermann Joseph Muller (1890-1967) et Lewis John Stadler +(1896-1954), en 1926, les mécanismes de la mutagenèse, mais peut être +considérée comme théoriquement achevée, dans le sens d’une conciliation avec +les développements mêmes de la recherche sur l’action des gènes : mise en +évidence du contrôle polygénique des caractères, étude expérimentale de la +sélection, découverte des gènes modificateurs, de l’influence de +l’environnement sur l’expression phénotypique, de la pléiotropie (un seul gène +contrôlant plusieurs caractères), du contrôle des caractères quantitatifs par +plusieurs paires de gènes exerçant une action allélique additionnelle, tous +éléments aboutissant à intégrer dans l’édifice mendélien des effets identiques +à ceux d’une hérédité mélangeante. Au début des années 1920, la contradiction +n’a plus lieu de subsister, et les conditions sont réunies pour une +reviviscence de la théorie sélective darwinienne intégrant les données de la +génétique. + +[1mIII. – La théorie synthétique de l’évolution [0m + +La fameuse « crise du transformisme » qui, dans un contexte de spécialisation +accélérée des disciplines biologiques, agita la communauté des chercheurs +concernés par l’évolution était donc le fruit de trois antagonismes majeurs : +celui qui opposait les néodarwiniens (lesquels, avec Weismann, rejetaient +l’hérédité des caractères acquis) aux néolamarckiens (qui la maintenaient comme +condition du changement évolutif) ; et celui qui opposait les partisans +mutationnistes de la génétique mendélienne aux « darwiniens » (principalement +les biométriciens issus de l’École de Galton et Pearson) adeptes de la +conception orthodoxe d’une évolution graduelle par sélection de petites +variations. Il est à peine besoin d’évoquer enfin l’opposition frontale entre +les néolamarckiens et les généticiens, les premiers usant leurs forces à tenter +de prouver expérimentalement l’hérédité de certaines caractéristiques ou +habitudes acquises. Le premier et le dernier de ces conflits devaient se +résoudre par la défaite objective du néolamarckisme devant la génétique. Le +second allait se résorber, comme nous venons de l’indiquer, par l’effet des +progrès de cette même science, notamment à travers l’avènement de la génétique +des populations ou génétique évolutive. + +Mais il importe de noter que le « néodarwinisme » de Weismann était déjà, à +travers sa rupture avec le néolamarckisme et à travers ses intuitions sur +l’origine de la variabilité dans la recombinaison sexuelle, en accord potentiel +avec la future science de l’hérédité. Le nom de « néodarwinisme », que l’on +étend souvent à la « théorie synthétique », au lieu d’être un simple +anachronisme, porte peut-être la trace de cette « préadaptation ». + +[1m1. Génétique des populations, biométrie, darwinisme : la synthèse +fishérienne [0m + +La génétique des populations étudie les fréquences des gènes nouveaux (allèles) +issus de mutations au sein de populations d’individus appartenant à une espèce, +et les variations éventuelles de ces fréquences, source de variabilité donnant +prise à la sélection. En 1908, sans communiquer entre eux, le mathématicien +anglais Godfrey Harold Hardy (1877-1947), sous l’impulsion du généticien +Reginald Crundall Punnett (1875-1967), et le gynécologue allemand Wilhelm +Weinberg (1862-1937) montrèrent par des calculs appropriés que dans une +population où la reproduction s’effectue au hasard (panmixie), et dans des +conditions idéales (effectif illimité, aucune mutation, aucune sélection, +aucune migration), la composition génétique des différentes générations +successives sera stable (équilibre de Hardy-Weinberg). Cette découverte ne sera +exploitée qu’à partir des années 1920, lorsque trois biologistes également +isolés, les Anglais Ronald Aylmer Fisher (1890-1962) et John Burdon Sanderson +Haldane (1892-1964), et l’Américain Sewall Wright (1889-1988), appliqueront +l’outil statistique à l’étude génétique des populations. + +En 1918, Ronald Aylmer Fisher, biométricien darwinien d’obédience galtonienne, +mais ouvert à la génétique mendélienne, publie un article qu’il s’est vu +refuser par Pearson pour la revue Biometrika. Dans cet article, Fisher +réinterprète l’évolution à partir du mendélisme. La suite de ses travaux +aboutit, en 1930, à leur synthèse dans l’ouvrage intitulé The Genetical Theory +of Natural Selection. C’est le mécanisme de l’hérédité mendélienne qui rend +compte de l’équilibre de Hardy-Weinberg (stabilité transgénérationnelle de la +fréquence des variants), lequel alimente la sélection. Le travail théorique de +Fisher embrassera une foule d’autres sujets, tous importants pour l’évolution : +évolution de la dominance sous l’action de gènes modificateurs, évolution du +sexe ; vigueur hybride ; génétique et écologie ; équation entre l’accroissement +de la valeur sélective (fitness) d’une population et sa variance génétique (« +théorème fondamental de la sélection naturelle ») ; mimétisme, etc. Sa passion +eugéniste, héritée de Galton, si elle a pu susciter nombre de ses découvertes, +a cependant jeté sur le caractère de sa pensée et de son travail un double +voile d’erreur et de discrédit. + +[1m2. L’apport de Haldane [0m + +Entre 1924 et 1934, le biochimiste, biométricien et généticien anglais John +Burdon Sanderson Haldane va, tout en confirmant les résultats principaux de +Fisher, leur apporter des correctifs et des développements dont certains +déboucheront sur le travail expérimental. Fisher estimait que les processus +évolutifs révélaient leur maximum d’efficacité à l’intérieur de vastes +populations. Haldane pensait au contraire que la spéciation présentait une plus +haute probabilité au sein des populations plus petites, position qui concordait +avec celle qu’allait argumenter de son côté S. Wright. Dans la première partie +(1924) de son long travail intitulé « A Mathematical Theory of Natural and +Artificial Selection », il s’attache à déterminer mathématiquement l’intensité +de sélection minimale requise pour assurer le mélanisme de la phalène du +bouleau (Biston betularia) dans les régions industrielles anglaises. Presque +trente ans plus tard, en 1953, l’entomologiste anglais H. Bernard D. Kettlewell +(1907-1978) publiera la vérification expérimentale de ces prévisions en mettant +en évidence par comptage le fait que les mutants noirs de ce papillon, moins +repérables par les oiseaux sur les supports noircis des zones industrielles, +survivaient en grand nombre, à l’inverse de la souche claire, plus repérable et +devenue victime de la prédation. L’observation naturaliste confirmait ainsi un +calcul mathématique et convergeait avec lui pour corroborer l’efficacité du +processus sélectif. Les travaux de Haldane sur la fixation d’un gène mutant +dans une population, sur les mutations délétères, sur la consanguinité et sur +le coût de la sélection constituent autant d’avancées remarquables du +darwinisme moderne, qui soumet à la sélection le matériau mutationnel. Membre +du parti communiste entre 1942 et 1950, il prit position dès 1938 contre le +racisme et mit en garde contre l’eugénisme. + +[1m3. Wright et la dérive génétique [0m + +D’abord spécialisé dans la génétique physiologique, formé à l’École morganienne +aux côtés de William Ernest Castle (1867-1962), Sewall Wright expérimente sur +le pelage du cochon d’Inde, matériel qui avait permis à ce dernier d’obtenir +une maîtrise génétique de la variation graduelle conduite par sélection +artificielle. Il publie sa thèse en 1916 et participe à la mise en évidence de +l’action des gènes modificateurs (1919). Mais il a sans doute compris avant +1915 l’importance de l’interaction génique. C’est comme généticien +zootechnicien attaché à l’amélioration du bétail qu’il examinera à partir de +cette date les questions de consanguinité (en particulier chez les bœufs +shorthorn, qui intéressaient déjà Darwin). Il prend connaissance des travaux de +Fisher, mesure des corrélations gamétiques, évalue l’effet alterné de la +reproduction consanguine et des croisements. Il doute alors de la pertinence de +la représentation fishérienne des grandes populations panmictiques comme cadre +de prédilection de l’action sélective dans la nature et construit sa propre +théorie entre 1925 et 1931. Elle requiert des populations de taille plus +restreinte, au sein desquelles et entre lesquelles opèrent les phénomènes de +consanguinité, de croisement, de mutation, de sélection, de migration et de +dérive aléatoire (fluctuation au hasard aboutissant à la fixation ou à la +disparition d’un allèle indépendamment de sa valeur adaptative). C’est un +équilibre mobile entre ces facteurs qui assure à chaque instant la structure +génétique des populations, la sélection agissant à l’intérieur des groupes et +entre les groupes en intégrant le facteur de la dérive. C’est la shifting +balance theory (« théorie de l’équilibre fluctuant »). Ce modèle inspirera +aussi bien les créateurs darwiniens de la théorie synthétique, tels Theodosius +Dobzhansky (1900-1975), Ernst Mayr (1904-2005) et George Gaylord Simpson +(1902-1984), que leurs adversaires neutralistes (James Franklin Crow [né en +1916], Motoo Kimura [1924-2000], Jack Lester King [1934-1983] et Thomas H. +Jukes [1906-1999]), les paléontologues promoteurs du modèle des équilibres +intermittents (punctuated equilibria), Niles Eldredge (né en 1943) et Stephen +Jay Gould (1941-2002), partisans d’une alternance de périodes d’évolution +accélérée et de périodes de stase longue, ou encore les travaux du Français +Maxime Lamotte sur les populations de l’escargot des haies Cepaea nemoralis. + +[1m4. Teissier, L’Héritier et les cages à population [0m + +En 1933, deux biologistes français, Georges Teissier (1900-1971) et Philippe +L’Héritier (1906-1994), mettent au point le dispositif expérimental qui va leur +permettre de confirmer la validité de la théorie sélective darwinienne tout en +vérifiant son accord avec les calculs de la génétique mathématique des +populations. + +Ils fabriquent des cages dans lesquelles ils élèvent des mouches du vinaigre +(drosophiles) en ajustant la quantité de nourriture à la survie de 3 000 +individus. La drosophile se reproduit en trois semaines et chaque couple +produit plus de 100 œufs. La mortalité doit donc être très forte à chaque +génération. On mêle alors deux populations différant seulement par une mutation +aisément repérable : une population « œil Bar » (diminution du nombre des +facettes oculaires), et une population « sauvage » composée d’individus « œil +normal ». Si la mortalité n’est pas le fruit du hasard, on doit pouvoir +assister à un phénomène de survie différentielle et à la mise en évidence d’un +avantage vital propre à l’une des deux populations en présence. On assiste +alors à la disparition tendancielle du caractère « Bar ». Pour d’autres +mutations, telle « ebony » (pigmentation noire), on assiste à l’établissement +d’une quasi-stabilité alors que le nombre des mutants est encore élevé. + +La sélection naturelle ne fait donc pas qu’éliminer les déviants, mais peut +dans certains cas maintenir la variabilité génétique. Dobzhansky en tirera son +idée de polymorphisme équilibré, assurant une sélection adaptative constante. +Teissier et L’Héritier, au terme d’autres expériences, mirent également en +évidence le fait que les chances de survie des larves « Bar » augmentaient +sensiblement lorsque celles-ci étaient devenues rares dans la population de la +cage. C’est le fameux « avantage du rare » dont l’interprétation biologique fut +développée ensuite par Claudine Petit qui, comparant les courbes théoriques et +celles issues des résultats obtenus en laboratoire, mit en lumière d’une part +l’existence parallèle d’une sélection larvaire et d’une sélection sexuelle en +faveur des mâles « sauvages » aux fréquences élevées, et d’autre part une +sélection sexuelle plus favorable aux « Bar » devenus rares. L’ensemble du +travail scientifique développé autour des cages à populations (ou démomètres) +dans le monde entier donnait ainsi une formidable consistance aux concepts +darwiniens, tout en activant la convergence entre l’observation naturaliste, +l’expérimentation de laboratoire et la mathématisation de la biologie, sur la +voie d’un darwinisme moderne. + +[1m5. La « synthèse moderne » [0m + +Évoquant le conflit entre, d’une part, l’option saltationniste des premiers +généticiens (William Bateson, Hugo De Vries, Wilhelm Johannsen), leur +réductionnisme et leur rejet de la sélection naturelle, et d’autre part les +choix holistes (c’est-à-dire considérant la totalité, en l’occurrence +l’organisme individuel ou les systèmes écologiques) des naturalistes, Ernst +Mayr, l’un des artisans de la « théorie synthétique de l’évolution », résume +ainsi son émergence comme reconnaissance et ajustement mutuels des disciplines +appelées à collaborer dans l’explication des phénomènes évolutifs : + +« Il a semblé pendant longtemps qu’aucune compréhension mutuelle ne pouvait +s’établir entre les camps opposés, en particulier les généticiens +réductionnistes et les naturalistes, plutôt holistes. Pourtant, en quelques +années, de 1936 à 1950, un large accord fut réalisé. Ce fut possible lorsque +plusieurs groupes de généticiens (Morgan, East, Baur) insistèrent sur +l’importance des très petites mutations, tandis que d’autres (Fisher, Haldane +et Wright) démontraient leur valeur sélective, et que des naturalistes +systématiciens adoptaient la génétique et introduisaient les idées de la +systématique populationnelle dans la génétique évolutive (Četverikov et ses +étudiants, Timofeev-Resovskij, Dobzhansky). Le moment était venu d’une +réconciliation, et l’on parvint à une sorte de consensus parmi les disciplines +naguère rivales. Dans les pays anglophones, le nouveau mouvement fut impulsé +par la publication en 1937 de l’ouvrage de Dobzhansky, Genetics and the Origin +of Species, suivie des travaux de Julian Sorell Huxley (1942), Ernst Mayr +(1942), George Gaylord Simpson (1944) et George Ledyard Stebbins (1950). Dans +les pays de langue allemande, un mouvement parallèle fut conduit par les +publications de Nikolaj Vladimirovič Timofeev-Resovskij (1942) et de Bernhard +Rensch (1947). + +« Il en résulta ce que Huxley appela la synthetic theory. C’était simplement +une confirmation des principes de base du darwinisme, non pas une révolution, +mais plutôt l’unification d’un champ de bataille, jusqu’alors sévèrement +divisé, par l’information mutuelle et le développement d’une matrice +interdisciplinaire. Parmi les doctrines les plus caractéristiques de la théorie +synthétique, citons : l’hérédité est particulaire (non mélangeante) et +d’origine exclusivement génétique (pas d’hérédité des caractères acquis) ; il y +a une énorme variabilité dans les populations naturelles ; l’évolution se +déroule dans des populations distribuées géographiquement ; l’évolution procède +par modification graduelle des populations ; les changements dans les +populations sont le résultat de la sélection naturelle ; les différences +observées entre des organismes sont, pour une grande part, des adaptations ; la +macro-évolution n’est que la prolongation avec le temps de ces mêmes processus +qui contrôlent l’évolution des populations. + +« [3mL’acceptation de ces idées nécessitait la réfutation des trois théories +antidarwiniennes les plus répandues : saltationnisme, lamarckisme et +orthogenèse [0m [2]…» + +La sélection naturelle fut considérée comme un processus en deux temps. Dans le +premier temps, une quantité considérable de variations génétiques est produite +par mutation et recombinaison, tandis que le second temps, la sélection, au +sens restreint, consiste en la survivance et la reproduction des quelques +individus les mieux adaptés. + +« [3mLa sélection n’est ni un processus finalisé ni un processus +déterministe, mais elle est influencée aux deux étapes par des processus +stochastiques (hasard). La variation génétique est aléatoire en ce sens [0m +[3mqu’elle n’est ni induite par des conditions d’environnement spécifiques, +ni une réponse aux besoins de l’organisme. Cependant, bien entendu, elle est +étroitement contrainte, car il y a peu de degrés de [0m [3mliberté dans le +type de variation génétique possible pour n’importe quelle sorte d’organisme à +n’importe quel moment. [0m[3mLe pluralisme, c’est-à-dire le fait de trouver +plusieurs solutions différentes aux mêmes [0m [3mbesoins adaptatifs d’un +organisme, est tout à fait caractéristique de l’évolution darwinienne. La +sélection naturelle utilise toujours les variations disponibles dans le +génotype qui seraient les plus utiles. [0m [3mComme l’a dit F. Jacob, +l’évolution “bricole”. Cependant, à tous les changements de l’environnement +(arrivée de nouveaux ennemis, ouverture de nouvelles niches, apparition de +nouveaux organismes pathogènes, [0m [3metc.), il est répondu par des +changements adaptatifs. [0m» [3] + +Sous l’influence du travail de Mayr, en dépit de nombreuses discussions, les « +synthéticiens » tendront à privilégier une conception de la formation de +nouvelles espèces (spéciation) liée à l’existence de barrières géographiques. +C’est le mécanisme, considéré comme dominant, de spéciation allopatrique (ou +géographique), dont l’idée avait déjà été défendue par le naturaliste et +géographe allemand Moritz Wagner (1813-1887) dès 1841, puis en 1868 dans un +ouvrage (Migrationsgesetz) où il exposait sa « loi de migration », suivant +laquelle une nouvelle espèce ne pouvait naître que d’une portion détachée d’une +population de l’espèce mère (une « population fondatrice ») se développant à +l’abri d’une barrière d’isolement. Les données de la génétique moderne des +populations montrent que l’échantillon fondateur ne pouvant, du fait de sa +petite taille, être totalement représentatif de la diversité génétique de +l’ensemble de la population souche, il s’ensuivra un effet de « révolution +génétique » producteur de la divergence évolutive. + +Le « concept biologique de l’espèce » popularisé par Mayr et qui, pour ce qui +le concerne, remonte (au moins) à Buffon, met lui aussi l’accent sur +l’isolement reproductif, dont les mécanismes sont d’ailleurs multiples. + +Dans une période plus récente, les acquis remarquables de la biologie +moléculaire ont conduit à une confirmation générale du transformisme comme +théorie de la descendance modifiée : « L’analyse phylogénétique des familles de +macromolécules, écrit Vadim A. Ratner, telles que les fragments d’adn (gènes, +espaceurs…), les arn, les peptides et les protéines, est un moyen extrêmement +puissant pour étudier la microévolution moléculaire et pour établir la +taxonomie moléculaire en biologie. Les méthodes de cette analyse sont +suffisamment justifiées et élaborées. Elles nous donnent la possibilité +d’édifier des arbres phylogénétiques commençant par quelques individus +apparentés, puis reliant des espèces proches, et finissant par englober la +taxonomie générale du monde vivant. » [4] + +[1mIV. – L’objection neutraliste [0m + +La théorie synthétique a été discutée au cours des années 1980 à travers une +argumentation essentiellement mathématique qui s’est intitulée « théorie +neutraliste de l’évolution moléculaire ». Ses fondements empruntent du reste +aux mêmes théoriciens qui ont permis de construire la théorie synthétique. + +R. Fisher, reprenant l’idée de A. L. Hagedoorn, démontre que, dans une petite +population, la succession de la ségrégation mendélienne, lors de la méiose, et +de la fusion des gamètes entraîne la perte aléatoire de certains allèles par +rapport à d’autres (1921). S. Wright trouve la loi de la distribution +asymptotique des fréquences géniques de ces allèles (1931, 1937, 1938). Le +Français Gustave Malécot (1911-1998) traduit cette dynamique génique par un +processus stochastique (i.e. aléatoire) et donne sa solution en fonction du +temps (1945). Cette interprétation néodarwinienne décrit les conséquences +dynamiques des liens qui existent entre l’enrichissement de la variabilité +génétique par les mutations et son appauvrissement par la sélection et la +dérive génique fortuite. + +La très grande variabilité détectée au niveau des séquences peptidiques +(protéines) et des séquences nucléotidiques (adn) conduit Motoo Kimura à +privilégier extrêmement le rôle de la dérive génique fortuite (1969). Il s’en +explique dans The Neutral Theory of Molecular Evolution (1983). Son propos est +clair : « I would like to emphasize the importance of random genetic drift as a +major cause of evolution. We must be liberated, so to speak, from the selective +constraint posed by the neo-Darwinian (or synthetic) theory of evolution » [« +J’aimerais souligner l’importance de la dérive génique fortuite comme cause +majeure de l’évolution. Il nous faut être libérés, pour ainsi dire, de la +contrainte sélective postulée par la théorie néodarwinienne (ou synthétique) de +l’évolution »]. + +Le succès idéologique de cette théorie a été tout aussi important qu’il fut +éphémère, et sa réfutation scientifique aisée. + +John H. Gillespie montre qu’il s’agit d’un scénario mathématique parmi d’autres +également explicatifs (1984). Michael George Bulmer (1973) tente d’appliquer la +théorie neutraliste à l’interprétation de la structure génique d’une grande +population naturelle de drosophiles. Il trouve des incohérences. Aussi, +utilisant l’étude de la migration isotropique (déplacement équivalent des +individus dans tous les sens) de G. Malécot (1967), Bulmer conclut que +l’hypothèse du nombre infini de sites mutationnels et celle, associée, de la +dérive génique fortuite ne peuvent à elles seules rendre compte des faits +observés. L’échec de la théorie neutraliste tient à une raison essentielle : +la modélisation mathématique conçue par Kimura est insuffisante. Ainsi, Malécot +démontre qu’il n’y a pas de séparation tranchée entre les différents types de +populations infinies ou finies, structurées en sous-populations (ou dèmes), +contrairement aux présupposés de Kimura (1981). Michel Gillois relève +l’incohérence des définitions des classes de gènes identiques (copies +biochimiques sans mutation d’un même gène ancêtre) et isoactifs (gènes allèles +qui ont le même niveau de fonction sur le même caractère, étant identiques ou +non), et de celles des types de mutation, conduisant à des dynamiques +irréalistes, ainsi que l’absence d’une prise en compte rationnelle de +l’expression génique (1964, 1987, 1991). + +L’épisode médiatique de la théorie neutraliste de l’évolution moléculaire est +clos. La théorie synthétique de l’évolution s’éloigne à la fois du pansélectif +de Fisher et de Haldane et du neutralisme de Kimura. Elle trouve aujourd’hui +dans les travaux de Wright et de Malécot des raisons nouvelles de son pouvoir +explicatif. + +[1mNotes [0m + +[1] Comptes rendus de l’Académie des sciences, 130, 1900, p. 845-848. + +[2] Évolution rectilinéaire dirigée avec ou sans finalisme. + +[3] E. Mayr, art. « Théorie synthétique de l’évolution » du Dictionnaire du +darwinisme et de l’évolution, Paris, PUF, 1996. + +[4] V. A. Ratner, art. « Biologie moléculaire et évolution » du Dictionnaire du +darwinisme et de l’évolution, Paris, PUF, 1996. + +[1mConclusion [0m + +Le darwinisme est jeune. L’actuelle théorie synthétique – non plus que la +répétition souvent inféconde de la formule classique de Jacques Monod combinant +le « hasard » et la « nécessité » – ne saurait être son dernier mot. Le +formidable programme interdisciplinaire inscrit dans l’ensemble de l’œuvre de +Darwin et des principaux acteurs de la biologie de l’évolution n’est certes pas +près d’être épuisé. Pour l’heure, on peut dire qu’aucune des grandes divisions +des sciences naturelles, de l’anatomie comparée à la biochimie moléculaire en +passant par la paléontologie et la génétique, de l’écologie à l’éthologie en +passant par l’étude de la vie microbienne et virale, n’est étrangère à +l’héritage darwinien, ni ne saurait se soustraire à la production convergente +de confirmations nouvelles de la dynamique darwinienne des changements +évolutifs. + +Les modes antidarwiniennes, voire antitransformistes (tels que la reviviscence +du vieux discours créationniste sous des travestissements indéfiniment +remaillés) reviennent périodiquement, ce qui signe leur caractère idéologique, +en avançant des objections plus que centenaires. Si leur niveau scientifique +inexistant les condamne aux yeux des spécialistes, le soutien qu’elles puisent +dans des résistances ancestrales et organisées leur permet d’exercer néanmoins +leur influence – en particulier aux États-Unis – sur un public insuffisamment +instruit ou qui ne souhaite pas l’être. + +Une mise en scène rendue courante par certaines évolutions « spectaculaires » +de l’information scientifique consiste à opposer des antidarwiniens plus ou +moins ouvertement créationnistes à de faux darwiniens (de type très +généralement « sociobiologiste ») censés représenter dans cette controverse +l’engagement évolutionniste le plus authentique. La plupart des scientifiques +savent naturellement que les véritables débats sont ailleurs. + +Dans l’univers scientifique, l’approbation multidisciplinaire des principes +fondamentaux de l’évolution darwinienne devrait entraîner une conséquence +épistémologiquement nécessaire : la théorie de l’évolution étant une théorie +phylogénétique – soulignant de ce fait la nécessité du raisonnement +phylogénétique lui-même comme voie unique de compréhension profonde des +phénomènes biologiques observables –, l’ensemble de la biologie moderne, en +tant qu’étudiant dans chacune de ses régions un certain ordre de conséquences +de l’évolution, devrait reposer sur la connaissance et sur la théorie de +celle-ci comme sur son socle intellectuel indispensable et légitime. + +Cette perspective, qui s’oppose au réductionnisme impliqué par les +cloisonnements trop souvent installés entre des disciplines biologiques dont la +destination devrait être, à l’inverse, une compénétration mutuelle productive, +est celle qu’illustre l’œuvre considérable, qui demeure en grande partie à +découvrir, du grand biochimiste espagnol Faustino Cordón (1909-1999), lequel, à +partir et autour de l’étude de l’évolution du métabolisme cellulaire, a +construit, en termes d’émergences évolutives sélectionnées, la plus forte +théorisation actuelle des niveaux d’intégration du vivant. + +[1mBibliographie [0m + +Bocquet Ch., Génermont J., Lamotte M. (dir.), « Les problèmes de l’espèce dans +le règne animal », Mémoires de la Société zoologique de France, 1976, nos +38-40, 1977, 1980. + +Chiesura G., Charles Darwin geologo, Benevento, Hevelius Edizioni, 2002-2004, 2 +vol. + +Cordón F., Tratado evolucionista de biología, Madrid, Anthropos, 1990. + +Cunchillos Ch., Les Voies de l’émergence. Introduction à la théorie des unités +de niveau d’intégration, Paris, Belin, 2014. + +Darwin Ch., The Works, Eds. Pickering & Chatto, 29 vol. + +– Première esquisse au crayon de ma théorie des espèces, Tort (éd.), Genève, +icdi/Slatkine, 2007. + +– L’Origine des espèces, Tort (éd.), Paris, icdi/Champion Classiques, 2009. + +– La Variation des animaux et des plantes à l’état domestique, Tort (éd.), +Genève, icdi/Slatkine, 2008 ; Paris, ICDI/Champion Classiques, 2014. + +– Journal de bord du voyage du Beagle, Tort (éd.), Genève, icdi/Slatkine, 2010 +; Paris, icdi/Champion Classiques, 2011. + +– La Filiation de l’Homme et la sélection liée au sexe, Genève, icdi/Slatkine, +2012 ; Paris, icdi/Champion Classiques, 2013. + +– Zoologie du voyage du H.M.S. Beagle. Première partie : Mammifères fossiles, +Tort (éd.), Genève, ICDI/Slatkine, 2013. + +Dobzhansky Th., Genetics and the Origin of Species, New York, Columbia Univ. +Press, 1937. + +Fisher R. A., The Genetical Theory of Natural Selection, Oxford, Clarendon +Press, 1930. + +Galton F., « Hereditary Talent and Character », MacMillan’s Magazine, juin et +août 1865. + +– Hereditary Genius, Londres, MacMillan, 1869. + +Haldane J. B. S., « A Mathematical Theory of Natural and Artificial Selection +», Trans., puis Proc. of the Cambridge Philos. Soc., 1924-1934. + +Huxley J. S., Evolution: the Modern Synthesis, Londres, Allen & Unwin, 1942. + +Malécot G., Les Mathématiques de l’hérédité, Paris, Masson, 1948. + +Mayr E., Animal Species and Evolution, Cambridge (Mass.), Harvard Univ. Press, +1963. + +Mivart St G. J., Genesis of Species, 1871. + +Simpson G. G., Tempo and Mode in Evolution, New York, Columbia University +Press, 1944. + +Spencer H., Autobiographie, précédée de « Spencer et le système des sciences », +par P. Tort, Paris, PUF, 1987. + +Tort P., La Pensée hiérarchique et l’évolution, Paris, Aubier, 1983. + +Tort P. (dir.), Dictionnaire du darwinisme et de l’évolution, Paris, PUF, 1996, +3 vol., XIV-4862 p. Ouvrage couronné par l’Académie des sciences. Articles : +Altruisme (G. Guille-Escuret/J. Gervet), Antidarwinisme (P. J. Bowler), +Anthropologie darwinienne (P. T.), Bateson (P. T.), Baur (B. Rupp-Eisenreich), +Biologie moléculaire et évolution (V. A. Ratner), Biogéographie (Ph. Janvier), +Biométrie (J. Gayon), Bricolage de l’évolution (F. Jacob), Carrel (P. T.), +Četverikov (V. Babkov), Civilisation (P. T.), Compensation – Technologies de – +(P. T.), Consanguinité (P. T./M. Gillois), Continuisme/Discontinuisme (P. T.), +Cordón (Ch. Cunchillos), Correns (B. Rupp-Eisenreich), Croisements (P. T.), +Darwin (M. Ghiselin), Darwin – Chronobibliographie de – (P. T.), Darwinisme +allemand (B. Rupp-Eisenreich), Darwinisme anglo-saxon (M. Di Gregorio), +Darwinisme espagnol (D. Núñez), Darwinisme français (G. Molina), Darwinisme +italien (G. Landucci), Darwinisme russe (V. Babkov), Darwinisme et +évolutionnisme philosophique (P. T.), Darwinisme social (D. Becquemont), De +Vries (Ch. Lenay), Dobzhansky (D. Dreuil), Domestication (P. T.), Écologie (V. +Labeyrie), Effet réversif de l’évolution (P. T.), Équilibres ponctués (Ch. +Devillers), Espèce (E. Mayr), Espinas (A. La Vergata), Eugénisme (D. +Becquemont), Évolution – Système de l’ – (P. T.), Évolution des organismes +(J.-P. Gasc), Fisher (J. Gayon), Fitness (M. Gillois), Fondateur – Principe du +– (J. Génermont), Fréquences géniques – Variations des – (M. Gillois), Galton +(Ch. Lenay), Génétique (G. Montalenti), Génétique des populations (J. +Génermont), Génétique des populations et darwinisme en France (Cl. Petit), +Gradualisme (P. T./Y. Carton), Haeckel (B. Rupp-Eisenreich), Haeckel – +Chronobibliographie de – (P. T.), Haldane (F. Zimmermann), Hardy-Weinberg – +Loi, principe ou équilibre de – (J. Génermont), Henslow (P. T.), Hooker (P. +T.), Hunt (B. Rupp-Eisenreich), Huxley – T. H. – (Di Gregorio), Huxley – J. S. +– (D. Dreuil), Instincts sociaux (P. T.), Jaeger (B. Rupp-Eisenreich), James +(P. T.), Johannsen (P. T.), Lamarck (G. Laurent), Lutte pour l’existence (P. +T.), Lyell (G. Laurent), Lyssenko (J.-M. Goux, P. Roubaud), Malécot (M. +Gillois), Malthus (J. Dupâquier) et Addition (P. T.), Manuscrits de Darwin +(J.-M. Goux), Marx-Engels et Darwin (B. Naccache), Mayr (Ch. Devillers), +Mélanisme industriel (Ch. Devillers/Y. Guy), Mendel (Ch. Lenay), Mimétisme (M. +Boulard), Monisme (P. T.), Morgan (P. T.), Muller (D. Dreuil), Müller (P. T.), +Nägeli (Ch. Lenay), Nature/Culture (Y. Quiniou) et Addition (P. T.), +Néodarwinisme (D. Dreuil), Néolamarckisme (A. La Vergata), Neutralisme (M. +Gillois), Œil (M. Delsol), Organicisme (P. T.), Owen (Di Gregorio), Pearson (D. +Becquemont/J. Gayon), Progrès (P. T.), Récifs coralliens (Ch. Devillers), +Religion (P. T.), Rensch (B. Rupp-Eisenreich), Révolution génétique (J. +Génermont/M. Delsol), Romanes (P. T.), Sélection artificielle (P. T.), +Sélection naturelle (P. T.), Sélection sexuelle (P. T.), Simpson (Ch. +Devillers), Sociobiologie (G. Guille-Escuret), Spéciation (J. Génermont), +Spencer (P. T.), Superorganisme (G. Guille-Escuret), Survivance du plus apte +(D. Becquemont), Sympathie (P. T.), Théorie synthétique de l’évolution (E. +Mayr), Timofeev-Resovskij (V. Babkov), Transformisme (P. T.), Tschermak (B. +Rupp-Eisenreich), Vacher de Lapouge (A. Béjin), Variation (P. T.), Voyage d’un +naturaliste (P. T.), Wagner (E. Mayr), Wallace (G. Molina), Weismann (Ch. +Lenay), Wright (J. Gayon), Zoologie (Ch. Devillers). + +Tort P., Darwin et la science de l’évolution, Paris, Gallimard, coll. « +Découvertes », 2000. + +Tort P., La Seconde Révolution darwinienne (Biologie évolutive et théorie de la +civilisation), Paris, Kimé, 2002. + +Tort P., Darwin et la Philosophie (Religion, morale, matérialisme), Paris, +Kimé, 2004. + +Tort P., L’Effet Darwin (Sélection naturelle et naissance de la civilisation), +Paris, Le Seuil, 2008. + +Tort P., Darwin n’est pas celui qu’on croit, Paris, Le Cavalier Bleu, 2010. + +Tort P., Darwin et la Religion, Ellipses, 2011. + +Tort P., Darwinisme et Marxisme, Paris, Arkhê, 2012. + +Wright S., Evolution and the Genetics of Populations, Univ. of Chicago Press, +1978. + +Les textes de Darwin figurant dans ce volume ont été traduits par l’auteur. +Site Web de l’institut Charles-Darwin international : www.darwinisme.org + diff --git a/darwin/biblio/Darwin et l’après Darwin_ Une histoire de l’hypothèse de -- Jean Gayon -- Histoire des idées, Paris (2, Impasse des Peintres 75002), -- EDITIONS KIME -- 9782841748372 -- 2a79d021c197ad6d3c.ds8skgMI.epub.part b/darwin/biblio/Darwin et l’après Darwin_ Une histoire de l’hypothèse de -- Jean Gayon -- Histoire des idées, Paris (2, Impasse des Peintres 75002), -- EDITIONS KIME -- 9782841748372 -- 2a79d021c197ad6d3c.ds8skgMI.epub.part Binary files differ. diff --git a/darwin/biblio/Darwin et l’après Darwin_ Une histoire de l’hypothèse de -- Jean Gayon -- Histoire des idées, Paris (2, Impasse des Peintres 75002), -- EDITIONS KIME -- 9782841748372 -- 2a79d021c197ad6d3c.epub b/darwin/biblio/Darwin et l’après Darwin_ Une histoire de l’hypothèse de -- Jean Gayon -- Histoire des idées, Paris (2, Impasse des Peintres 75002), -- EDITIONS KIME -- 9782841748372 -- 2a79d021c197ad6d3c.epub diff --git a/darwin/biblio/Inventing Temperature_ Measurement and Scientific Progress -- Chang, Hasok -- Oxford studies in philosophy of science, 2nd print, Oxford, -- Oxford -- 9780195171273 -- 5c30ea556f8973567f152d23a67.epub b/darwin/biblio/Inventing Temperature_ Measurement and Scientific Progress -- Chang, Hasok -- Oxford studies in philosophy of science, 2nd print, Oxford, -- Oxford -- 9780195171273 -- 5c30ea556f8973567f152d23a67.epub Binary files differ. diff --git a/darwin/biblio/Qu'est-ce que le matérialisme _ _ introduction à l'analyse -- Tort, Patrick -- 2017 -- Humensis -- 9782410000498 -- 3c89bc9d1ac41987c86d182947d344bd -- Anna’s Archive.epub b/darwin/biblio/Qu'est-ce que le matérialisme _ _ introduction à l'analyse -- Tort, Patrick -- 2017 -- Humensis -- 9782410000498 -- 3c89bc9d1ac41987c86d182947d344bd -- Anna’s Archive.epub Binary files differ. diff --git a/darwin/biblio/Seeking Wisdom_ From Darwin to Munger, 3rd Edition -- Bevelin, Peter -- Third edition, 2018;2007 -- PCA Publications L_L_C_; PCA Publications -- 9781578644285 -- fc29e83e825ec3045c15f31e225e2324 .epub b/darwin/biblio/Seeking Wisdom_ From Darwin to Munger, 3rd Edition -- Bevelin, Peter -- Third edition, 2018;2007 -- PCA Publications L_L_C_; PCA Publications -- 9781578644285 -- fc29e83e825ec3045c15f31e225e2324 .epub Binary files differ. diff --git a/darwin/biblio_darwin b/darwin/biblio_darwin @@ -45,3 +45,5 @@ Bouveresse, Jacques; *Robert Musil,l'homme probable, le hasard, la moyenne et l' julia voss Darwin's blder et drama fin +Waquet, Françoise L'ordre Matériel du savoir: Comment les savants travaillent + diff --git a/darwin/biographyDarwin.txt b/darwin/biographyDarwin.txt @@ -0,0 +1,2512 @@ +The Project Gutenberg eBook of The Autobiography of Charles Darwin + +This ebook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this ebook or online +at www.gutenberg.org. If you are not located in the United States, +you will have to check the laws of the country where you are located +before using this eBook. + +Title: The Autobiography of Charles Darwin + +Author: Charles Darwin + +Editor: Sir Francis Darwin + +Release date: December 1, 1999 [eBook #2010] + Most recently updated: April 26, 2022 + +Language: English + +Credits: Sue Asscher + + +*** START OF THE PROJECT GUTENBERG EBOOK THE AUTOBIOGRAPHY OF CHARLES DARWIN *** + + + + +THE AUTOBIOGRAPHY OF +CHARLES DARWIN + +From The Life and Letters of Charles Darwin + +By Charles Darwin + +Edited by his Son Francis Darwin + + +CONTENTS + +CAMBRIDGE 1828-1831. + “VOYAGE OF THE ‘BEAGLE’ FROM DECEMBER 27, 1831, TO OCTOBER 2, 1836.” + FROM MY RETURN TO ENGLAND (OCTOBER 2, 1836) TO MY MARRIAGE (JANUARY 29, 1839.) + FROM MY MARRIAGE, JANUARY 29, 1839, AND RESIDENCE IN UPPER GOWER STREET, TO OUR LEAVING LONDON AND SETTLING AT DOWN, SEPTEMBER 14, 1842. + RESIDENCE AT DOWN FROM SEPTEMBER 14, 1842, TO THE PRESENT TIME, 1876. + MY SEVERAL PUBLICATIONS. + WRITTEN MAY 1ST, 1881. + + + + +[My father’s autobiographical recollections, given in the present +chapter, were written for his children,—and written without any thought +that they would ever be published. To many this may seem an +impossibility; but those who knew my father will understand how it was +not only possible, but natural. The autobiography bears the heading, +‘Recollections of the Development of my Mind and Character,’ and end +with the following note:—“Aug. 3, 1876. This sketch of my life was +begun about May 28th at Hopedene (Mr. Hensleigh Wedgwood’s house in +Surrey.), and since then I have written for nearly an hour on most +afternoons.” It will easily be understood that, in a narrative of a +personal and intimate kind written for his wife and children, passages +should occur which must here be omitted; and I have not thought it +necessary to indicate where such omissions are made. It has been found +necessary to make a few corrections of obvious verbal slips, but the +number of such alterations has been kept down to the minimum.—F.D.] + + +A German Editor having written to me for an account of the development +of my mind and character with some sketch of my autobiography, I have +thought that the attempt would amuse me, and might possibly interest my +children or their children. I know that it would have interested me +greatly to have read even so short and dull a sketch of the mind of my +grandfather, written by himself, and what he thought and did, and how +he worked. I have attempted to write the following account of myself, +as if I were a dead man in another world looking back at my own life. +Nor have I found this difficult, for life is nearly over with me. I +have taken no pains about my style of writing. + +I was born at Shrewsbury on February 12th, 1809, and my earliest +recollection goes back only to when I was a few months over four years +old, when we went to near Abergele for sea-bathing, and I recollect +some events and places there with some little distinctness. + +My mother died in July 1817, when I was a little over eight years old, +and it is odd that I can remember hardly anything about her except her +death-bed, her black velvet gown, and her curiously constructed +work-table. In the spring of this same year I was sent to a day-school +in Shrewsbury, where I stayed a year. I have been told that I was much +slower in learning than my younger sister Catherine, and I believe that +I was in many ways a naughty boy. + +By the time I went to this day-school (Kept by Rev. G. Case, minister +of the Unitarian Chapel in the High Street. Mrs. Darwin was a Unitarian +and attended Mr. Case’s chapel, and my father as a little boy went +there with his elder sisters. But both he and his brother were +christened and intended to belong to the Church of England; and after +his early boyhood he seems usually to have gone to church and not to +Mr. Case’s. It appears (“St. James’ Gazette”, Dec. 15, 1883) that a +mural tablet has been erected to his memory in the chapel, which is now +known as the ‘Free Christian Church.’) my taste for natural history, +and more especially for collecting, was well developed. I tried to make +out the names of plants (Rev. W.A. Leighton, who was a schoolfellow of +my father’s at Mr. Case’s school, remembers his bringing a flower to +school and saying that his mother had taught him how by looking at the +inside of the blossom the name of the plant could be discovered. Mr. +Leighton goes on, “This greatly roused my attention and curiosity, and +I enquired of him repeatedly how this could be done?”—but his lesson +was naturally enough not transmissible.—F.D.), and collected all sorts +of things, shells, seals, franks, coins, and minerals. The passion for +collecting which leads a man to be a systematic naturalist, a virtuoso, +or a miser, was very strong in me, and was clearly innate, as none of +my sisters or brother ever had this taste. + +One little event during this year has fixed itself very firmly in my +mind, and I hope that it has done so from my conscience having been +afterwards sorely troubled by it; it is curious as showing that +apparently I was interested at this early age in the variability of +plants! I told another little boy (I believe it was Leighton, who +afterwards became a well-known lichenologist and botanist), that I +could produce variously coloured polyanthuses and primroses by watering +them with certain coloured fluids, which was of course a monstrous +fable, and had never been tried by me. I may here also confess that as +a little boy I was much given to inventing deliberate falsehoods, and +this was always done for the sake of causing excitement. For instance, +I once gathered much valuable fruit from my father’s trees and hid it +in the shrubbery, and then ran in breathless haste to spread the news +that I had discovered a hoard of stolen fruit. + +I must have been a very simple little fellow when I first went to the +school. A boy of the name of Garnett took me into a cake shop one day, +and bought some cakes for which he did not pay, as the shopman trusted +him. When we came out I asked him why he did not pay for them, and he +instantly answered, “Why, do you not know that my uncle left a great +sum of money to the town on condition that every tradesman should give +whatever was wanted without payment to any one who wore his old hat and +moved [it] in a particular manner?” and he then showed me how it was +moved. He then went into another shop where he was trusted, and asked +for some small article, moving his hat in the proper manner, and of +course obtained it without payment. When we came out he said, “Now if +you like to go by yourself into that cake-shop (how well I remember its +exact position) I will lend you my hat, and you can get whatever you +like if you move the hat on your head properly.” I gladly accepted the +generous offer, and went in and asked for some cakes, moved the old hat +and was walking out of the shop, when the shopman made a rush at me, so +I dropped the cakes and ran for dear life, and was astonished by being +greeted with shouts of laughter by my false friend Garnett. + +I can say in my own favour that I was as a boy humane, but I owed this +entirely to the instruction and example of my sisters. I doubt indeed +whether humanity is a natural or innate quality. I was very fond of +collecting eggs, but I never took more than a single egg out of a +bird’s nest, except on one single occasion, when I took all, not for +their value, but from a sort of bravado. + +I had a strong taste for angling, and would sit for any number of hours +on the bank of a river or pond watching the float; when at Maer (The +house of his uncle, Josiah Wedgwood.) I was told that I could kill the +worms with salt and water, and from that day I never spitted a living +worm, though at the expense probably of some loss of success. + +Once as a very little boy whilst at the day school, or before that +time, I acted cruelly, for I beat a puppy, I believe, simply from +enjoying the sense of power; but the beating could not have been +severe, for the puppy did not howl, of which I feel sure, as the spot +was near the house. This act lay heavily on my conscience, as is shown +by my remembering the exact spot where the crime was committed. It +probably lay all the heavier from my love of dogs being then, and for a +long time afterwards, a passion. Dogs seemed to know this, for I was an +adept in robbing their love from their masters. + +I remember clearly only one other incident during this year whilst at +Mr. Case’s daily school,—namely, the burial of a dragoon soldier; and +it is surprising how clearly I can still see the horse with the man’s +empty boots and carbine suspended to the saddle, and the firing over +the grave. This scene deeply stirred whatever poetic fancy there was in +me. + +In the summer of 1818 I went to Dr. Butler’s great school in +Shrewsbury, and remained there for seven years till Midsummer 1825, +when I was sixteen years old. I boarded at this school, so that I had +the great advantage of living the life of a true schoolboy; but as the +distance was hardly more than a mile to my home, I very often ran there +in the longer intervals between the callings over and before locking up +at night. This, I think, was in many ways advantageous to me by keeping +up home affections and interests. I remember in the early part of my +school life that I often had to run very quickly to be in time, and +from being a fleet runner was generally successful; but when in doubt I +prayed earnestly to God to help me, and I well remember that I +attributed my success to the prayers and not to my quick running, and +marvelled how generally I was aided. + +I have heard my father and elder sister say that I had, as a very young +boy, a strong taste for long solitary walks; but what I thought about I +know not. I often became quite absorbed, and once, whilst returning to +school on the summit of the old fortifications round Shrewsbury, which +had been converted into a public foot-path with no parapet on one side, +I walked off and fell to the ground, but the height was only seven or +eight feet. Nevertheless the number of thoughts which passed through my +mind during this very short, but sudden and wholly unexpected fall, was +astonishing, and seem hardly compatible with what physiologists have, I +believe, proved about each thought requiring quite an appreciable +amount of time. + +Nothing could have been worse for the development of my mind than Dr. +Butler’s school, as it was strictly classical, nothing else being +taught, except a little ancient geography and history. The school as a +means of education to me was simply a blank. During my whole life I +have been singularly incapable of mastering any language. Especial +attention was paid to verse-making, and this I could never do well. I +had many friends, and got together a good collection of old verses, +which by patching together, sometimes aided by other boys, I could work +into any subject. Much attention was paid to learning by heart the +lessons of the previous day; this I could effect with great facility, +learning forty or fifty lines of Virgil or Homer, whilst I was in +morning chapel; but this exercise was utterly useless, for every verse +was forgotten in forty-eight hours. I was not idle, and with the +exception of versification, generally worked conscientiously at my +classics, not using cribs. The sole pleasure I ever received from such +studies, was from some of the odes of Horace, which I admired greatly. + +When I left the school I was for my age neither high nor low in it; and +I believe that I was considered by all my masters and by my father as a +very ordinary boy, rather below the common standard in intellect. To my +deep mortification my father once said to me, “You care for nothing but +shooting, dogs, and rat-catching, and you will be a disgrace to +yourself and all your family.” But my father, who was the kindest man I +ever knew and whose memory I love with all my heart, must have been +angry and somewhat unjust when he used such words. + +Looking back as well as I can at my character during my school life, +the only qualities which at this period promised well for the future, +were, that I had strong and diversified tastes, much zeal for whatever +interested me, and a keen pleasure in understanding any complex subject +or thing. I was taught Euclid by a private tutor, and I distinctly +remember the intense satisfaction which the clear geometrical proofs +gave me. I remember, with equal distinctness, the delight which my +uncle gave me (the father of Francis Galton) by explaining the +principle of the vernier of a barometer with respect to diversified +tastes, independently of science, I was fond of reading various books, +and I used to sit for hours reading the historical plays of +Shakespeare, generally in an old window in the thick walls of the +school. I read also other poetry, such as Thomson’s ‘Seasons,’ and the +recently published poems of Byron and Scott. I mention this because +later in life I wholly lost, to my great regret, all pleasure from +poetry of any kind, including Shakespeare. In connection with pleasure +from poetry, I may add that in 1822 a vivid delight in scenery was +first awakened in my mind, during a riding tour on the borders of +Wales, and this has lasted longer than any other aesthetic pleasure. + +Early in my school days a boy had a copy of the ‘Wonders of the World,’ +which I often read, and disputed with other boys about the veracity of +some of the statements; and I believe that this book first gave me a +wish to travel in remote countries, which was ultimately fulfilled by +the voyage of the “Beagle”. In the latter part of my school life I +became passionately fond of shooting; I do not believe that any one +could have shown more zeal for the most holy cause than I did for +shooting birds. How well I remember killing my first snipe, and my +excitement was so great that I had much difficulty in reloading my gun +from the trembling of my hands. This taste long continued, and I became +a very good shot. When at Cambridge I used to practise throwing up my +gun to my shoulder before a looking-glass to see that I threw it up +straight. Another and better plan was to get a friend to wave about a +lighted candle, and then to fire at it with a cap on the nipple, and if +the aim was accurate the little puff of air would blow out the candle. +The explosion of the cap caused a sharp crack, and I was told that the +tutor of the college remarked, “What an extraordinary thing it is, Mr. +Darwin seems to spend hours in cracking a horse-whip in his room, for I +often hear the crack when I pass under his windows.” + +I had many friends amongst the schoolboys, whom I loved dearly, and I +think that my disposition was then very affectionate. + +With respect to science, I continued collecting minerals with much +zeal, but quite unscientifically—all that I cared about was a +new-_named_ mineral, and I hardly attempted to classify them. I must +have observed insects with some little care, for when ten years old +(1819) I went for three weeks to Plas Edwards on the sea-coast in +Wales, I was very much interested and surprised at seeing a large black +and scarlet Hemipterous insect, many moths (Zygaena), and a Cicindela +which are not found in Shropshire. I almost made up my mind to begin +collecting all the insects which I could find dead, for on consulting +my sister I concluded that it was not right to kill insects for the +sake of making a collection. From reading White’s ‘Selborne,’ I took +much pleasure in watching the habits of birds, and even made notes on +the subject. In my simplicity I remember wondering why every gentleman +did not become an ornithologist. + +Towards the close of my school life, my brother worked hard at +chemistry, and made a fair laboratory with proper apparatus in the +tool-house in the garden, and I was allowed to aid him as a servant in +most of his experiments. He made all the gases and many compounds, and +I read with great care several books on chemistry, such as Henry and +Parkes’ ‘Chemical Catechism.’ The subject interested me greatly, and we +often used to go on working till rather late at night. This was the +best part of my education at school, for it showed me practically the +meaning of experimental science. The fact that we worked at chemistry +somehow got known at school, and as it was an unprecedented fact, I was +nicknamed “Gas.” I was also once publicly rebuked by the head-master, +Dr. Butler, for thus wasting my time on such useless subjects; and he +called me very unjustly a “poco curante,” and as I did not understand +what he meant, it seemed to me a fearful reproach. + +As I was doing no good at school, my father wisely took me away at a +rather earlier age than usual, and sent me (Oct. 1825) to Edinburgh +University with my brother, where I stayed for two years or sessions. +My brother was completing his medical studies, though I do not believe +he ever really intended to practise, and I was sent there to commence +them. But soon after this period I became convinced from various small +circumstances that my father would leave me property enough to subsist +on with some comfort, though I never imagined that I should be so rich +a man as I am; but my belief was sufficient to check any strenuous +efforts to learn medicine. + +The instruction at Edinburgh was altogether by lectures, and these were +intolerably dull, with the exception of those on chemistry by Hope; but +to my mind there are no advantages and many disadvantages in lectures +compared with reading. Dr. Duncan’s lectures on Materia Medica at 8 +o’clock on a winter’s morning are something fearful to remember. Dr.—— +made his lectures on human anatomy as dull as he was himself, and the +subject disgusted me. It has proved one of the greatest evils in my +life that I was not urged to practise dissection, for I should soon +have got over my disgust; and the practice would have been invaluable +for all my future work. This has been an irremediable evil, as well as +my incapacity to draw. I also attended regularly the clinical wards in +the hospital. Some of the cases distressed me a good deal, and I still +have vivid pictures before me of some of them; but I was not so foolish +as to allow this to lessen my attendance. I cannot understand why this +part of my medical course did not interest me in a greater degree; for +during the summer before coming to Edinburgh I began attending some of +the poor people, chiefly children and women in Shrewsbury: I wrote down +as full an account as I could of the case with all the symptoms, and +read them aloud to my father, who suggested further inquiries and +advised me what medicines to give, which I made up myself. At one time +I had at least a dozen patients, and I felt a keen interest in the +work. My father, who was by far the best judge of character whom I ever +knew, declared that I should make a successful physician,—meaning by +this one who would get many patients. He maintained that the chief +element of success was exciting confidence; but what he saw in me which +convinced him that I should create confidence I know not. I also +attended on two occasions the operating theatre in the hospital at +Edinburgh, and saw two very bad operations, one on a child, but I +rushed away before they were completed. Nor did I ever attend again, +for hardly any inducement would have been strong enough to make me do +so; this being long before the blessed days of chloroform. The two +cases fairly haunted me for many a long year. + +My brother stayed only one year at the University, so that during the +second year I was left to my own resources; and this was an advantage, +for I became well acquainted with several young men fond of natural +science. One of these was Ainsworth, who afterwards published his +travels in Assyria; he was a Wernerian geologist, and knew a little +about many subjects. Dr. Coldstream was a very different young man, +prim, formal, highly religious, and most kind-hearted; he afterwards +published some good zoological articles. A third young man was Hardie, +who would, I think, have made a good botanist, but died early in India. +Lastly, Dr. Grant, my senior by several years, but how I became +acquainted with him I cannot remember; he published some first-rate +zoological papers, but after coming to London as Professor in +University College, he did nothing more in science, a fact which has +always been inexplicable to me. I knew him well; he was dry and formal +in manner, with much enthusiasm beneath this outer crust. He one day, +when we were walking together, burst forth in high admiration of +Lamarck and his views on evolution. I listened in silent astonishment, +and as far as I can judge without any effect on my mind. I had +previously read the ‘Zoonomia’ of my grandfather, in which similar +views are maintained, but without producing any effect on me. +Nevertheless it is probable that the hearing rather early in life such +views maintained and praised may have favoured my upholding them under +a different form in my ‘Origin of Species.’ At this time I admired +greatly the ‘Zoonomia;’ but on reading it a second time after an +interval of ten or fifteen years, I was much disappointed; the +proportion of speculation being so large to the facts given. + +Drs. Grant and Coldstream attended much to marine Zoology, and I often +accompanied the former to collect animals in the tidal pools, which I +dissected as well as I could. I also became friends with some of the +Newhaven fishermen, and sometimes accompanied them when they trawled +for oysters, and thus got many specimens. But from not having had any +regular practice in dissection, and from possessing only a wretched +microscope, my attempts were very poor. Nevertheless I made one +interesting little discovery, and read, about the beginning of the year +1826, a short paper on the subject before the Plinian Society. This was +that the so-called ova of Flustra had the power of independent movement +by means of cilia, and were in fact larvae. In another short paper I +showed that the little globular bodies which had been supposed to be +the young state of Fucus loreus were the egg-cases of the wormlike +Pontobdella muricata. + +The Plinian Society was encouraged and, I believe, founded by Professor +Jameson: it consisted of students and met in an underground room in the +University for the sake of reading papers on natural science and +discussing them. I used regularly to attend, and the meetings had a +good effect on me in stimulating my zeal and giving me new congenial +acquaintances. One evening a poor young man got up, and after +stammering for a prodigious length of time, blushing crimson, he at +last slowly got out the words, “Mr. President, I have forgotten what I +was going to say.” The poor fellow looked quite overwhelmed, and all +the members were so surprised that no one could think of a word to say +to cover his confusion. The papers which were read to our little +society were not printed, so that I had not the satisfaction of seeing +my paper in print; but I believe Dr. Grant noticed my small discovery +in his excellent memoir on Flustra. + +I was also a member of the Royal Medical Society, and attended pretty +regularly; but as the subjects were exclusively medical, I did not much +care about them. Much rubbish was talked there, but there were some +good speakers, of whom the best was the present Sir J. +Kay-Shuttleworth. Dr. Grant took me occasionally to the meetings of the +Wernerian Society, where various papers on natural history were read, +discussed, and afterwards published in the ‘Transactions.’ I heard +Audubon deliver there some interesting discourses on the habits of N. +American birds, sneering somewhat unjustly at Waterton. By the way, a +negro lived in Edinburgh, who had travelled with Waterton, and gained +his livelihood by stuffing birds, which he did excellently: he gave me +lessons for payment, and I used often to sit with him, for he was a +very pleasant and intelligent man. + +Mr. Leonard Horner also took me once to a meeting of the Royal Society +of Edinburgh, where I saw Sir Walter Scott in the chair as President, +and he apologised to the meeting as not feeling fitted for such a +position. I looked at him and at the whole scene with some awe and +reverence, and I think it was owing to this visit during my youth, and +to my having attended the Royal Medical Society, that I felt the honour +of being elected a few years ago an honorary member of both these +Societies, more than any other similar honour. If I had been told at +that time that I should one day have been thus honoured, I declare that +I should have thought it as ridiculous and improbable, as if I had been +told that I should be elected King of England. + +During my second year at Edinburgh I attended ——’s lectures on Geology +and Zoology, but they were incredibly dull. The sole effect they +produced on me was the determination never as long as I lived to read a +book on Geology, or in any way to study the science. Yet I feel sure +that I was prepared for a philosophical treatment of the subject; for +an old Mr. Cotton in Shropshire, who knew a good deal about rocks, had +pointed out to me two or three years previously a well-known large +erratic boulder in the town of Shrewsbury, called the “bell-stone”; he +told me that there was no rock of the same kind nearer than Cumberland +or Scotland, and he solemnly assured me that the world would come to an +end before any one would be able to explain how this stone came where +it now lay. This produced a deep impression on me, and I meditated over +this wonderful stone. So that I felt the keenest delight when I first +read of the action of icebergs in transporting boulders, and I gloried +in the progress of Geology. Equally striking is the fact that I, though +now only sixty-seven years old, heard the Professor, in a field lecture +at Salisbury Craigs, discoursing on a trapdyke, with amygdaloidal +margins and the strata indurated on each side, with volcanic rocks all +around us, say that it was a fissure filled with sediment from above, +adding with a sneer that there were men who maintained that it had been +injected from beneath in a molten condition. When I think of this +lecture, I do not wonder that I determined never to attend to Geology. + +From attending ——’s lectures, I became acquainted with the curator of +the museum, Mr. Macgillivray, who afterwards published a large and +excellent book on the birds of Scotland. I had much interesting +natural-history talk with him, and he was very kind to me. He gave me +some rare shells, for I at that time collected marine mollusca, but +with no great zeal. + +My summer vacations during these two years were wholly given up to +amusements, though I always had some book in hand, which I read with +interest. During the summer of 1826 I took a long walking tour with two +friends with knapsacks on our backs through North Wales. We walked +thirty miles most days, including one day the ascent of Snowdon. I also +went with my sister a riding tour in North Wales, a servant with +saddle-bags carrying our clothes. The autumns were devoted to shooting +chiefly at Mr. Owen’s, at Woodhouse, and at my Uncle Jos’s (Josiah +Wedgwood, the son of the founder of the Etruria Works.) at Maer. My +zeal was so great that I used to place my shooting-boots open by my +bed-side when I went to bed, so as not to lose half a minute in putting +them on in the morning; and on one occasion I reached a distant part of +the Maer estate, on the 20th of August for black-game shooting, before +I could see: I then toiled on with the game-keeper the whole day +through thick heath and young Scotch firs. + +I kept an exact record of every bird which I shot throughout the whole +season. One day when shooting at Woodhouse with Captain Owen, the +eldest son, and Major Hill, his cousin, afterwards Lord Berwick, both +of whom I liked very much, I thought myself shamefully used, for every +time after I had fired and thought that I had killed a bird, one of the +two acted as if loading his gun, and cried out, “You must not count +that bird, for I fired at the same time,” and the gamekeeper, +perceiving the joke, backed them up. After some hours they told me the +joke, but it was no joke to me, for I had shot a large number of birds, +but did not know how many, and could not add them to my list, which I +used to do by making a knot in a piece of string tied to a button-hole. +This my wicked friends had perceived. + +How I did enjoy shooting! But I think that I must have been +half-consciously ashamed of my zeal, for I tried to persuade myself +that shooting was almost an intellectual employment; it required so +much skill to judge where to find most game and to hunt the dogs well. + +One of my autumnal visits to Maer in 1827 was memorable from meeting +there Sir J. Mackintosh, who was the best converser I ever listened to. +I heard afterwards with a glow of pride that he had said, “There is +something in that young man that interests me.” This must have been +chiefly due to his perceiving that I listened with much interest to +everything which he said, for I was as ignorant as a pig about his +subjects of history, politics, and moral philosophy. To hear of praise +from an eminent person, though no doubt apt or certain to excite +vanity, is, I think, good for a young man, as it helps to keep him in +the right course. + +My visits to Maer during these two or three succeeding years were quite +delightful, independently of the autumnal shooting. Life there was +perfectly free; the country was very pleasant for walking or riding; +and in the evening there was much very agreeable conversation, not so +personal as it generally is in large family parties, together with +music. In the summer the whole family used often to sit on the steps of +the old portico, with the flower-garden in front, and with the steep +wooded bank opposite the house reflected in the lake, with here and +there a fish rising or a water-bird paddling about. Nothing has left a +more vivid picture on my mind than these evenings at Maer. I was also +attached to and greatly revered my Uncle Jos; he was silent and +reserved, so as to be a rather awful man; but he sometimes talked +openly with me. He was the very type of an upright man, with the +clearest judgment. I do not believe that any power on earth could have +made him swerve an inch from what he considered the right course. I +used to apply to him in my mind the well-known ode of Horace, now +forgotten by me, in which the words “nec vultus tyranni,* etc.,” come +in. + +* Justum et tenacem propositi virum +Non civium ardor prava jubentium +Non vultus instantis tyranni +Mente quatit solida. + + + + +CAMBRIDGE 1828-1831. + + +After having spent two sessions in Edinburgh, my father perceived, or +he heard from my sisters, that I did not like the thought of being a +physician, so he proposed that I should become a clergyman. He was very +properly vehement against my turning into an idle sporting man, which +then seemed my probable destination. I asked for some time to consider, +as from what little I had heard or thought on the subject I had +scruples about declaring my belief in all the dogmas of the Church of +England; though otherwise I liked the thought of being a country +clergyman. Accordingly I read with care ‘Pearson on the Creed,’ and a +few other books on divinity; and as I did not then in the least doubt +the strict and literal truth of every word in the Bible, I soon +persuaded myself that our Creed must be fully accepted. + +Considering how fiercely I have been attacked by the orthodox, it seems +ludicrous that I once intended to be a clergyman. Nor was this +intention and my father’s wish ever formerly given up, but died a +natural death when, on leaving Cambridge, I joined the “Beagle” as +naturalist. If the phrenologists are to be trusted, I was well fitted +in one respect to be a clergyman. A few years ago the secretaries of a +German psychological society asked me earnestly by letter for a +photograph of myself; and some time afterwards I received the +proceedings of one of the meetings, in which it seemed that the shape +of my head had been the subject of a public discussion, and one of the +speakers declared that I had the bump of reverence developed enough for +ten priests. + +As it was decided that I should be a clergyman, it was necessary that I +should go to one of the English universities and take a degree; but as +I had never opened a classical book since leaving school, I found to my +dismay, that in the two intervening years I had actually forgotten, +incredible as it may appear, almost everything which I had learnt, even +to some few of the Greek letters. I did not therefore proceed to +Cambridge at the usual time in October, but worked with a private tutor +in Shrewsbury, and went to Cambridge after the Christmas vacation, +early in 1828. I soon recovered my school standard of knowledge, and +could translate easy Greek books, such as Homer and the Greek +Testament, with moderate facility. + +During the three years which I spent at Cambridge my time was wasted, +as far as the academical studies were concerned, as completely as at +Edinburgh and at school. I attempted mathematics, and even went during +the summer of 1828 with a private tutor (a very dull man) to Barmouth, +but I got on very slowly. The work was repugnant to me, chiefly from my +not being able to see any meaning in the early steps in algebra. This +impatience was very foolish, and in after years I have deeply regretted +that I did not proceed far enough at least to understand something of +the great leading principles of mathematics, for men thus endowed seem +to have an extra sense. But I do not believe that I should ever have +succeeded beyond a very low grade. With respect to Classics I did +nothing except attend a few compulsory college lectures, and the +attendance was almost nominal. In my second year I had to work for a +month or two to pass the Little-Go, which I did easily. Again, in my +last year I worked with some earnestness for my final degree of B.A., +and brushed up my Classics, together with a little Algebra and Euclid, +which latter gave me much pleasure, as it did at school. In order to +pass the B.A. examination, it was also necessary to get up Paley’s +‘Evidences of Christianity,’ and his ‘Moral Philosophy.’ This was done +in a thorough manner, and I am convinced that I could have written out +the whole of the ‘Evidences’ with perfect correctness, but not of +course in the clear language of Paley. The logic of this book and, as I +may add, of his ‘Natural Theology,’ gave me as much delight as did +Euclid. The careful study of these works, without attempting to learn +any part by rote, was the only part of the academical course which, as +I then felt and as I still believe, was of the least use to me in the +education of my mind. I did not at that time trouble myself about +Paley’s premises; and taking these on trust, I was charmed and +convinced by the long line of argumentation. By answering well the +examination questions in Paley, by doing Euclid well, and by not +failing miserably in Classics, I gained a good place among the oi +polloi or crowd of men who do not go in for honours. Oddly enough, I +cannot remember how high I stood, and my memory fluctuates between the +fifth, tenth, or twelfth, name on the list. (Tenth in the list of +January 1831.) + +Public lectures on several branches were given in the University, +attendance being quite voluntary; but I was so sickened with lectures +at Edinburgh that I did not even attend Sedgwick’s eloquent and +interesting lectures. Had I done so I should probably have become a +geologist earlier than I did. I attended, however, Henslow’s lectures +on Botany, and liked them much for their extreme clearness, and the +admirable illustrations; but I did not study botany. Henslow used to +take his pupils, including several of the older members of the +University, field excursions, on foot or in coaches, to distant places, +or in a barge down the river, and lectured on the rarer plants and +animals which were observed. These excursions were delightful. + +Although, as we shall presently see, there were some redeeming features +in my life at Cambridge, my time was sadly wasted there, and worse than +wasted. From my passion for shooting and for hunting, and, when this +failed, for riding across country, I got into a sporting set, including +some dissipated low-minded young men. We used often to dine together in +the evening, though these dinners often included men of a higher stamp, +and we sometimes drank too much, with jolly singing and playing at +cards afterwards. I know that I ought to feel ashamed of days and +evenings thus spent, but as some of my friends were very pleasant, and +we were all in the highest spirits, I cannot help looking back to these +times with much pleasure. + +But I am glad to think that I had many other friends of a widely +different nature. I was very intimate with Whitley (Rev. C. Whitley, +Hon. Canon of Durham, formerly Reader in Natural Philosophy in Durham +University.), who was afterwards Senior Wrangler, and we used +continually to take long walks together. He inoculated me with a taste +for pictures and good engravings, of which I bought some. I frequently +went to the Fitzwilliam Gallery, and my taste must have been fairly +good, for I certainly admired the best pictures, which I discussed with +the old curator. I read also with much interest Sir Joshua Reynolds’ +book. This taste, though not natural to me, lasted for several years, +and many of the pictures in the National Gallery in London gave me much +pleasure; that of Sebastian del Piombo exciting in me a sense of +sublimity. + +I also got into a musical set, I believe by means of my warm-hearted +friend, Herbert (The late John Maurice Herbert, County Court Judge of +Cardiff and the Monmouth Circuit.), who took a high wrangler’s degree. +From associating with these men, and hearing them play, I acquired a +strong taste for music, and used very often to time my walks so as to +hear on week days the anthem in King’s College Chapel. This gave me +intense pleasure, so that my backbone would sometimes shiver. I am sure +that there was no affectation or mere imitation in this taste, for I +used generally to go by myself to King’s College, and I sometimes hired +the chorister boys to sing in my rooms. Nevertheless I am so utterly +destitute of an ear, that I cannot perceive a discord, or keep time and +hum a tune correctly; and it is a mystery how I could possibly have +derived pleasure from music. + +My musical friends soon perceived my state, and sometimes amused +themselves by making me pass an examination, which consisted in +ascertaining how many tunes I could recognise when they were played +rather more quickly or slowly than usual. ‘God save the King,’ when +thus played, was a sore puzzle. There was another man with almost as +bad an ear as I had, and strange to say he played a little on the +flute. Once I had the triumph of beating him in one of our musical +examinations. + +But no pursuit at Cambridge was followed with nearly so much eagerness +or gave me so much pleasure as collecting beetles. It was the mere +passion for collecting, for I did not dissect them, and rarely compared +their external characters with published descriptions, but got them +named anyhow. I will give a proof of my zeal: one day, on tearing off +some old bark, I saw two rare beetles, and seized one in each hand; +then I saw a third and new kind, which I could not bear to lose, so +that I popped the one which I held in my right hand into my mouth. +Alas! it ejected some intensely acrid fluid, which burnt my tongue so +that I was forced to spit the beetle out, which was lost, as was the +third one. + +I was very successful in collecting, and invented two new methods; I +employed a labourer to scrape during the winter, moss off old trees and +place it in a large bag, and likewise to collect the rubbish at the +bottom of the barges in which reeds are brought from the fens, and thus +I got some very rare species. No poet ever felt more delighted at +seeing his first poem published than I did at seeing, in Stephens’ +‘Illustrations of British Insects,’ the magic words, “captured by C. +Darwin, Esq.” I was introduced to entomology by my second cousin W. +Darwin Fox, a clever and most pleasant man, who was then at Christ’s +College, and with whom I became extremely intimate. Afterwards I became +well acquainted, and went out collecting, with Albert Way of Trinity, +who in after years became a well-known archaeologist; also with H. +Thompson of the same College, afterwards a leading agriculturist, +chairman of a great railway, and Member of Parliament. It seems +therefore that a taste for collecting beetles is some indication of +future success in life! + +I am surprised what an indelible impression many of the beetles which I +caught at Cambridge have left on my mind. I can remember the exact +appearance of certain posts, old trees and banks where I made a good +capture. The pretty Panagaeus crux-major was a treasure in those days, +and here at Down I saw a beetle running across a walk, and on picking +it up instantly perceived that it differed slightly from P. crux-major, +and it turned out to be P. quadripunctatus, which is only a variety or +closely allied species, differing from it very slightly in outline. I +had never seen in those old days Licinus alive, which to an uneducated +eye hardly differs from many of the black Carabidous beetles; but my +sons found here a specimen, and I instantly recognised that it was new +to me; yet I had not looked at a British beetle for the last twenty +years. + +I have not as yet mentioned a circumstance which influenced my whole +career more than any other. This was my friendship with Professor +Henslow. Before coming up to Cambridge, I had heard of him from my +brother as a man who knew every branch of science, and I was +accordingly prepared to reverence him. He kept open house once every +week when all undergraduates, and some older members of the University, +who were attached to science, used to meet in the evening. I soon got, +through Fox, an invitation, and went there regularly. Before long I +became well acquainted with Henslow, and during the latter half of my +time at Cambridge took long walks with him on most days; so that I was +called by some of the dons “the man who walks with Henslow;” and in the +evening I was very often asked to join his family dinner. His knowledge +was great in botany, entomology, chemistry, mineralogy, and geology. +His strongest taste was to draw conclusions from long-continued minute +observations. His judgment was excellent, and his whole mind well +balanced; but I do not suppose that any one would say that he possessed +much original genius. He was deeply religious, and so orthodox that he +told me one day he should be grieved if a single word of the +Thirty-nine Articles were altered. His moral qualities were in every +way admirable. He was free from every tinge of vanity or other petty +feeling; and I never saw a man who thought so little about himself or +his own concerns. His temper was imperturbably good, with the most +winning and courteous manners; yet, as I have seen, he could be roused +by any bad action to the warmest indignation and prompt action. + +I once saw in his company in the streets of Cambridge almost as horrid +a scene as could have been witnessed during the French Revolution. Two +body-snatchers had been arrested, and whilst being taken to prison had +been torn from the constable by a crowd of the roughest men, who +dragged them by their legs along the muddy and stony road. They were +covered from head to foot with mud, and their faces were bleeding +either from having been kicked or from the stones; they looked like +corpses, but the crowd was so dense that I got only a few momentary +glimpses of the wretched creatures. Never in my life have I seen such +wrath painted on a man’s face as was shown by Henslow at this horrid +scene. He tried repeatedly to penetrate the mob; but it was simply +impossible. He then rushed away to the mayor, telling me not to follow +him, but to get more policemen. I forget the issue, except that the two +men were got into the prison without being killed. + +Henslow’s benevolence was unbounded, as he proved by his many excellent +schemes for his poor parishioners, when in after years he held the +living of Hitcham. My intimacy with such a man ought to have been, and +I hope was, an inestimable benefit. I cannot resist mentioning a +trifling incident, which showed his kind consideration. Whilst +examining some pollen-grains on a damp surface, I saw the tubes +exserted, and instantly rushed off to communicate my surprising +discovery to him. Now I do not suppose any other professor of botany +could have helped laughing at my coming in such a hurry to make such a +communication. But he agreed how interesting the phenomenon was, and +explained its meaning, but made me clearly understand how well it was +known; so I left him not in the least mortified, but well pleased at +having discovered for myself so remarkable a fact, but determined not +to be in such a hurry again to communicate my discoveries. + +Dr. Whewell was one of the older and distinguished men who sometimes +visited Henslow, and on several occasions I walked home with him at +night. Next to Sir J. Mackintosh he was the best converser on grave +subjects to whom I ever listened. Leonard Jenyns (The well-known Soame +Jenyns was cousin to Mr. Jenyns’ father.), who afterwards published +some good essays in Natural History (Mr. Jenyns (now Blomefield) +described the fish for the Zoology of the “Beagle”; and is author of a +long series of papers, chiefly Zoological.), often stayed with Henslow, +who was his brother-in-law. I visited him at his parsonage on the +borders of the Fens [Swaffham Bulbeck], and had many a good walk and +talk with him about Natural History. I became also acquainted with +several other men older than me, who did not care much about science, +but were friends of Henslow. One was a Scotchman, brother of Sir +Alexander Ramsay, and tutor of Jesus College: he was a delightful man, +but did not live for many years. Another was Mr. Dawes, afterwards Dean +of Hereford, and famous for his success in the education of the poor. +These men and others of the same standing, together with Henslow, used +sometimes to take distant excursions into the country, which I was +allowed to join, and they were most agreeable. + +Looking back, I infer that there must have been something in me a +little superior to the common run of youths, otherwise the +above-mentioned men, so much older than me and higher in academical +position, would never have allowed me to associate with them. Certainly +I was not aware of any such superiority, and I remember one of my +sporting friends, Turner, who saw me at work with my beetles, saying +that I should some day be a Fellow of the Royal Society, and the notion +seemed to me preposterous. + +During my last year at Cambridge, I read with care and profound +interest Humboldt’s ‘Personal Narrative.’ This work, and Sir J. +Herschel’s ‘Introduction to the Study of Natural Philosophy,’ stirred +up in me a burning zeal to add even the most humble contribution to the +noble structure of Natural Science. No one or a dozen other books +influenced me nearly so much as these two. I copied out from Humboldt +long passages about Teneriffe, and read them aloud on one of the +above-mentioned excursions, to (I think) Henslow, Ramsay, and Dawes, +for on a previous occasion I had talked about the glories of Teneriffe, +and some of the party declared they would endeavour to go there; but I +think that they were only half in earnest. I was, however, quite in +earnest, and got an introduction to a merchant in London to enquire +about ships; but the scheme was, of course, knocked on the head by the +voyage of the “Beagle”. + +My summer vacations were given up to collecting beetles, to some +reading, and short tours. In the autumn my whole time was devoted to +shooting, chiefly at Woodhouse and Maer, and sometimes with young Eyton +of Eyton. Upon the whole the three years which I spent at Cambridge +were the most joyful in my happy life; for I was then in excellent +health, and almost always in high spirits. + +As I had at first come up to Cambridge at Christmas, I was forced to +keep two terms after passing my final examination, at the commencement +of 1831; and Henslow then persuaded me to begin the study of geology. +Therefore on my return to Shropshire I examined sections, and coloured +a map of parts round Shrewsbury. Professor Sedgwick intended to visit +North Wales in the beginning of August to pursue his famous geological +investigations amongst the older rocks, and Henslow asked him to allow +me to accompany him. (In connection with this tour my father used to +tell a story about Sedgwick: they had started from their inn one +morning, and had walked a mile or two, when Sedgwick suddenly stopped, +and vowed that he would return, being certain “that damned scoundrel” +(the waiter) had not given the chambermaid the sixpence intrusted to +him for the purpose. He was ultimately persuaded to give up the +project, seeing that there was no reason for suspecting the waiter of +especial perfidy.—F.D.) Accordingly he came and slept at my father’s +house. + +A short conversation with him during this evening produced a strong +impression on my mind. Whilst examining an old gravel-pit near +Shrewsbury, a labourer told me that he had found in it a large worn +tropical Volute shell, such as may be seen on the chimney-pieces of +cottages; and as he would not sell the shell, I was convinced that he +had really found it in the pit. I told Sedgwick of the fact, and he at +once said (no doubt truly) that it must have been thrown away by some +one into the pit; but then added, if really embedded there it would be +the greatest misfortune to geology, as it would overthrow all that we +know about the superficial deposits of the Midland Counties. These +gravel-beds belong in fact to the glacial period, and in after years I +found in them broken arctic shells. But I was then utterly astonished +at Sedgwick not being delighted at so wonderful a fact as a tropical +shell being found near the surface in the middle of England. Nothing +before had ever made me thoroughly realise, though I had read various +scientific books, that science consists in grouping facts so that +general laws or conclusions may be drawn from them. + +Next morning we started for Llangollen, Conway, Bangor, and Capel +Curig. This tour was of decided use in teaching me a little how to make +out the geology of a country. Sedgwick often sent me on a line parallel +to his, telling me to bring back specimens of the rocks and to mark the +stratification on a map. I have little doubt that he did this for my +good, as I was too ignorant to have aided him. On this tour I had a +striking instance of how easy it is to overlook phenomena, however +conspicuous, before they have been observed by any one. We spent many +hours in Cwm Idwal, examining all the rocks with extreme care, as +Sedgwick was anxious to find fossils in them; but neither of us saw a +trace of the wonderful glacial phenomena all around us; we did not +notice the plainly scored rocks, the perched boulders, the lateral and +terminal moraines. Yet these phenomena are so conspicuous that, as I +declared in a paper published many years afterwards in the +‘Philosophical Magazine’ (‘Philosophical Magazine,’ 1842.), a house +burnt down by fire did not tell its story more plainly than did this +valley. If it had still been filled by a glacier, the phenomena would +have been less distinct than they now are. + +At Capel Curig I left Sedgwick and went in a straight line by compass +and map across the mountains to Barmouth, never following any track +unless it coincided with my course. I thus came on some strange wild +places, and enjoyed much this manner of travelling. I visited Barmouth +to see some Cambridge friends who were reading there, and thence +returned to Shrewsbury and to Maer for shooting; for at that time I +should have thought myself mad to give up the first days of +partridge-shooting for geology or any other science. + + + + +“VOYAGE OF THE ‘BEAGLE’ FROM DECEMBER 27, 1831, TO OCTOBER 2, 1836.” + + +On returning home from my short geological tour in North Wales, I found +a letter from Henslow, informing me that Captain Fitz-Roy was willing +to give up part of his own cabin to any young man who would volunteer +to go with him without pay as naturalist to the Voyage of the “Beagle”. +I have given, as I believe, in my MS. Journal an account of all the +circumstances which then occurred; I will here only say that I was +instantly eager to accept the offer, but my father strongly objected, +adding the words, fortunate for me, “If you can find any man of common +sense who advises you to go I will give my consent.” So I wrote that +evening and refused the offer. On the next morning I went to Maer to be +ready for September 1st, and, whilst out shooting, my uncle (Josiah +Wedgwood.) sent for me, offering to drive me over to Shrewsbury and +talk with my father, as my uncle thought it would be wise in me to +accept the offer. My father always maintained that he was one of the +most sensible men in the world, and he at once consented in the kindest +manner. I had been rather extravagant at Cambridge, and to console my +father, said, “that I should be deuced clever to spend more than my +allowance whilst on board the ‘Beagle’;” but he answered with a smile, +“But they tell me you are very clever.” + +Next day I started for Cambridge to see Henslow, and thence to London +to see Fitz-Roy, and all was soon arranged. Afterwards, on becoming +very intimate with Fitz-Roy, I heard that I had run a very narrow risk +of being rejected, on account of the shape of my nose! He was an ardent +disciple of Lavater, and was convinced that he could judge of a man’s +character by the outline of his features; and he doubted whether any +one with my nose could possess sufficient energy and determination for +the voyage. But I think he was afterwards well satisfied that my nose +had spoken falsely. + +Fitz-Roy’s character was a singular one, with very many noble features: +he was devoted to his duty, generous to a fault, bold, determined, and +indomitably energetic, and an ardent friend to all under his sway. He +would undertake any sort of trouble to assist those whom he thought +deserved assistance. He was a handsome man, strikingly like a +gentleman, with highly courteous manners, which resembled those of his +maternal uncle, the famous Lord Castlereagh, as I was told by the +Minister at Rio. Nevertheless he must have inherited much in his +appearance from Charles II., for Dr. Wallich gave me a collection of +photographs which he had made, and I was struck with the resemblance of +one to Fitz-Roy; and on looking at the name, I found it Ch. E. Sobieski +Stuart, Count d’Albanie, a descendant of the same monarch. + +Fitz-Roy’s temper was a most unfortunate one. It was usually worst in +the early morning, and with his eagle eye he could generally detect +something amiss about the ship, and was then unsparing in his blame. He +was very kind to me, but was a man very difficult to live with on the +intimate terms which necessarily followed from our messing by ourselves +in the same cabin. We had several quarrels; for instance, early in the +voyage at Bahia, in Brazil, he defended and praised slavery, which I +abominated, and told me that he had just visited a great slave-owner, +who had called up many of his slaves and asked them whether they were +happy, and whether they wished to be free, and all answered “No.” I +then asked him, perhaps with a sneer, whether he thought that the +answer of slaves in the presence of their master was worth anything? +This made him excessively angry, and he said that as I doubted his word +we could not live any longer together. I thought that I should have +been compelled to leave the ship; but as soon as the news spread, which +it did quickly, as the captain sent for the first lieutenant to assuage +his anger by abusing me, I was deeply gratified by receiving an +invitation from all the gun-room officers to mess with them. But after +a few hours Fitz-Roy showed his usual magnanimity by sending an officer +to me with an apology and a request that I would continue to live with +him. + +His character was in several respects one of the most noble which I +have ever known. + +The voyage of the “Beagle” has been by far the most important event in +my life, and has determined my whole career; yet it depended on so +small a circumstance as my uncle offering to drive me thirty miles to +Shrewsbury, which few uncles would have done, and on such a trifle as +the shape of my nose. I have always felt that I owe to the voyage the +first real training or education of my mind; I was led to attend +closely to several branches of natural history, and thus my powers of +observation were improved, though they were always fairly developed. + +The investigation of the geology of all the places visited was far more +important, as reasoning here comes into play. On first examining a new +district nothing can appear more hopeless than the chaos of rocks; but +by recording the stratification and nature of the rocks and fossils at +many points, always reasoning and predicting what will be found +elsewhere, light soon begins to dawn on the district, and the structure +of the whole becomes more or less intelligible. I had brought with me +the first volume of Lyell’s ‘Principles of Geology,’ which I studied +attentively; and the book was of the highest service to me in many +ways. The very first place which I examined, namely St. Jago in the +Cape de Verde islands, showed me clearly the wonderful superiority of +Lyell’s manner of treating geology, compared with that of any other +author, whose works I had with me or ever afterwards read. + +Another of my occupations was collecting animals of all classes, +briefly describing and roughly dissecting many of the marine ones; but +from not being able to draw, and from not having sufficient anatomical +knowledge, a great pile of MS. which I made during the voyage has +proved almost useless. I thus lost much time, with the exception of +that spent in acquiring some knowledge of the Crustaceans, as this was +of service when in after years I undertook a monograph of the +Cirripedia. + +During some part of the day I wrote my Journal, and took much pains in +describing carefully and vividly all that I had seen; and this was good +practice. My Journal served also, in part, as letters to my home, and +portions were sent to England whenever there was an opportunity. + +The above various special studies were, however, of no importance +compared with the habit of energetic industry and of concentrated +attention to whatever I was engaged in, which I then acquired. +Everything about which I thought or read was made to bear directly on +what I had seen or was likely to see; and this habit of mind was +continued during the five years of the voyage. I feel sure that it was +this training which has enabled me to do whatever I have done in +science. + +Looking backwards, I can now perceive how my love for science gradually +preponderated over every other taste. During the first two years my old +passion for shooting survived in nearly full force, and I shot myself +all the birds and animals for my collection; but gradually I gave up my +gun more and more, and finally altogether, to my servant, as shooting +interfered with my work, more especially with making out the geological +structure of a country. I discovered, though unconsciously and +insensibly, that the pleasure of observing and reasoning was a much +higher one than that of skill and sport. That my mind became developed +through my pursuits during the voyage is rendered probable by a remark +made by my father, who was the most acute observer whom I ever saw, of +a sceptical disposition, and far from being a believer in phrenology; +for on first seeing me after the voyage, he turned round to my sisters, +and exclaimed, “Why, the shape of his head is quite altered.” + +To return to the voyage. On September 11th (1831), I paid a flying +visit with Fitz-Roy to the “Beagle” at Plymouth. Thence to Shrewsbury +to wish my father and sisters a long farewell. On October 24th I took +up my residence at Plymouth, and remained there until December 27th, +when the “Beagle” finally left the shores of England for her +circumnavigation of the world. We made two earlier attempts to sail, +but were driven back each time by heavy gales. These two months at +Plymouth were the most miserable which I ever spent, though I exerted +myself in various ways. I was out of spirits at the thought of leaving +all my family and friends for so long a time, and the weather seemed to +me inexpressibly gloomy. I was also troubled with palpitation and pain +about the heart, and like many a young ignorant man, especially one +with a smattering of medical knowledge, was convinced that I had heart +disease. I did not consult any doctor, as I fully expected to hear the +verdict that I was not fit for the voyage, and I was resolved to go at +all hazards. + +I need not here refer to the events of the voyage—where we went and +what we did—as I have given a sufficiently full account in my published +Journal. The glories of the vegetation of the Tropics rise before my +mind at the present time more vividly than anything else; though the +sense of sublimity, which the great deserts of Patagonia and the +forest-clad mountains of Tierra del Fuego excited in me, has left an +indelible impression on my mind. The sight of a naked savage in his +native land is an event which can never be forgotten. Many of my +excursions on horseback through wild countries, or in the boats, some +of which lasted several weeks, were deeply interesting: their +discomfort and some degree of danger were at that time hardly a +drawback, and none at all afterwards. I also reflect with high +satisfaction on some of my scientific work, such as solving the problem +of coral islands, and making out the geological structure of certain +islands, for instance, St. Helena. Nor must I pass over the discovery +of the singular relations of the animals and plants inhabiting the +several islands of the Galapagos archipelago, and of all of them to the +inhabitants of South America. + +As far as I can judge of myself, I worked to the utmost during the +voyage from the mere pleasure of investigation, and from my strong +desire to add a few facts to the great mass of facts in Natural +Science. But I was also ambitious to take a fair place among scientific +men,—whether more ambitious or less so than most of my fellow-workers, +I can form no opinion. + +The geology of St. Jago is very striking, yet simple: a stream of lava +formerly flowed over the bed of the sea, formed of triturated recent +shells and corals, which it has baked into a hard white rock. Since +then the whole island has been upheaved. But the line of white rock +revealed to me a new and important fact, namely, that there had been +afterwards subsidence round the craters, which had since been in +action, and had poured forth lava. It then first dawned on me that I +might perhaps write a book on the geology of the various countries +visited, and this made me thrill with delight. That was a memorable +hour to me, and how distinctly I can call to mind the low cliff of lava +beneath which I rested, with the sun glaring hot, a few strange desert +plants growing near, and with living corals in the tidal pools at my +feet. Later in the voyage, Fitz-Roy asked me to read some of my +Journal, and declared it would be worth publishing; so here was a +second book in prospect! + +Towards the close of our voyage I received a letter whilst at +Ascension, in which my sisters told me that Sedgwick had called on my +father, and said that I should take a place among the leading +scientific men. I could not at the time understand how he could have +learnt anything of my proceedings, but I heard (I believe afterwards) +that Henslow had read some of the letters which I wrote to him before +the Philosophical Society of Cambridge (Read at the meeting held +November 16, 1835, and printed in a pamphlet of 31 pages for +distribution among the members of the Society.), and had printed them +for private distribution. My collection of fossil bones, which had been +sent to Henslow, also excited considerable attention amongst +palaeontologists. After reading this letter, I clambered over the +mountains of Ascension with a bounding step, and made the volcanic +rocks resound under my geological hammer. All this shows how ambitious +I was; but I think that I can say with truth that in after years, +though I cared in the highest degree for the approbation of such men as +Lyell and Hooker, who were my friends, I did not care much about the +general public. I do not mean to say that a favourable review or a +large sale of my books did not please me greatly, but the pleasure was +a fleeting one, and I am sure that I have never turned one inch out of +my course to gain fame. + + + + +FROM MY RETURN TO ENGLAND (OCTOBER 2, 1836) TO MY MARRIAGE (JANUARY 29, +1839.) + +These two years and three months were the most active ones which I ever +spent, though I was occasionally unwell, and so lost some time. After +going backwards and forwards several times between Shrewsbury, Maer, +Cambridge, and London, I settled in lodgings at Cambridge (In +Fitzwilliam Street.) on December 13th, where all my collections were +under the care of Henslow. I stayed here three months, and got my +minerals and rocks examined by the aid of Professor Miller. + +I began preparing my ‘Journal of Travels,’ which was not hard work, as +my MS. Journal had been written with care, and my chief labour was +making an abstract of my more interesting scientific results. I sent +also, at the request of Lyell, a short account of my observations on +the elevation of the coast of Chile to the Geological Society. +(‘Geolog. Soc. Proc. ii. 1838, pages 446-449.) + +On March 7th, 1837, I took lodgings in Great Marlborough Street in +London, and remained there for nearly two years, until I was married. +During these two years I finished my Journal, read several papers +before the Geological Society, began preparing the MS. for my +‘Geological Observations,’ and arranged for the publication of the +‘Zoology of the Voyage of the “Beagle”.’ In July I opened my first +note-book for facts in relation to the Origin of Species, about which I +had long reflected, and never ceased working for the next twenty years. + +During these two years I also went a little into society, and acted as +one of the honorary secretaries of the Geological Society. I saw a +great deal of Lyell. One of his chief characteristics was his sympathy +with the work of others, and I was as much astonished as delighted at +the interest which he showed when, on my return to England, I explained +to him my views on coral reefs. This encouraged me greatly, and his +advice and example had much influence on me. During this time I saw +also a good deal of Robert Brown; I used often to call and sit with him +during his breakfast on Sunday mornings, and he poured forth a rich +treasure of curious observations and acute remarks, but they almost +always related to minute points, and he never with me discussed large +or general questions in science. + +During these two years I took several short excursions as a relaxation, +and one longer one to the Parallel Roads of Glen Roy, an account of +which was published in the ‘Philosophical Transactions.’ (1839, pages +39-82.) This paper was a great failure, and I am ashamed of it. Having +been deeply impressed with what I had seen of the elevation of the land +of South America, I attributed the parallel lines to the action of the +sea; but I had to give up this view when Agassiz propounded his +glacier-lake theory. Because no other explanation was possible under +our then state of knowledge, I argued in favour of sea-action; and my +error has been a good lesson to me never to trust in science to the +principle of exclusion. + +As I was not able to work all day at science, I read a good deal during +these two years on various subjects, including some metaphysical books; +but I was not well fitted for such studies. About this time I took much +delight in Wordsworth’s and Coleridge’s poetry; and can boast that I +read the ‘Excursion’ twice through. Formerly Milton’s ‘Paradise Lost’ +had been my chief favourite, and in my excursions during the voyage of +the “Beagle”, when I could take only a single volume, I always chose +Milton. + + + + +FROM MY MARRIAGE, JANUARY 29, 1839, AND RESIDENCE IN UPPER GOWER +STREET, TO OUR LEAVING LONDON AND SETTLING AT DOWN, SEPTEMBER 14, +1842. + + +(After speaking of his happy married life, and of his children, he +continues:—) + +During the three years and eight months whilst we resided in London, I +did less scientific work, though I worked as hard as I possibly could, +than during any other equal length of time in my life. This was owing +to frequently recurring unwellness, and to one long and serious +illness. The greater part of my time, when I could do anything, was +devoted to my work on ‘Coral Reefs,’ which I had begun before my +marriage, and of which the last proof-sheet was corrected on May 6th, +1842. This book, though a small one, cost me twenty months of hard +work, as I had to read every work on the islands of the Pacific and to +consult many charts. It was thought highly of by scientific men, and +the theory therein given is, I think, now well established. + +No other work of mine was begun in so deductive a spirit as this, for +the whole theory was thought out on the west coast of South America, +before I had seen a true coral reef. I had therefore only to verify and +extend my views by a careful examination of living reefs. But it should +be observed that I had during the two previous years been incessantly +attending to the effects on the shores of South America of the +intermittent elevation of the land, together with denudation and the +deposition of sediment. This necessarily led me to reflect much on the +effects of subsidence, and it was easy to replace in imagination the +continued deposition of sediment by the upward growth of corals. To do +this was to form my theory of the formation of barrier-reefs and +atolls. + +Besides my work on coral-reefs, during my residence in London, I read +before the Geological Society papers on the Erratic Boulders of South +America (‘Geolog. Soc. Proc.’ iii. 1842.), on Earthquakes (‘Geolog. +Trans. v. 1840.), and on the Formation by the Agency of Earth-worms of +Mould. (‘Geolog. Soc. Proc. ii. 1838.) I also continued to superintend +the publication of the ‘Zoology of the Voyage of the “Beagle”.’ Nor did +I ever intermit collecting facts bearing on the origin of species; and +I could sometimes do this when I could do nothing else from illness. + +In the summer of 1842 I was stronger than I had been for some time, and +took a little tour by myself in North Wales, for the sake of observing +the effects of the old glaciers which formerly filled all the larger +valleys. I published a short account of what I saw in the +‘Philosophical Magazine.’ (‘Philosophical Magazine,’ 1842.) This +excursion interested me greatly, and it was the last time I was ever +strong enough to climb mountains or to take long walks such as are +necessary for geological work. + +During the early part of our life in London, I was strong enough to go +into general society, and saw a good deal of several scientific men, +and other more or less distinguished men. I will give my impressions +with respect to some of them, though I have little to say worth saying. + +I saw more of Lyell than of any other man, both before and after my +marriage. His mind was characterised, as it appeared to me, by +clearness, caution, sound judgment, and a good deal of originality. +When I made any remark to him on Geology, he never rested until he saw +the whole case clearly, and often made me see it more clearly than I +had done before. He would advance all possible objections to my +suggestion, and even after these were exhausted would long remain +dubious. A second characteristic was his hearty sympathy with the work +of other scientific men. (The slight repetition here observable is +accounted for by the notes on Lyell, etc., having been added in April, +1881, a few years after the rest of the ‘Recollections’ were written.) + +On my return from the voyage of the “Beagle”, I explained to him my +views on coral-reefs, which differed from his, and I was greatly +surprised and encouraged by the vivid interest which he showed. His +delight in science was ardent, and he felt the keenest interest in the +future progress of mankind. He was very kind-hearted, and thoroughly +liberal in his religious beliefs, or rather disbeliefs; but he was a +strong theist. His candour was highly remarkable. He exhibited this by +becoming a convert to the Descent theory, though he had gained much +fame by opposing Lamarck’s views, and this after he had grown old. He +reminded me that I had many years before said to him, when discussing +the opposition of the old school of geologists to his new views, “What +a good thing it would be if every scientific man was to die when sixty +years old, as afterwards he would be sure to oppose all new doctrines.” +But he hoped that now he might be allowed to live. + +The science of Geology is enormously indebted to Lyell—more so, as I +believe, than to any other man who ever lived. When [I was] starting on +the voyage of the “Beagle”, the sagacious Henslow, who, like all other +geologists, believed at that time in successive cataclysms, advised me +to get and study the first volume of the ‘Principles,’ which had then +just been published, but on no account to accept the views therein +advocated. How differently would anyone now speak of the ‘Principles’! +I am proud to remember that the first place, namely, St. Jago, in the +Cape de Verde archipelago, in which I geologised, convinced me of the +infinite superiority of Lyell’s views over those advocated in any other +work known to me. + +The powerful effects of Lyell’s works could formerly be plainly seen in +the different progress of the science in France and England. The +present total oblivion of Elie de Beaumont’s wild hypotheses, such as +his ‘Craters of Elevation’ and ‘Lines of Elevation’ (which latter +hypothesis I heard Sedgwick at the Geological Society lauding to the +skies), may be largely attributed to Lyell. + +I saw a good deal of Robert Brown, “facile Princeps Botanicorum,” as he +was called by Humboldt. He seemed to me to be chiefly remarkable for +the minuteness of his observations, and their perfect accuracy. His +knowledge was extraordinarily great, and much died with him, owing to +his excessive fear of ever making a mistake. He poured out his +knowledge to me in the most unreserved manner, yet was strangely +jealous on some points. I called on him two or three times before the +voyage of the “Beagle”, and on one occasion he asked me to look through +a microscope and describe what I saw. This I did, and believe now that +it was the marvellous currents of protoplasm in some vegetable cell. I +then asked him what I had seen; but he answered me, “That is my little +secret.” + +He was capable of the most generous actions. When old, much out of +health, and quite unfit for any exertion, he daily visited (as Hooker +told me) an old man-servant, who lived at a distance (and whom he +supported), and read aloud to him. This is enough to make up for any +degree of scientific penuriousness or jealousy. + +I may here mention a few other eminent men, whom I have occasionally +seen, but I have little to say about them worth saying. I felt a high +reverence for Sir J. Herschel, and was delighted to dine with him at +his charming house at the Cape of Good Hope, and afterwards at his +London house. I saw him, also, on a few other occasions. He never +talked much, but every word which he uttered was worth listening to. + +I once met at breakfast at Sir R. Murchison’s house the illustrious +Humboldt, who honoured me by expressing a wish to see me. I was a +little disappointed with the great man, but my anticipations probably +were too high. I can remember nothing distinctly about our interview, +except that Humboldt was very cheerful and talked much. + +—reminds me of Buckle whom I once met at Hensleigh Wedgwood’s. I was +very glad to learn from him his system of collecting facts. He told me +that he bought all the books which he read, and made a full index, to +each, of the facts which he thought might prove serviceable to him, and +that he could always remember in what book he had read anything, for +his memory was wonderful. I asked him how at first he could judge what +facts would be serviceable, and he answered that he did not know, but +that a sort of instinct guided him. From this habit of making indices, +he was enabled to give the astonishing number of references on all +sorts of subjects, which may be found in his ‘History of Civilisation.’ +This book I thought most interesting, and read it twice, but I doubt +whether his generalisations are worth anything. Buckle was a great +talker, and I listened to him saying hardly a word, nor indeed could I +have done so for he left no gaps. When Mrs. Farrer began to sing, I +jumped up and said that I must listen to her; after I had moved away he +turned around to a friend and said (as was overheard by my brother), +“Well, Mr. Darwin’s books are much better than his conversation.” + +Of other great literary men, I once met Sydney Smith at Dean Milman’s +house. There was something inexplicably amusing in every word which he +uttered. Perhaps this was partly due to the expectation of being +amused. He was talking about Lady Cork, who was then extremely old. +This was the lady who, as he said, was once so much affected by one of +his charity sermons, that she _borrowed_ a guinea from a friend to put +in the plate. He now said “It is generally believed that my dear old +friend Lady Cork has been overlooked,” and he said this in such a +manner that no one could for a moment doubt that he meant that his dear +old friend had been overlooked by the devil. How he managed to express +this I know not. + +I likewise once met Macaulay at Lord Stanhope’s (the historian’s) +house, and as there was only one other man at dinner, I had a grand +opportunity of hearing him converse, and he was very agreeable. He did +not talk at all too much; nor indeed could such a man talk too much, as +long as he allowed others to turn the stream of his conversation, and +this he did allow. + +Lord Stanhope once gave me a curious little proof of the accuracy and +fulness of Macaulay’s memory: many historians used often to meet at +Lord Stanhope’s house, and in discussing various subjects they would +sometimes differ from Macaulay, and formerly they often referred to +some book to see who was right; but latterly, as Lord Stanhope noticed, +no historian ever took this trouble, and whatever Macaulay said was +final. + +On another occasion I met at Lord Stanhope’s house, one of his parties +of historians and other literary men, and amongst them were Motley and +Grote. After luncheon I walked about Chevening Park for nearly an hour +with Grote, and was much interested by his conversation and pleased by +the simplicity and absence of all pretension in his manners. + +Long ago I dined occasionally with the old Earl, the father of the +historian; he was a strange man, but what little I knew of him I liked +much. He was frank, genial, and pleasant. He had strongly marked +features, with a brown complexion, and his clothes, when I saw him, +were all brown. He seemed to believe in everything which was to others +utterly incredible. He said one day to me, “Why don’t you give up your +fiddle-faddle of geology and zoology, and turn to the occult sciences!” +The historian, then Lord Mahon, seemed shocked at such a speech to me, +and his charming wife much amused. + +The last man whom I will mention is Carlyle, seen by me several times +at my brother’s house, and two or three times at my own house. His talk +was very racy and interesting, just like his writings, but he sometimes +went on too long on the same subject. I remember a funny dinner at my +brother’s, where, amongst a few others, were Babbage and Lyell, both of +whom liked to talk. Carlyle, however, silenced every one by haranguing +during the whole dinner on the advantages of silence. After dinner +Babbage, in his grimmest manner, thanked Carlyle for his very +interesting lecture on silence. + +Carlyle sneered at almost every one: one day in my house he called +Grote’s ‘History’ “a fetid quagmire, with nothing spiritual about it.” +I always thought, until his ‘Reminiscences’ appeared, that his sneers +were partly jokes, but this now seems rather doubtful. His expression +was that of a depressed, almost despondent yet benevolent man; and it +is notorious how heartily he laughed. I believe that his benevolence +was real, though stained by not a little jealousy. No one can doubt +about his extraordinary power of drawing pictures of things and men—far +more vivid, as it appears to me, than any drawn by Macaulay. Whether +his pictures of men were true ones is another question. + +He has been all-powerful in impressing some grand moral truths on the +minds of men. On the other hand, his views about slavery were +revolting. In his eyes might was right. His mind seemed to me a very +narrow one; even if all branches of science, which he despised, are +excluded. It is astonishing to me that Kingsley should have spoken of +him as a man well fitted to advance science. He laughed to scorn the +idea that a mathematician, such as Whewell, could judge, as I +maintained he could, of Goethe’s views on light. He thought it a most +ridiculous thing that any one should care whether a glacier moved a +little quicker or a little slower, or moved at all. As far as I could +judge, I never met a man with a mind so ill adapted for scientific +research. + +Whilst living in London, I attended as regularly as I could the +meetings of several scientific societies, and acted as secretary to the +Geological Society. But such attendance, and ordinary society, suited +my health so badly that we resolved to live in the country, which we +both preferred and have never repented of. + + + + +RESIDENCE AT DOWN FROM SEPTEMBER 14, 1842, TO THE PRESENT TIME, 1876. + + +After several fruitless searches in Surrey and elsewhere, we found this +house and purchased it. I was pleased with the diversified appearance +of vegetation proper to a chalk district, and so unlike what I had been +accustomed to in the Midland counties; and still more pleased with the +extreme quietness and rusticity of the place. It is not, however, quite +so retired a place as a writer in a German periodical makes it, who +says that my house can be approached only by a mule-track! Our fixing +ourselves here has answered admirably in one way, which we did not +anticipate, namely, by being very convenient for frequent visits from +our children. + +Few persons can have lived a more retired life than we have done. +Besides short visits to the houses of relations, and occasionally to +the seaside or elsewhere, we have gone nowhere. During the first part +of our residence we went a little into society, and received a few +friends here; but my health almost always suffered from the excitement, +violent shivering and vomiting attacks being thus brought on. I have +therefore been compelled for many years to give up all dinner-parties; +and this has been somewhat of a deprivation to me, as such parties +always put me into high spirits. From the same cause I have been able +to invite here very few scientific acquaintances. + +My chief enjoyment and sole employment throughout life has been +scientific work; and the excitement from such work makes me for the +time forget, or drives quite away, my daily discomfort. I have +therefore nothing to record during the rest of my life, except the +publication of my several books. Perhaps a few details how they arose +may be worth giving. + + + + +MY SEVERAL PUBLICATIONS. + + +In the early part of 1844, my observations on the volcanic islands +visited during the voyage of the “Beagle” were published. In 1845, I +took much pains in correcting a new edition of my ‘Journal of +Researches,’ which was originally published in 1839 as part of +Fitz-Roy’s work. The success of this, my first literary child, always +tickles my vanity more than that of any of my other books. Even to this +day it sells steadily in England and the United States, and has been +translated for the second time into German, and into French and other +languages. This success of a book of travels, especially of a +scientific one, so many years after its first publication, is +surprising. Ten thousand copies have been sold in England of the second +edition. In 1846 my ‘Geological Observations on South America’ were +published. I record in a little diary, which I have always kept, that +my three geological books (‘Coral Reefs’ included) consumed four and a +half years’ steady work; “and now it is ten years since my return to +England. How much time have I lost by illness?” I have nothing to say +about these three books except that to my surprise new editions have +lately been called for. (‘Geological Observations,’ 2nd Edit.1876. +‘Coral Reefs,’ 2nd Edit. 1874.) + +In October, 1846, I began to work on ‘Cirripedia.’ When on the coast of +Chile, I found a most curious form, which burrowed into the shells of +Concholepas, and which differed so much from all other Cirripedes that +I had to form a new sub-order for its sole reception. Lately an allied +burrowing genus has been found on the shores of Portugal. To understand +the structure of my new Cirripede I had to examine and dissect many of +the common forms; and this gradually led me on to take up the whole +group. I worked steadily on this subject for the next eight years, and +ultimately published two thick volumes (Published by the Ray Society.), +describing all the known living species, and two thin quartos on the +extinct species. I do not doubt that Sir E. Lytton Bulwer had me in his +mind when he introduced in one of his novels a Professor Long, who had +written two huge volumes on limpets. + +Although I was employed during eight years on this work, yet I record +in my diary that about two years out of this time was lost by illness. +On this account I went in 1848 for some months to Malvern for +hydropathic treatment, which did me much good, so that on my return +home I was able to resume work. So much was I out of health that when +my dear father died on November 13th, 1848, I was unable to attend his +funeral or to act as one of his executors. + +My work on the Cirripedia possesses, I think, considerable value, as +besides describing several new and remarkable forms, I made out the +homologies of the various parts—I discovered the cementing apparatus, +though I blundered dreadfully about the cement glands—and lastly I +proved the existence in certain genera of minute males complemental to +and parasitic on the hermaphrodites. This latter discovery has at last +been fully confirmed; though at one time a German writer was pleased to +attribute the whole account to my fertile imagination. The Cirripedes +form a highly varying and difficult group of species to class; and my +work was of considerable use to me, when I had to discuss in the +‘Origin of Species’ the principles of a natural classification. +Nevertheless, I doubt whether the work was worth the consumption of so +much time. + +From September 1854 I devoted my whole time to arranging my huge pile +of notes, to observing, and to experimenting in relation to the +transmutation of species. During the voyage of the “Beagle” I had been +deeply impressed by discovering in the Pampean formation great fossil +animals covered with armour like that on the existing armadillos; +secondly, by the manner in which closely allied animals replace one +another in proceeding southwards over the Continent; and thirdly, by +the South American character of most of the productions of the +Galapagos archipelago, and more especially by the manner in which they +differ slightly on each island of the group; none of the islands +appearing to be very ancient in a geological sense. + +It was evident that such facts as these, as well as many others, could +only be explained on the supposition that species gradually become +modified; and the subject haunted me. But it was equally evident that +neither the action of the surrounding conditions, nor the will of the +organisms (especially in the case of plants) could account for the +innumerable cases in which organisms of every kind are beautifully +adapted to their habits of life—for instance, a woodpecker or a +tree-frog to climb trees, or a seed for dispersal by hooks or plumes. I +had always been much struck by such adaptations, and until these could +be explained it seemed to me almost useless to endeavour to prove by +indirect evidence that species have been modified. + +After my return to England it appeared to me that by following the +example of Lyell in Geology, and by collecting all facts which bore in +any way on the variation of animals and plants under domestication and +nature, some light might perhaps be thrown on the whole subject. My +first note-book was opened in July 1837. I worked on true Baconian +principles, and without any theory collected facts on a wholesale +scale, more especially with respect to domesticated productions, by +printed enquiries, by conversation with skilful breeders and gardeners, +and by extensive reading. When I see the list of books of all kinds +which I read and abstracted, including whole series of Journals and +Transactions, I am surprised at my industry. I soon perceived that +selection was the keystone of man’s success in making useful races of +animals and plants. But how selection could be applied to organisms +living in a state of nature remained for some time a mystery to me. + +In October 1838, that is, fifteen months after I had begun my +systematic enquiry, I happened to read for amusement ‘Malthus on +Population,’ and being well prepared to appreciate the struggle for +existence which everywhere goes on from long-continued observation of +the habits of animals and plants, it at once struck me that under these +circumstances favourable variations would tend to be preserved, and +unfavourable ones to be destroyed. The result of this would be the +formation of new species. Here then I had at last got a theory by which +to work; but I was so anxious to avoid prejudice, that I determined not +for some time to write even the briefest sketch of it. In June 1842 I +first allowed myself the satisfaction of writing a very brief abstract +of my theory in pencil in 35 pages; and this was enlarged during the +summer of 1844 into one of 230 pages, which I had fairly copied out and +still possess. + +But at that time I overlooked one problem of great importance; and it +is astonishing to me, except on the principle of Columbus and his egg, +how I could have overlooked it and its solution. This problem is the +tendency in organic beings descended from the same stock to diverge in +character as they become modified. That they have diverged greatly is +obvious from the manner in which species of all kinds can be classed +under genera, genera under families, families under sub-orders and so +forth; and I can remember the very spot in the road, whilst in my +carriage, when to my joy the solution occurred to me; and this was long +after I had come to Down. The solution, as I believe, is that the +modified offspring of all dominant and increasing forms tend to become +adapted to many and highly diversified places in the economy of nature. + +Early in 1856 Lyell advised me to write out my views pretty fully, and +I began at once to do so on a scale three or four times as extensive as +that which was afterwards followed in my ‘Origin of Species;’ yet it +was only an abstract of the materials which I had collected, and I got +through about half the work on this scale. But my plans were +overthrown, for early in the summer of 1858 Mr. Wallace, who was then +in the Malay archipelago, sent me an essay “On the Tendency of +Varieties to depart indefinitely from the Original Type;” and this +essay contained exactly the same theory as mine. Mr. Wallace expressed +the wish that if I thought well of his essay, I should sent it to Lyell +for perusal. + +The circumstances under which I consented at the request of Lyell and +Hooker to allow of an abstract from my MS., together with a letter to +Asa Gray, dated September 5, 1857, to be published at the same time +with Wallace’s Essay, are given in the ‘Journal of the Proceedings of +the Linnean Society,’ 1858, page 45. I was at first very unwilling to +consent, as I thought Mr. Wallace might consider my doing so +unjustifiable, for I did not then know how generous and noble was his +disposition. The extract from my MS. and the letter to Asa Gray had +neither been intended for publication, and were badly written. Mr. +Wallace’s essay, on the other hand, was admirably expressed and quite +clear. Nevertheless, our joint productions excited very little +attention, and the only published notice of them which I can remember +was by Professor Haughton of Dublin, whose verdict was that all that +was new in them was false, and what was true was old. This shows how +necessary it is that any new view should be explained at considerable +length in order to arouse public attention. + +In September 1858 I set to work by the strong advice of Lyell and +Hooker to prepare a volume on the transmutation of species, but was +often interrupted by ill-health, and short visits to Dr. Lane’s +delightful hydropathic establishment at Moor Park. I abstracted the MS. +begun on a much larger scale in 1856, and completed the volume on the +same reduced scale. It cost me thirteen months and ten days’ hard +labour. It was published under the title of the ‘Origin of Species,’ in +November 1859. Though considerably added to and corrected in the later +editions, it has remained substantially the same book. + +It is no doubt the chief work of my life. It was from the first highly +successful. The first small edition of 1250 copies was sold on the day +of publication, and a second edition of 3000 copies soon afterwards. +Sixteen thousand copies have now (1876) been sold in England; and +considering how stiff a book it is, this is a large sale. It has been +translated into almost every European tongue, even into such languages +as Spanish, Bohemian, Polish, and Russian. It has also, according to +Miss Bird, been translated into Japanese (Miss Bird is mistaken, as I +learn from Prof. Mitsukuri.—F.D.), and is there much studied. Even an +essay in Hebrew has appeared on it, showing that the theory is +contained in the Old Testament! The reviews were very numerous; for +some time I collected all that appeared on the ‘Origin’ and on my +related books, and these amount (excluding newspaper reviews) to 265; +but after a time I gave up the attempt in despair. Many separate essays +and books on the subject have appeared; and in Germany a catalogue or +bibliography on “Darwinismus” has appeared every year or two. + +The success of the ‘Origin’ may, I think, be attributed in large part +to my having long before written two condensed sketches, and to my +having finally abstracted a much larger manuscript, which was itself an +abstract. By this means I was enabled to select the more striking facts +and conclusions. I had, also, during many years followed a golden rule, +namely, that whenever a published fact, a new observation or thought +came across me, which was opposed to my general results, to make a +memorandum of it without fail and at once; for I had found by +experience that such facts and thoughts were far more apt to escape +from the memory than favourable ones. Owing to this habit, very few +objections were raised against my views which I had not at least +noticed and attempted to answer. + +It has sometimes been said that the success of the ‘Origin’ proved +“that the subject was in the air,” or “that men’s minds were prepared +for it.” I do not think that this is strictly true, for I occasionally +sounded not a few naturalists, and never happened to come across a +single one who seemed to doubt about the permanence of species. Even +Lyell and Hooker, though they would listen with interest to me, never +seemed to agree. I tried once or twice to explain to able men what I +meant by Natural Selection, but signally failed. What I believe was +strictly true is that innumerable well-observed facts were stored in +the minds of naturalists ready to take their proper places as soon as +any theory which would receive them was sufficiently explained. Another +element in the success of the book was its moderate size; and this I +owe to the appearance of Mr. Wallace’s essay; had I published on the +scale in which I began to write in 1856, the book would have been four +or five times as large as the ‘Origin,’ and very few would have had the +patience to read it. + +I gained much by my delay in publishing from about 1839, when the +theory was clearly conceived, to 1859; and I lost nothing by it, for I +cared very little whether men attributed most originality to me or +Wallace; and his essay no doubt aided in the reception of the theory. I +was forestalled in only one important point, which my vanity has always +made me regret, namely, the explanation by means of the Glacial period +of the presence of the same species of plants and of some few animals +on distant mountain summits and in the arctic regions. This view +pleased me so much that I wrote it out in extenso, and I believe that +it was read by Hooker some years before E. Forbes published his +celebrated memoir (‘Geolog. Survey Mem.,’ 1846.) on the subject. In the +very few points in which we differed, I still think that I was in the +right. I have never, of course, alluded in print to my having +independently worked out this view. + +Hardly any point gave me so much satisfaction when I was at work on the +‘Origin,’ as the explanation of the wide difference in many classes +between the embryo and the adult animal, and of the close resemblance +of the embryos within the same class. No notice of this point was +taken, as far as I remember, in the early reviews of the ‘Origin,’ and +I recollect expressing my surprise on this head in a letter to Asa +Gray. Within late years several reviewers have given the whole credit +to Fritz Muller and Hackel, who undoubtedly have worked it out much +more fully, and in some respects more correctly than I did. I had +materials for a whole chapter on the subject, and I ought to have made +the discussion longer; for it is clear that I failed to impress my +readers; and he who succeeds in doing so deserves, in my opinion, all +the credit. + +This leads me to remark that I have almost always been treated honestly +by my reviewers, passing over those without scientific knowledge as not +worthy of notice. My views have often been grossly misrepresented, +bitterly opposed and ridiculed, but this has been generally done, as I +believe, in good faith. On the whole I do not doubt that my works have +been over and over again greatly overpraised. I rejoice that I have +avoided controversies, and this I owe to Lyell, who many years ago, in +reference to my geological works, strongly advised me never to get +entangled in a controversy, as it rarely did any good and caused a +miserable loss of time and temper. + +Whenever I have found out that I have blundered, or that my work has +been imperfect, and when I have been contemptuously criticised, and +even when I have been overpraised, so that I have felt mortified, it +has been my greatest comfort to say hundreds of times to myself that “I +have worked as hard and as well as I could, and no man can do more than +this.” I remember when in Good Success Bay, in Tierra del Fuego, +thinking (and, I believe, that I wrote home to the effect) that I could +not employ my life better than in adding a little to Natural Science. +This I have done to the best of my abilities, and critics may say what +they like, but they cannot destroy this conviction. + +During the two last months of 1859 I was fully occupied in preparing a +second edition of the ‘Origin,’ and by an enormous correspondence. On +January 1st, 1860, I began arranging my notes for my work on the +‘Variation of Animals and Plants under Domestication;’ but it was not +published until the beginning of 1868; the delay having been caused +partly by frequent illnesses, one of which lasted seven months, and +partly by being tempted to publish on other subjects which at the time +interested me more. + +On May 15th, 1862, my little book on the ‘Fertilisation of Orchids,’ +which cost me ten months’ work, was published: most of the facts had +been slowly accumulated during several previous years. During the +summer of 1839, and, I believe, during the previous summer, I was led +to attend to the cross-fertilisation of flowers by the aid of insects, +from having come to the conclusion in my speculations on the origin of +species, that crossing played an important part in keeping specific +forms constant. I attended to the subject more or less during every +subsequent summer; and my interest in it was greatly enhanced by having +procured and read in November 1841, through the advice of Robert Brown, +a copy of C.K. Sprengel’s wonderful book, ‘Das entdeckte Geheimniss der +Natur.’ For some years before 1862 I had specially attended to the +fertilisation of our British orchids; and it seemed to me the best plan +to prepare as complete a treatise on this group of plants as well as I +could, rather than to utilise the great mass of matter which I had +slowly collected with respect to other plants. + +My resolve proved a wise one; for since the appearance of my book, a +surprising number of papers and separate works on the fertilisation of +all kinds of flowers have appeared: and these are far better done than +I could possibly have effected. The merits of poor old Sprengel, so +long overlooked, are now fully recognised many years after his death. + +During the same year I published in the ‘Journal of the Linnean +Society’ a paper “On the Two Forms, or Dimorphic Condition of Primula,” +and during the next five years, five other papers on dimorphic and +trimorphic plants. I do not think anything in my scientific life has +given me so much satisfaction as making out the meaning of the +structure of these plants. I had noticed in 1838 or 1839 the dimorphism +of Linum flavum, and had at first thought that it was merely a case of +unmeaning variability. But on examining the common species of Primula I +found that the two forms were much too regular and constant to be thus +viewed. I therefore became almost convinced that the common cowslip and +primrose were on the high road to become dioecious;—that the short +pistil in the one form, and the short stamens in the other form were +tending towards abortion. The plants were therefore subjected under +this point of view to trial; but as soon as the flowers with short +pistils fertilised with pollen from the short stamens, were found to +yield more seeds than any other of the four possible unions, the +abortion-theory was knocked on the head. After some additional +experiment, it became evident that the two forms, though both were +perfect hermaphrodites, bore almost the same relation to one another as +do the two sexes of an ordinary animal. With Lythrum we have the still +more wonderful case of three forms standing in a similar relation to +one another. I afterwards found that the offspring from the union of +two plants belonging to the same forms presented a close and curious +analogy with hybrids from the union of two distinct species. + +In the autumn of 1864 I finished a long paper on ‘Climbing Plants,’ and +sent it to the Linnean Society. The writing of this paper cost me four +months; but I was so unwell when I received the proof-sheets that I was +forced to leave them very badly and often obscurely expressed. The +paper was little noticed, but when in 1875 it was corrected and +published as a separate book it sold well. I was led to take up this +subject by reading a short paper by Asa Gray, published in 1858. He +sent me seeds, and on raising some plants I was so much fascinated and +perplexed by the revolving movements of the tendrils and stems, which +movements are really very simple, though appearing at first sight very +complex, that I procured various other kinds of climbing plants, and +studied the whole subject. I was all the more attracted to it, from not +being at all satisfied with the explanation which Henslow gave us in +his lectures, about twining plants, namely, that they had a natural +tendency to grow up in a spire. This explanation proved quite +erroneous. Some of the adaptations displayed by Climbing Plants are as +beautiful as those of Orchids for ensuring cross-fertilisation. + +My ‘Variation of Animals and Plants under Domestication’ was begun, as +already stated, in the beginning of 1860, but was not published until +the beginning of 1868. It was a big book, and cost me four years and +two months’ hard labour. It gives all my observations and an immense +number of facts collected from various sources, about our domestic +productions. In the second volume the causes and laws of variation, +inheritance, etc., are discussed as far as our present state of +knowledge permits. Towards the end of the work I give my well-abused +hypothesis of Pangenesis. An unverified hypothesis is of little or no +value; but if anyone should hereafter be led to make observations by +which some such hypothesis could be established, I shall have done good +service, as an astonishing number of isolated facts can be thus +connected together and rendered intelligible. In 1875 a second and +largely corrected edition, which cost me a good deal of labour, was +brought out. + +My ‘Descent of Man’ was published in February, 1871. As soon as I had +become, in the year 1837 or 1838, convinced that species were mutable +productions, I could not avoid the belief that man must come under the +same law. Accordingly I collected notes on the subject for my own +satisfaction, and not for a long time with any intention of publishing. +Although in the ‘Origin of Species’ the derivation of any particular +species is never discussed, yet I thought it best, in order that no +honourable man should accuse me of concealing my views, to add that by +the work “light would be thrown on the origin of man and his history.” +It would have been useless and injurious to the success of the book to +have paraded, without giving any evidence, my conviction with respect +to his origin. + +But when I found that many naturalists fully accepted the doctrine of +the evolution of species, it seemed to me advisable to work up such +notes as I possessed, and to publish a special treatise on the origin +of man. I was the more glad to do so, as it gave me an opportunity of +fully discussing sexual selection—a subject which had always greatly +interested me. This subject, and that of the variation of our domestic +productions, together with the causes and laws of variation, +inheritance, and the intercrossing of plants, are the sole subjects +which I have been able to write about in full, so as to use all the +materials which I have collected. The ‘Descent of Man’ took me three +years to write, but then as usual some of this time was lost by ill +health, and some was consumed by preparing new editions and other minor +works. A second and largely corrected edition of the ‘Descent’ appeared +in 1874. + +My book on the ‘Expression of the Emotions in Men and Animals’ was +published in the autumn of 1872. I had intended to give only a chapter +on the subject in the ‘Descent of Man,’ but as soon as I began to put +my notes together, I saw that it would require a separate treatise. + +My first child was born on December 27th, 1839, and I at once commenced +to make notes on the first dawn of the various expressions which he +exhibited, for I felt convinced, even at this early period, that the +most complex and fine shades of expression must all have had a gradual +and natural origin. During the summer of the following year, 1840, I +read Sir C. Bell’s admirable work on expression, and this greatly +increased the interest which I felt in the subject, though I could not +at all agree with his belief that various muscles had been specially +created for the sake of expression. From this time forward I +occasionally attended to the subject, both with respect to man and our +domesticated animals. My book sold largely; 5267 copies having been +disposed of on the day of publication. + +In the summer of 1860 I was idling and resting near Hartfield, where +two species of Drosera abound; and I noticed that numerous insects had +been entrapped by the leaves. I carried home some plants, and on giving +them insects saw the movements of the tentacles, and this made me think +it probable that the insects were caught for some special purpose. +Fortunately a crucial test occurred to me, that of placing a large +number of leaves in various nitrogenous and non-nitrogenous fluids of +equal density; and as soon as I found that the former alone excited +energetic movements, it was obvious that here was a fine new field for +investigation. + +During subsequent years, whenever I had leisure, I pursued my +experiments, and my book on ‘Insectivorous Plants’ was published in +July 1875—that is, sixteen years after my first observations. The delay +in this case, as with all my other books, has been a great advantage to +me; for a man after a long interval can criticise his own work, almost +as well as if it were that of another person. The fact that a plant +should secrete, when properly excited, a fluid containing an acid and +ferment, closely analogous to the digestive fluid of an animal, was +certainly a remarkable discovery. + +During this autumn of 1876 I shall publish on the ‘Effects of Cross and +Self-Fertilisation in the Vegetable Kingdom.’ This book will form a +complement to that on the ‘Fertilisation of Orchids,’ in which I showed +how perfect were the means for cross-fertilisation, and here I shall +show how important are the results. I was led to make, during eleven +years, the numerous experiments recorded in this volume, by a mere +accidental observation; and indeed it required the accident to be +repeated before my attention was thoroughly aroused to the remarkable +fact that seedlings of self-fertilised parentage are inferior, even in +the first generation, in height and vigour to seedlings of +cross-fertilised parentage. I hope also to republish a revised edition +of my book on Orchids, and hereafter my papers on dimorphic and +trimorphic plants, together with some additional observations on allied +points which I never have had time to arrange. My strength will then +probably be exhausted, and I shall be ready to exclaim “Nunc dimittis.” + + + + +WRITTEN MAY 1ST, 1881. + + +‘The Effects of Cross and Self-Fertilisation’ was published in the +autumn of 1876; and the results there arrived at explain, as I believe, +the endless and wonderful contrivances for the transportal of pollen +from one plant to another of the same species. I now believe, however, +chiefly from the observations of Hermann Muller, that I ought to have +insisted more strongly than I did on the many adaptations for +self-fertilisation; though I was well aware of many such adaptations. A +much enlarged edition of my ‘Fertilisation of Orchids’ was published in +1877. + +In this same year ‘The Different Forms of Flowers, etc.,’ appeared, and +in 1880 a second edition. This book consists chiefly of the several +papers on Heterostyled flowers originally published by the Linnean +Society, corrected, with much new matter added, together with +observations on some other cases in which the same plant bears two +kinds of flowers. As before remarked, no little discovery of mine ever +gave me so much pleasure as the making out the meaning of heterostyled +flowers. The results of crossing such flowers in an illegitimate +manner, I believe to be very important, as bearing on the sterility of +hybrids; although these results have been noticed by only a few +persons. + +In 1879, I had a translation of Dr. Ernst Krause’s ‘Life of Erasmus +Darwin’ published, and I added a sketch of his character and habits +from material in my possession. Many persons have been much interested +by this little life, and I am surprised that only 800 or 900 copies +were sold. + +In 1880 I published, with [my son] Frank’s assistance, our ‘Power of +Movement in Plants.’ This was a tough piece of work. The book bears +somewhat the same relation to my little book on ‘Climbing Plants,’ +which ‘Cross-Fertilisation’ did to the ‘Fertilisation of Orchids;’ for +in accordance with the principle of evolution it was impossible to +account for climbing plants having been developed in so many widely +different groups unless all kinds of plants possess some slight power +of movement of an analogous kind. This I proved to be the case; and I +was further led to a rather wide generalisation, viz. that the great +and important classes of movements, excited by light, the attraction of +gravity, etc., are all modified forms of the fundamental movement of +circumnutation. It has always pleased me to exalt plants in the scale +of organised beings; and I therefore felt an especial pleasure in +showing how many and what admirably well adapted movements the tip of a +root possesses. + +I have now (May 1, 1881) sent to the printers the MS. of a little book +on ‘The Formation of Vegetable Mould, through the Action of Worms.’ +This is a subject of but small importance; and I know not whether it +will interest any readers (Between November 1881 and February 1884, +8500 copies have been sold.), but it has interested me. It is the +completion of a short paper read before the Geological Society more +than forty years ago, and has revived old geological thoughts. + +I have now mentioned all the books which I have published, and these +have been the milestones in my life, so that little remains to be said. +I am not conscious of any change in my mind during the last thirty +years, excepting in one point presently to be mentioned; nor, indeed, +could any change have been expected unless one of general +deterioration. But my father lived to his eighty-third year with his +mind as lively as ever it was, and all his faculties undimmed; and I +hope that I may die before my mind fails to a sensible extent. I think +that I have become a little more skilful in guessing right explanations +and in devising experimental tests; but this may probably be the result +of mere practice, and of a larger store of knowledge. I have as much +difficulty as ever in expressing myself clearly and concisely; and this +difficulty has caused me a very great loss of time; but it has had the +compensating advantage of forcing me to think long and intently about +every sentence, and thus I have been led to see errors in reasoning and +in my own observations or those of others. + +There seems to be a sort of fatality in my mind leading me to put at +first my statement or proposition in a wrong or awkward form. Formerly +I used to think about my sentences before writing them down; but for +several years I have found that it saves time to scribble in a vile +hand whole pages as quickly as I possibly can, contracting half the +words; and then correct deliberately. Sentences thus scribbled down are +often better ones than I could have written deliberately. + +Having said thus much about my manner of writing, I will add that with +my large books I spend a good deal of time over the general arrangement +of the matter. I first make the rudest outline in two or three pages, +and then a larger one in several pages, a few words or one word +standing for a whole discussion or series of facts. Each one of these +headings is again enlarged and often transferred before I begin to +write in extenso. As in several of my books facts observed by others +have been very extensively used, and as I have always had several quite +distinct subjects in hand at the same time, I may mention that I keep +from thirty to forty large portfolios, in cabinets with labelled +shelves, into which I can at once put a detached reference or +memorandum. I have bought many books, and at their ends I make an index +of all the facts that concern my work; or, if the book is not my own, +write out a separate abstract, and of such abstracts I have a large +drawer full. Before beginning on any subject I look to all the short +indexes and make a general and classified index, and by taking the one +or more proper portfolios I have all the information collected during +my life ready for use. + +I have said that in one respect my mind has changed during the last +twenty or thirty years. Up to the age of thirty, or beyond it, poetry +of many kinds, such as the works of Milton, Gray, Byron, Wordsworth, +Coleridge, and Shelley, gave me great pleasure, and even as a schoolboy +I took intense delight in Shakespeare, especially in the historical +plays. I have also said that formerly pictures gave me considerable, +and music very great delight. But now for many years I cannot endure to +read a line of poetry: I have tried lately to read Shakespeare, and +found it so intolerably dull that it nauseated me. I have also almost +lost my taste for pictures or music. Music generally sets me thinking +too energetically on what I have been at work on, instead of giving me +pleasure. I retain some taste for fine scenery, but it does not cause +me the exquisite delight which it formerly did. On the other hand, +novels which are works of the imagination, though not of a very high +order, have been for years a wonderful relief and pleasure to me, and I +often bless all novelists. A surprising number have been read aloud to +me, and I like all if moderately good, and if they do not end +unhappily—against which a law ought to be passed. A novel, according to +my taste, does not come into the first class unless it contains some +person whom one can thoroughly love, and if a pretty woman all the +better. + +This curious and lamentable loss of the higher aesthetic tastes is all +the odder, as books on history, biographies, and travels (independently +of any scientific facts which they may contain), and essays on all +sorts of subjects interest me as much as ever they did. My mind seems +to have become a kind of machine for grinding general laws out of large +collections of facts, but why this should have caused the atrophy of +that part of the brain alone, on which the higher tastes depend, I +cannot conceive. A man with a mind more highly organised or better +constituted than mine, would not, I suppose, have thus suffered; and if +I had to live my life again, I would have made a rule to read some +poetry and listen to some music at least once every week; for perhaps +the parts of my brain now atrophied would thus have been kept active +through use. The loss of these tastes is a loss of happiness, and may +possibly be injurious to the intellect, and more probably to the moral +character, by enfeebling the emotional part of our nature. + +My books have sold largely in England, have been translated into many +languages, and passed through several editions in foreign countries. I +have heard it said that the success of a work abroad is the best test +of its enduring value. I doubt whether this is at all trustworthy; but +judged by this standard my name ought to last for a few years. +Therefore it may be worth while to try to analyse the mental qualities +and the conditions on which my success has depended; though I am aware +that no man can do this correctly. + +I have no great quickness of apprehension or wit which is so remarkable +in some clever men, for instance, Huxley. I am therefore a poor critic: +a paper or book, when first read, generally excites my admiration, and +it is only after considerable reflection that I perceive the weak +points. My power to follow a long and purely abstract train of thought +is very limited; and therefore I could never have succeeded with +metaphysics or mathematics. My memory is extensive, yet hazy: it +suffices to make me cautious by vaguely telling me that I have observed +or read something opposed to the conclusion which I am drawing, or on +the other hand in favour of it; and after a time I can generally +recollect where to search for my authority. So poor in one sense is my +memory, that I have never been able to remember for more than a few +days a single date or a line of poetry. + +Some of my critics have said, “Oh, he is a good observer, but he has no +power of reasoning!” I do not think that this can be true, for the +‘Origin of Species’ is one long argument from the beginning to the end, +and it has convinced not a few able men. No one could have written it +without having some power of reasoning. I have a fair share of +invention, and of common sense or judgment, such as every fairly +successful lawyer or doctor must have, but not, I believe, in any +higher degree. + +On the favourable side of the balance, I think that I am superior to +the common run of men in noticing things which easily escape attention, +and in observing them carefully. My industry has been nearly as great +as it could have been in the observation and collection of facts. What +is far more important, my love of natural science has been steady and +ardent. + +This pure love has, however, been much aided by the ambition to be +esteemed by my fellow naturalists. From my early youth I have had the +strongest desire to understand or explain whatever I observed,—that is, +to group all facts under some general laws. These causes combined have +given me the patience to reflect or ponder for any number of years over +any unexplained problem. As far as I can judge, I am not apt to follow +blindly the lead of other men. I have steadily endeavoured to keep my +mind free so as to give up any hypothesis, however much beloved (and I +cannot resist forming one on every subject), as soon as facts are shown +to be opposed to it. Indeed, I have had no choice but to act in this +manner, for with the exception of the Coral Reefs, I cannot remember a +single first-formed hypothesis which had not after a time to be given +up or greatly modified. This has naturally led me to distrust greatly +deductive reasoning in the mixed sciences. On the other hand, I am not +very sceptical,—a frame of mind which I believe to be injurious to the +progress of science. A good deal of scepticism in a scientific man is +advisable to avoid much loss of time, but I have met with not a few +men, who, I feel sure, have often thus been deterred from experiment or +observations, which would have proved directly or indirectly +serviceable. + +In illustration, I will give the oddest case which I have known. A +gentleman (who, as I afterwards heard, is a good local botanist) wrote +to me from the Eastern counties that the seed or beans of the common +field-bean had this year everywhere grown on the wrong side of the pod. +I wrote back, asking for further information, as I did not understand +what was meant; but I did not receive any answer for a very long time. +I then saw in two newspapers, one published in Kent and the other in +Yorkshire, paragraphs stating that it was a most remarkable fact that +“the beans this year had all grown on the wrong side.” So I thought +there must be some foundation for so general a statement. Accordingly, +I went to my gardener, an old Kentish man, and asked him whether he had +heard anything about it, and he answered, “Oh, no, sir, it must be a +mistake, for the beans grow on the wrong side only on leap-year, and +this is not leap-year.” I then asked him how they grew in common years +and how on leap-years, but soon found that he knew absolutely nothing +of how they grew at any time, but he stuck to his belief. + +After a time I heard from my first informant, who, with many apologies, +said that he should not have written to me had he not heard the +statement from several intelligent farmers; but that he had since +spoken again to every one of them, and not one knew in the least what +he had himself meant. So that here a belief—if indeed a statement with +no definite idea attached to it can be called a belief—had spread over +almost the whole of England without any vestige of evidence. + +I have known in the course of my life only three intentionally +falsified statements, and one of these may have been a hoax (and there +have been several scientific hoaxes) which, however, took in an +American Agricultural Journal. It related to the formation in Holland +of a new breed of oxen by the crossing of distinct species of Bos (some +of which I happen to know are sterile together), and the author had the +impudence to state that he had corresponded with me, and that I had +been deeply impressed with the importance of his result. The article +was sent to me by the editor of an English Agricultural Journal, asking +for my opinion before republishing it. + +A second case was an account of several varieties, raised by the author +from several species of Primula, which had spontaneously yielded a full +complement of seed, although the parent plants had been carefully +protected from the access of insects. This account was published before +I had discovered the meaning of heterostylism, and the whole statement +must have been fraudulent, or there was neglect in excluding insects so +gross as to be scarcely credible. + +The third case was more curious: Mr. Huth published in his book on +‘Consanguineous Marriage’ some long extracts from a Belgian author, who +stated that he had interbred rabbits in the closest manner for very +many generations, without the least injurious effects. The account was +published in a most respectable Journal, that of the Royal Society of +Belgium; but I could not avoid feeling doubts—I hardly know why, except +that there were no accidents of any kind, and my experience in breeding +animals made me think this very improbable. + +So with much hesitation I wrote to Professor Van Beneden, asking him +whether the author was a trustworthy man. I soon heard in answer that +the Society had been greatly shocked by discovering that the whole +account was a fraud. (The falseness of the published statements on +which Mr. Huth relied has been pointed out by himself in a slip +inserted in all the copies of his book which then remained unsold.) The +writer had been publicly challenged in the Journal to say where he had +resided and kept his large stock of rabbits while carrying on his +experiments, which must have consumed several years, and no answer +could be extracted from him. + +My habits are methodical, and this has been of not a little use for my +particular line of work. Lastly, I have had ample leisure from not +having to earn my own bread. Even ill-health, though it has annihilated +several years of my life, has saved me from the distractions of society +and amusement. + +Therefore my success as a man of science, whatever this may have +amounted to, has been determined, as far as I can judge, by complex and +diversified mental qualities and conditions. Of these, the most +important have been—the love of science—unbounded patience in long +reflecting over any subject—industry in observing and collecting +facts—and a fair share of invention as well as of common sense. With +such moderate abilities as I possess, it is truly surprising that I +should have influenced to a considerable extent the belief of +scientific men on some important points. + + + + + + +*** END OF THE PROJECT GUTENBERG EBOOK THE AUTOBIOGRAPHY OF CHARLES DARWIN *** + + + + +Updated editions will replace the previous one—the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. Special rules, set forth in the General Terms of Use part +of this license, apply to copying and distributing Project +Gutenberg™ electronic works to protect the PROJECT GUTENBERG™ +concept and trademark. 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Owing to this habit, very few +objections were raised against my views which I had not at least +noticed and attempted to answer. diff --git a/darwin/corpus/.OS.txt.swp b/darwin/corpus/.OS.txt.swp Binary files differ. diff --git a/darwin/corpus/OS.txt b/darwin/corpus/OS.txt @@ -0,0 +1,14562 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 1228 *** + +There are several editions of this ebook in the Project Gutenberg collection. +Various characteristics of each ebook are listed to aid in selecting the +preferred file. +Click on any of the filenumbers below to quickly view each ebook. + +┏━━━━━━━┯━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓ +┃ 1228 │ 1859, First Edition ┃ +┠───────┼─────────────────────────────────────────────────────────┨ +┃ 22764 │ 1860, Second Edition ┃ +┠───────┼─────────────────────────────────────────────────────────┨ +┃ 2009 │ 1872, Sixth Edition, considered the definitive edition. ┃ +┗━━━━━━━┷━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛ + +On +the Origin of Species + +BY MEANS OF NATURAL SELECTION, + +OR THE +PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE. + +By Charles Darwin, M.A., + +Fellow Of The Royal, Geological, Linnæan, Etc., Societies; +Author Of ‘Journal Of Researches During H.M.S. Beagle’s Voyage Round The +World.’ + + +LONDON: +JOHN MURRAY, ALBEMARLE STREET. +1859. + +━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ + +“But with regard to the material world, we can at least go so far as this—we +can perceive that events are brought about not by insulated interpositions of +Divine power, exerted in each particular case, but by the establishment of +general laws.” + +W. WHEWELL: Bridgewater Treatise. + +“To conclude, therefore, let no man out of a weak conceit of sobriety, or an +ill-applied moderation, think or maintain, that a man can search too far or be +too well studied in the book of God’s word, or in the book of God’s works; +divinity or philosophy; but rather let men endeavour an endless progress or +proficience in both.” + +BACON: Advancement of Learning. + +Down, Bromley, Kent, + October, 1st, 1859. + +━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ + +Contents + +INTRODUCTION. +1. VARIATION UNDER DOMESTICATION. +2. VARIATION UNDER NATURE. +3. STRUGGLE FOR EXISTENCE. +4. NATURAL SELECTION. +5. LAWS OF VARIATION. +6. DIFFICULTIES ON THEORY. +7. INSTINCT. +8. HYBRIDISM. +9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. +10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. +11. GEOGRAPHICAL DISTRIBUTION. +12. GEOGRAPHICAL DISTRIBUTION—continued. +13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: +14. RECAPITULATION AND CONCLUSION. +INDEX + +DETEAILED CONTENTS. ON THE ORIGIN OF SPECIES. + +INTRODUCTION. + + +CHAPTER I. VARIATION UNDER DOMESTICATION. + +Causes of Variability. +Effects of Habit. +Correlation of Growth. +Inheritance. +Character of Domestic Varieties. +Difficulty of distinguishing between Varieties and Species. +Origin of Domestic Varieties from one or more Species. +Domestic Pigeons, their Differences and Origin. +Principle of Selection anciently followed, its Effects. +Methodical and Unconscious Selection. +Unknown Origin of our Domestic Productions. +Circumstances favourable to Man’s power of Selection. + + +CHAPTER 2. VARIATION UNDER NATURE. + +Variability. +Individual Differences. +Doubtful species. +Wide ranging, much diffused, and common species vary most. +Species of the larger genera in any country vary more than the species of the +smaller genera. +Many of the species of the larger genera resemble varieties in being very +closely, but unequally, related to each other, and in having restricted ranges. + + +CHAPTER 3. STRUGGLE FOR EXISTENCE. + +Bears on natural selection. +The term used in a wide sense. +Geometrical powers of increase. +Rapid increase of naturalised animals and plants. +Nature of the checks to increase. +Competition universal. +Effects of climate. +Protection from the number of individuals. +Complex relations of all animals and plants throughout nature. +Struggle for life most severe between individuals and varieties of the same +species; often severe between species of the same genus. +The relation of organism to organism the most important of all relations. + + +CHAPTER 4. NATURAL SELECTION. + +Natural Selection: its power compared with man’s selection, its power on +characters of trifling importance, its power at all ages and on both sexes. +Sexual Selection. +On the generality of intercrosses between individuals of the same species. +Circumstances favourable and unfavourable to Natural Selection, +namely, intercrossing, isolation, number of individuals. +Slow action. +Extinction caused by Natural Selection. +Divergence of Character, related to the diversity of inhabitants of any small +area, and to naturalisation. +Action of Natural Selection, through Divergence of Character and +Extinction, on the descendants from a common parent. +Explains the Grouping of all organic beings. + + +CHAPTER 5. LAWS OF VARIATION. + +Effects of external conditions. +Use and disuse, combined with natural selection; organs of flight and of +vision. +Acclimatisation. +Correlation of growth. +Compensation and economy of growth. +False correlations. +Multiple, rudimentary, and lowly organised structures variable. +Parts developed in an unusual manner are highly variable: specific characters +more variable than generic: secondary sexual characters variable. +Species of the same genus vary in an analogous manner. +Reversions to long-lost characters. +Summary. + + +CHAPTER 6. DIFFICULTIES ON THEORY. + +Difficulties on the theory of descent with modification. +Transitions. +Absence or rarity of transitional varieties. +Transitions in habits of life. +Diversified habits in the same species. +Species with habits widely different from those of their allies. +Organs of extreme perfection. +Means of transition. +Cases of difficulty. +Natura non facit saltum. +Organs of small importance. +Organs not in all cases absolutely perfect. +The law of Unity of Type and of the Conditions of Existence embraced by the +theory of Natural Selection. + + +CHAPTER 7. INSTINCT. + +Instincts comparable with habits, but different in their origin. +Instincts graduated. +Aphides and ants. +Instincts variable. +Domestic instincts, their origin. +Natural instincts of the cuckoo, ostrich, and parasitic bees. +Slave-making ants. +Hive-bee, its cell-making instinct. +Difficulties on the theory of the Natural Selection of instincts. +Neuter or sterile insects. + +Summary. + +CHAPTER 8. HYBRIDISM. + +Distinction between the sterility of first crosses and of hybrids. +Sterility various in degree, not universal, affected by close interbreeding, +removed by domestication. +Laws governing the sterility of hybrids. +Sterility not a special endowment, but incidental on other differences. +Causes of the sterility of first crosses and of hybrids. +Parallelism between the effects of changed conditions of life and crossing. +Fertility of varieties when crossed and of their mongrel offspring not +universal. +Hybrids and mongrels compared independently of their fertility. +Summary. + + +CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + +On the absence of intermediate varieties at the present day. +On the nature of extinct intermediate varieties; on their number. +On the vast lapse of time, as inferred from the rate of deposition and of +denudation. +On the poorness of our palæontological collections. +On the intermittence of geological formations. +On the absence of intermediate varieties in any one formation. +On the sudden appearance of groups of species. +On their sudden appearance in the lowest known fossiliferous strata. + + +CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + +On the slow and successive appearance of new species. +On their different rates of change. +Species once lost do not reappear. +Groups of species follow the same general rules in their appearance and +disappearance as do single species. +On Extinction. +On simultaneous changes in the forms of life throughout the world. +On the affinities of extinct species to each other and to living species. +On the state of development of ancient forms. +On the succession of the same types within the same areas. +Summary of preceding and present chapters. + + +CHAPTER 11. GEOGRAPHICAL DISTRIBUTION. + +Present distribution cannot be accounted for by differences in physical +conditions. +Importance of barriers. +Affinity of the productions of the same continent. +Centres of creation. +Means of dispersal, by changes of climate and of the level of the land, and by +occasional means. +Dispersal during the Glacial period co-extensive with the world. + + +CHAPTER 12. GEOGRAPHICAL DISTRIBUTION—continued. + +Distribution of fresh-water productions. +On the inhabitants of oceanic islands. +Absence of Batrachians and of terrestrial Mammals. +On the relation of the inhabitants of islands to those of the nearest mainland. +On colonisation from the nearest source with subsequent modification. +Summary of the last and present chapters. + + +CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: +RUDIMENTARY ORGANS. + +CLASSIFICATION, groups subordinate to groups. +Natural system. +Rules and difficulties in classification, explained on the theory of descent +with modification. +Classification of varieties. +Descent always used in classification. +Analogical or adaptive characters. +Affinities, general, complex and radiating. +Extinction separates and defines groups. +MORPHOLOGY, between members of the same class, between parts of the same +individual. +EMBRYOLOGY, laws of, explained by variations not supervening at an early age, +and being inherited at a corresponding age. +RUDIMENTARY ORGANS; their origin explained. +Summary. + + +CHAPTER 14. RECAPITULATION AND CONCLUSION. + +Recapitulation of the difficulties on the theory of Natural Selection. +Recapitulation of the general and special circumstances in its favour. +Causes of the general belief in the immutability of species. +How far the theory of natural selection may be extended. +Effects of its adoption on the study of Natural history. +Concluding remarks. + +ON THE ORIGIN OF SPECIES. + +INTRODUCTION. + +When on board H.M.S. ‘Beagle,’ as naturalist, I was much struck with certain +facts in the distribution of the inhabitants of South America, and in the +geological relations of the present to the past inhabitants of that continent. +These facts seemed to me to throw some light on the origin of species—that +mystery of mysteries, as it has been called by one of our greatest +philosophers. On my return home, it occurred to me, in 1837, that something +might perhaps be made out on this question by patiently accumulating and +reflecting on all sorts of facts which could possibly have any bearing on it. +After five years’ work I allowed myself to speculate on the subject, and drew +up some short notes; these I enlarged in 1844 into a sketch of the conclusions, +which then seemed to me probable: from that period to the present day I have +steadily pursued the same object. I hope that I may be excused for entering on +these personal details, as I give them to show that I have not been hasty in +coming to a decision. + +My work is now nearly finished; but as it will take me two or three more years +to complete it, and as my health is far from strong, I have been urged to +publish this Abstract. I have more especially been induced to do this, as Mr. +Wallace, who is now studying the natural history of the Malay archipelago, has +arrived at almost exactly the same general conclusions that I have on the +origin of species. Last year he sent to me a memoir on this subject, with a +request that I would forward it to Sir Charles Lyell, who sent it to the +Linnean Society, and it is published in the third volume of the Journal of that +Society. Sir C. Lyell and Dr. Hooker, who both knew of my work—the latter +having read my sketch of 1844—honoured me by thinking it advisable to publish, +with Mr. Wallace’s excellent memoir, some brief extracts from my manuscripts. + +This Abstract, which I now publish, must necessarily be imperfect. I cannot +here give references and authorities for my several statements; and I must +trust to the reader reposing some confidence in my accuracy. No doubt errors +will have crept in, though I hope I have always been cautious in trusting to +good authorities alone. I can here give only the general conclusions at which I +have arrived, with a few facts in illustration, but which, I hope, in most +cases will suffice. No one can feel more sensible than I do of the necessity of +hereafter publishing in detail all the facts, with references, on which my +conclusions have been grounded; and I hope in a future work to do this. For I +am well aware that scarcely a single point is discussed in this volume on which +facts cannot be adduced, often apparently leading to conclusions directly +opposite to those at which I have arrived. A fair result can be obtained only +by fully stating and balancing the facts and arguments on both sides of each +question; and this cannot possibly be here done. + +I much regret that want of space prevents my having the satisfaction of +acknowledging the generous assistance which I have received from very many +naturalists, some of them personally unknown to me. I cannot, however, let this +opportunity pass without expressing my deep obligations to Dr. Hooker, who for +the last fifteen years has aided me in every possible way by his large stores +of knowledge and his excellent judgment. + +In considering the Origin of Species, it is quite conceivable that a +naturalist, reflecting on the mutual affinities of organic beings, on their +embryological relations, their geographical distribution, geological +succession, and other such facts, might come to the conclusion that each +species had not been independently created, but had descended, like varieties, +from other species. Nevertheless, such a conclusion, even if well founded, +would be unsatisfactory, until it could be shown how the innumerable species +inhabiting this world have been modified, so as to acquire that perfection of +structure and coadaptation which most justly excites our admiration. +Naturalists continually refer to external conditions, such as climate, food, +etc., as the only possible cause of variation. In one very limited sense, as we +shall hereafter see, this may be true; but it is preposterous to attribute to +mere external conditions, the structure, for instance, of the woodpecker, with +its feet, tail, beak, and tongue, so admirably adapted to catch insects under +the bark of trees. In the case of the misseltoe, which draws its nourishment +from certain trees, which has seeds that must be transported by certain birds, +and which has flowers with separate sexes absolutely requiring the agency of +certain insects to bring pollen from one flower to the other, it is equally +preposterous to account for the structure of this parasite, with its relations +to several distinct organic beings, by the effects of external conditions, or +of habit, or of the volition of the plant itself. + +The author of the ‘Vestiges of Creation’ would, I presume, say that, after a +certain unknown number of generations, some bird had given birth to a +woodpecker, and some plant to the misseltoe, and that these had been produced +perfect as we now see them; but this assumption seems to me to be no +explanation, for it leaves the case of the coadaptations of organic beings to +each other and to their physical conditions of life, untouched and unexplained. + +It is, therefore, of the highest importance to gain a clear insight into the +means of modification and coadaptation. At the commencement of my observations +it seemed to me probable that a careful study of domesticated animals and of +cultivated plants would offer the best chance of making out this obscure +problem. Nor have I been disappointed; in this and in all other perplexing +cases I have invariably found that our knowledge, imperfect though it be, of +variation under domestication, afforded the best and safest clue. I may venture +to express my conviction of the high value of such studies, although they have +been very commonly neglected by naturalists. + +From these considerations, I shall devote the first chapter of this Abstract to +Variation under Domestication. We shall thus see that a large amount of +hereditary modification is at least possible, and, what is equally or more +important, we shall see how great is the power of man in accumulating by his +Selection successive slight variations. I will then pass on to the variability +of species in a state of nature; but I shall, unfortunately, be compelled to +treat this subject far too briefly, as it can be treated properly only by +giving long catalogues of facts. We shall, however, be enabled to discuss what +circumstances are most favourable to variation. In the next chapter the +Struggle for Existence amongst all organic beings throughout the world, which +inevitably follows from their high geometrical powers of increase, will be +treated of. This is the doctrine of Malthus, applied to the whole animal and +vegetable kingdoms. As many more individuals of each species are born than can +possibly survive; and as, consequently, there is a frequently recurring +struggle for existence, it follows that any being, if it vary however slightly +in any manner profitable to itself, under the complex and sometimes varying +conditions of life, will have a better chance of surviving, and thus be +naturally selected. From the strong principle of inheritance, any selected +variety will tend to propagate its new and modified form. + +This fundamental subject of Natural Selection will be treated at some length in +the fourth chapter; and we shall then see how Natural Selection almost +inevitably causes much Extinction of the less improved forms of life and +induces what I have called Divergence of Character. In the next chapter I shall +discuss the complex and little known laws of variation and of correlation of +growth. In the four succeeding chapters, the most apparent and gravest +difficulties on the theory will be given: namely, first, the difficulties of +transitions, or in understanding how a simple being or a simple organ can be +changed and perfected into a highly developed being or elaborately constructed +organ; secondly the subject of Instinct, or the mental powers of animals, +thirdly, Hybridism, or the infertility of species and the fertility of +varieties when intercrossed; and fourthly, the imperfection of the Geological +Record. In the next chapter I shall consider the geological succession of +organic beings throughout time; in the eleventh and twelfth, their geographical +distribution throughout space; in the thirteenth, their classification or +mutual affinities, both when mature and in an embryonic condition. In the last +chapter I shall give a brief recapitulation of the whole work, and a few +concluding remarks. + +No one ought to feel surprise at much remaining as yet unexplained in regard to +the origin of species and varieties, if he makes due allowance for our profound +ignorance in regard to the mutual relations of all the beings which live around +us. Who can explain why one species ranges widely and is very numerous, and why +another allied species has a narrow range and is rare? Yet these relations are +of the highest importance, for they determine the present welfare, and, as I +believe, the future success and modification of every inhabitant of this world. +Still less do we know of the mutual relations of the innumerable inhabitants of +the world during the many past geological epochs in its history. Although much +remains obscure, and will long remain obscure, I can entertain no doubt, after +the most deliberate study and dispassionate judgment of which I am capable, +that the view which most naturalists entertain, and which I formerly +entertained—namely, that each species has been independently created—is +erroneous. I am fully convinced that species are not immutable; but that those +belonging to what are called the same genera are lineal descendants of some +other and generally extinct species, in the same manner as the acknowledged +varieties of any one species are the descendants of that species. Furthermore, +I am convinced that Natural Selection has been the main but not exclusive means +of modification. + +CHAPTER I. +VARIATION UNDER DOMESTICATION. + +Causes of Variability. Effects of Habit. Correlation of Growth. Inheritance. +Character of Domestic Varieties. Difficulty of distinguishing between Varieties +and Species. Origin of Domestic Varieties from one or more Species. Domestic +Pigeons, their Differences and Origin. Principle of Selection anciently +followed, its Effects. Methodical and Unconscious Selection. Unknown Origin of +our Domestic Productions. Circumstances favourable to Man’s power of Selection. + +When we look to the individuals of the same variety or sub-variety of our older +cultivated plants and animals, one of the first points which strikes us, is, +that they generally differ much more from each other, than do the individuals +of any one species or variety in a state of nature. When we reflect on the vast +diversity of the plants and animals which have been cultivated, and which have +varied during all ages under the most different climates and treatment, I think +we are driven to conclude that this greater variability is simply due to our +domestic productions having been raised under conditions of life not so uniform +as, and somewhat different from, those to which the parent-species have been +exposed under nature. There is, also, I think, some probability in the view +propounded by Andrew Knight, that this variability may be partly connected with +excess of food. It seems pretty clear that organic beings must be exposed +during several generations to the new conditions of life to cause any +appreciable amount of variation; and that when the organisation has once begun +to vary, it generally continues to vary for many generations. No case is on +record of a variable being ceasing to be variable under cultivation. Our oldest +cultivated plants, such as wheat, still often yield new varieties: our oldest +domesticated animals are still capable of rapid improvement or modification. + +It has been disputed at what period of life the causes of variability, whatever +they may be, generally act; whether during the early or late period of +development of the embryo, or at the instant of conception. Geoffroy St. +Hilaire’s experiments show that unnatural treatment of the embryo causes +monstrosities; and monstrosities cannot be separated by any clear line of +distinction from mere variations. But I am strongly inclined to suspect that +the most frequent cause of variability may be attributed to the male and female +reproductive elements having been affected prior to the act of conception. +Several reasons make me believe in this; but the chief one is the remarkable +effect which confinement or cultivation has on the functions of the +reproductive system; this system appearing to be far more susceptible than any +other part of the organisation, to the action of any change in the conditions +of life. Nothing is more easy than to tame an animal, and few things more +difficult than to get it to breed freely under confinement, even in the many +cases when the male and female unite. How many animals there are which will not +breed, though living long under not very close confinement in their native +country! This is generally attributed to vitiated instincts; but how many +cultivated plants display the utmost vigour, and yet rarely or never seed! In +some few such cases it has been found out that very trifling changes, such as a +little more or less water at some particular period of growth, will determine +whether or not the plant sets a seed. I cannot here enter on the copious +details which I have collected on this curious subject; but to show how +singular the laws are which determine the reproduction of animals under +confinement, I may just mention that carnivorous animals, even from the +tropics, breed in this country pretty freely under confinement, with the +exception of the plantigrades or bear family; whereas, carnivorous birds, with +the rarest exceptions, hardly ever lay fertile eggs. Many exotic plants have +pollen utterly worthless, in the same exact condition as in the most sterile +hybrids. When, on the one hand, we see domesticated animals and plants, though +often weak and sickly, yet breeding quite freely under confinement; and when, +on the other hand, we see individuals, though taken young from a state of +nature, perfectly tamed, long-lived, and healthy (of which I could give +numerous instances), yet having their reproductive system so seriously affected +by unperceived causes as to fail in acting, we need not be surprised at this +system, when it does act under confinement, acting not quite regularly, and +producing offspring not perfectly like their parents or variable. + +Sterility has been said to be the bane of horticulture; but on this view we owe +variability to the same cause which produces sterility; and variability is the +source of all the choicest productions of the garden. I may add, that as some +organisms will breed most freely under the most unnatural conditions (for +instance, the rabbit and ferret kept in hutches), showing that their +reproductive system has not been thus affected; so will some animals and plants +withstand domestication or cultivation, and vary very slightly—perhaps hardly +more than in a state of nature. + +A long list could easily be given of “sporting plants;” by this term gardeners +mean a single bud or offset, which suddenly assumes a new and sometimes very +different character from that of the rest of the plant. Such buds can be +propagated by grafting, etc., and sometimes by seed. These “sports” are +extremely rare under nature, but far from rare under cultivation; and in this +case we see that the treatment of the parent has affected a bud or offset, and +not the ovules or pollen. But it is the opinion of most physiologists that +there is no essential difference between a bud and an ovule in their earliest +stages of formation; so that, in fact, “sports” support my view, that +variability may be largely attributed to the ovules or pollen, or to both, +having been affected by the treatment of the parent prior to the act of +conception. These cases anyhow show that variation is not necessarily +connected, as some authors have supposed, with the act of generation. + +Seedlings from the same fruit, and the young of the same litter, sometimes +differ considerably from each other, though both the young and the parents, as +Müller has remarked, have apparently been exposed to exactly the same +conditions of life; and this shows how unimportant the direct effects of the +conditions of life are in comparison with the laws of reproduction, and of +growth, and of inheritance; for had the action of the conditions been direct, +if any of the young had varied, all would probably have varied in the same +manner. To judge how much, in the case of any variation, we should attribute to +the direct action of heat, moisture, light, food, etc., is most difficult: my +impression is, that with animals such agencies have produced very little direct +effect, though apparently more in the case of plants. Under this point of view, +Mr. Buckman’s recent experiments on plants seem extremely valuable. When all or +nearly all the individuals exposed to certain conditions are affected in the +same way, the change at first appears to be directly due to such conditions; +but in some cases it can be shown that quite opposite conditions produce +similar changes of structure. Nevertheless some slight amount of change may, I +think, be attributed to the direct action of the conditions of life—as, in some +cases, increased size from amount of food, colour from particular kinds of food +and from light, and perhaps the thickness of fur from climate. + +Habit also has a decided influence, as in the period of flowering with plants +when transported from one climate to another. In animals it has a more marked +effect; for instance, I find in the domestic duck that the bones of the wing +weigh less and the bones of the leg more, in proportion to the whole skeleton, +than do the same bones in the wild-duck; and I presume that this change may be +safely attributed to the domestic duck flying much less, and walking more, than +its wild parent. The great and inherited development of the udders in cows and +goats in countries where they are habitually milked, in comparison with the +state of these organs in other countries, is another instance of the effect of +use. Not a single domestic animal can be named which has not in some country +drooping ears; and the view suggested by some authors, that the drooping is due +to the disuse of the muscles of the ear, from the animals not being much +alarmed by danger, seems probable. + +There are many laws regulating variation, some few of which can be dimly seen, +and will be hereafter briefly mentioned. I will here only allude to what may be +called correlation of growth. Any change in the embryo or larva will almost +certainly entail changes in the mature animal. In monstrosities, the +correlations between quite distinct parts are very curious; and many instances +are given in Isidore Geoffroy St. Hilaire’s great work on this subject. +Breeders believe that long limbs are almost always accompanied by an elongated +head. Some instances of correlation are quite whimsical; thus cats with blue +eyes are invariably deaf; colour and constitutional peculiarities go together, +of which many remarkable cases could be given amongst animals and plants. From +the facts collected by Heusinger, it appears that white sheep and pigs are +differently affected from coloured individuals by certain vegetable poisons. +Hairless dogs have imperfect teeth; long-haired and coarse-haired animals are +apt to have, as is asserted, long or many horns; pigeons with feathered feet +have skin between their outer toes; pigeons with short beaks have small feet, +and those with long beaks large feet. Hence, if man goes on selecting, and thus +augmenting, any peculiarity, he will almost certainly unconsciously modify +other parts of the structure, owing to the mysterious laws of the correlation +of growth. + +The result of the various, quite unknown, or dimly seen laws of variation is +infinitely complex and diversified. It is well worth while carefully to study +the several treatises published on some of our old cultivated plants, as on the +hyacinth, potato, even the dahlia, etc.; and it is really surprising to note +the endless points in structure and constitution in which the varieties and +sub-varieties differ slightly from each other. The whole organisation seems to +have become plastic, and tends to depart in some small degree from that of the +parental type. + +Any variation which is not inherited is unimportant for us. But the number and +diversity of inheritable deviations of structure, both those of slight and +those of considerable physiological importance, is endless. Dr. Prosper Lucas’s +treatise, in two large volumes, is the fullest and the best on this subject. No +breeder doubts how strong is the tendency to inheritance: like produces like is +his fundamental belief: doubts have been thrown on this principle by +theoretical writers alone. When a deviation appears not unfrequently, and we +see it in the father and child, we cannot tell whether it may not be due to the +same original cause acting on both; but when amongst individuals, apparently +exposed to the same conditions, any very rare deviation, due to some +extraordinary combination of circumstances, appears in the parent—say, once +amongst several million individuals—and it reappears in the child, the mere +doctrine of chances almost compels us to attribute its reappearance to +inheritance. Every one must have heard of cases of albinism, prickly skin, +hairy bodies, etc., appearing in several members of the same family. If strange +and rare deviations of structure are truly inherited, less strange and commoner +deviations may be freely admitted to be inheritable. Perhaps the correct way of +viewing the whole subject, would be, to look at the inheritance of every +character whatever as the rule, and non-inheritance as the anomaly. + +The laws governing inheritance are quite unknown; no one can say why the same +peculiarity in different individuals of the same species, and in individuals of +different species, is sometimes inherited and sometimes not so; why the child +often reverts in certain characters to its grandfather or grandmother or other +much more remote ancestor; why a peculiarity is often transmitted from one sex +to both sexes or to one sex alone, more commonly but not exclusively to the +like sex. It is a fact of some little importance to us, that peculiarities +appearing in the males of our domestic breeds are often transmitted either +exclusively, or in a much greater degree, to males alone. A much more important +rule, which I think may be trusted, is that, at whatever period of life a +peculiarity first appears, it tends to appear in the offspring at a +corresponding age, though sometimes earlier. In many cases this could not be +otherwise: thus the inherited peculiarities in the horns of cattle could appear +only in the offspring when nearly mature; peculiarities in the silkworm are +known to appear at the corresponding caterpillar or cocoon stage. But +hereditary diseases and some other facts make me believe that the rule has a +wider extension, and that when there is no apparent reason why a peculiarity +should appear at any particular age, yet that it does tend to appear in the +offspring at the same period at which it first appeared in the parent. I +believe this rule to be of the highest importance in explaining the laws of +embryology. These remarks are of course confined to the first appearance of the +peculiarity, and not to its primary cause, which may have acted on the ovules +or male element; in nearly the same manner as in the crossed offspring from a +short-horned cow by a long-horned bull, the greater length of horn, though +appearing late in life, is clearly due to the male element. + +Having alluded to the subject of reversion, I may here refer to a statement +often made by naturalists—namely, that our domestic varieties, when run wild, +gradually but certainly revert in character to their aboriginal stocks. Hence +it has been argued that no deductions can be drawn from domestic races to +species in a state of nature. I have in vain endeavoured to discover on what +decisive facts the above statement has so often and so boldly been made. There +would be great difficulty in proving its truth: we may safely conclude that +very many of the most strongly-marked domestic varieties could not possibly +live in a wild state. In many cases we do not know what the aboriginal stock +was, and so could not tell whether or not nearly perfect reversion had ensued. +It would be quite necessary, in order to prevent the effects of intercrossing, +that only a single variety should be turned loose in its new home. +Nevertheless, as our varieties certainly do occasionally revert in some of +their characters to ancestral forms, it seems to me not improbable, that if we +could succeed in naturalising, or were to cultivate, during many generations, +the several races, for instance, of the cabbage, in very poor soil (in which +case, however, some effect would have to be attributed to the direct action of +the poor soil), that they would to a large extent, or even wholly, revert to +the wild aboriginal stock. Whether or not the experiment would succeed, is not +of great importance for our line of argument; for by the experiment itself the +conditions of life are changed. If it could be shown that our domestic +varieties manifested a strong tendency to reversion,—that is, to lose their +acquired characters, whilst kept under unchanged conditions, and whilst kept in +a considerable body, so that free intercrossing might check, by blending +together, any slight deviations of structure, in such case, I grant that we +could deduce nothing from domestic varieties in regard to species. But there is +not a shadow of evidence in favour of this view: to assert that we could not +breed our cart and race-horses, long and short-horned cattle, and poultry of +various breeds, and esculent vegetables, for an almost infinite number of +generations, would be opposed to all experience. I may add, that when under +nature the conditions of life do change, variations and reversions of character +probably do occur; but natural selection, as will hereafter be explained, will +determine how far the new characters thus arising shall be preserved. + +When we look to the hereditary varieties or races of our domestic animals and +plants, and compare them with species closely allied together, we generally +perceive in each domestic race, as already remarked, less uniformity of +character than in true species. Domestic races of the same species, also, often +have a somewhat monstrous character; by which I mean, that, although differing +from each other, and from the other species of the same genus, in several +trifling respects, they often differ in an extreme degree in some one part, +both when compared one with another, and more especially when compared with all +the species in nature to which they are nearest allied. With these exceptions +(and with that of the perfect fertility of varieties when crossed,—a subject +hereafter to be discussed), domestic races of the same species differ from each +other in the same manner as, only in most cases in a lesser degree than, do +closely-allied species of the same genus in a state of nature. I think this +must be admitted, when we find that there are hardly any domestic races, either +amongst animals or plants, which have not been ranked by some competent judges +as mere varieties, and by other competent judges as the descendants of +aboriginally distinct species. If any marked distinction existed between +domestic races and species, this source of doubt could not so perpetually +recur. It has often been stated that domestic races do not differ from each +other in characters of generic value. I think it could be shown that this +statement is hardly correct; but naturalists differ most widely in determining +what characters are of generic value; all such valuations being at present +empirical. Moreover, on the view of the origin of genera which I shall +presently give, we have no right to expect often to meet with generic +differences in our domesticated productions. + +When we attempt to estimate the amount of structural difference between the +domestic races of the same species, we are soon involved in doubt, from not +knowing whether they have descended from one or several parent-species. This +point, if it could be cleared up, would be interesting; if, for instance, it +could be shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, +which we all know propagate their kind so truly, were the offspring of any +single species, then such facts would have great weight in making us doubt +about the immutability of the many very closely allied and natural species—for +instance, of the many foxes—inhabiting different quarters of the world. I do +not believe, as we shall presently see, that all our dogs have descended from +any one wild species; but, in the case of some other domestic races, there is +presumptive, or even strong, evidence in favour of this view. + +It has often been assumed that man has chosen for domestication animals and +plants having an extraordinary inherent tendency to vary, and likewise to +withstand diverse climates. I do not dispute that these capacities have added +largely to the value of most of our domesticated productions; but how could a +savage possibly know, when he first tamed an animal, whether it would vary in +succeeding generations, and whether it would endure other climates? Has the +little variability of the ass or guinea-fowl, or the small power of endurance +of warmth by the rein-deer, or of cold by the common camel, prevented their +domestication? I cannot doubt that if other animals and plants, equal in number +to our domesticated productions, and belonging to equally diverse classes and +countries, were taken from a state of nature, and could be made to breed for an +equal number of generations under domestication, they would vary on an average +as largely as the parent species of our existing domesticated productions have +varied. + +In the case of most of our anciently domesticated animals and plants, I do not +think it is possible to come to any definite conclusion, whether they have +descended from one or several species. The argument mainly relied on by those +who believe in the multiple origin of our domestic animals is, that we find in +the most ancient records, more especially on the monuments of Egypt, much +diversity in the breeds; and that some of the breeds closely resemble, perhaps +are identical with, those still existing. Even if this latter fact were found +more strictly and generally true than seems to me to be the case, what does it +show, but that some of our breeds originated there, four or five thousand years +ago? But Mr. Horner’s researches have rendered it in some degree probable that +man sufficiently civilized to have manufactured pottery existed in the valley +of the Nile thirteen or fourteen thousand years ago; and who will pretend to +say how long before these ancient periods, savages, like those of Tierra del +Fuego or Australia, who possess a semi-domestic dog, may not have existed in +Egypt? + +The whole subject must, I think, remain vague; nevertheless, I may, without +here entering on any details, state that, from geographical and other +considerations, I think it highly probable that our domestic dogs have +descended from several wild species. In regard to sheep and goats I can form no +opinion. I should think, from facts communicated to me by Mr. Blyth, on the +habits, voice, and constitution, etc., of the humped Indian cattle, that these +had descended from a different aboriginal stock from our European cattle; and +several competent judges believe that these latter have had more than one wild +parent. With respect to horses, from reasons which I cannot give here, I am +doubtfully inclined to believe, in opposition to several authors, that all the +races have descended from one wild stock. Mr. Blyth, whose opinion, from his +large and varied stores of knowledge, I should value more than that of almost +any one, thinks that all the breeds of poultry have proceeded from the common +wild Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the breeds +of which differ considerably from each other in structure, I do not doubt that +they all have descended from the common wild duck and rabbit. + +The doctrine of the origin of our several domestic races from several +aboriginal stocks, has been carried to an absurd extreme by some authors. They +believe that every race which breeds true, let the distinctive characters be +ever so slight, has had its wild prototype. At this rate there must have +existed at least a score of species of wild cattle, as many sheep, and several +goats in Europe alone, and several even within Great Britain. One author +believes that there formerly existed in Great Britain eleven wild species of +sheep peculiar to it! When we bear in mind that Britain has now hardly one +peculiar mammal, and France but few distinct from those of Germany and +conversely, and so with Hungary, Spain, etc., but that each of these kingdoms +possesses several peculiar breeds of cattle, sheep, etc., we must admit that +many domestic breeds have originated in Europe; for whence could they have been +derived, as these several countries do not possess a number of peculiar species +as distinct parent-stocks? So it is in India. Even in the case of the domestic +dogs of the whole world, which I fully admit have probably descended from +several wild species, I cannot doubt that there has been an immense amount of +inherited variation. Who can believe that animals closely resembling the +Italian greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.—so +unlike all wild Canidæ—ever existed freely in a state of nature? It has often +been loosely said that all our races of dogs have been produced by the crossing +of a few aboriginal species; but by crossing we can get only forms in some +degree intermediate between their parents; and if we account for our several +domestic races by this process, we must admit the former existence of the most +extreme forms, as the Italian greyhound, bloodhound, bull-dog, etc., in the +wild state. Moreover, the possibility of making distinct races by crossing has +been greatly exaggerated. There can be no doubt that a race may be modified by +occasional crosses, if aided by the careful selection of those individual +mongrels, which present any desired character; but that a race could be +obtained nearly intermediate between two extremely different races or species, +I can hardly believe. Sir J. Sebright expressly experimentised for this object, +and failed. The offspring from the first cross between two pure breeds is +tolerably and sometimes (as I have found with pigeons) extremely uniform, and +everything seems simple enough; but when these mongrels are crossed one with +another for several generations, hardly two of them will be alike, and then the +extreme difficulty, or rather utter hopelessness, of the task becomes apparent. +Certainly, a breed intermediate between two very distinct breeds could not be +got without extreme care and long-continued selection; nor can I find a single +case on record of a permanent race having been thus formed. + +On the Breeds of the Domestic Pigeon.—Believing that it is always best to study +some special group, I have, after deliberation, taken up domestic pigeons. I +have kept every breed which I could purchase or obtain, and have been most +kindly favoured with skins from several quarters of the world, more especially +by the Honourable W. Elliot from India, and by the Honourable C. Murray from +Persia. Many treatises in different languages have been published on pigeons, +and some of them are very important, as being of considerable antiquity. I have +associated with several eminent fanciers, and have been permitted to join two +of the London Pigeon Clubs. The diversity of the breeds is something +astonishing. Compare the English carrier and the short-faced tumbler, and see +the wonderful difference in their beaks, entailing corresponding differences in +their skulls. The carrier, more especially the male bird, is also remarkable +from the wonderful development of the carunculated skin about the head, and +this is accompanied by greatly elongated eyelids, very large external orifices +to the nostrils, and a wide gape of mouth. The short-faced tumbler has a beak +in outline almost like that of a finch; and the common tumbler has the singular +and strictly inherited habit of flying at a great height in a compact flock, +and tumbling in the air head over heels. The runt is a bird of great size, with +long, massive beak and large feet; some of the sub-breeds of runts have very +long necks, others very long wings and tails, others singularly short tails. +The barb is allied to the carrier, but, instead of a very long beak, has a very +short and very broad one. The pouter has a much elongated body, wings, and +legs; and its enormously developed crop, which it glories in inflating, may +well excite astonishment and even laughter. The turbit has a very short and +conical beak, with a line of reversed feathers down the breast; and it has the +habit of continually expanding slightly the upper part of the oesophagus. The +Jacobin has the feathers so much reversed along the back of the neck that they +form a hood, and it has, proportionally to its size, much elongated wing and +tail feathers. The trumpeter and laugher, as their names express, utter a very +different coo from the other breeds. The fantail has thirty or even forty +tail-feathers, instead of twelve or fourteen, the normal number in all members +of the great pigeon family; and these feathers are kept expanded, and are +carried so erect that in good birds the head and tail touch; the oil-gland is +quite aborted. Several other less distinct breeds might have been specified. + +In the skeletons of the several breeds, the development of the bones of the +face in length and breadth and curvature differs enormously. The shape, as well +as the breadth and length of the ramus of the lower jaw, varies in a highly +remarkable manner. The number of the caudal and sacral vertebræ vary; as does +the number of the ribs, together with their relative breadth and the presence +of processes. The size and shape of the apertures in the sternum are highly +variable; so is the degree of divergence and relative size of the two arms of +the furcula. The proportional width of the gape of mouth, the proportional +length of the eyelids, of the orifice of the nostrils, of the tongue (not +always in strict correlation with the length of beak), the size of the crop and +of the upper part of the oesophagus; the development and abortion of the +oil-gland; the number of the primary wing and caudal feathers; the relative +length of wing and tail to each other and to the body; the relative length of +leg and of the feet; the number of scutellæ on the toes, the development of +skin between the toes, are all points of structure which are variable. The +period at which the perfect plumage is acquired varies, as does the state of +the down with which the nestling birds are clothed when hatched. The shape and +size of the eggs vary. The manner of flight differs remarkably; as does in some +breeds the voice and disposition. Lastly, in certain breeds, the males and +females have come to differ to a slight degree from each other. + +Altogether at least a score of pigeons might be chosen, which if shown to an +ornithologist, and he were told that they were wild birds, would certainly, I +think, be ranked by him as well-defined species. Moreover, I do not believe +that any ornithologist would place the English carrier, the short-faced +tumbler, the runt, the barb, pouter, and fantail in the same genus; more +especially as in each of these breeds several truly-inherited sub-breeds, or +species as he might have called them, could be shown him. + +Great as the differences are between the breeds of pigeons, I am fully +convinced that the common opinion of naturalists is correct, namely, that all +have descended from the rock-pigeon (Columba livia), including under this term +several geographical races or sub-species, which differ from each other in the +most trifling respects. As several of the reasons which have led me to this +belief are in some degree applicable in other cases, I will here briefly give +them. If the several breeds are not varieties, and have not proceeded from the +rock-pigeon, they must have descended from at least seven or eight aboriginal +stocks; for it is impossible to make the present domestic breeds by the +crossing of any lesser number: how, for instance, could a pouter be produced by +crossing two breeds unless one of the parent-stocks possessed the +characteristic enormous crop? The supposed aboriginal stocks must all have been +rock-pigeons, that is, not breeding or willingly perching on trees. But besides +C. livia, with its geographical sub-species, only two or three other species of +rock-pigeons are known; and these have not any of the characters of the +domestic breeds. Hence the supposed aboriginal stocks must either still exist +in the countries where they were originally domesticated, and yet be unknown to +ornithologists; and this, considering their size, habits, and remarkable +characters, seems very improbable; or they must have become extinct in the wild +state. But birds breeding on precipices, and good fliers, are unlikely to be +exterminated; and the common rock-pigeon, which has the same habits with the +domestic breeds, has not been exterminated even on several of the smaller +British islets, or on the shores of the Mediterranean. Hence the supposed +extermination of so many species having similar habits with the rock-pigeon +seems to me a very rash assumption. Moreover, the several above-named +domesticated breeds have been transported to all parts of the world, and, +therefore, some of them must have been carried back again into their native +country; but not one has ever become wild or feral, though the dovecot-pigeon, +which is the rock-pigeon in a very slightly altered state, has become feral in +several places. Again, all recent experience shows that it is most difficult to +get any wild animal to breed freely under domestication; yet on the hypothesis +of the multiple origin of our pigeons, it must be assumed that at least seven +or eight species were so thoroughly domesticated in ancient times by +half-civilized man, as to be quite prolific under confinement. + +An argument, as it seems to me, of great weight, and applicable in several +other cases, is, that the above-specified breeds, though agreeing generally in +constitution, habits, voice, colouring, and in most parts of their structure, +with the wild rock-pigeon, yet are certainly highly abnormal in other parts of +their structure: we may look in vain throughout the whole great family of +Columbidæ for a beak like that of the English carrier, or that of the +short-faced tumbler, or barb; for reversed feathers like those of the jacobin; +for a crop like that of the pouter; for tail-feathers like those of the +fantail. Hence it must be assumed not only that half-civilized man succeeded in +thoroughly domesticating several species, but that he intentionally or by +chance picked out extraordinarily abnormal species; and further, that these +very species have since all become extinct or unknown. So many strange +contingencies seem to me improbable in the highest degree. + +Some facts in regard to the colouring of pigeons well deserve consideration. +The rock-pigeon is of a slaty-blue, and has a white rump (the Indian +sub-species, C. intermedia of Strickland, having it bluish); the tail has a +terminal dark bar, with the bases of the outer feathers externally edged with +white; the wings have two black bars; some semi-domestic breeds and some +apparently truly wild breeds have, besides the two black bars, the wings +chequered with black. These several marks do not occur together in any other +species of the whole family. Now, in every one of the domestic breeds, taking +thoroughly well-bred birds, all the above marks, even to the white edging of +the outer tail-feathers, sometimes concur perfectly developed. Moreover, when +two birds belonging to two distinct breeds are crossed, neither of which is +blue or has any of the above-specified marks, the mongrel offspring are very +apt suddenly to acquire these characters; for instance, I crossed some +uniformly white fantails with some uniformly black barbs, and they produced +mottled brown and black birds; these I again crossed together, and one +grandchild of the pure white fantail and pure black barb was of as beautiful a +blue colour, with the white rump, double black wing-bar, and barred and +white-edged tail-feathers, as any wild rock-pigeon! We can understand these +facts, on the well-known principle of reversion to ancestral characters, if all +the domestic breeds have descended from the rock-pigeon. But if we deny this, +we must make one of the two following highly improbable suppositions. Either, +firstly, that all the several imagined aboriginal stocks were coloured and +marked like the rock-pigeon, although no other existing species is thus +coloured and marked, so that in each separate breed there might be a tendency +to revert to the very same colours and markings. Or, secondly, that each breed, +even the purest, has within a dozen or, at most, within a score of generations, +been crossed by the rock-pigeon: I say within a dozen or twenty generations, +for we know of no fact countenancing the belief that the child ever reverts to +some one ancestor, removed by a greater number of generations. In a breed which +has been crossed only once with some distinct breed, the tendency to reversion +to any character derived from such cross will naturally become less and less, +as in each succeeding generation there will be less of the foreign blood; but +when there has been no cross with a distinct breed, and there is a tendency in +both parents to revert to a character, which has been lost during some former +generation, this tendency, for all that we can see to the contrary, may be +transmitted undiminished for an indefinite number of generations. These two +distinct cases are often confounded in treatises on inheritance. + +Lastly, the hybrids or mongrels from between all the domestic breeds of pigeons +are perfectly fertile. I can state this from my own observations, purposely +made on the most distinct breeds. Now, it is difficult, perhaps impossible, to +bring forward one case of the hybrid offspring of two animals clearly distinct +being themselves perfectly fertile. Some authors believe that long-continued +domestication eliminates this strong tendency to sterility: from the history of +the dog I think there is some probability in this hypothesis, if applied to +species closely related together, though it is unsupported by a single +experiment. But to extend the hypothesis so far as to suppose that species, +aboriginally as distinct as carriers, tumblers, pouters, and fantails now are, +should yield offspring perfectly fertile, inter se, seems to me rash in the +extreme. + +From these several reasons, namely, the improbability of man having formerly +got seven or eight supposed species of pigeons to breed freely under +domestication; these supposed species being quite unknown in a wild state, and +their becoming nowhere feral; these species having very abnormal characters in +certain respects, as compared with all other Columbidæ, though so like in most +other respects to the rock-pigeon; the blue colour and various marks +occasionally appearing in all the breeds, both when kept pure and when crossed; +the mongrel offspring being perfectly fertile;—from these several reasons, +taken together, I can feel no doubt that all our domestic breeds have descended +from the Columba livia with its geographical sub-species. + +In favour of this view, I may add, firstly, that C. livia, or the rock-pigeon, +has been found capable of domestication in Europe and in India; and that it +agrees in habits and in a great number of points of structure with all the +domestic breeds. Secondly, although an English carrier or short-faced tumbler +differs immensely in certain characters from the rock-pigeon, yet by comparing +the several sub-breeds of these breeds, more especially those brought from +distant countries, we can make an almost perfect series between the extremes of +structure. Thirdly, those characters which are mainly distinctive of each +breed, for instance the wattle and length of beak of the carrier, the shortness +of that of the tumbler, and the number of tail-feathers in the fantail, are in +each breed eminently variable; and the explanation of this fact will be obvious +when we come to treat of selection. Fourthly, pigeons have been watched, and +tended with the utmost care, and loved by many people. They have been +domesticated for thousands of years in several quarters of the world; the +earliest known record of pigeons is in the fifth Aegyptian dynasty, about 3000 +B.C., as was pointed out to me by Professor Lepsius; but Mr. Birch informs me +that pigeons are given in a bill of fare in the previous dynasty. In the time +of the Romans, as we hear from Pliny, immense prices were given for pigeons; +“nay, they are come to this pass, that they can reckon up their pedigree and +race.” Pigeons were much valued by Akber Khan in India, about the year 1600; +never less than 20,000 pigeons were taken with the court. “The monarchs of Iran +and Turan sent him some very rare birds;” and, continues the courtly historian, +“His Majesty by crossing the breeds, which method was never practised before, +has improved them astonishingly.” About this same period the Dutch were as +eager about pigeons as were the old Romans. The paramount importance of these +considerations in explaining the immense amount of variation which pigeons have +undergone, will be obvious when we treat of Selection. We shall then, also, see +how it is that the breeds so often have a somewhat monstrous character. It is +also a most favourable circumstance for the production of distinct breeds, that +male and female pigeons can be easily mated for life; and thus different breeds +can be kept together in the same aviary. + +I have discussed the probable origin of domestic pigeons at some, yet quite +insufficient, length; because when I first kept pigeons and watched the several +kinds, knowing well how true they bred, I felt fully as much difficulty in +believing that they could ever have descended from a common parent, as any +naturalist could in coming to a similar conclusion in regard to the many +species of finches, or other large groups of birds, in nature. One circumstance +has struck me much; namely, that all the breeders of the various domestic +animals and the cultivators of plants, with whom I have ever conversed, or +whose treatises I have read, are firmly convinced that the several breeds to +which each has attended, are descended from so many aboriginally distinct +species. Ask, as I have asked, a celebrated raiser of Hereford cattle, whether +his cattle might not have descended from long horns, and he will laugh you to +scorn. I have never met a pigeon, or poultry, or duck, or rabbit fancier, who +was not fully convinced that each main breed was descended from a distinct +species. Van Mons, in his treatise on pears and apples, shows how utterly he +disbelieves that the several sorts, for instance a Ribston-pippin or +Codlin-apple, could ever have proceeded from the seeds of the same tree. +Innumerable other examples could be given. The explanation, I think, is simple: +from long-continued study they are strongly impressed with the differences +between the several races; and though they well know that each race varies +slightly, for they win their prizes by selecting such slight differences, yet +they ignore all general arguments, and refuse to sum up in their minds slight +differences accumulated during many successive generations. May not those +naturalists who, knowing far less of the laws of inheritance than does the +breeder, and knowing no more than he does of the intermediate links in the long +lines of descent, yet admit that many of our domestic races have descended from +the same parents—may they not learn a lesson of caution, when they deride the +idea of species in a state of nature being lineal descendants of other species? + +Selection.—Let us now briefly consider the steps by which domestic races have +been produced, either from one or from several allied species. Some little +effect may, perhaps, be attributed to the direct action of the external +conditions of life, and some little to habit; but he would be a bold man who +would account by such agencies for the differences of a dray and race horse, a +greyhound and bloodhound, a carrier and tumbler pigeon. One of the most +remarkable features in our domesticated races is that we see in them +adaptation, not indeed to the animal’s or plant’s own good, but to man’s use or +fancy. Some variations useful to him have probably arisen suddenly, or by one +step; many botanists, for instance, believe that the fuller’s teazle, with its +hooks, which cannot be rivalled by any mechanical contrivance, is only a +variety of the wild Dipsacus; and this amount of change may have suddenly +arisen in a seedling. So it has probably been with the turnspit dog; and this +is known to have been the case with the ancon sheep. But when we compare the +dray-horse and race-horse, the dromedary and camel, the various breeds of sheep +fitted either for cultivated land or mountain pasture, with the wool of one +breed good for one purpose, and that of another breed for another purpose; when +we compare the many breeds of dogs, each good for man in very different ways; +when we compare the game-cock, so pertinacious in battle, with other breeds so +little quarrelsome, with “everlasting layers” which never desire to sit, and +with the bantam so small and elegant; when we compare the host of agricultural, +culinary, orchard, and flower-garden races of plants, most useful to man at +different seasons and for different purposes, or so beautiful in his eyes, we +must, I think, look further than to mere variability. We cannot suppose that +all the breeds were suddenly produced as perfect and as useful as we now see +them; indeed, in several cases, we know that this has not been their history. +The key is man’s power of accumulative selection: nature gives successive +variations; man adds them up in certain directions useful to him. In this sense +he may be said to make for himself useful breeds. + +The great power of this principle of selection is not hypothetical. It is +certain that several of our eminent breeders have, even within a single +lifetime, modified to a large extent some breeds of cattle and sheep. In order +fully to realise what they have done, it is almost necessary to read several of +the many treatises devoted to this subject, and to inspect the animals. +Breeders habitually speak of an animal’s organisation as something quite +plastic, which they can model almost as they please. If I had space I could +quote numerous passages to this effect from highly competent authorities. +Youatt, who was probably better acquainted with the works of agriculturalists +than almost any other individual, and who was himself a very good judge of an +animal, speaks of the principle of selection as “that which enables the +agriculturist, not only to modify the character of his flock, but to change it +altogether. It is the magician’s wand, by means of which he may summon into +life whatever form and mould he pleases.” Lord Somerville, speaking of what +breeders have done for sheep, says:—“It would seem as if they had chalked out +upon a wall a form perfect in itself, and then had given it existence.” That +most skilful breeder, Sir John Sebright, used to say, with respect to pigeons, +that “he would produce any given feather in three years, but it would take him +six years to obtain head and beak.” In Saxony the importance of the principle +of selection in regard to merino sheep is so fully recognised, that men follow +it as a trade: the sheep are placed on a table and are studied, like a picture +by a connoisseur; this is done three times at intervals of months, and the +sheep are each time marked and classed, so that the very best may ultimately be +selected for breeding. + +What English breeders have actually effected is proved by the enormous prices +given for animals with a good pedigree; and these have now been exported to +almost every quarter of the world. The improvement is by no means generally due +to crossing different breeds; all the best breeders are strongly opposed to +this practice, except sometimes amongst closely allied sub-breeds. And when a +cross has been made, the closest selection is far more indispensable even than +in ordinary cases. If selection consisted merely in separating some very +distinct variety, and breeding from it, the principle would be so obvious as +hardly to be worth notice; but its importance consists in the great effect +produced by the accumulation in one direction, during successive generations, +of differences absolutely inappreciable by an uneducated eye—differences which +I for one have vainly attempted to appreciate. Not one man in a thousand has +accuracy of eye and judgment sufficient to become an eminent breeder. If gifted +with these qualities, and he studies his subject for years, and devotes his +lifetime to it with indomitable perseverance, he will succeed, and may make +great improvements; if he wants any of these qualities, he will assuredly fail. +Few would readily believe in the natural capacity and years of practice +requisite to become even a skilful pigeon-fancier. + +The same principles are followed by horticulturists; but the variations are +here often more abrupt. No one supposes that our choicest productions have been +produced by a single variation from the aboriginal stock. We have proofs that +this is not so in some cases, in which exact records have been kept; thus, to +give a very trifling instance, the steadily-increasing size of the common +gooseberry may be quoted. We see an astonishing improvement in many florists’ +flowers, when the flowers of the present day are compared with drawings made +only twenty or thirty years ago. When a race of plants is once pretty well +established, the seed-raisers do not pick out the best plants, but merely go +over their seed-beds, and pull up the “rogues,” as they call the plants that +deviate from the proper standard. With animals this kind of selection is, in +fact, also followed; for hardly any one is so careless as to allow his worst +animals to breed. + +In regard to plants, there is another means of observing the accumulated +effects of selection—namely, by comparing the diversity of flowers in the +different varieties of the same species in the flower-garden; the diversity of +leaves, pods, or tubers, or whatever part is valued, in the kitchen-garden, in +comparison with the flowers of the same varieties; and the diversity of fruit +of the same species in the orchard, in comparison with the leaves and flowers +of the same set of varieties. See how different the leaves of the cabbage are, +and how extremely alike the flowers; how unlike the flowers of the heartsease +are, and how alike the leaves; how much the fruit of the different kinds of +gooseberries differ in size, colour, shape, and hairiness, and yet the flowers +present very slight differences. It is not that the varieties which differ +largely in some one point do not differ at all in other points; this is hardly +ever, perhaps never, the case. The laws of correlation of growth, the +importance of which should never be overlooked, will ensure some differences; +but, as a general rule, I cannot doubt that the continued selection of slight +variations, either in the leaves, the flowers, or the fruit, will produce races +differing from each other chiefly in these characters. + +It may be objected that the principle of selection has been reduced to +methodical practice for scarcely more than three-quarters of a century; it has +certainly been more attended to of late years, and many treatises have been +published on the subject; and the result, I may add, has been, in a +corresponding degree, rapid and important. But it is very far from true that +the principle is a modern discovery. I could give several references to the +full acknowledgment of the importance of the principle in works of high +antiquity. In rude and barbarous periods of English history choice animals were +often imported, and laws were passed to prevent their exportation: the +destruction of horses under a certain size was ordered, and this may be +compared to the “roguing” of plants by nurserymen. The principle of selection I +find distinctly given in an ancient Chinese encyclopædia. Explicit rules are +laid down by some of the Roman classical writers. From passages in Genesis, it +is clear that the colour of domestic animals was at that early period attended +to. Savages now sometimes cross their dogs with wild canine animals, to improve +the breed, and they formerly did so, as is attested by passages in Pliny. The +savages in South Africa match their draught cattle by colour, as do some of the +Esquimaux their teams of dogs. Livingstone shows how much good domestic breeds +are valued by the negroes of the interior of Africa who have not associated +with Europeans. Some of these facts do not show actual selection, but they show +that the breeding of domestic animals was carefully attended to in ancient +times, and is now attended to by the lowest savages. It would, indeed, have +been a strange fact, had attention not been paid to breeding, for the +inheritance of good and bad qualities is so obvious. + +At the present time, eminent breeders try by methodical selection, with a +distinct object in view, to make a new strain or sub-breed, superior to +anything existing in the country. But, for our purpose, a kind of Selection, +which may be called Unconscious, and which results from every one trying to +possess and breed from the best individual animals, is more important. Thus, a +man who intends keeping pointers naturally tries to get as good dogs as he can, +and afterwards breeds from his own best dogs, but he has no wish or expectation +of permanently altering the breed. Nevertheless I cannot doubt that this +process, continued during centuries, would improve and modify any breed, in the +same way as Bakewell, Collins, etc., by this very same process, only carried on +more methodically, did greatly modify, even during their own lifetimes, the +forms and qualities of their cattle. Slow and insensible changes of this kind +could never be recognised unless actual measurements or careful drawings of the +breeds in question had been made long ago, which might serve for comparison. In +some cases, however, unchanged or but little changed individuals of the same +breed may be found in less civilised districts, where the breed has been less +improved. There is reason to believe that King Charles’s spaniel has been +unconsciously modified to a large extent since the time of that monarch. Some +highly competent authorities are convinced that the setter is directly derived +from the spaniel, and has probably been slowly altered from it. It is known +that the English pointer has been greatly changed within the last century, and +in this case the change has, it is believed, been chiefly effected by crosses +with the fox-hound; but what concerns us is, that the change has been effected +unconsciously and gradually, and yet so effectually, that, though the old +Spanish pointer certainly came from Spain, Mr. Borrow has not seen, as I am +informed by him, any native dog in Spain like our pointer. + +By a similar process of selection, and by careful training, the whole body of +English racehorses have come to surpass in fleetness and size the parent Arab +stock, so that the latter, by the regulations for the Goodwood Races, are +favoured in the weights they carry. Lord Spencer and others have shown how the +cattle of England have increased in weight and in early maturity, compared with +the stock formerly kept in this country. By comparing the accounts given in old +pigeon treatises of carriers and tumblers with these breeds as now existing in +Britain, India, and Persia, we can, I think, clearly trace the stages through +which they have insensibly passed, and come to differ so greatly from the +rock-pigeon. + +Youatt gives an excellent illustration of the effects of a course of selection, +which may be considered as unconsciously followed, in so far that the breeders +could never have expected or even have wished to have produced the result which +ensued—namely, the production of two distinct strains. The two flocks of +Leicester sheep kept by Mr. Buckley and Mr. Burgess, as Mr. Youatt remarks, +“have been purely bred from the original stock of Mr. Bakewell for upwards of +fifty years. There is not a suspicion existing in the mind of any one at all +acquainted with the subject that the owner of either of them has deviated in +any one instance from the pure blood of Mr. Bakewell’s flock, and yet the +difference between the sheep possessed by these two gentlemen is so great that +they have the appearance of being quite different varieties.” + +If there exist savages so barbarous as never to think of the inherited +character of the offspring of their domestic animals, yet any one animal +particularly useful to them, for any special purpose, would be carefully +preserved during famines and other accidents, to which savages are so liable, +and such choice animals would thus generally leave more offspring than the +inferior ones; so that in this case there would be a kind of unconscious +selection going on. We see the value set on animals even by the barbarians of +Tierra del Fuego, by their killing and devouring their old women, in times of +dearth, as of less value than their dogs. + +In plants the same gradual process of improvement, through the occasional +preservation of the best individuals, whether or not sufficiently distinct to +be ranked at their first appearance as distinct varieties, and whether or not +two or more species or races have become blended together by crossing, may +plainly be recognised in the increased size and beauty which we now see in the +varieties of the heartsease, rose, pelargonium, dahlia, and other plants, when +compared with the older varieties or with their parent-stocks. No one would +ever expect to get a first-rate heartsease or dahlia from the seed of a wild +plant. No one would expect to raise a first-rate melting pear from the seed of +a wild pear, though he might succeed from a poor seedling growing wild, if it +had come from a garden-stock. The pear, though cultivated in classical times, +appears, from Pliny’s description, to have been a fruit of very inferior +quality. I have seen great surprise expressed in horticultural works at the +wonderful skill of gardeners, in having produced such splendid results from +such poor materials; but the art, I cannot doubt, has been simple, and, as far +as the final result is concerned, has been followed almost unconsciously. It +has consisted in always cultivating the best known variety, sowing its seeds, +and, when a slightly better variety has chanced to appear, selecting it, and so +onwards. But the gardeners of the classical period, who cultivated the best +pear they could procure, never thought what splendid fruit we should eat; +though we owe our excellent fruit, in some small degree, to their having +naturally chosen and preserved the best varieties they could anywhere find. + +A large amount of change in our cultivated plants, thus slowly and +unconsciously accumulated, explains, as I believe, the well-known fact, that in +a vast number of cases we cannot recognise, and therefore do not know, the wild +parent-stocks of the plants which have been longest cultivated in our flower +and kitchen gardens. If it has taken centuries or thousands of years to improve +or modify most of our plants up to their present standard of usefulness to man, +we can understand how it is that neither Australia, the Cape of Good Hope, nor +any other region inhabited by quite uncivilised man, has afforded us a single +plant worth culture. It is not that these countries, so rich in species, do not +by a strange chance possess the aboriginal stocks of any useful plants, but +that the native plants have not been improved by continued selection up to a +standard of perfection comparable with that given to the plants in countries +anciently civilised. + +In regard to the domestic animals kept by uncivilised man, it should not be +overlooked that they almost always have to struggle for their own food, at +least during certain seasons. And in two countries very differently +circumstanced, individuals of the same species, having slightly different +constitutions or structure, would often succeed better in the one country than +in the other, and thus by a process of “natural selection,” as will hereafter +be more fully explained, two sub-breeds might be formed. This, perhaps, partly +explains what has been remarked by some authors, namely, that the varieties +kept by savages have more of the character of species than the varieties kept +in civilised countries. + +On the view here given of the all-important part which selection by man has +played, it becomes at once obvious, how it is that our domestic races show +adaptation in their structure or in their habits to man’s wants or fancies. We +can, I think, further understand the frequently abnormal character of our +domestic races, and likewise their differences being so great in external +characters and relatively so slight in internal parts or organs. Man can hardly +select, or only with much difficulty, any deviation of structure excepting such +as is externally visible; and indeed he rarely cares for what is internal. He +can never act by selection, excepting on variations which are first given to +him in some slight degree by nature. No man would ever try to make a fantail, +till he saw a pigeon with a tail developed in some slight degree in an unusual +manner, or a pouter till he saw a pigeon with a crop of somewhat unusual size; +and the more abnormal or unusual any character was when it first appeared, the +more likely it would be to catch his attention. But to use such an expression +as trying to make a fantail, is, I have no doubt, in most cases, utterly +incorrect. The man who first selected a pigeon with a slightly larger tail, +never dreamed what the descendants of that pigeon would become through +long-continued, partly unconscious and partly methodical selection. Perhaps the +parent bird of all fantails had only fourteen tail-feathers somewhat expanded, +like the present Java fantail, or like individuals of other and distinct +breeds, in which as many as seventeen tail-feathers have been counted. Perhaps +the first pouter-pigeon did not inflate its crop much more than the turbit now +does the upper part of its oesophagus,—a habit which is disregarded by all +fanciers, as it is not one of the points of the breed. + +Nor let it be thought that some great deviation of structure would be necessary +to catch the fancier’s eye: he perceives extremely small differences, and it is +in human nature to value any novelty, however slight, in one’s own possession. +Nor must the value which would formerly be set on any slight differences in the +individuals of the same species, be judged of by the value which would now be +set on them, after several breeds have once fairly been established. Many +slight differences might, and indeed do now, arise amongst pigeons, which are +rejected as faults or deviations from the standard of perfection of each breed. +The common goose has not given rise to any marked varieties; hence the +Thoulouse and the common breed, which differ only in colour, that most fleeting +of characters, have lately been exhibited as distinct at our poultry-shows. + +I think these views further explain what has sometimes been noticed—namely that +we know nothing about the origin or history of any of our domestic breeds. But, +in fact, a breed, like a dialect of a language, can hardly be said to have had +a definite origin. A man preserves and breeds from an individual with some +slight deviation of structure, or takes more care than usual in matching his +best animals and thus improves them, and the improved individuals slowly spread +in the immediate neighbourhood. But as yet they will hardly have a distinct +name, and from being only slightly valued, their history will be disregarded. +When further improved by the same slow and gradual process, they will spread +more widely, and will get recognised as something distinct and valuable, and +will then probably first receive a provincial name. In semi-civilised +countries, with little free communication, the spreading and knowledge of any +new sub-breed will be a slow process. As soon as the points of value of the new +sub-breed are once fully acknowledged, the principle, as I have called it, of +unconscious selection will always tend,—perhaps more at one period than at +another, as the breed rises or falls in fashion,—perhaps more in one district +than in another, according to the state of civilisation of the +inhabitants—slowly to add to the characteristic features of the breed, whatever +they may be. But the chance will be infinitely small of any record having been +preserved of such slow, varying, and insensible changes. + +I must now say a few words on the circumstances, favourable, or the reverse, to +man’s power of selection. A high degree of variability is obviously favourable, +as freely giving the materials for selection to work on; not that mere +individual differences are not amply sufficient, with extreme care, to allow of +the accumulation of a large amount of modification in almost any desired +direction. But as variations manifestly useful or pleasing to man appear only +occasionally, the chance of their appearance will be much increased by a large +number of individuals being kept; and hence this comes to be of the highest +importance to success. On this principle Marshall has remarked, with respect to +the sheep of parts of Yorkshire, that “as they generally belong to poor people, +and are mostly in small lots, they never can be improved.” On the other hand, +nurserymen, from raising large stocks of the same plants, are generally far +more successful than amateurs in getting new and valuable varieties. The +keeping of a large number of individuals of a species in any country requires +that the species should be placed under favourable conditions of life, so as to +breed freely in that country. When the individuals of any species are scanty, +all the individuals, whatever their quality may be, will generally be allowed +to breed, and this will effectually prevent selection. But probably the most +important point of all, is, that the animal or plant should be so highly useful +to man, or so much valued by him, that the closest attention should be paid to +even the slightest deviation in the qualities or structure of each individual. +Unless such attention be paid nothing can be effected. I have seen it gravely +remarked, that it was most fortunate that the strawberry began to vary just +when gardeners began to attend closely to this plant. No doubt the strawberry +had always varied since it was cultivated, but the slight varieties had been +neglected. As soon, however, as gardeners picked out individual plants with +slightly larger, earlier, or better fruit, and raised seedlings from them, and +again picked out the best seedlings and bred from them, then, there appeared +(aided by some crossing with distinct species) those many admirable varieties +of the strawberry which have been raised during the last thirty or forty years. + +In the case of animals with separate sexes, facility in preventing crosses is +an important element of success in the formation of new races,—at least, in a +country which is already stocked with other races. In this respect enclosure of +the land plays a part. Wandering savages or the inhabitants of open plains +rarely possess more than one breed of the same species. Pigeons can be mated +for life, and this is a great convenience to the fancier, for thus many races +may be kept true, though mingled in the same aviary; and this circumstance must +have largely favoured the improvement and formation of new breeds. Pigeons, I +may add, can be propagated in great numbers and at a very quick rate, and +inferior birds may be freely rejected, as when killed they serve for food. On +the other hand, cats, from their nocturnal rambling habits, cannot be matched, +and, although so much valued by women and children, we hardly ever see a +distinct breed kept up; such breeds as we do sometimes see are almost always +imported from some other country, often from islands. Although I do not doubt +that some domestic animals vary less than others, yet the rarity or absence of +distinct breeds of the cat, the donkey, peacock, goose, etc., may be attributed +in main part to selection not having been brought into play: in cats, from the +difficulty in pairing them; in donkeys, from only a few being kept by poor +people, and little attention paid to their breeding; in peacocks, from not +being very easily reared and a large stock not kept; in geese, from being +valuable only for two purposes, food and feathers, and more especially from no +pleasure having been felt in the display of distinct breeds. + +To sum up on the origin of our Domestic Races of animals and plants. I believe +that the conditions of life, from their action on the reproductive system, are +so far of the highest importance as causing variability. I do not believe that +variability is an inherent and necessary contingency, under all circumstances, +with all organic beings, as some authors have thought. The effects of +variability are modified by various degrees of inheritance and of reversion. +Variability is governed by many unknown laws, more especially by that of +correlation of growth. Something may be attributed to the direct action of the +conditions of life. Something must be attributed to use and disuse. The final +result is thus rendered infinitely complex. In some cases, I do not doubt that +the intercrossing of species, aboriginally distinct, has played an important +part in the origin of our domestic productions. When in any country several +domestic breeds have once been established, their occasional intercrossing, +with the aid of selection, has, no doubt, largely aided in the formation of new +sub-breeds; but the importance of the crossing of varieties has, I believe, +been greatly exaggerated, both in regard to animals and to those plants which +are propagated by seed. In plants which are temporarily propagated by cuttings, +buds, etc., the importance of the crossing both of distinct species and of +varieties is immense; for the cultivator here quite disregards the extreme +variability both of hybrids and mongrels, and the frequent sterility of +hybrids; but the cases of plants not propagated by seed are of little +importance to us, for their endurance is only temporary. Over all these causes +of Change I am convinced that the accumulative action of Selection, whether +applied methodically and more quickly, or unconsciously and more slowly, but +more efficiently, is by far the predominant Power. + +CHAPTER II. +VARIATION UNDER NATURE. + +Variability. Individual differences. Doubtful species. Wide ranging, much +diffused, and common species vary most. Species of the larger genera in any +country vary more than the species of the smaller genera. Many of the species +of the larger genera resemble varieties in being very closely, but unequally, +related to each other, and in having restricted ranges. + +Before applying the principles arrived at in the last chapter to organic beings +in a state of nature, we must briefly discuss whether these latter are subject +to any variation. To treat this subject at all properly, a long catalogue of +dry facts should be given; but these I shall reserve for my future work. Nor +shall I here discuss the various definitions which have been given of the term +species. No one definition has as yet satisfied all naturalists; yet every +naturalist knows vaguely what he means when he speaks of a species. Generally +the term includes the unknown element of a distinct act of creation. The term +“variety” is almost equally difficult to define; but here community of descent +is almost universally implied, though it can rarely be proved. We have also +what are called monstrosities; but they graduate into varieties. By a +monstrosity I presume is meant some considerable deviation of structure in one +part, either injurious to or not useful to the species, and not generally +propagated. Some authors use the term “variation” in a technical sense, as +implying a modification directly due to the physical conditions of life; and +“variations” in this sense are supposed not to be inherited: but who can say +that the dwarfed condition of shells in the brackish waters of the Baltic, or +dwarfed plants on Alpine summits, or the thicker fur of an animal from far +northwards, would not in some cases be inherited for at least some few +generations? and in this case I presume that the form would be called a +variety. + +Again, we have many slight differences which may be called individual +differences, such as are known frequently to appear in the offspring from the +same parents, or which may be presumed to have thus arisen, from being +frequently observed in the individuals of the same species inhabiting the same +confined locality. No one supposes that all the individuals of the same species +are cast in the very same mould. These individual differences are highly +important for us, as they afford materials for natural selection to accumulate, +in the same manner as man can accumulate in any given direction individual +differences in his domesticated productions. These individual differences +generally affect what naturalists consider unimportant parts; but I could show +by a long catalogue of facts, that parts which must be called important, +whether viewed under a physiological or classificatory point of view, sometimes +vary in the individuals of the same species. I am convinced that the most +experienced naturalist would be surprised at the number of the cases of +variability, even in important parts of structure, which he could collect on +good authority, as I have collected, during a course of years. It should be +remembered that systematists are far from pleased at finding variability in +important characters, and that there are not many men who will laboriously +examine internal and important organs, and compare them in many specimens of +the same species. I should never have expected that the branching of the main +nerves close to the great central ganglion of an insect would have been +variable in the same species; I should have expected that changes of this +nature could have been effected only by slow degrees: yet quite recently Mr. +Lubbock has shown a degree of variability in these main nerves in Coccus, which +may almost be compared to the irregular branching of the stem of a tree. This +philosophical naturalist, I may add, has also quite recently shown that the +muscles in the larvæ of certain insects are very far from uniform. Authors +sometimes argue in a circle when they state that important organs never vary; +for these same authors practically rank that character as important (as some +few naturalists have honestly confessed) which does not vary; and, under this +point of view, no instance of an important part varying will ever be found: but +under any other point of view many instances assuredly can be given. + +There is one point connected with individual differences, which seems to me +extremely perplexing: I refer to those genera which have sometimes been called +“protean” or “polymorphic,” in which the species present an inordinate amount +of variation; and hardly two naturalists can agree which forms to rank as +species and which as varieties. We may instance Rubus, Rosa, and Hieracium +amongst plants, several genera of insects, and several genera of Brachiopod +shells. In most polymorphic genera some of the species have fixed and definite +characters. Genera which are polymorphic in one country seem to be, with some +few exceptions, polymorphic in other countries, and likewise, judging from +Brachiopod shells, at former periods of time. These facts seem to be very +perplexing, for they seem to show that this kind of variability is independent +of the conditions of life. I am inclined to suspect that we see in these +polymorphic genera variations in points of structure which are of no service or +disservice to the species, and which consequently have not been seized on and +rendered definite by natural selection, as hereafter will be explained. + +Those forms which possess in some considerable degree the character of species, +but which are so closely similar to some other forms, or are so closely linked +to them by intermediate gradations, that naturalists do not like to rank them +as distinct species, are in several respects the most important for us. We have +every reason to believe that many of these doubtful and closely-allied forms +have permanently retained their characters in their own country for a long +time; for as long, as far as we know, as have good and true species. +Practically, when a naturalist can unite two forms together by others having +intermediate characters, he treats the one as a variety of the other, ranking +the most common, but sometimes the one first described, as the species, and the +other as the variety. But cases of great difficulty, which I will not here +enumerate, sometimes occur in deciding whether or not to rank one form as a +variety of another, even when they are closely connected by intermediate links; +nor will the commonly-assumed hybrid nature of the intermediate links always +remove the difficulty. In very many cases, however, one form is ranked as a +variety of another, not because the intermediate links have actually been +found, but because analogy leads the observer to suppose either that they do +now somewhere exist, or may formerly have existed; and here a wide door for the +entry of doubt and conjecture is opened. + +Hence, in determining whether a form should be ranked as a species or a +variety, the opinion of naturalists having sound judgment and wide experience +seems the only guide to follow. We must, however, in many cases, decide by a +majority of naturalists, for few well-marked and well-known varieties can be +named which have not been ranked as species by at least some competent judges. + +That varieties of this doubtful nature are far from uncommon cannot be +disputed. Compare the several floras of Great Britain, of France or of the +United States, drawn up by different botanists, and see what a surprising +number of forms have been ranked by one botanist as good species, and by +another as mere varieties. Mr. H. C. Watson, to whom I lie under deep +obligation for assistance of all kinds, has marked for me 182 British plants, +which are generally considered as varieties, but which have all been ranked by +botanists as species; and in making this list he has omitted many trifling +varieties, but which nevertheless have been ranked by some botanists as +species, and he has entirely omitted several highly polymorphic genera. Under +genera, including the most polymorphic forms, Mr. Babington gives 251 species, +whereas Mr. Bentham gives only 112,—a difference of 139 doubtful forms! Amongst +animals which unite for each birth, and which are highly locomotive, doubtful +forms, ranked by one zoologist as a species and by another as a variety, can +rarely be found within the same country, but are common in separated areas. How +many of those birds and insects in North America and Europe, which differ very +slightly from each other, have been ranked by one eminent naturalist as +undoubted species, and by another as varieties, or, as they are often called, +as geographical races! Many years ago, when comparing, and seeing others +compare, the birds from the separate islands of the Galapagos Archipelago, both +one with another, and with those from the American mainland, I was much struck +how entirely vague and arbitrary is the distinction between species and +varieties. On the islets of the little Madeira group there are many insects +which are characterized as varieties in Mr. Wollaston’s admirable work, but +which it cannot be doubted would be ranked as distinct species by many +entomologists. Even Ireland has a few animals, now generally regarded as +varieties, but which have been ranked as species by some zoologists. Several +most experienced ornithologists consider our British red grouse as only a +strongly-marked race of a Norwegian species, whereas the greater number rank it +as an undoubted species peculiar to Great Britain. A wide distance between the +homes of two doubtful forms leads many naturalists to rank both as distinct +species; but what distance, it has been well asked, will suffice? if that +between America and Europe is ample, will that between the Continent and the +Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It must be +admitted that many forms, considered by highly-competent judges as varieties, +have so perfectly the character of species that they are ranked by other +highly-competent judges as good and true species. But to discuss whether they +are rightly called species or varieties, before any definition of these terms +has been generally accepted, is vainly to beat the air. + +Many of the cases of strongly-marked varieties or doubtful species well deserve +consideration; for several interesting lines of argument, from geographical +distribution, analogical variation, hybridism, etc., have been brought to bear +on the attempt to determine their rank. I will here give only a single +instance,—the well-known one of the primrose and cowslip, or Primula veris and +elatior. These plants differ considerably in appearance; they have a different +flavour and emit a different odour; they flower at slightly different periods; +they grow in somewhat different stations; they ascend mountains to different +heights; they have different geographical ranges; and lastly, according to very +numerous experiments made during several years by that most careful observer +Gärtner, they can be crossed only with much difficulty. We could hardly wish +for better evidence of the two forms being specifically distinct. On the other +hand, they are united by many intermediate links, and it is very doubtful +whether these links are hybrids; and there is, as it seems to me, an +overwhelming amount of experimental evidence, showing that they descend from +common parents, and consequently must be ranked as varieties. + +Close investigation, in most cases, will bring naturalists to an agreement how +to rank doubtful forms. Yet it must be confessed, that it is in the best-known +countries that we find the greatest number of forms of doubtful value. I have +been struck with the fact, that if any animal or plant in a state of nature be +highly useful to man, or from any cause closely attract his attention, +varieties of it will almost universally be found recorded. These varieties, +moreover, will be often ranked by some authors as species. Look at the common +oak, how closely it has been studied; yet a German author makes more than a +dozen species out of forms, which are very generally considered as varieties; +and in this country the highest botanical authorities and practical men can be +quoted to show that the sessile and pedunculated oaks are either good and +distinct species or mere varieties. + +When a young naturalist commences the study of a group of organisms quite +unknown to him, he is at first much perplexed to determine what differences to +consider as specific, and what as varieties; for he knows nothing of the amount +and kind of variation to which the group is subject; and this shows, at least, +how very generally there is some variation. But if he confine his attention to +one class within one country, he will soon make up his mind how to rank most of +the doubtful forms. His general tendency will be to make many species, for he +will become impressed, just like the pigeon or poultry-fancier before alluded +to, with the amount of difference in the forms which he is continually +studying; and he has little general knowledge of analogical variation in other +groups and in other countries, by which to correct his first impressions. As he +extends the range of his observations, he will meet with more cases of +difficulty; for he will encounter a greater number of closely-allied forms. But +if his observations be widely extended, he will in the end generally be enabled +to make up his own mind which to call varieties and which species; but he will +succeed in this at the expense of admitting much variation,—and the truth of +this admission will often be disputed by other naturalists. When, moreover, he +comes to study allied forms brought from countries not now continuous, in which +case he can hardly hope to find the intermediate links between his doubtful +forms, he will have to trust almost entirely to analogy, and his difficulties +will rise to a climax. + +Certainly no clear line of demarcation has as yet been drawn between species +and sub-species—that is, the forms which in the opinion of some naturalists +come very near to, but do not quite arrive at the rank of species; or, again, +between sub-species and well-marked varieties, or between lesser varieties and +individual differences. These differences blend into each other in an +insensible series; and a series impresses the mind with the idea of an actual +passage. + +Hence I look at individual differences, though of small interest to the +systematist, as of high importance for us, as being the first step towards such +slight varieties as are barely thought worth recording in works on natural +history. And I look at varieties which are in any degree more distinct and +permanent, as steps leading to more strongly marked and more permanent +varieties; and at these latter, as leading to sub-species, and to species. The +passage from one stage of difference to another and higher stage may be, in +some cases, due merely to the long-continued action of different physical +conditions in two different regions; but I have not much faith in this view; +and I attribute the passage of a variety, from a state in which it differs very +slightly from its parent to one in which it differs more, to the action of +natural selection in accumulating (as will hereafter be more fully explained) +differences of structure in certain definite directions. Hence I believe a +well-marked variety may be justly called an incipient species; but whether this +belief be justifiable must be judged of by the general weight of the several +facts and views given throughout this work. + +It need not be supposed that all varieties or incipient species necessarily +attain the rank of species. They may whilst in this incipient state become +extinct, or they may endure as varieties for very long periods, as has been +shown to be the case by Mr. Wollaston with the varieties of certain fossil +land-shells in Madeira. If a variety were to flourish so as to exceed in +numbers the parent species, it would then rank as the species, and the species +as the variety; or it might come to supplant and exterminate the parent +species; or both might co-exist, and both rank as independent species. But we +shall hereafter have to return to this subject. + +From these remarks it will be seen that I look at the term species, as one +arbitrarily given for the sake of convenience to a set of individuals closely +resembling each other, and that it does not essentially differ from the term +variety, which is given to less distinct and more fluctuating forms. The term +variety, again, in comparison with mere individual differences, is also applied +arbitrarily, and for mere convenience sake. + +Guided by theoretical considerations, I thought that some interesting results +might be obtained in regard to the nature and relations of the species which +vary most, by tabulating all the varieties in several well-worked floras. At +first this seemed a simple task; but Mr. H. C. Watson, to whom I am much +indebted for valuable advice and assistance on this subject, soon convinced me +that there were many difficulties, as did subsequently Dr. Hooker, even in +stronger terms. I shall reserve for my future work the discussion of these +difficulties, and the tables themselves of the proportional numbers of the +varying species. Dr. Hooker permits me to add, that after having carefully read +my manuscript, and examined the tables, he thinks that the following statements +are fairly well established. The whole subject, however, treated as it +necessarily here is with much brevity, is rather perplexing, and allusions +cannot be avoided to the “struggle for existence,” “divergence of character,” +and other questions, hereafter to be discussed. + +Alph. De Candolle and others have shown that plants which have very wide ranges +generally present varieties; and this might have been expected, as they become +exposed to diverse physical conditions, and as they come into competition +(which, as we shall hereafter see, is a far more important circumstance) with +different sets of organic beings. But my tables further show that, in any +limited country, the species which are most common, that is abound most in +individuals, and the species which are most widely diffused within their own +country (and this is a different consideration from wide range, and to a +certain extent from commonness), often give rise to varieties sufficiently +well-marked to have been recorded in botanical works. Hence it is the most +flourishing, or, as they may be called, the dominant species,—those which range +widely over the world, are the most diffused in their own country, and are the +most numerous in individuals,—which oftenest produce well-marked varieties, or, +as I consider them, incipient species. And this, perhaps, might have been +anticipated; for, as varieties, in order to become in any degree permanent, +necessarily have to struggle with the other inhabitants of the country, the +species which are already dominant will be the most likely to yield offspring +which, though in some slight degree modified, will still inherit those +advantages that enabled their parents to become dominant over their +compatriots. + +If the plants inhabiting a country and described in any Flora be divided into +two equal masses, all those in the larger genera being placed on one side, and +all those in the smaller genera on the other side, a somewhat larger number of +the very common and much diffused or dominant species will be found on the side +of the larger genera. This, again, might have been anticipated; for the mere +fact of many species of the same genus inhabiting any country, shows that there +is something in the organic or inorganic conditions of that country favourable +to the genus; and, consequently, we might have expected to have found in the +larger genera, or those including many species, a large proportional number of +dominant species. But so many causes tend to obscure this result, that I am +surprised that my tables show even a small majority on the side of the larger +genera. I will here allude to only two causes of obscurity. Fresh-water and +salt-loving plants have generally very wide ranges and are much diffused, but +this seems to be connected with the nature of the stations inhabited by them, +and has little or no relation to the size of the genera to which the species +belong. Again, plants low in the scale of organisation are generally much more +widely diffused than plants higher in the scale; and here again there is no +close relation to the size of the genera. The cause of lowly-organised plants +ranging widely will be discussed in our chapter on geographical distribution. + +From looking at species as only strongly-marked and well-defined varieties, I +was led to anticipate that the species of the larger genera in each country +would oftener present varieties, than the species of the smaller genera; for +wherever many closely related species (i.e. species of the same genus) have +been formed, many varieties or incipient species ought, as a general rule, to +be now forming. Where many large trees grow, we expect to find saplings. Where +many species of a genus have been formed through variation, circumstances have +been favourable for variation; and hence we might expect that the circumstances +would generally be still favourable to variation. On the other hand, if we look +at each species as a special act of creation, there is no apparent reason why +more varieties should occur in a group having many species, than in one having +few. + +To test the truth of this anticipation I have arranged the plants of twelve +countries, and the coleopterous insects of two districts, into two nearly equal +masses, the species of the larger genera on one side, and those of the smaller +genera on the other side, and it has invariably proved to be the case that a +larger proportion of the species on the side of the larger genera present +varieties, than on the side of the smaller genera. Moreover, the species of the +large genera which present any varieties, invariably present a larger average +number of varieties than do the species of the small genera. Both these results +follow when another division is made, and when all the smallest genera, with +from only one to four species, are absolutely excluded from the tables. These +facts are of plain signification on the view that species are only strongly +marked and permanent varieties; for wherever many species of the same genus +have been formed, or where, if we may use the expression, the manufactory of +species has been active, we ought generally to find the manufactory still in +action, more especially as we have every reason to believe the process of +manufacturing new species to be a slow one. And this certainly is the case, if +varieties be looked at as incipient species; for my tables clearly show as a +general rule that, wherever many species of a genus have been formed, the +species of that genus present a number of varieties, that is of incipient +species, beyond the average. It is not that all large genera are now varying +much, and are thus increasing in the number of their species, or that no small +genera are now varying and increasing; for if this had been so, it would have +been fatal to my theory; inasmuch as geology plainly tells us that small genera +have in the lapse of time often increased greatly in size; and that large +genera have often come to their maxima, declined, and disappeared. All that we +want to show is, that where many species of a genus have been formed, on an +average many are still forming; and this holds good. + +There are other relations between the species of large genera and their +recorded varieties which deserve notice. We have seen that there is no +infallible criterion by which to distinguish species and well-marked varieties; +and in those cases in which intermediate links have not been found between +doubtful forms, naturalists are compelled to come to a determination by the +amount of difference between them, judging by analogy whether or not the amount +suffices to raise one or both to the rank of species. Hence the amount of +difference is one very important criterion in settling whether two forms should +be ranked as species or varieties. Now Fries has remarked in regard to plants, +and Westwood in regard to insects, that in large genera the amount of +difference between the species is often exceedingly small. I have endeavoured +to test this numerically by averages, and, as far as my imperfect results go, +they always confirm the view. I have also consulted some sagacious and most +experienced observers, and, after deliberation, they concur in this view. In +this respect, therefore, the species of the larger genera resemble varieties, +more than do the species of the smaller genera. Or the case may be put in +another way, and it may be said, that in the larger genera, in which a number +of varieties or incipient species greater than the average are now +manufacturing, many of the species already manufactured still to a certain +extent resemble varieties, for they differ from each other by a less than usual +amount of difference. + +Moreover, the species of the large genera are related to each other, in the +same manner as the varieties of any one species are related to each other. No +naturalist pretends that all the species of a genus are equally distinct from +each other; they may generally be divided into sub-genera, or sections, or +lesser groups. As Fries has well remarked, little groups of species are +generally clustered like satellites around certain other species. And what are +varieties but groups of forms, unequally related to each other, and clustered +round certain forms—that is, round their parent-species? Undoubtedly there is +one most important point of difference between varieties and species; namely, +that the amount of difference between varieties, when compared with each other +or with their parent-species, is much less than that between the species of the +same genus. But when we come to discuss the principle, as I call it, of +Divergence of Character, we shall see how this may be explained, and how the +lesser differences between varieties will tend to increase into the greater +differences between species. + +There is one other point which seems to me worth notice. Varieties generally +have much restricted ranges: this statement is indeed scarcely more than a +truism, for if a variety were found to have a wider range than that of its +supposed parent-species, their denominations ought to be reversed. But there is +also reason to believe, that those species which are very closely allied to +other species, and in so far resemble varieties, often have much restricted +ranges. For instance, Mr. H. C. Watson has marked for me in the well-sifted +London Catalogue of plants (4th edition) 63 plants which are therein ranked as +species, but which he considers as so closely allied to other species as to be +of doubtful value: these 63 reputed species range on an average over 6.9 of the +provinces into which Mr. Watson has divided Great Britain. Now, in this same +catalogue, 53 acknowledged varieties are recorded, and these range over 7.7 +provinces; whereas, the species to which these varieties belong range over 14.3 +provinces. So that the acknowledged varieties have very nearly the same +restricted average range, as have those very closely allied forms, marked for +me by Mr. Watson as doubtful species, but which are almost universally ranked +by British botanists as good and true species. + +Finally, then, varieties have the same general characters as species, for they +cannot be distinguished from species,—except, firstly, by the discovery of +intermediate linking forms, and the occurrence of such links cannot affect the +actual characters of the forms which they connect; and except, secondly, by a +certain amount of difference, for two forms, if differing very little, are +generally ranked as varieties, notwithstanding that intermediate linking forms +have not been discovered; but the amount of difference considered necessary to +give to two forms the rank of species is quite indefinite. In genera having +more than the average number of species in any country, the species of these +genera have more than the average number of varieties. In large genera the +species are apt to be closely, but unequally, allied together, forming little +clusters round certain species. Species very closely allied to other species +apparently have restricted ranges. In all these several respects the species of +large genera present a strong analogy with varieties. And we can clearly +understand these analogies, if species have once existed as varieties, and have +thus originated: whereas, these analogies are utterly inexplicable if each +species has been independently created. + +We have, also, seen that it is the most flourishing and dominant species of the +larger genera which on an average vary most; and varieties, as we shall +hereafter see, tend to become converted into new and distinct species. The +larger genera thus tend to become larger; and throughout nature the forms of +life which are now dominant tend to become still more dominant by leaving many +modified and dominant descendants. But by steps hereafter to be explained, the +larger genera also tend to break up into smaller genera. And thus, the forms of +life throughout the universe become divided into groups subordinate to groups. + +CHAPTER III. +STRUGGLE FOR EXISTENCE. + +Bears on natural selection. The term used in a wide sense. Geometrical powers +of increase. Rapid increase of naturalised animals and plants. Nature of the +checks to increase. Competition universal. Effects of climate. Protection from +the number of individuals. Complex relations of all animals and plants +throughout nature. Struggle for life most severe between individuals and +varieties of the same species; often severe between species of the same genus. +The relation of organism to organism the most important of all relations. + +Before entering on the subject of this chapter, I must make a few preliminary +remarks, to show how the struggle for existence bears on Natural Selection. It +has been seen in the last chapter that amongst organic beings in a state of +nature there is some individual variability; indeed I am not aware that this +has ever been disputed. It is immaterial for us whether a multitude of doubtful +forms be called species or sub-species or varieties; what rank, for instance, +the two or three hundred doubtful forms of British plants are entitled to hold, +if the existence of any well-marked varieties be admitted. But the mere +existence of individual variability and of some few well-marked varieties, +though necessary as the foundation for the work, helps us but little in +understanding how species arise in nature. How have all those exquisite +adaptations of one part of the organisation to another part, and to the +conditions of life, and of one distinct organic being to another being, been +perfected? We see these beautiful co-adaptations most plainly in the woodpecker +and missletoe; and only a little less plainly in the humblest parasite which +clings to the hairs of a quadruped or feathers of a bird; in the structure of +the beetle which dives through the water; in the plumed seed which is wafted by +the gentlest breeze; in short, we see beautiful adaptations everywhere and in +every part of the organic world. + +Again, it may be asked, how is it that varieties, which I have called incipient +species, become ultimately converted into good and distinct species, which in +most cases obviously differ from each other far more than do the varieties of +the same species? How do those groups of species, which constitute what are +called distinct genera, and which differ from each other more than do the +species of the same genus, arise? All these results, as we shall more fully see +in the next chapter, follow inevitably from the struggle for life. Owing to +this struggle for life, any variation, however slight and from whatever cause +proceeding, if it be in any degree profitable to an individual of any species, +in its infinitely complex relations to other organic beings and to external +nature, will tend to the preservation of that individual, and will generally be +inherited by its offspring. The offspring, also, will thus have a better chance +of surviving, for, of the many individuals of any species which are +periodically born, but a small number can survive. I have called this +principle, by which each slight variation, if useful, is preserved, by the term +of Natural Selection, in order to mark its relation to man’s power of +selection. We have seen that man by selection can certainly produce great +results, and can adapt organic beings to his own uses, through the accumulation +of slight but useful variations, given to him by the hand of Nature. But +Natural Selection, as we shall hereafter see, is a power incessantly ready for +action, and is as immeasurably superior to man’s feeble efforts, as the works +of Nature are to those of Art. + +We will now discuss in a little more detail the struggle for existence. In my +future work this subject shall be treated, as it well deserves, at much greater +length. The elder De Candolle and Lyell have largely and philosophically shown +that all organic beings are exposed to severe competition. In regard to plants, +no one has treated this subject with more spirit and ability than W. Herbert, +Dean of Manchester, evidently the result of his great horticultural knowledge. +Nothing is easier than to admit in words the truth of the universal struggle +for life, or more difficult—at least I have found it so—than constantly to bear +this conclusion in mind. Yet unless it be thoroughly engrained in the mind, I +am convinced that the whole economy of nature, with every fact on distribution, +rarity, abundance, extinction, and variation, will be dimly seen or quite +misunderstood. We behold the face of nature bright with gladness, we often see +superabundance of food; we do not see, or we forget, that the birds which are +idly singing round us mostly live on insects or seeds, and are thus constantly +destroying life; or we forget how largely these songsters, or their eggs, or +their nestlings, are destroyed by birds and beasts of prey; we do not always +bear in mind, that though food may be now superabundant, it is not so at all +seasons of each recurring year. + +I should premise that I use the term Struggle for Existence in a large and +metaphorical sense, including dependence of one being on another, and including +(which is more important) not only the life of the individual, but success in +leaving progeny. Two canine animals in a time of dearth, may be truly said to +struggle with each other which shall get food and live. But a plant on the edge +of a desert is said to struggle for life against the drought, though more +properly it should be said to be dependent on the moisture. A plant which +annually produces a thousand seeds, of which on an average only one comes to +maturity, may be more truly said to struggle with the plants of the same and +other kinds which already clothe the ground. The missletoe is dependent on the +apple and a few other trees, but can only in a far-fetched sense be said to +struggle with these trees, for if too many of these parasites grow on the same +tree, it will languish and die. But several seedling missletoes, growing close +together on the same branch, may more truly be said to struggle with each +other. As the missletoe is disseminated by birds, its existence depends on +birds; and it may metaphorically be said to struggle with other fruit-bearing +plants, in order to tempt birds to devour and thus disseminate its seeds rather +than those of other plants. In these several senses, which pass into each +other, I use for convenience sake the general term of struggle for existence. + +A struggle for existence inevitably follows from the high rate at which all +organic beings tend to increase. Every being, which during its natural lifetime +produces several eggs or seeds, must suffer destruction during some period of +its life, and during some season or occasional year, otherwise, on the +principle of geometrical increase, its numbers would quickly become so +inordinately great that no country could support the product. Hence, as more +individuals are produced than can possibly survive, there must in every case be +a struggle for existence, either one individual with another of the same +species, or with the individuals of distinct species, or with the physical +conditions of life. It is the doctrine of Malthus applied with manifold force +to the whole animal and vegetable kingdoms; for in this case there can be no +artificial increase of food, and no prudential restraint from marriage. +Although some species may be now increasing, more or less rapidly, in numbers, +all cannot do so, for the world would not hold them. + +There is no exception to the rule that every organic being naturally increases +at so high a rate, that if not destroyed, the earth would soon be covered by +the progeny of a single pair. Even slow-breeding man has doubled in twenty-five +years, and at this rate, in a few thousand years, there would literally not be +standing room for his progeny. Linnæus has calculated that if an annual plant +produced only two seeds—and there is no plant so unproductive as this—and their +seedlings next year produced two, and so on, then in twenty years there would +be a million plants. The elephant is reckoned to be the slowest breeder of all +known animals, and I have taken some pains to estimate its probable minimum +rate of natural increase: it will be under the mark to assume that it breeds +when thirty years old, and goes on breeding till ninety years old, bringing +forth three pair of young in this interval; if this be so, at the end of the +fifth century there would be alive fifteen million elephants, descended from +the first pair. + +But we have better evidence on this subject than mere theoretical calculations, +namely, the numerous recorded cases of the astonishingly rapid increase of +various animals in a state of nature, when circumstances have been favourable +to them during two or three following seasons. Still more striking is the +evidence from our domestic animals of many kinds which have run wild in several +parts of the world: if the statements of the rate of increase of slow-breeding +cattle and horses in South America, and latterly in Australia, had not been +well authenticated, they would have been quite incredible. So it is with +plants: cases could be given of introduced plants which have become common +throughout whole islands in a period of less than ten years. Several of the +plants now most numerous over the wide plains of La Plata, clothing square +leagues of surface almost to the exclusion of all other plants, have been +introduced from Europe; and there are plants which now range in India, as I +hear from Dr. Falconer, from Cape Comorin to the Himalaya, which have been +imported from America since its discovery. In such cases, and endless instances +could be given, no one supposes that the fertility of these animals or plants +has been suddenly and temporarily increased in any sensible degree. The obvious +explanation is that the conditions of life have been very favourable, and that +there has consequently been less destruction of the old and young, and that +nearly all the young have been enabled to breed. In such cases the geometrical +ratio of increase, the result of which never fails to be surprising, simply +explains the extraordinarily rapid increase and wide diffusion of naturalised +productions in their new homes. + +In a state of nature almost every plant produces seed, and amongst animals +there are very few which do not annually pair. Hence we may confidently assert, +that all plants and animals are tending to increase at a geometrical ratio, +that all would most rapidly stock every station in which they could any how +exist, and that the geometrical tendency to increase must be checked by +destruction at some period of life. Our familiarity with the larger domestic +animals tends, I think, to mislead us: we see no great destruction falling on +them, and we forget that thousands are annually slaughtered for food, and that +in a state of nature an equal number would have somehow to be disposed of. + +The only difference between organisms which annually produce eggs or seeds by +the thousand, and those which produce extremely few, is, that the slow-breeders +would require a few more years to people, under favourable conditions, a whole +district, let it be ever so large. The condor lays a couple of eggs and the +ostrich a score, and yet in the same country the condor may be the more +numerous of the two: the Fulmar petrel lays but one egg, yet it is believed to +be the most numerous bird in the world. One fly deposits hundreds of eggs, and +another, like the hippobosca, a single one; but this difference does not +determine how many individuals of the two species can be supported in a +district. A large number of eggs is of some importance to those species, which +depend on a rapidly fluctuating amount of food, for it allows them rapidly to +increase in number. But the real importance of a large number of eggs or seeds +is to make up for much destruction at some period of life; and this period in +the great majority of cases is an early one. If an animal can in any way +protect its own eggs or young, a small number may be produced, and yet the +average stock be fully kept up; but if many eggs or young are destroyed, many +must be produced, or the species will become extinct. It would suffice to keep +up the full number of a tree, which lived on an average for a thousand years, +if a single seed were produced once in a thousand years, supposing that this +seed were never destroyed, and could be ensured to germinate in a fitting +place. So that in all cases, the average number of any animal or plant depends +only indirectly on the number of its eggs or seeds. + +In looking at Nature, it is most necessary to keep the foregoing considerations +always in mind—never to forget that every single organic being around us may be +said to be striving to the utmost to increase in numbers; that each lives by a +struggle at some period of its life; that heavy destruction inevitably falls +either on the young or old, during each generation or at recurrent intervals. +Lighten any check, mitigate the destruction ever so little, and the number of +the species will almost instantaneously increase to any amount. The face of +Nature may be compared to a yielding surface, with ten thousand sharp wedges +packed close together and driven inwards by incessant blows, sometimes one +wedge being struck, and then another with greater force. + +What checks the natural tendency of each species to increase in number is most +obscure. Look at the most vigorous species; by as much as it swarms in numbers, +by so much will its tendency to increase be still further increased. We know +not exactly what the checks are in even one single instance. Nor will this +surprise any one who reflects how ignorant we are on this head, even in regard +to mankind, so incomparably better known than any other animal. This subject +has been ably treated by several authors, and I shall, in my future work, +discuss some of the checks at considerable length, more especially in regard to +the feral animals of South America. Here I will make only a few remarks, just +to recall to the reader’s mind some of the chief points. Eggs or very young +animals seem generally to suffer most, but this is not invariably the case. +With plants there is a vast destruction of seeds, but, from some observations +which I have made, I believe that it is the seedlings which suffer most from +germinating in ground already thickly stocked with other plants. Seedlings, +also, are destroyed in vast numbers by various enemies; for instance, on a +piece of ground three feet long and two wide, dug and cleared, and where there +could be no choking from other plants, I marked all the seedlings of our native +weeds as they came up, and out of the 357 no less than 295 were destroyed, +chiefly by slugs and insects. If turf which has long been mown, and the case +would be the same with turf closely browsed by quadrupeds, be let to grow, the +more vigorous plants gradually kill the less vigorous, though fully grown, +plants: thus out of twenty species growing on a little plot of turf (three feet +by four) nine species perished from the other species being allowed to grow up +freely. + +The amount of food for each species of course gives the extreme limit to which +each can increase; but very frequently it is not the obtaining food, but the +serving as prey to other animals, which determines the average numbers of a +species. Thus, there seems to be little doubt that the stock of partridges, +grouse, and hares on any large estate depends chiefly on the destruction of +vermin. If not one head of game were shot during the next twenty years in +England, and, at the same time, if no vermin were destroyed, there would, in +all probability, be less game than at present, although hundreds of thousands +of game animals are now annually killed. On the other hand, in some cases, as +with the elephant and rhinoceros, none are destroyed by beasts of prey: even +the tiger in India most rarely dares to attack a young elephant protected by +its dam. + +Climate plays an important part in determining the average numbers of a +species, and periodical seasons of extreme cold or drought, I believe to be the +most effective of all checks. I estimated that the winter of 1854-55 destroyed +four-fifths of the birds in my own grounds; and this is a tremendous +destruction, when we remember that ten per cent. is an extraordinarily severe +mortality from epidemics with man. The action of climate seems at first sight +to be quite independent of the struggle for existence; but in so far as climate +chiefly acts in reducing food, it brings on the most severe struggle between +the individuals, whether of the same or of distinct species, which subsist on +the same kind of food. Even when climate, for instance extreme cold, acts +directly, it will be the least vigorous, or those which have got least food +through the advancing winter, which will suffer most. When we travel from south +to north, or from a damp region to a dry, we invariably see some species +gradually getting rarer and rarer, and finally disappearing; and the change of +climate being conspicuous, we are tempted to attribute the whole effect to its +direct action. But this is a very false view: we forget that each species, even +where it most abounds, is constantly suffering enormous destruction at some +period of its life, from enemies or from competitors for the same place and +food; and if these enemies or competitors be in the least degree favoured by +any slight change of climate, they will increase in numbers, and, as each area +is already fully stocked with inhabitants, the other species will decrease. +When we travel southward and see a species decreasing in numbers, we may feel +sure that the cause lies quite as much in other species being favoured, as in +this one being hurt. So it is when we travel northward, but in a somewhat +lesser degree, for the number of species of all kinds, and therefore of +competitors, decreases northwards; hence in going northward, or in ascending a +mountain, we far oftener meet with stunted forms, due to the directly injurious +action of climate, than we do in proceeding southwards or in descending a +mountain. When we reach the Arctic regions, or snow-capped summits, or absolute +deserts, the struggle for life is almost exclusively with the elements. + +That climate acts in main part indirectly by favouring other species, we may +clearly see in the prodigious number of plants in our gardens which can +perfectly well endure our climate, but which never become naturalised, for they +cannot compete with our native plants, nor resist destruction by our native +animals. + +When a species, owing to highly favourable circumstances, increases +inordinately in numbers in a small tract, epidemics—at least, this seems +generally to occur with our game animals—often ensue: and here we have a +limiting check independent of the struggle for life. But even some of these +so-called epidemics appear to be due to parasitic worms, which have from some +cause, possibly in part through facility of diffusion amongst the crowded +animals, been disproportionably favoured: and here comes in a sort of struggle +between the parasite and its prey. + +On the other hand, in many cases, a large stock of individuals of the same +species, relatively to the numbers of its enemies, is absolutely necessary for +its preservation. Thus we can easily raise plenty of corn and rape-seed, etc., +in our fields, because the seeds are in great excess compared with the number +of birds which feed on them; nor can the birds, though having a superabundance +of food at this one season, increase in number proportionally to the supply of +seed, as their numbers are checked during winter: but any one who has tried, +knows how troublesome it is to get seed from a few wheat or other such plants +in a garden; I have in this case lost every single seed. This view of the +necessity of a large stock of the same species for its preservation, explains, +I believe, some singular facts in nature, such as that of very rare plants +being sometimes extremely abundant in the few spots where they do occur; and +that of some social plants being social, that is, abounding in individuals, +even on the extreme confines of their range. For in such cases, we may believe, +that a plant could exist only where the conditions of its life were so +favourable that many could exist together, and thus save each other from utter +destruction. I should add that the good effects of frequent intercrossing, and +the ill effects of close interbreeding, probably come into play in some of +these cases; but on this intricate subject I will not here enlarge. + +Many cases are on record showing how complex and unexpected are the checks and +relations between organic beings, which have to struggle together in the same +country. I will give only a single instance, which, though a simple one, has +interested me. In Staffordshire, on the estate of a relation where I had ample +means of investigation, there was a large and extremely barren heath, which had +never been touched by the hand of man; but several hundred acres of exactly the +same nature had been enclosed twenty-five years previously and planted with +Scotch fir. The change in the native vegetation of the planted part of the +heath was most remarkable, more than is generally seen in passing from one +quite different soil to another: not only the proportional numbers of the +heath-plants were wholly changed, but twelve species of plants (not counting +grasses and carices) flourished in the plantations, which could not be found on +the heath. The effect on the insects must have been still greater, for six +insectivorous birds were very common in the plantations, which were not to be +seen on the heath; and the heath was frequented by two or three distinct +insectivorous birds. Here we see how potent has been the effect of the +introduction of a single tree, nothing whatever else having been done, with the +exception that the land had been enclosed, so that cattle could not enter. But +how important an element enclosure is, I plainly saw near Farnham, in Surrey. +Here there are extensive heaths, with a few clumps of old Scotch firs on the +distant hill-tops: within the last ten years large spaces have been enclosed, +and self-sown firs are now springing up in multitudes, so close together that +all cannot live. When I ascertained that these young trees had not been sown or +planted, I was so much surprised at their numbers that I went to several points +of view, whence I could examine hundreds of acres of the unenclosed heath, and +literally I could not see a single Scotch fir, except the old planted clumps. +But on looking closely between the stems of the heath, I found a multitude of +seedlings and little trees, which had been perpetually browsed down by the +cattle. In one square yard, at a point some hundred yards distant from one of +the old clumps, I counted thirty-two little trees; and one of them, judging +from the rings of growth, had during twenty-six years tried to raise its head +above the stems of the heath, and had failed. No wonder that, as soon as the +land was enclosed, it became thickly clothed with vigorously growing young +firs. Yet the heath was so extremely barren and so extensive that no one would +ever have imagined that cattle would have so closely and effectually searched +it for food. + +Here we see that cattle absolutely determine the existence of the Scotch fir; +but in several parts of the world insects determine the existence of cattle. +Perhaps Paraguay offers the most curious instance of this; for here neither +cattle nor horses nor dogs have ever run wild, though they swarm southward and +northward in a feral state; and Azara and Rengger have shown that this is +caused by the greater number in Paraguay of a certain fly, which lays its eggs +in the navels of these animals when first born. The increase of these flies, +numerous as they are, must be habitually checked by some means, probably by +birds. Hence, if certain insectivorous birds (whose numbers are probably +regulated by hawks or beasts of prey) were to increase in Paraguay, the flies +would decrease—then cattle and horses would become feral, and this would +certainly greatly alter (as indeed I have observed in parts of South America) +the vegetation: this again would largely affect the insects; and this, as we +just have seen in Staffordshire, the insectivorous birds, and so onwards in +ever-increasing circles of complexity. We began this series by insectivorous +birds, and we have ended with them. Not that in nature the relations can ever +be as simple as this. Battle within battle must ever be recurring with varying +success; and yet in the long-run the forces are so nicely balanced, that the +face of nature remains uniform for long periods of time, though assuredly the +merest trifle would often give the victory to one organic being over another. +Nevertheless so profound is our ignorance, and so high our presumption, that we +marvel when we hear of the extinction of an organic being; and as we do not see +the cause, we invoke cataclysms to desolate the world, or invent laws on the +duration of the forms of life! + +I am tempted to give one more instance showing how plants and animals, most +remote in the scale of nature, are bound together by a web of complex +relations. I shall hereafter have occasion to show that the exotic Lobelia +fulgens, in this part of England, is never visited by insects, and +consequently, from its peculiar structure, never can set a seed. Many of our +orchidaceous plants absolutely require the visits of moths to remove their +pollen-masses and thus to fertilise them. I have, also, reason to believe that +humble-bees are indispensable to the fertilisation of the heartsease (Viola +tricolor), for other bees do not visit this flower. From experiments which I +have tried, I have found that the visits of bees, if not indispensable, are at +least highly beneficial to the fertilisation of our clovers; but humble-bees +alone visit the common red clover (Trifolium pratense), as other bees cannot +reach the nectar. Hence I have very little doubt, that if the whole genus of +humble-bees became extinct or very rare in England, the heartsease and red +clover would become very rare, or wholly disappear. The number of humble-bees +in any district depends in a great degree on the number of field-mice, which +destroy their combs and nests; and Mr. H. Newman, who has long attended to the +habits of humble-bees, believes that “more than two thirds of them are thus +destroyed all over England.” Now the number of mice is largely dependent, as +every one knows, on the number of cats; and Mr. Newman says, “Near villages and +small towns I have found the nests of humble-bees more numerous than elsewhere, +which I attribute to the number of cats that destroy the mice.” Hence it is +quite credible that the presence of a feline animal in large numbers in a +district might determine, through the intervention first of mice and then of +bees, the frequency of certain flowers in that district! + +In the case of every species, many different checks, acting at different +periods of life, and during different seasons or years, probably come into +play; some one check or some few being generally the most potent, but all +concurring in determining the average number or even the existence of the +species. In some cases it can be shown that widely-different checks act on the +same species in different districts. When we look at the plants and bushes +clothing an entangled bank, we are tempted to attribute their proportional +numbers and kinds to what we call chance. But how false a view is this! Every +one has heard that when an American forest is cut down, a very different +vegetation springs up; but it has been observed that the trees now growing on +the ancient Indian mounds, in the Southern United States, display the same +beautiful diversity and proportion of kinds as in the surrounding virgin +forests. What a struggle between the several kinds of trees must here have gone +on during long centuries, each annually scattering its seeds by the thousand; +what war between insect and insect—between insects, snails, and other animals +with birds and beasts of prey—all striving to increase, and all feeding on each +other or on the trees or their seeds and seedlings, or on the other plants +which first clothed the ground and thus checked the growth of the trees! Throw +up a handful of feathers, and all must fall to the ground according to definite +laws; but how simple is this problem compared to the action and reaction of the +innumerable plants and animals which have determined, in the course of +centuries, the proportional numbers and kinds of trees now growing on the old +Indian ruins! + +The dependency of one organic being on another, as of a parasite on its prey, +lies generally between beings remote in the scale of nature. This is often the +case with those which may strictly be said to struggle with each other for +existence, as in the case of locusts and grass-feeding quadrupeds. But the +struggle almost invariably will be most severe between the individuals of the +same species, for they frequent the same districts, require the same food, and +are exposed to the same dangers. In the case of varieties of the same species, +the struggle will generally be almost equally severe, and we sometimes see the +contest soon decided: for instance, if several varieties of wheat be sown +together, and the mixed seed be resown, some of the varieties which best suit +the soil or climate, or are naturally the most fertile, will beat the others +and so yield more seed, and will consequently in a few years quite supplant the +other varieties. To keep up a mixed stock of even such extremely close +varieties as the variously coloured sweet-peas, they must be each year +harvested separately, and the seed then mixed in due proportion, otherwise the +weaker kinds will steadily decrease in numbers and disappear. So again with the +varieties of sheep: it has been asserted that certain mountain-varieties will +starve out other mountain-varieties, so that they cannot be kept together. The +same result has followed from keeping together different varieties of the +medicinal leech. It may even be doubted whether the varieties of any one of our +domestic plants or animals have so exactly the same strength, habits, and +constitution, that the original proportions of a mixed stock could be kept up +for half a dozen generations, if they were allowed to struggle together, like +beings in a state of nature, and if the seed or young were not annually sorted. + +As species of the same genus have usually, though by no means invariably, some +similarity in habits and constitution, and always in structure, the struggle +will generally be more severe between species of the same genus, when they come +into competition with each other, than between species of distinct genera. We +see this in the recent extension over parts of the United States of one species +of swallow having caused the decrease of another species. The recent increase +of the missel-thrush in parts of Scotland has caused the decrease of the +song-thrush. How frequently we hear of one species of rat taking the place of +another species under the most different climates! In Russia the small Asiatic +cockroach has everywhere driven before it its great congener. One species of +charlock will supplant another, and so in other cases. We can dimly see why the +competition should be most severe between allied forms, which fill nearly the +same place in the economy of nature; but probably in no one case could we +precisely say why one species has been victorious over another in the great +battle of life. + +A corollary of the highest importance may be deduced from the foregoing +remarks, namely, that the structure of every organic being is related, in the +most essential yet often hidden manner, to that of all other organic beings, +with which it comes into competition for food or residence, or from which it +has to escape, or on which it preys. This is obvious in the structure of the +teeth and talons of the tiger; and in that of the legs and claws of the +parasite which clings to the hair on the tiger’s body. But in the beautifully +plumed seed of the dandelion, and in the flattened and fringed legs of the +water-beetle, the relation seems at first confined to the elements of air and +water. Yet the advantage of plumed seeds no doubt stands in the closest +relation to the land being already thickly clothed by other plants; so that the +seeds may be widely distributed and fall on unoccupied ground. In the +water-beetle, the structure of its legs, so well adapted for diving, allows it +to compete with other aquatic insects, to hunt for its own prey, and to escape +serving as prey to other animals. + +The store of nutriment laid up within the seeds of many plants seems at first +sight to have no sort of relation to other plants. But from the strong growth +of young plants produced from such seeds (as peas and beans), when sown in the +midst of long grass, I suspect that the chief use of the nutriment in the seed +is to favour the growth of the young seedling, whilst struggling with other +plants growing vigorously all around. + +Look at a plant in the midst of its range, why does it not double or quadruple +its numbers? We know that it can perfectly well withstand a little more heat or +cold, dampness or dryness, for elsewhere it ranges into slightly hotter or +colder, damper or drier districts. In this case we can clearly see that if we +wished in imagination to give the plant the power of increasing in number, we +should have to give it some advantage over its competitors, or over the animals +which preyed on it. On the confines of its geographical range, a change of +constitution with respect to climate would clearly be an advantage to our +plant; but we have reason to believe that only a few plants or animals range so +far, that they are destroyed by the rigour of the climate alone. Not until we +reach the extreme confines of life, in the arctic regions or on the borders of +an utter desert, will competition cease. The land may be extremely cold or dry, +yet there will be competition between some few species, or between the +individuals of the same species, for the warmest or dampest spots. + +Hence, also, we can see that when a plant or animal is placed in a new country +amongst new competitors, though the climate may be exactly the same as in its +former home, yet the conditions of its life will generally be changed in an +essential manner. If we wished to increase its average numbers in its new home, +we should have to modify it in a different way to what we should have done in +its native country; for we should have to give it some advantage over a +different set of competitors or enemies. + +It is good thus to try in our imagination to give any form some advantage over +another. Probably in no single instance should we know what to do, so as to +succeed. It will convince us of our ignorance on the mutual relations of all +organic beings; a conviction as necessary, as it seems to be difficult to +acquire. All that we can do, is to keep steadily in mind that each organic +being is striving to increase at a geometrical ratio; that each at some period +of its life, during some season of the year, during each generation or at +intervals, has to struggle for life, and to suffer great destruction. When we +reflect on this struggle, we may console ourselves with the full belief, that +the war of nature is not incessant, that no fear is felt, that death is +generally prompt, and that the vigorous, the healthy, and the happy survive and +multiply. + +CHAPTER IV. +NATURAL SELECTION. + +Natural Selection: its power compared with man’s selection, its power on +characters of trifling importance, its power at all ages and on both sexes. +Sexual Selection. On the generality of intercrosses between individuals of the +same species. Circumstances favourable and unfavourable to Natural Selection, +namely, intercrossing, isolation, number of individuals. Slow action. +Extinction caused by Natural Selection. Divergence of Character, related to the +diversity of inhabitants of any small area, and to naturalisation. Action of +Natural Selection, through Divergence of Character and Extinction, on the +descendants from a common parent. Explains the Grouping of all organic beings. + +How will the struggle for existence, discussed too briefly in the last chapter, +act in regard to variation? Can the principle of selection, which we have seen +is so potent in the hands of man, apply in nature? I think we shall see that it +can act most effectually. Let it be borne in mind in what an endless number of +strange peculiarities our domestic productions, and, in a lesser degree, those +under nature, vary; and how strong the hereditary tendency is. Under +domestication, it may be truly said that the whole organisation becomes in some +degree plastic. Let it be borne in mind how infinitely complex and +close-fitting are the mutual relations of all organic beings to each other and +to their physical conditions of life. Can it, then, be thought improbable, +seeing that variations useful to man have undoubtedly occurred, that other +variations useful in some way to each being in the great and complex battle of +life, should sometimes occur in the course of thousands of generations? If such +do occur, can we doubt (remembering that many more individuals are born than +can possibly survive) that individuals having any advantage, however slight, +over others, would have the best chance of surviving and of procreating their +kind? On the other hand, we may feel sure that any variation in the least +degree injurious would be rigidly destroyed. This preservation of favourable +variations and the rejection of injurious variations, I call Natural Selection. +Variations neither useful nor injurious would not be affected by natural +selection, and would be left a fluctuating element, as perhaps we see in the +species called polymorphic. + +We shall best understand the probable course of natural selection by taking the +case of a country undergoing some physical change, for instance, of climate. +The proportional numbers of its inhabitants would almost immediately undergo a +change, and some species might become extinct. We may conclude, from what we +have seen of the intimate and complex manner in which the inhabitants of each +country are bound together, that any change in the numerical proportions of +some of the inhabitants, independently of the change of climate itself, would +most seriously affect many of the others. If the country were open on its +borders, new forms would certainly immigrate, and this also would seriously +disturb the relations of some of the former inhabitants. Let it be remembered +how powerful the influence of a single introduced tree or mammal has been shown +to be. But in the case of an island, or of a country partly surrounded by +barriers, into which new and better adapted forms could not freely enter, we +should then have places in the economy of nature which would assuredly be +better filled up, if some of the original inhabitants were in some manner +modified; for, had the area been open to immigration, these same places would +have been seized on by intruders. In such case, every slight modification, +which in the course of ages chanced to arise, and which in any way favoured the +individuals of any of the species, by better adapting them to their altered +conditions, would tend to be preserved; and natural selection would thus have +free scope for the work of improvement. + +We have reason to believe, as stated in the first chapter, that a change in the +conditions of life, by specially acting on the reproductive system, causes or +increases variability; and in the foregoing case the conditions of life are +supposed to have undergone a change, and this would manifestly be favourable to +natural selection, by giving a better chance of profitable variations +occurring; and unless profitable variations do occur, natural selection can do +nothing. Not that, as I believe, any extreme amount of variability is +necessary; as man can certainly produce great results by adding up in any given +direction mere individual differences, so could Nature, but far more easily, +from having incomparably longer time at her disposal. Nor do I believe that any +great physical change, as of climate, or any unusual degree of isolation to +check immigration, is actually necessary to produce new and unoccupied places +for natural selection to fill up by modifying and improving some of the varying +inhabitants. For as all the inhabitants of each country are struggling together +with nicely balanced forces, extremely slight modifications in the structure or +habits of one inhabitant would often give it an advantage over others; and +still further modifications of the same kind would often still further increase +the advantage. No country can be named in which all the native inhabitants are +now so perfectly adapted to each other and to the physical conditions under +which they live, that none of them could anyhow be improved; for in all +countries, the natives have been so far conquered by naturalised productions, +that they have allowed foreigners to take firm possession of the land. And as +foreigners have thus everywhere beaten some of the natives, we may safely +conclude that the natives might have been modified with advantage, so as to +have better resisted such intruders. + +As man can produce and certainly has produced a great result by his methodical +and unconscious means of selection, what may not nature effect? Man can act +only on external and visible characters: nature cares nothing for appearances, +except in so far as they may be useful to any being. She can act on every +internal organ, on every shade of constitutional difference, on the whole +machinery of life. Man selects only for his own good; Nature only for that of +the being which she tends. Every selected character is fully exercised by her; +and the being is placed under well-suited conditions of life. Man keeps the +natives of many climates in the same country; he seldom exercises each selected +character in some peculiar and fitting manner; he feeds a long and a short +beaked pigeon on the same food; he does not exercise a long-backed or +long-legged quadruped in any peculiar manner; he exposes sheep with long and +short wool to the same climate. He does not allow the most vigorous males to +struggle for the females. He does not rigidly destroy all inferior animals, but +protects during each varying season, as far as lies in his power, all his +productions. He often begins his selection by some half-monstrous form; or at +least by some modification prominent enough to catch his eye, or to be plainly +useful to him. Under nature, the slightest difference of structure or +constitution may well turn the nicely-balanced scale in the struggle for life, +and so be preserved. How fleeting are the wishes and efforts of man! how short +his time! and consequently how poor will his products be, compared with those +accumulated by nature during whole geological periods. Can we wonder, then, +that nature’s productions should be far “truer” in character than man’s +productions; that they should be infinitely better adapted to the most complex +conditions of life, and should plainly bear the stamp of far higher +workmanship? + +It may be said that natural selection is daily and hourly scrutinising, +throughout the world, every variation, even the slightest; rejecting that which +is bad, preserving and adding up all that is good; silently and insensibly +working, whenever and wherever opportunity offers, at the improvement of each +organic being in relation to its organic and inorganic conditions of life. We +see nothing of these slow changes in progress, until the hand of time has +marked the long lapse of ages, and then so imperfect is our view into long past +geological ages, that we only see that the forms of life are now different from +what they formerly were. + +Although natural selection can act only through and for the good of each being, +yet characters and structures, which we are apt to consider as of very trifling +importance, may thus be acted on. When we see leaf-eating insects green, and +bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse +the colour of heather, and the black-grouse that of peaty earth, we must +believe that these tints are of service to these birds and insects in +preserving them from danger. Grouse, if not destroyed at some period of their +lives, would increase in countless numbers; they are known to suffer largely +from birds of prey; and hawks are guided by eyesight to their prey,—so much so, +that on parts of the Continent persons are warned not to keep white pigeons, as +being the most liable to destruction. Hence I can see no reason to doubt that +natural selection might be most effective in giving the proper colour to each +kind of grouse, and in keeping that colour, when once acquired, true and +constant. Nor ought we to think that the occasional destruction of an animal of +any particular colour would produce little effect: we should remember how +essential it is in a flock of white sheep to destroy every lamb with the +faintest trace of black. In plants the down on the fruit and the colour of the +flesh are considered by botanists as characters of the most trifling +importance: yet we hear from an excellent horticulturist, Downing, that in the +United States smooth-skinned fruits suffer far more from a beetle, a curculio, +than those with down; that purple plums suffer far more from a certain disease +than yellow plums; whereas another disease attacks yellow-fleshed peaches far +more than those with other coloured flesh. If, with all the aids of art, these +slight differences make a great difference in cultivating the several +varieties, assuredly, in a state of nature, where the trees would have to +struggle with other trees and with a host of enemies, such differences would +effectually settle which variety, whether a smooth or downy, a yellow or purple +fleshed fruit, should succeed. + +In looking at many small points of difference between species, which, as far as +our ignorance permits us to judge, seem to be quite unimportant, we must not +forget that climate, food, etc., probably produce some slight and direct +effect. It is, however, far more necessary to bear in mind that there are many +unknown laws of correlation of growth, which, when one part of the organisation +is modified through variation, and the modifications are accumulated by natural +selection for the good of the being, will cause other modifications, often of +the most unexpected nature. + +As we see that those variations which under domestication appear at any +particular period of life, tend to reappear in the offspring at the same +period;—for instance, in the seeds of the many varieties of our culinary and +agricultural plants; in the caterpillar and cocoon stages of the varieties of +the silkworm; in the eggs of poultry, and in the colour of the down of their +chickens; in the horns of our sheep and cattle when nearly adult;—so in a state +of nature, natural selection will be enabled to act on and modify organic +beings at any age, by the accumulation of profitable variations at that age, +and by their inheritance at a corresponding age. If it profit a plant to have +its seeds more and more widely disseminated by the wind, I can see no greater +difficulty in this being effected through natural selection, than in the +cotton-planter increasing and improving by selection the down in the pods on +his cotton-trees. Natural selection may modify and adapt the larva of an insect +to a score of contingencies, wholly different from those which concern the +mature insect. These modifications will no doubt affect, through the laws of +correlation, the structure of the adult; and probably in the case of those +insects which live only for a few hours, and which never feed, a large part of +their structure is merely the correlated result of successive changes in the +structure of their larvæ. So, conversely, modifications in the adult will +probably often affect the structure of the larva; but in all cases natural +selection will ensure that modifications consequent on other modifications at a +different period of life, shall not be in the least degree injurious: for if +they became so, they would cause the extinction of the species. + +Natural selection will modify the structure of the young in relation to the +parent, and of the parent in relation to the young. In social animals it will +adapt the structure of each individual for the benefit of the community; if +each in consequence profits by the selected change. What natural selection +cannot do, is to modify the structure of one species, without giving it any +advantage, for the good of another species; and though statements to this +effect may be found in works of natural history, I cannot find one case which +will bear investigation. A structure used only once in an animal’s whole life, +if of high importance to it, might be modified to any extent by natural +selection; for instance, the great jaws possessed by certain insects, and used +exclusively for opening the cocoon—or the hard tip to the beak of nestling +birds, used for breaking the egg. It has been asserted, that of the best +short-beaked tumbler-pigeons more perish in the egg than are able to get out of +it; so that fanciers assist in the act of hatching. Now, if nature had to make +the beak of a full-grown pigeon very short for the bird’s own advantage, the +process of modification would be very slow, and there would be simultaneously +the most rigorous selection of the young birds within the egg, which had the +most powerful and hardest beaks, for all with weak beaks would inevitably +perish: or, more delicate and more easily broken shells might be selected, the +thickness of the shell being known to vary like every other structure. + +Sexual Selection.—Inasmuch as peculiarities often appear under domestication in +one sex and become hereditarily attached to that sex, the same fact probably +occurs under nature, and if so, natural selection will be able to modify one +sex in its functional relations to the other sex, or in relation to wholly +different habits of life in the two sexes, as is sometimes the case with +insects. And this leads me to say a few words on what I call Sexual Selection. +This depends, not on a struggle for existence, but on a struggle between the +males for possession of the females; the result is not death to the +unsuccessful competitor, but few or no offspring. Sexual selection is, +therefore, less rigorous than natural selection. Generally, the most vigorous +males, those which are best fitted for their places in nature, will leave most +progeny. But in many cases, victory will depend not on general vigour, but on +having special weapons, confined to the male sex. A hornless stag or spurless +cock would have a poor chance of leaving offspring. Sexual selection by always +allowing the victor to breed might surely give indomitable courage, length to +the spur, and strength to the wing to strike in the spurred leg, as well as the +brutal cock-fighter, who knows well that he can improve his breed by careful +selection of the best cocks. How low in the scale of nature this law of battle +descends, I know not; male alligators have been described as fighting, +bellowing, and whirling round, like Indians in a war-dance, for the possession +of the females; male salmons have been seen fighting all day long; male +stag-beetles often bear wounds from the huge mandibles of other males. The war +is, perhaps, severest between the males of polygamous animals, and these seem +oftenest provided with special weapons. The males of carnivorous animals are +already well armed; though to them and to others, special means of defence may +be given through means of sexual selection, as the mane to the lion, the +shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield +may be as important for victory, as the sword or spear. + +Amongst birds, the contest is often of a more peaceful character. All those who +have attended to the subject, believe that there is the severest rivalry +between the males of many species to attract by singing the females. The +rock-thrush of Guiana, birds of Paradise, and some others, congregate; and +successive males display their gorgeous plumage and perform strange antics +before the females, which standing by as spectators, at last choose the most +attractive partner. Those who have closely attended to birds in confinement +well know that they often take individual preferences and dislikes: thus Sir R. +Heron has described how one pied peacock was eminently attractive to all his +hen birds. It may appear childish to attribute any effect to such apparently +weak means: I cannot here enter on the details necessary to support this view; +but if man can in a short time give elegant carriage and beauty to his bantams, +according to his standard of beauty, I can see no good reason to doubt that +female birds, by selecting, during thousands of generations, the most melodious +or beautiful males, according to their standard of beauty, might produce a +marked effect. I strongly suspect that some well-known laws with respect to the +plumage of male and female birds, in comparison with the plumage of the young, +can be explained on the view of plumage having been chiefly modified by sexual +selection, acting when the birds have come to the breeding age or during the +breeding season; the modifications thus produced being inherited at +corresponding ages or seasons, either by the males alone, or by the males and +females; but I have not space here to enter on this subject. + +Thus it is, as I believe, that when the males and females of any animal have +the same general habits of life, but differ in structure, colour, or ornament, +such differences have been mainly caused by sexual selection; that is, +individual males have had, in successive generations, some slight advantage +over other males, in their weapons, means of defence, or charms; and have +transmitted these advantages to their male offspring. Yet, I would not wish to +attribute all such sexual differences to this agency: for we see peculiarities +arising and becoming attached to the male sex in our domestic animals (as the +wattle in male carriers, horn-like protuberances in the cocks of certain fowls, +etc.), which we cannot believe to be either useful to the males in battle, or +attractive to the females. We see analogous cases under nature, for instance, +the tuft of hair on the breast of the turkey-cock, which can hardly be either +useful or ornamental to this bird;—indeed, had the tuft appeared under +domestication, it would have been called a monstrosity. + +Illustrations of the action of Natural Selection.—In order to make it clear +how, as I believe, natural selection acts, I must beg permission to give one or +two imaginary illustrations. Let us take the case of a wolf, which preys on +various animals, securing some by craft, some by strength, and some by +fleetness; and let us suppose that the fleetest prey, a deer for instance, had +from any change in the country increased in numbers, or that other prey had +decreased in numbers, during that season of the year when the wolf is hardest +pressed for food. I can under such circumstances see no reason to doubt that +the swiftest and slimmest wolves would have the best chance of surviving, and +so be preserved or selected,—provided always that they retained strength to +master their prey at this or at some other period of the year, when they might +be compelled to prey on other animals. I can see no more reason to doubt this, +than that man can improve the fleetness of his greyhounds by careful and +methodical selection, or by that unconscious selection which results from each +man trying to keep the best dogs without any thought of modifying the breed. + +Even without any change in the proportional numbers of the animals on which our +wolf preyed, a cub might be born with an innate tendency to pursue certain +kinds of prey. Nor can this be thought very improbable; for we often observe +great differences in the natural tendencies of our domestic animals; one cat, +for instance, taking to catch rats, another mice; one cat, according to Mr. St. +John, bringing home winged game, another hares or rabbits, and another hunting +on marshy ground and almost nightly catching woodcocks or snipes. The tendency +to catch rats rather than mice is known to be inherited. Now, if any slight +innate change of habit or of structure benefited an individual wolf, it would +have the best chance of surviving and of leaving offspring. Some of its young +would probably inherit the same habits or structure, and by the repetition of +this process, a new variety might be formed which would either supplant or +coexist with the parent-form of wolf. Or, again, the wolves inhabiting a +mountainous district, and those frequenting the lowlands, would naturally be +forced to hunt different prey; and from the continued preservation of the +individuals best fitted for the two sites, two varieties might slowly be +formed. These varieties would cross and blend where they met; but to this +subject of intercrossing we shall soon have to return. I may add, that, +according to Mr. Pierce, there are two varieties of the wolf inhabiting the +Catskill Mountains in the United States, one with a light greyhound-like form, +which pursues deer, and the other more bulky, with shorter legs, which more +frequently attacks the shepherd’s flocks. + +Let us now take a more complex case. Certain plants excrete a sweet juice, +apparently for the sake of eliminating something injurious from their sap: this +is effected by glands at the base of the stipules in some Leguminosæ, and at +the back of the leaf of the common laurel. This juice, though small in +quantity, is greedily sought by insects. Let us now suppose a little sweet +juice or nectar to be excreted by the inner bases of the petals of a flower. In +this case insects in seeking the nectar would get dusted with pollen, and would +certainly often transport the pollen from one flower to the stigma of another +flower. The flowers of two distinct individuals of the same species would thus +get crossed; and the act of crossing, we have good reason to believe (as will +hereafter be more fully alluded to), would produce very vigorous seedlings, +which consequently would have the best chance of flourishing and surviving. +Some of these seedlings would probably inherit the nectar-excreting power. +Those individual flowers which had the largest glands or nectaries, and which +excreted most nectar, would be oftenest visited by insects, and would be +oftenest crossed; and so in the long-run would gain the upper hand. Those +flowers, also, which had their stamens and pistils placed, in relation to the +size and habits of the particular insects which visited them, so as to favour +in any degree the transportal of their pollen from flower to flower, would +likewise be favoured or selected. We might have taken the case of insects +visiting flowers for the sake of collecting pollen instead of nectar; and as +pollen is formed for the sole object of fertilisation, its destruction appears +a simple loss to the plant; yet if a little pollen were carried, at first +occasionally and then habitually, by the pollen-devouring insects from flower +to flower, and a cross thus effected, although nine-tenths of the pollen were +destroyed, it might still be a great gain to the plant; and those individuals +which produced more and more pollen, and had larger and larger anthers, would +be selected. + +When our plant, by this process of the continued preservation or natural +selection of more and more attractive flowers, had been rendered highly +attractive to insects, they would, unintentionally on their part, regularly +carry pollen from flower to flower; and that they can most effectually do this, +I could easily show by many striking instances. I will give only one—not as a +very striking case, but as likewise illustrating one step in the separation of +the sexes of plants, presently to be alluded to. Some holly-trees bear only +male flowers, which have four stamens producing rather a small quantity of +pollen, and a rudimentary pistil; other holly-trees bear only female flowers; +these have a full-sized pistil, and four stamens with shrivelled anthers, in +which not a grain of pollen can be detected. Having found a female tree exactly +sixty yards from a male tree, I put the stigmas of twenty flowers, taken from +different branches, under the microscope, and on all, without exception, there +were pollen-grains, and on some a profusion of pollen. As the wind had set for +several days from the female to the male tree, the pollen could not thus have +been carried. The weather had been cold and boisterous, and therefore not +favourable to bees, nevertheless every female flower which I examined had been +effectually fertilised by the bees, accidentally dusted with pollen, having +flown from tree to tree in search of nectar. But to return to our imaginary +case: as soon as the plant had been rendered so highly attractive to insects +that pollen was regularly carried from flower to flower, another process might +commence. No naturalist doubts the advantage of what has been called the +“physiological division of labour;” hence we may believe that it would be +advantageous to a plant to produce stamens alone in one flower or on one whole +plant, and pistils alone in another flower or on another plant. In plants under +culture and placed under new conditions of life, sometimes the male organs and +sometimes the female organs become more or less impotent; now if we suppose +this to occur in ever so slight a degree under nature, then as pollen is +already carried regularly from flower to flower, and as a more complete +separation of the sexes of our plant would be advantageous on the principle of +the division of labour, individuals with this tendency more and more increased, +would be continually favoured or selected, until at last a complete separation +of the sexes would be effected. + +Let us now turn to the nectar-feeding insects in our imaginary case: we may +suppose the plant of which we have been slowly increasing the nectar by +continued selection, to be a common plant; and that certain insects depended in +main part on its nectar for food. I could give many facts, showing how anxious +bees are to save time; for instance, their habit of cutting holes and sucking +the nectar at the bases of certain flowers, which they can, with a very little +more trouble, enter by the mouth. Bearing such facts in mind, I can see no +reason to doubt that an accidental deviation in the size and form of the body, +or in the curvature and length of the proboscis, etc., far too slight to be +appreciated by us, might profit a bee or other insect, so that an individual so +characterised would be able to obtain its food more quickly, and so have a +better chance of living and leaving descendants. Its descendants would probably +inherit a tendency to a similar slight deviation of structure. The tubes of the +corollas of the common red and incarnate clovers (Trifolium pratense and +incarnatum) do not on a hasty glance appear to differ in length; yet the +hive-bee can easily suck the nectar out of the incarnate clover, but not out of +the common red clover, which is visited by humble-bees alone; so that whole +fields of the red clover offer in vain an abundant supply of precious nectar to +the hive-bee. Thus it might be a great advantage to the hive-bee to have a +slightly longer or differently constructed proboscis. On the other hand, I have +found by experiment that the fertility of clover greatly depends on bees +visiting and moving parts of the corolla, so as to push the pollen on to the +stigmatic surface. Hence, again, if humble-bees were to become rare in any +country, it might be a great advantage to the red clover to have a shorter or +more deeply divided tube to its corolla, so that the hive-bee could visit its +flowers. Thus I can understand how a flower and a bee might slowly become, +either simultaneously or one after the other, modified and adapted in the most +perfect manner to each other, by the continued preservation of individuals +presenting mutual and slightly favourable deviations of structure. + +I am well aware that this doctrine of natural selection, exemplified in the +above imaginary instances, is open to the same objections which were at first +urged against Sir Charles Lyell’s noble views on “the modern changes of the +earth, as illustrative of geology;” but we now very seldom hear the action, for +instance, of the coast-waves, called a trifling and insignificant cause, when +applied to the excavation of gigantic valleys or to the formation of the +longest lines of inland cliffs. Natural selection can act only by the +preservation and accumulation of infinitesimally small inherited modifications, +each profitable to the preserved being; and as modern geology has almost +banished such views as the excavation of a great valley by a single diluvial +wave, so will natural selection, if it be a true principle, banish the belief +of the continued creation of new organic beings, or of any great and sudden +modification in their structure. + +On the Intercrossing of Individuals.—I must here introduce a short digression. +In the case of animals and plants with separated sexes, it is of course obvious +that two individuals must always unite for each birth; but in the case of +hermaphrodites this is far from obvious. Nevertheless I am strongly inclined to +believe that with all hermaphrodites two individuals, either occasionally or +habitually, concur for the reproduction of their kind. This view, I may add, +was first suggested by Andrew Knight. We shall presently see its importance; +but I must here treat the subject with extreme brevity, though I have the +materials prepared for an ample discussion. All vertebrate animals, all +insects, and some other large groups of animals, pair for each birth. Modern +research has much diminished the number of supposed hermaphrodites, and of real +hermaphrodites a large number pair; that is, two individuals regularly unite +for reproduction, which is all that concerns us. But still there are many +hermaphrodite animals which certainly do not habitually pair, and a vast +majority of plants are hermaphrodites. What reason, it may be asked, is there +for supposing in these cases that two individuals ever concur in reproduction? +As it is impossible here to enter on details, I must trust to some general +considerations alone. + +In the first place, I have collected so large a body of facts, showing, in +accordance with the almost universal belief of breeders, that with animals and +plants a cross between different varieties, or between individuals of the same +variety but of another strain, gives vigour and fertility to the offspring; and +on the other hand, that close interbreeding diminishes vigour and fertility; +that these facts alone incline me to believe that it is a general law of nature +(utterly ignorant though we be of the meaning of the law) that no organic being +self-fertilises itself for an eternity of generations; but that a cross with +another individual is occasionally—perhaps at very long +intervals—indispensable. + +On the belief that this is a law of nature, we can, I think, understand several +large classes of facts, such as the following, which on any other view are +inexplicable. Every hybridizer knows how unfavourable exposure to wet is to the +fertilisation of a flower, yet what a multitude of flowers have their anthers +and stigmas fully exposed to the weather! but if an occasional cross be +indispensable, the fullest freedom for the entrance of pollen from another +individual will explain this state of exposure, more especially as the plant’s +own anthers and pistil generally stand so close together that +self-fertilisation seems almost inevitable. Many flowers, on the other hand, +have their organs of fructification closely enclosed, as in the great +papilionaceous or pea-family; but in several, perhaps in all, such flowers, +there is a very curious adaptation between the structure of the flower and the +manner in which bees suck the nectar; for, in doing this, they either push the +flower’s own pollen on the stigma, or bring pollen from another flower. So +necessary are the visits of bees to papilionaceous flowers, that I have found, +by experiments published elsewhere, that their fertility is greatly diminished +if these visits be prevented. Now, it is scarcely possible that bees should fly +from flower to flower, and not carry pollen from one to the other, to the great +good, as I believe, of the plant. Bees will act like a camel-hair pencil, and +it is quite sufficient just to touch the anthers of one flower and then the +stigma of another with the same brush to ensure fertilisation; but it must not +be supposed that bees would thus produce a multitude of hybrids between +distinct species; for if you bring on the same brush a plant’s own pollen and +pollen from another species, the former will have such a prepotent effect, that +it will invariably and completely destroy, as has been shown by Gärtner, any +influence from the foreign pollen. + +When the stamens of a flower suddenly spring towards the pistil, or slowly move +one after the other towards it, the contrivance seems adapted solely to ensure +self-fertilisation; and no doubt it is useful for this end: but, the agency of +insects is often required to cause the stamens to spring forward, as Kölreuter +has shown to be the case with the barberry; and curiously in this very genus, +which seems to have a special contrivance for self-fertilisation, it is well +known that if very closely-allied forms or varieties are planted near each +other, it is hardly possible to raise pure seedlings, so largely do they +naturally cross. In many other cases, far from there being any aids for +self-fertilisation, there are special contrivances, as I could show from the +writings of C. C. Sprengel and from my own observations, which effectually +prevent the stigma receiving pollen from its own flower: for instance, in +Lobelia fulgens, there is a really beautiful and elaborate contrivance by which +every one of the infinitely numerous pollen-granules are swept out of the +conjoined anthers of each flower, before the stigma of that individual flower +is ready to receive them; and as this flower is never visited, at least in my +garden, by insects, it never sets a seed, though by placing pollen from one +flower on the stigma of another, I raised plenty of seedlings; and whilst +another species of Lobelia growing close by, which is visited by bees, seeds +freely. In very many other cases, though there be no special mechanical +contrivance to prevent the stigma of a flower receiving its own pollen, yet, as +C. C. Sprengel has shown, and as I can confirm, either the anthers burst before +the stigma is ready for fertilisation, or the stigma is ready before the pollen +of that flower is ready, so that these plants have in fact separated sexes, and +must habitually be crossed. How strange are these facts! How strange that the +pollen and stigmatic surface of the same flower, though placed so close +together, as if for the very purpose of self-fertilisation, should in so many +cases be mutually useless to each other! How simply are these facts explained +on the view of an occasional cross with a distinct individual being +advantageous or indispensable! + +If several varieties of the cabbage, radish, onion, and of some other plants, +be allowed to seed near each other, a large majority, as I have found, of the +seedlings thus raised will turn out mongrels: for instance, I raised 233 +seedling cabbages from some plants of different varieties growing near each +other, and of these only 78 were true to their kind, and some even of these +were not perfectly true. Yet the pistil of each cabbage-flower is surrounded +not only by its own six stamens, but by those of the many other flowers on the +same plant. How, then, comes it that such a vast number of the seedlings are +mongrelized? I suspect that it must arise from the pollen of a distinct variety +having a prepotent effect over a flower’s own pollen; and that this is part of +the general law of good being derived from the intercrossing of distinct +individuals of the same species. When distinct species are crossed the case is +directly the reverse, for a plant’s own pollen is always prepotent over foreign +pollen; but to this subject we shall return in a future chapter. + +In the case of a gigantic tree covered with innumerable flowers, it may be +objected that pollen could seldom be carried from tree to tree, and at most +only from flower to flower on the same tree, and that flowers on the same tree +can be considered as distinct individuals only in a limited sense. I believe +this objection to be valid, but that nature has largely provided against it by +giving to trees a strong tendency to bear flowers with separated sexes. When +the sexes are separated, although the male and female flowers may be produced +on the same tree, we can see that pollen must be regularly carried from flower +to flower; and this will give a better chance of pollen being occasionally +carried from tree to tree. That trees belonging to all Orders have their sexes +more often separated than other plants, I find to be the case in this country; +and at my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa +Gray those of the United States, and the result was as I anticipated. On the +other hand, Dr. Hooker has recently informed me that he finds that the rule +does not hold in Australia; and I have made these few remarks on the sexes of +trees simply to call attention to the subject. + +Turning for a very brief space to animals: on the land there are some +hermaphrodites, as land-mollusca and earth-worms; but these all pair. As yet I +have not found a single case of a terrestrial animal which fertilises itself. +We can understand this remarkable fact, which offers so strong a contrast with +terrestrial plants, on the view of an occasional cross being indispensable, by +considering the medium in which terrestrial animals live, and the nature of the +fertilising element; for we know of no means, analogous to the action of +insects and of the wind in the case of plants, by which an occasional cross +could be effected with terrestrial animals without the concurrence of two +individuals. Of aquatic animals, there are many self-fertilising +hermaphrodites; but here currents in the water offer an obvious means for an +occasional cross. And, as in the case of flowers, I have as yet failed, after +consultation with one of the highest authorities, namely, Professor Huxley, to +discover a single case of an hermaphrodite animal with the organs of +reproduction so perfectly enclosed within the body, that access from without +and the occasional influence of a distinct individual can be shown to be +physically impossible. Cirripedes long appeared to me to present a case of very +great difficulty under this point of view; but I have been enabled, by a +fortunate chance, elsewhere to prove that two individuals, though both are +self-fertilising hermaphrodites, do sometimes cross. + +It must have struck most naturalists as a strange anomaly that, in the case of +both animals and plants, species of the same family and even of the same genus, +though agreeing closely with each other in almost their whole organisation, yet +are not rarely, some of them hermaphrodites, and some of them unisexual. But +if, in fact, all hermaphrodites do occasionally intercross with other +individuals, the difference between hermaphrodites and unisexual species, as +far as function is concerned, becomes very small. + +From these several considerations and from the many special facts which I have +collected, but which I am not here able to give, I am strongly inclined to +suspect that, both in the vegetable and animal kingdoms, an occasional +intercross with a distinct individual is a law of nature. I am well aware that +there are, on this view, many cases of difficulty, some of which I am trying to +investigate. Finally then, we may conclude that in many organic beings, a cross +between two individuals is an obvious necessity for each birth; in many others +it occurs perhaps only at long intervals; but in none, as I suspect, can +self-fertilisation go on for perpetuity. + +Circumstances favourable to Natural Selection.—This is an extremely intricate +subject. A large amount of inheritable and diversified variability is +favourable, but I believe mere individual differences suffice for the work. A +large number of individuals, by giving a better chance for the appearance +within any given period of profitable variations, will compensate for a lesser +amount of variability in each individual, and is, I believe, an extremely +important element of success. Though nature grants vast periods of time for the +work of natural selection, she does not grant an indefinite period; for as all +organic beings are striving, it may be said, to seize on each place in the +economy of nature, if any one species does not become modified and improved in +a corresponding degree with its competitors, it will soon be exterminated. + +In man’s methodical selection, a breeder selects for some definite object, and +free intercrossing will wholly stop his work. But when many men, without +intending to alter the breed, have a nearly common standard of perfection, and +all try to get and breed from the best animals, much improvement and +modification surely but slowly follow from this unconscious process of +selection, notwithstanding a large amount of crossing with inferior animals. +Thus it will be in nature; for within a confined area, with some place in its +polity not so perfectly occupied as might be, natural selection will always +tend to preserve all the individuals varying in the right direction, though in +different degrees, so as better to fill up the unoccupied place. But if the +area be large, its several districts will almost certainly present different +conditions of life; and then if natural selection be modifying and improving a +species in the several districts, there will be intercrossing with the other +individuals of the same species on the confines of each. And in this case the +effects of intercrossing can hardly be counterbalanced by natural selection +always tending to modify all the individuals in each district in exactly the +same manner to the conditions of each; for in a continuous area, the conditions +will generally graduate away insensibly from one district to another. The +intercrossing will most affect those animals which unite for each birth, which +wander much, and which do not breed at a very quick rate. Hence in animals of +this nature, for instance in birds, varieties will generally be confined to +separated countries; and this I believe to be the case. In hermaphrodite +organisms which cross only occasionally, and likewise in animals which unite +for each birth, but which wander little and which can increase at a very rapid +rate, a new and improved variety might be quickly formed on any one spot, and +might there maintain itself in a body, so that whatever intercrossing took +place would be chiefly between the individuals of the same new variety. A local +variety when once thus formed might subsequently slowly spread to other +districts. On the above principle, nurserymen always prefer getting seed from a +large body of plants of the same variety, as the chance of intercrossing with +other varieties is thus lessened. + +Even in the case of slow-breeding animals, which unite for each birth, we must +not overrate the effects of intercrosses in retarding natural selection; for I +can bring a considerable catalogue of facts, showing that within the same area, +varieties of the same animal can long remain distinct, from haunting different +stations, from breeding at slightly different seasons, or from varieties of the +same kind preferring to pair together. + +Intercrossing plays a very important part in nature in keeping the individuals +of the same species, or of the same variety, true and uniform in character. It +will obviously thus act far more efficiently with those animals which unite for +each birth; but I have already attempted to show that we have reason to believe +that occasional intercrosses take place with all animals and with all plants. +Even if these take place only at long intervals, I am convinced that the young +thus produced will gain so much in vigour and fertility over the offspring from +long-continued self-fertilisation, that they will have a better chance of +surviving and propagating their kind; and thus, in the long run, the influence +of intercrosses, even at rare intervals, will be great. If there exist organic +beings which never intercross, uniformity of character can be retained amongst +them, as long as their conditions of life remain the same, only through the +principle of inheritance, and through natural selection destroying any which +depart from the proper type; but if their conditions of life change and they +undergo modification, uniformity of character can be given to their modified +offspring, solely by natural selection preserving the same favourable +variations. + +Isolation, also, is an important element in the process of natural selection. +In a confined or isolated area, if not very large, the organic and inorganic +conditions of life will generally be in a great degree uniform; so that natural +selection will tend to modify all the individuals of a varying species +throughout the area in the same manner in relation to the same conditions. +Intercrosses, also, with the individuals of the same species, which otherwise +would have inhabited the surrounding and differently circumstanced districts, +will be prevented. But isolation probably acts more efficiently in checking the +immigration of better adapted organisms, after any physical change, such as of +climate or elevation of the land, etc.; and thus new places in the natural +economy of the country are left open for the old inhabitants to struggle for, +and become adapted to, through modifications in their structure and +constitution. Lastly, isolation, by checking immigration and consequently +competition, will give time for any new variety to be slowly improved; and this +may sometimes be of importance in the production of new species. If, however, +an isolated area be very small, either from being surrounded by barriers, or +from having very peculiar physical conditions, the total number of the +individuals supported on it will necessarily be very small; and fewness of +individuals will greatly retard the production of new species through natural +selection, by decreasing the chance of the appearance of favourable variations. + +If we turn to nature to test the truth of these remarks, and look at any small +isolated area, such as an oceanic island, although the total number of the +species inhabiting it, will be found to be small, as we shall see in our +chapter on geographical distribution; yet of these species a very large +proportion are endemic,—that is, have been produced there, and nowhere else. +Hence an oceanic island at first sight seems to have been highly favourable for +the production of new species. But we may thus greatly deceive ourselves, for +to ascertain whether a small isolated area, or a large open area like a +continent, has been most favourable for the production of new organic forms, we +ought to make the comparison within equal times; and this we are incapable of +doing. + +Although I do not doubt that isolation is of considerable importance in the +production of new species, on the whole I am inclined to believe that largeness +of area is of more importance, more especially in the production of species, +which will prove capable of enduring for a long period, and of spreading +widely. Throughout a great and open area, not only will there be a better +chance of favourable variations arising from the large number of individuals of +the same species there supported, but the conditions of life are infinitely +complex from the large number of already existing species; and if some of these +many species become modified and improved, others will have to be improved in a +corresponding degree or they will be exterminated. Each new form, also, as soon +as it has been much improved, will be able to spread over the open and +continuous area, and will thus come into competition with many others. Hence +more new places will be formed, and the competition to fill them will be more +severe, on a large than on a small and isolated area. Moreover, great areas, +though now continuous, owing to oscillations of level, will often have recently +existed in a broken condition, so that the good effects of isolation will +generally, to a certain extent, have concurred. Finally, I conclude that, +although small isolated areas probably have been in some respects highly +favourable for the production of new species, yet that the course of +modification will generally have been more rapid on large areas; and what is +more important, that the new forms produced on large areas, which already have +been victorious over many competitors, will be those that will spread most +widely, will give rise to most new varieties and species, and will thus play an +important part in the changing history of the organic world. + +We can, perhaps, on these views, understand some facts which will be again +alluded to in our chapter on geographical distribution; for instance, that the +productions of the smaller continent of Australia have formerly yielded, and +apparently are now yielding, before those of the larger Europæo-Asiatic area. +Thus, also, it is that continental productions have everywhere become so +largely naturalised on islands. On a small island, the race for life will have +been less severe, and there will have been less modification and less +extermination. Hence, perhaps, it comes that the flora of Madeira, according to +Oswald Heer, resembles the extinct tertiary flora of Europe. All fresh-water +basins, taken together, make a small area compared with that of the sea or of +the land; and, consequently, the competition between fresh-water productions +will have been less severe than elsewhere; new forms will have been more slowly +formed, and old forms more slowly exterminated. And it is in fresh water that +we find seven genera of Ganoid fishes, remnants of a once preponderant order: +and in fresh water we find some of the most anomalous forms now known in the +world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect to +a certain extent orders now widely separated in the natural scale. These +anomalous forms may almost be called living fossils; they have endured to the +present day, from having inhabited a confined area, and from having thus been +exposed to less severe competition. + +To sum up the circumstances favourable and unfavourable to natural selection, +as far as the extreme intricacy of the subject permits. I conclude, looking to +the future, that for terrestrial productions a large continental area, which +will probably undergo many oscillations of level, and which consequently will +exist for long periods in a broken condition, will be the most favourable for +the production of many new forms of life, likely to endure long and to spread +widely. For the area will first have existed as a continent, and the +inhabitants, at this period numerous in individuals and kinds, will have been +subjected to very severe competition. When converted by subsidence into large +separate islands, there will still exist many individuals of the same species +on each island: intercrossing on the confines of the range of each species will +thus be checked: after physical changes of any kind, immigration will be +prevented, so that new places in the polity of each island will have to be +filled up by modifications of the old inhabitants; and time will be allowed for +the varieties in each to become well modified and perfected. When, by renewed +elevation, the islands shall be re-converted into a continental area, there +will again be severe competition: the most favoured or improved varieties will +be enabled to spread: there will be much extinction of the less improved forms, +and the relative proportional numbers of the various inhabitants of the renewed +continent will again be changed; and again there will be a fair field for +natural selection to improve still further the inhabitants, and thus produce +new species. + +That natural selection will always act with extreme slowness, I fully admit. +Its action depends on there being places in the polity of nature, which can be +better occupied by some of the inhabitants of the country undergoing +modification of some kind. The existence of such places will often depend on +physical changes, which are generally very slow, and on the immigration of +better adapted forms having been checked. But the action of natural selection +will probably still oftener depend on some of the inhabitants becoming slowly +modified; the mutual relations of many of the other inhabitants being thus +disturbed. Nothing can be effected, unless favourable variations occur, and +variation itself is apparently always a very slow process. The process will +often be greatly retarded by free intercrossing. Many will exclaim that these +several causes are amply sufficient wholly to stop the action of natural +selection. I do not believe so. On the other hand, I do believe that natural +selection will always act very slowly, often only at long intervals of time, +and generally on only a very few of the inhabitants of the same region at the +same time. I further believe, that this very slow, intermittent action of +natural selection accords perfectly well with what geology tells us of the rate +and manner at which the inhabitants of this world have changed. + +Slow though the process of selection may be, if feeble man can do much by his +powers of artificial selection, I can see no limit to the amount of change, to +the beauty and infinite complexity of the coadaptations between all organic +beings, one with another and with their physical conditions of life, which may +be effected in the long course of time by nature’s power of selection. + +Extinction.—This subject will be more fully discussed in our chapter on +Geology; but it must be here alluded to from being intimately connected with +natural selection. Natural selection acts solely through the preservation of +variations in some way advantageous, which consequently endure. But as from the +high geometrical powers of increase of all organic beings, each area is already +fully stocked with inhabitants, it follows that as each selected and favoured +form increases in number, so will the less favoured forms decrease and become +rare. Rarity, as geology tells us, is the precursor to extinction. We can, +also, see that any form represented by few individuals will, during +fluctuations in the seasons or in the number of its enemies, run a good chance +of utter extinction. But we may go further than this; for as new forms are +continually and slowly being produced, unless we believe that the number of +specific forms goes on perpetually and almost indefinitely increasing, numbers +inevitably must become extinct. That the number of specific forms has not +indefinitely increased, geology shows us plainly; and indeed we can see reason +why they should not have thus increased, for the number of places in the polity +of nature is not indefinitely great,—not that we have any means of knowing that +any one region has as yet got its maximum of species. Probably no region is as +yet fully stocked, for at the Cape of Good Hope, where more species of plants +are crowded together than in any other quarter of the world, some foreign +plants have become naturalised, without causing, as far as we know, the +extinction of any natives. + +Furthermore, the species which are most numerous in individuals will have the +best chance of producing within any given period favourable variations. We have +evidence of this, in the facts given in the second chapter, showing that it is +the common species which afford the greatest number of recorded varieties, or +incipient species. Hence, rare species will be less quickly modified or +improved within any given period, and they will consequently be beaten in the +race for life by the modified descendants of the commoner species. + +From these several considerations I think it inevitably follows, that as new +species in the course of time are formed through natural selection, others will +become rarer and rarer, and finally extinct. The forms which stand in closest +competition with those undergoing modification and improvement, will naturally +suffer most. And we have seen in the chapter on the Struggle for Existence that +it is the most closely-allied forms,—varieties of the same species, and species +of the same genus or of related genera,—which, from having nearly the same +structure, constitution, and habits, generally come into the severest +competition with each other. Consequently, each new variety or species, during +the progress of its formation, will generally press hardest on its nearest +kindred, and tend to exterminate them. We see the same process of extermination +amongst our domesticated productions, through the selection of improved forms +by man. Many curious instances could be given showing how quickly new breeds of +cattle, sheep, and other animals, and varieties of flowers, take the place of +older and inferior kinds. In Yorkshire, it is historically known that the +ancient black cattle were displaced by the long-horns, and that these “were +swept away by the short-horns” (I quote the words of an agricultural writer) +“as if by some murderous pestilence.” + +Divergence of Character.—The principle, which I have designated by this term, +is of high importance on my theory, and explains, as I believe, several +important facts. In the first place, varieties, even strongly-marked ones, +though having somewhat of the character of species—as is shown by the hopeless +doubts in many cases how to rank them—yet certainly differ from each other far +less than do good and distinct species. Nevertheless, according to my view, +varieties are species in the process of formation, or are, as I have called +them, incipient species. How, then, does the lesser difference between +varieties become augmented into the greater difference between species? That +this does habitually happen, we must infer from most of the innumerable species +throughout nature presenting well-marked differences; whereas varieties, the +supposed prototypes and parents of future well-marked species, present slight +and ill-defined differences. Mere chance, as we may call it, might cause one +variety to differ in some character from its parents, and the offspring of this +variety again to differ from its parent in the very same character and in a +greater degree; but this alone would never account for so habitual and large an +amount of difference as that between varieties of the same species and species +of the same genus. + +As has always been my practice, let us seek light on this head from our +domestic productions. We shall here find something analogous. A fancier is +struck by a pigeon having a slightly shorter beak; another fancier is struck by +a pigeon having a rather longer beak; and on the acknowledged principle that +“fanciers do not and will not admire a medium standard, but like extremes,” +they both go on (as has actually occurred with tumbler-pigeons) choosing and +breeding from birds with longer and longer beaks, or with shorter and shorter +beaks. Again, we may suppose that at an early period one man preferred swifter +horses; another stronger and more bulky horses. The early differences would be +very slight; in the course of time, from the continued selection of swifter +horses by some breeders, and of stronger ones by others, the differences would +become greater, and would be noted as forming two sub-breeds; finally, after +the lapse of centuries, the sub-breeds would become converted into two +well-established and distinct breeds. As the differences slowly become greater, +the inferior animals with intermediate characters, being neither very swift nor +very strong, will have been neglected, and will have tended to disappear. Here, +then, we see in man’s productions the action of what may be called the +principle of divergence, causing differences, at first barely appreciable, +steadily to increase, and the breeds to diverge in character both from each +other and from their common parent. + +But how, it may be asked, can any analogous principle apply in nature? I +believe it can and does apply most efficiently, from the simple circumstance +that the more diversified the descendants from any one species become in +structure, constitution, and habits, by so much will they be better enabled to +seize on many and widely diversified places in the polity of nature, and so be +enabled to increase in numbers. + +We can clearly see this in the case of animals with simple habits. Take the +case of a carnivorous quadruped, of which the number that can be supported in +any country has long ago arrived at its full average. If its natural powers of +increase be allowed to act, it can succeed in increasing (the country not +undergoing any change in its conditions) only by its varying descendants +seizing on places at present occupied by other animals: some of them, for +instance, being enabled to feed on new kinds of prey, either dead or alive; +some inhabiting new stations, climbing trees, frequenting water, and some +perhaps becoming less carnivorous. The more diversified in habits and structure +the descendants of our carnivorous animal became, the more places they would be +enabled to occupy. What applies to one animal will apply throughout all time to +all animals—that is, if they vary—for otherwise natural selection can do +nothing. So it will be with plants. It has been experimentally proved, that if +a plot of ground be sown with one species of grass, and a similar plot be sown +with several distinct genera of grasses, a greater number of plants and a +greater weight of dry herbage can thus be raised. The same has been found to +hold good when first one variety and then several mixed varieties of wheat have +been sown on equal spaces of ground. Hence, if any one species of grass were to +go on varying, and those varieties were continually selected which differed +from each other in at all the same manner as distinct species and genera of +grasses differ from each other, a greater number of individual plants of this +species of grass, including its modified descendants, would succeed in living +on the same piece of ground. And we well know that each species and each +variety of grass is annually sowing almost countless seeds; and thus, as it may +be said, is striving its utmost to increase its numbers. Consequently, I cannot +doubt that in the course of many thousands of generations, the most distinct +varieties of any one species of grass would always have the best chance of +succeeding and of increasing in numbers, and thus of supplanting the less +distinct varieties; and varieties, when rendered very distinct from each other, +take the rank of species. + +The truth of the principle, that the greatest amount of life can be supported +by great diversification of structure, is seen under many natural +circumstances. In an extremely small area, especially if freely open to +immigration, and where the contest between individual and individual must be +severe, we always find great diversity in its inhabitants. For instance, I +found that a piece of turf, three feet by four in size, which had been exposed +for many years to exactly the same conditions, supported twenty species of +plants, and these belonged to eighteen genera and to eight orders, which shows +how much these plants differed from each other. So it is with the plants and +insects on small and uniform islets; and so in small ponds of fresh water. +Farmers find that they can raise most food by a rotation of plants belonging to +the most different orders: nature follows what may be called a simultaneous +rotation. Most of the animals and plants which live close round any small piece +of ground, could live on it (supposing it not to be in any way peculiar in its +nature), and may be said to be striving to the utmost to live there; but, it is +seen, that where they come into the closest competition with each other, the +advantages of diversification of structure, with the accompanying differences +of habit and constitution, determine that the inhabitants, which thus jostle +each other most closely, shall, as a general rule, belong to what we call +different genera and orders. + +The same principle is seen in the naturalisation of plants through man’s agency +in foreign lands. It might have been expected that the plants which have +succeeded in becoming naturalised in any land would generally have been closely +allied to the indigenes; for these are commonly looked at as specially created +and adapted for their own country. It might, also, perhaps have been expected +that naturalised plants would have belonged to a few groups more especially +adapted to certain stations in their new homes. But the case is very different; +and Alph. De Candolle has well remarked in his great and admirable work, that +floras gain by naturalisation, proportionally with the number of the native +genera and species, far more in new genera than in new species. To give a +single instance: in the last edition of Dr. Asa Gray’s ‘Manual of the Flora of +the Northern United States,’ 260 naturalised plants are enumerated, and these +belong to 162 genera. We thus see that these naturalised plants are of a highly +diversified nature. They differ, moreover, to a large extent from the +indigenes, for out of the 162 genera, no less than 100 genera are not there +indigenous, and thus a large proportional addition is made to the genera of +these States. + +By considering the nature of the plants or animals which have struggled +successfully with the indigenes of any country, and have there become +naturalised, we can gain some crude idea in what manner some of the natives +would have had to be modified, in order to have gained an advantage over the +other natives; and we may, I think, at least safely infer that diversification +of structure, amounting to new generic differences, would have been profitable +to them. + +The advantage of diversification in the inhabitants of the same region is, in +fact, the same as that of the physiological division of labour in the organs of +the same individual body—a subject so well elucidated by Milne Edwards. No +physiologist doubts that a stomach by being adapted to digest vegetable matter +alone, or flesh alone, draws most nutriment from these substances. So in the +general economy of any land, the more widely and perfectly the animals and +plants are diversified for different habits of life, so will a greater number +of individuals be capable of there supporting themselves. A set of animals, +with their organisation but little diversified, could hardly compete with a set +more perfectly diversified in structure. It may be doubted, for instance, +whether the Australian marsupials, which are divided into groups differing but +little from each other, and feebly representing, as Mr. Waterhouse and others +have remarked, our carnivorous, ruminant, and rodent mammals, could +successfully compete with these well-pronounced orders. In the Australian +mammals, we see the process of diversification in an early and incomplete stage +of development. After the foregoing discussion, which ought to have been much +amplified, we may, I think, assume that the modified descendants of any one +species will succeed by so much the better as they become more diversified in +structure, and are thus enabled to encroach on places occupied by other beings. +Now let us see how this principle of great benefit being derived from +divergence of character, combined with the principles of natural selection and +of extinction, will tend to act. + +The accompanying diagram will aid us in understanding this rather perplexing +subject. Let A to L represent the species of a genus large in its own country; +these species are supposed to resemble each other in unequal degrees, as is so +generally the case in nature, and as is represented in the diagram by the +letters standing at unequal distances. I have said a large genus, because we +have seen in the second chapter, that on an average more of the species of +large genera vary than of small genera; and the varying species of the large +genera present a greater number of varieties. We have, also, seen that the +species, which are the commonest and the most widely-diffused, vary more than +rare species with restricted ranges. Let (A) be a common, widely-diffused, and +varying species, belonging to a genus large in its own country. The little fan +of diverging dotted lines of unequal lengths proceeding from (A), may represent +its varying offspring. The variations are supposed to be extremely slight, but +of the most diversified nature; they are not supposed all to appear +simultaneously, but often after long intervals of time; nor are they all +supposed to endure for equal periods. Only those variations which are in some +way profitable will be preserved or naturally selected. And here the importance +of the principle of benefit being derived from divergence of character comes +in; for this will generally lead to the most different or divergent variations +(represented by the outer dotted lines) being preserved and accumulated by +natural selection. When a dotted line reaches one of the horizontal lines, and +is there marked by a small numbered letter, a sufficient amount of variation is +supposed to have been accumulated to have formed a fairly well-marked variety, +such as would be thought worthy of record in a systematic work. + +The intervals between the horizontal lines in the diagram, may represent each a +thousand generations; but it would have been better if each had represented ten +thousand generations. After a thousand generations, species (A) is supposed to +have produced two fairly well-marked varieties, namely a^1 and m^1. These two +varieties will generally continue to be exposed to the same conditions which +made their parents variable, and the tendency to variability is in itself +hereditary, consequently they will tend to vary, and generally to vary in +nearly the same manner as their parents varied. Moreover, these two varieties, +being only slightly modified forms, will tend to inherit those advantages which +made their common parent (A) more numerous than most of the other inhabitants +of the same country; they will likewise partake of those more general +advantages which made the genus to which the parent-species belonged, a large +genus in its own country. And these circumstances we know to be favourable to +the production of new varieties. + +If, then, these two varieties be variable, the most divergent of their +variations will generally be preserved during the next thousand generations. +And after this interval, variety a^1 is supposed in the diagram to have +produced variety a^2, which will, owing to the principle of divergence, differ +more from (A) than did variety a^1. Variety m^1 is supposed to have produced +two varieties, namely m^2 and s^2, differing from each other, and more +considerably from their common parent (A). We may continue the process by +similar steps for any length of time; some of the varieties, after each +thousand generations, producing only a single variety, but in a more and more +modified condition, some producing two or three varieties, and some failing to +produce any. Thus the varieties or modified descendants, proceeding from the +common parent (A), will generally go on increasing in number and diverging in +character. In the diagram the process is represented up to the ten-thousandth +generation, and under a condensed and simplified form up to the +fourteen-thousandth generation. + +But I must here remark that I do not suppose that the process ever goes on so +regularly as is represented in the diagram, though in itself made somewhat +irregular. I am far from thinking that the most divergent varieties will +invariably prevail and multiply: a medium form may often long endure, and may +or may not produce more than one modified descendant; for natural selection +will always act according to the nature of the places which are either +unoccupied or not perfectly occupied by other beings; and this will depend on +infinitely complex relations. But as a general rule, the more diversified in +structure the descendants from any one species can be rendered, the more places +they will be enabled to seize on, and the more their modified progeny will be +increased. In our diagram the line of succession is broken at regular intervals +by small numbered letters marking the successive forms which have become +sufficiently distinct to be recorded as varieties. But these breaks are +imaginary, and might have been inserted anywhere, after intervals long enough +to have allowed the accumulation of a considerable amount of divergent +variation. + +As all the modified descendants from a common and widely-diffused species, +belonging to a large genus, will tend to partake of the same advantages which +made their parent successful in life, they will generally go on multiplying in +number as well as diverging in character: this is represented in the diagram by +the several divergent branches proceeding from (A). The modified offspring from +the later and more highly improved branches in the lines of descent, will, it +is probable, often take the place of, and so destroy, the earlier and less +improved branches: this is represented in the diagram by some of the lower +branches not reaching to the upper horizontal lines. In some cases I do not +doubt that the process of modification will be confined to a single line of +descent, and the number of the descendants will not be increased; although the +amount of divergent modification may have been increased in the successive +generations. This case would be represented in the diagram, if all the lines +proceeding from (A) were removed, excepting that from a^1 to a^10. In the same +way, for instance, the English race-horse and English pointer have apparently +both gone on slowly diverging in character from their original stocks, without +either having given off any fresh branches or races. + +After ten thousand generations, species (A) is supposed to have produced three +forms, a^10, f^10, and m^10, which, from having diverged in character during +the successive generations, will have come to differ largely, but perhaps +unequally, from each other and from their common parent. If we suppose the +amount of change between each horizontal line in our diagram to be excessively +small, these three forms may still be only well-marked varieties; or they may +have arrived at the doubtful category of sub-species; but we have only to +suppose the steps in the process of modification to be more numerous or greater +in amount, to convert these three forms into well-defined species: thus the +diagram illustrates the steps by which the small differences distinguishing +varieties are increased into the larger differences distinguishing species. By +continuing the same process for a greater number of generations (as shown in +the diagram in a condensed and simplified manner), we get eight species, marked +by the letters between a^14 and m^14, all descended from (A). Thus, as I +believe, species are multiplied and genera are formed. + +In a large genus it is probable that more than one species would vary. In the +diagram I have assumed that a second species (I) has produced, by analogous +steps, after ten thousand generations, either two well-marked varieties (w^10 +and z^10) or two species, according to the amount of change supposed to be +represented between the horizontal lines. After fourteen thousand generations, +six new species, marked by the letters n^14 to z^14, are supposed to have been +produced. In each genus, the species, which are already extremely different in +character, will generally tend to produce the greatest number of modified +descendants; for these will have the best chance of filling new and widely +different places in the polity of nature: hence in the diagram I have chosen +the extreme species (A), and the nearly extreme species (I), as those which +have largely varied, and have given rise to new varieties and species. The +other nine species (marked by capital letters) of our original genus, may for a +long period continue transmitting unaltered descendants; and this is shown in +the diagram by the dotted lines not prolonged far upwards from want of space. + +But during the process of modification, represented in the diagram, another of +our principles, namely that of extinction, will have played an important part. +As in each fully stocked country natural selection necessarily acts by the +selected form having some advantage in the struggle for life over other forms, +there will be a constant tendency in the improved descendants of any one +species to supplant and exterminate in each stage of descent their predecessors +and their original parent. For it should be remembered that the competition +will generally be most severe between those forms which are most nearly related +to each other in habits, constitution, and structure. Hence all the +intermediate forms between the earlier and later states, that is between the +less and more improved state of a species, as well as the original +parent-species itself, will generally tend to become extinct. So it probably +will be with many whole collateral lines of descent, which will be conquered by +later and improved lines of descent. If, however, the modified offspring of a +species get into some distinct country, or become quickly adapted to some quite +new station, in which child and parent do not come into competition, both may +continue to exist. + +If then our diagram be assumed to represent a considerable amount of +modification, species (A) and all the earlier varieties will have become +extinct, having been replaced by eight new species (a^14 to m^14); and (I) will +have been replaced by six (n^14 to z^14) new species. + +But we may go further than this. The original species of our genus were +supposed to resemble each other in unequal degrees, as is so generally the case +in nature; species (A) being more nearly related to B, C, and D, than to the +other species; and species (I) more to G, H, K, L, than to the others. These +two species (A) and (I), were also supposed to be very common and widely +diffused species, so that they must originally have had some advantage over +most of the other species of the genus. Their modified descendants, fourteen in +number at the fourteen-thousandth generation, will probably have inherited some +of the same advantages: they have also been modified and improved in a +diversified manner at each stage of descent, so as to have become adapted to +many related places in the natural economy of their country. It seems, +therefore, to me extremely probable that they will have taken the places of, +and thus exterminated, not only their parents (A) and (I), but likewise some of +the original species which were most nearly related to their parents. Hence +very few of the original species will have transmitted offspring to the +fourteen-thousandth generation. We may suppose that only one (F), of the two +species which were least closely related to the other nine original species, +has transmitted descendants to this late stage of descent. + +The new species in our diagram descended from the original eleven species, will +now be fifteen in number. Owing to the divergent tendency of natural selection, +the extreme amount of difference in character between species a^14 and z^14 +will be much greater than that between the most different of the original +eleven species. The new species, moreover, will be allied to each other in a +widely different manner. Of the eight descendants from (A) the three marked a^ +14, q^14, p^14, will be nearly related from having recently branched off from a +^10; b^14 and f^14, from having diverged at an earlier period from a^5, will be +in some degree distinct from the three first-named species; and lastly, o^14, e +^14, and m^14, will be nearly related one to the other, but from having +diverged at the first commencement of the process of modification, will be +widely different from the other five species, and may constitute a sub-genus or +even a distinct genus. + +The six descendants from (I) will form two sub-genera or even genera. But as +the original species (I) differed largely from (A), standing nearly at the +extreme points of the original genus, the six descendants from (I) will, owing +to inheritance, differ considerably from the eight descendants from (A); the +two groups, moreover, are supposed to have gone on diverging in different +directions. The intermediate species, also (and this is a very important +consideration), which connected the original species (A) and (I), have all +become, excepting (F), extinct, and have left no descendants. Hence the six new +species descended from (I), and the eight descended from (A), will have to be +ranked as very distinct genera, or even as distinct sub-families. + +Thus it is, as I believe, that two or more genera are produced by descent, with +modification, from two or more species of the same genus. And the two or more +parent-species are supposed to have descended from some one species of an +earlier genus. In our diagram, this is indicated by the broken lines, beneath +the capital letters, converging in sub-branches downwards towards a single +point; this point representing a single species, the supposed single parent of +our several new sub-genera and genera. + +It is worth while to reflect for a moment on the character of the new species F +^14, which is supposed not to have diverged much in character, but to have +retained the form of (F), either unaltered or altered only in a slight degree. +In this case, its affinities to the other fourteen new species will be of a +curious and circuitous nature. Having descended from a form which stood between +the two parent-species (A) and (I), now supposed to be extinct and unknown, it +will be in some degree intermediate in character between the two groups +descended from these species. But as these two groups have gone on diverging in +character from the type of their parents, the new species (F^14) will not be +directly intermediate between them, but rather between types of the two groups; +and every naturalist will be able to bring some such case before his mind. + +In the diagram, each horizontal line has hitherto been supposed to represent a +thousand generations, but each may represent a million or hundred million +generations, and likewise a section of the successive strata of the earth’s +crust including extinct remains. We shall, when we come to our chapter on +Geology, have to refer again to this subject, and I think we shall then see +that the diagram throws light on the affinities of extinct beings, which, +though generally belonging to the same orders, or families, or genera, with +those now living, yet are often, in some degree, intermediate in character +between existing groups; and we can understand this fact, for the extinct +species lived at very ancient epochs when the branching lines of descent had +diverged less. + +I see no reason to limit the process of modification, as now explained, to the +formation of genera alone. If, in our diagram, we suppose the amount of change +represented by each successive group of diverging dotted lines to be very +great, the forms marked a^14 to p^14, those marked b^14 and f^14, and those +marked o^14 to m^14, will form three very distinct genera. We shall also have +two very distinct genera descended from (I) and as these latter two genera, +both from continued divergence of character and from inheritance from a +different parent, will differ widely from the three genera descended from (A), +the two little groups of genera will form two distinct families, or even +orders, according to the amount of divergent modification supposed to be +represented in the diagram. And the two new families, or orders, will have +descended from two species of the original genus; and these two species are +supposed to have descended from one species of a still more ancient and unknown +genus. + +We have seen that in each country it is the species of the larger genera which +oftenest present varieties or incipient species. This, indeed, might have been +expected; for as natural selection acts through one form having some advantage +over other forms in the struggle for existence, it will chiefly act on those +which already have some advantage; and the largeness of any group shows that +its species have inherited from a common ancestor some advantage in common. +Hence, the struggle for the production of new and modified descendants, will +mainly lie between the larger groups, which are all trying to increase in +number. One large group will slowly conquer another large group, reduce its +numbers, and thus lessen its chance of further variation and improvement. +Within the same large group, the later and more highly perfected sub-groups, +from branching out and seizing on many new places in the polity of Nature, will +constantly tend to supplant and destroy the earlier and less improved +sub-groups. Small and broken groups and sub-groups will finally tend to +disappear. Looking to the future, we can predict that the groups of organic +beings which are now large and triumphant, and which are least broken up, that +is, which as yet have suffered least extinction, will for a long period +continue to increase. But which groups will ultimately prevail, no man can +predict; for we well know that many groups, formerly most extensively +developed, have now become extinct. Looking still more remotely to the future, +we may predict that, owing to the continued and steady increase of the larger +groups, a multitude of smaller groups will become utterly extinct, and leave no +modified descendants; and consequently that of the species living at any one +period, extremely few will transmit descendants to a remote futurity. I shall +have to return to this subject in the chapter on Classification, but I may add +that on this view of extremely few of the more ancient species having +transmitted descendants, and on the view of all the descendants of the same +species making a class, we can understand how it is that there exist but very +few classes in each main division of the animal and vegetable kingdoms. +Although extremely few of the most ancient species may now have living and +modified descendants, yet at the most remote geological period, the earth may +have been as well peopled with many species of many genera, families, orders, +and classes, as at the present day. + +Summary of the Chapter.—If during the long course of ages and under varying +conditions of life, organic beings vary at all in the several parts of their +organisation, and I think this cannot be disputed; if there be, owing to the +high geometrical powers of increase of each species, at some age, season, or +year, a severe struggle for life, and this certainly cannot be disputed; then, +considering the infinite complexity of the relations of all organic beings to +each other and to their conditions of existence, causing an infinite diversity +in structure, constitution, and habits, to be advantageous to them, I think it +would be a most extraordinary fact if no variation ever had occurred useful to +each being’s own welfare, in the same way as so many variations have occurred +useful to man. But if variations useful to any organic being do occur, +assuredly individuals thus characterised will have the best chance of being +preserved in the struggle for life; and from the strong principle of +inheritance they will tend to produce offspring similarly characterised. This +principle of preservation, I have called, for the sake of brevity, Natural +Selection. Natural selection, on the principle of qualities being inherited at +corresponding ages, can modify the egg, seed, or young, as easily as the adult. +Amongst many animals, sexual selection will give its aid to ordinary selection, +by assuring to the most vigorous and best adapted males the greatest number of +offspring. Sexual selection will also give characters useful to the males +alone, in their struggles with other males. + +Whether natural selection has really thus acted in nature, in modifying and +adapting the various forms of life to their several conditions and stations, +must be judged of by the general tenour and balance of evidence given in the +following chapters. But we already see how it entails extinction; and how +largely extinction has acted in the world’s history, geology plainly declares. +Natural selection, also, leads to divergence of character; for more living +beings can be supported on the same area the more they diverge in structure, +habits, and constitution, of which we see proof by looking at the inhabitants +of any small spot or at naturalised productions. Therefore during the +modification of the descendants of any one species, and during the incessant +struggle of all species to increase in numbers, the more diversified these +descendants become, the better will be their chance of succeeding in the battle +of life. Thus the small differences distinguishing varieties of the same +species, will steadily tend to increase till they come to equal the greater +differences between species of the same genus, or even of distinct genera. + +We have seen that it is the common, the widely-diffused, and widely-ranging +species, belonging to the larger genera, which vary most; and these will tend +to transmit to their modified offspring that superiority which now makes them +dominant in their own countries. Natural selection, as has just been remarked, +leads to divergence of character and to much extinction of the less improved +and intermediate forms of life. On these principles, I believe, the nature of +the affinities of all organic beings may be explained. It is a truly wonderful +fact—the wonder of which we are apt to overlook from familiarity—that all +animals and all plants throughout all time and space should be related to each +other in group subordinate to group, in the manner which we everywhere +behold—namely, varieties of the same species most closely related together, +species of the same genus less closely and unequally related together, forming +sections and sub-genera, species of distinct genera much less closely related, +and genera related in different degrees, forming sub-families, families, +orders, sub-classes, and classes. The several subordinate groups in any class +cannot be ranked in a single file, but seem rather to be clustered round +points, and these round other points, and so on in almost endless cycles. On +the view that each species has been independently created, I can see no +explanation of this great fact in the classification of all organic beings; +but, to the best of my judgment, it is explained through inheritance and the +complex action of natural selection, entailing extinction and divergence of +character, as we have seen illustrated in the diagram. + +The affinities of all the beings of the same class have sometimes been +represented by a great tree. I believe this simile largely speaks the truth. +The green and budding twigs may represent existing species; and those produced +during each former year may represent the long succession of extinct species. +At each period of growth all the growing twigs have tried to branch out on all +sides, and to overtop and kill the surrounding twigs and branches, in the same +manner as species and groups of species have tried to overmaster other species +in the great battle for life. The limbs divided into great branches, and these +into lesser and lesser branches, were themselves once, when the tree was small, +budding twigs; and this connexion of the former and present buds by ramifying +branches may well represent the classification of all extinct and living +species in groups subordinate to groups. Of the many twigs which flourished +when the tree was a mere bush, only two or three, now grown into great +branches, yet survive and bear all the other branches; so with the species +which lived during long-past geological periods, very few now have living and +modified descendants. From the first growth of the tree, many a limb and branch +has decayed and dropped off; and these lost branches of various sizes may +represent those whole orders, families, and genera which have now no living +representatives, and which are known to us only from having been found in a +fossil state. As we here and there see a thin straggling branch springing from +a fork low down in a tree, and which by some chance has been favoured and is +still alive on its summit, so we occasionally see an animal like the +Ornithorhynchus or Lepidosiren, which in some small degree connects by its +affinities two large branches of life, and which has apparently been saved from +fatal competition by having inhabited a protected station. As buds give rise by +growth to fresh buds, and these, if vigorous, branch out and overtop on all +sides many a feebler branch, so by generation I believe it has been with the +great Tree of Life, which fills with its dead and broken branches the crust of +the earth, and covers the surface with its ever branching and beautiful +ramifications. + +CHAPTER V. +LAWS OF VARIATION. + +Effects of external conditions. Use and disuse, combined with natural +selection; organs of flight and of vision. Acclimatisation. Correlation of +growth. Compensation and economy of growth. False correlations. Multiple, +rudimentary, and lowly organised structures variable. Parts developed in an +unusual manner are highly variable: specific characters more variable than +generic: secondary sexual characters variable. Species of the same genus vary +in an analogous manner. Reversions to long lost characters. Summary. + +I have hitherto sometimes spoken as if the variations—so common and multiform +in organic beings under domestication, and in a lesser degree in those in a +state of nature—had been due to chance. This, of course, is a wholly incorrect +expression, but it serves to acknowledge plainly our ignorance of the cause of +each particular variation. Some authors believe it to be as much the function +of the reproductive system to produce individual differences, or very slight +deviations of structure, as to make the child like its parents. But the much +greater variability, as well as the greater frequency of monstrosities, under +domestication or cultivation, than under nature, leads me to believe that +deviations of structure are in some way due to the nature of the conditions of +life, to which the parents and their more remote ancestors have been exposed +during several generations. I have remarked in the first chapter—but a long +catalogue of facts which cannot be here given would be necessary to show the +truth of the remark—that the reproductive system is eminently susceptible to +changes in the conditions of life; and to this system being functionally +disturbed in the parents, I chiefly attribute the varying or plastic condition +of the offspring. The male and female sexual elements seem to be affected +before that union takes place which is to form a new being. In the case of +“sporting” plants, the bud, which in its earliest condition does not apparently +differ essentially from an ovule, is alone affected. But why, because the +reproductive system is disturbed, this or that part should vary more or less, +we are profoundly ignorant. Nevertheless, we can here and there dimly catch a +faint ray of light, and we may feel sure that there must be some cause for each +deviation of structure, however slight. + +How much direct effect difference of climate, food, etc., produces on any being +is extremely doubtful. My impression is, that the effect is extremely small in +the case of animals, but perhaps rather more in that of plants. We may, at +least, safely conclude that such influences cannot have produced the many +striking and complex co-adaptations of structure between one organic being and +another, which we see everywhere throughout nature. Some little influence may +be attributed to climate, food, etc.: thus, E. Forbes speaks confidently that +shells at their southern limit, and when living in shallow water, are more +brightly coloured than those of the same species further north or from greater +depths. Gould believes that birds of the same species are more brightly +coloured under a clear atmosphere, than when living on islands or near the +coast. So with insects, Wollaston is convinced that residence near the sea +affects their colours. Moquin-Tandon gives a list of plants which when growing +near the sea-shore have their leaves in some degree fleshy, though not +elsewhere fleshy. Several other such cases could be given. + +The fact of varieties of one species, when they range into the zone of +habitation of other species, often acquiring in a very slight degree some of +the characters of such species, accords with our view that species of all kinds +are only well-marked and permanent varieties. Thus the species of shells which +are confined to tropical and shallow seas are generally brighter-coloured than +those confined to cold and deeper seas. The birds which are confined to +continents are, according to Mr. Gould, brighter-coloured than those of +islands. The insect-species confined to sea-coasts, as every collector knows, +are often brassy or lurid. Plants which live exclusively on the sea-side are +very apt to have fleshy leaves. He who believes in the creation of each +species, will have to say that this shell, for instance, was created with +bright colours for a warm sea; but that this other shell became bright-coloured +by variation when it ranged into warmer or shallower waters. + +When a variation is of the slightest use to a being, we cannot tell how much of +it to attribute to the accumulative action of natural selection, and how much +to the conditions of life. Thus, it is well known to furriers that animals of +the same species have thicker and better fur the more severe the climate is +under which they have lived; but who can tell how much of this difference may +be due to the warmest-clad individuals having been favoured and preserved +during many generations, and how much to the direct action of the severe +climate? for it would appear that climate has some direct action on the hair of +our domestic quadrupeds. + +Instances could be given of the same variety being produced under conditions of +life as different as can well be conceived; and, on the other hand, of +different varieties being produced from the same species under the same +conditions. Such facts show how indirectly the conditions of life must act. +Again, innumerable instances are known to every naturalist of species keeping +true, or not varying at all, although living under the most opposite climates. +Such considerations as these incline me to lay very little weight on the direct +action of the conditions of life. Indirectly, as already remarked, they seem to +play an important part in affecting the reproductive system, and in thus +inducing variability; and natural selection will then accumulate all profitable +variations, however slight, until they become plainly developed and appreciable +by us. + +Effects of Use and Disuse.—From the facts alluded to in the first chapter, I +think there can be little doubt that use in our domestic animals strengthens +and enlarges certain parts, and disuse diminishes them; and that such +modifications are inherited. Under free nature, we can have no standard of +comparison, by which to judge of the effects of long-continued use or disuse, +for we know not the parent-forms; but many animals have structures which can be +explained by the effects of disuse. As Professor Owen has remarked, there is no +greater anomaly in nature than a bird that cannot fly; yet there are several in +this state. The logger-headed duck of South America can only flap along the +surface of the water, and has its wings in nearly the same condition as the +domestic Aylesbury duck. As the larger ground-feeding birds seldom take flight +except to escape danger, I believe that the nearly wingless condition of +several birds, which now inhabit or have lately inhabited several oceanic +islands, tenanted by no beast of prey, has been caused by disuse. The ostrich +indeed inhabits continents and is exposed to danger from which it cannot escape +by flight, but by kicking it can defend itself from enemies, as well as any of +the smaller quadrupeds. We may imagine that the early progenitor of the ostrich +had habits like those of a bustard, and that as natural selection increased in +successive generations the size and weight of its body, its legs were used +more, and its wings less, until they became incapable of flight. + +Kirby has remarked (and I have observed the same fact) that the anterior tarsi, +or feet, of many male dung-feeding beetles are very often broken off; he +examined seventeen specimens in his own collection, and not one had even a +relic left. In the Onites apelles the tarsi are so habitually lost, that the +insect has been described as not having them. In some other genera they are +present, but in a rudimentary condition. In the Ateuchus or sacred beetle of +the Egyptians, they are totally deficient. There is not sufficient evidence to +induce us to believe that mutilations are ever inherited; and I should prefer +explaining the entire absence of the anterior tarsi in Ateuchus, and their +rudimentary condition in some other genera, by the long-continued effects of +disuse in their progenitors; for as the tarsi are almost always lost in many +dung-feeding beetles, they must be lost early in life, and therefore cannot be +much used by these insects. + +In some cases we might easily put down to disuse modifications of structure +which are wholly, or mainly, due to natural selection. Mr. Wollaston has +discovered the remarkable fact that 200 beetles, out of the 550 species +inhabiting Madeira, are so far deficient in wings that they cannot fly; and +that of the twenty-nine endemic genera, no less than twenty-three genera have +all their species in this condition! Several facts, namely, that beetles in +many parts of the world are very frequently blown to sea and perish; that the +beetles in Madeira, as observed by Mr. Wollaston, lie much concealed, until the +wind lulls and the sun shines; that the proportion of wingless beetles is +larger on the exposed Dezertas than in Madeira itself; and especially the +extraordinary fact, so strongly insisted on by Mr. Wollaston, of the almost +entire absence of certain large groups of beetles, elsewhere excessively +numerous, and which groups have habits of life almost necessitating frequent +flight;—these several considerations have made me believe that the wingless +condition of so many Madeira beetles is mainly due to the action of natural +selection, but combined probably with disuse. For during thousands of +successive generations each individual beetle which flew least, either from its +wings having been ever so little less perfectly developed or from indolent +habit, will have had the best chance of surviving from not being blown out to +sea; and, on the other hand, those beetles which most readily took to flight +will oftenest have been blown to sea and thus have been destroyed. + +The insects in Madeira which are not ground-feeders, and which, as the +flower-feeding coleoptera and lepidoptera, must habitually use their wings to +gain their subsistence, have, as Mr. Wollaston suspects, their wings not at all +reduced, but even enlarged. This is quite compatible with the action of natural +selection. For when a new insect first arrived on the island, the tendency of +natural selection to enlarge or to reduce the wings, would depend on whether a +greater number of individuals were saved by successfully battling with the +winds, or by giving up the attempt and rarely or never flying. As with mariners +shipwrecked near a coast, it would have been better for the good swimmers if +they had been able to swim still further, whereas it would have been better for +the bad swimmers if they had not been able to swim at all and had stuck to the +wreck. + +The eyes of moles and of some burrowing rodents are rudimentary in size, and in +some cases are quite covered up by skin and fur. This state of the eyes is +probably due to gradual reduction from disuse, but aided perhaps by natural +selection. In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is +even more subterranean in its habits than the mole; and I was assured by a +Spaniard, who had often caught them, that they were frequently blind; one which +I kept alive was certainly in this condition, the cause, as appeared on +dissection, having been inflammation of the nictitating membrane. As frequent +inflammation of the eyes must be injurious to any animal, and as eyes are +certainly not indispensable to animals with subterranean habits, a reduction in +their size with the adhesion of the eyelids and growth of fur over them, might +in such case be an advantage; and if so, natural selection would constantly aid +the effects of disuse. + +It is well known that several animals, belonging to the most different classes, +which inhabit the caves of Styria and of Kentucky, are blind. In some of the +crabs the foot-stalk for the eye remains, though the eye is gone; the stand for +the telescope is there, though the telescope with its glasses has been lost. As +it is difficult to imagine that eyes, though useless, could be in any way +injurious to animals living in darkness, I attribute their loss wholly to +disuse. In one of the blind animals, namely, the cave-rat, the eyes are of +immense size; and Professor Silliman thought that it regained, after living +some days in the light, some slight power of vision. In the same manner as in +Madeira the wings of some of the insects have been enlarged, and the wings of +others have been reduced by natural selection aided by use and disuse, so in +the case of the cave-rat natural selection seems to have struggled with the +loss of light and to have increased the size of the eyes; whereas with all the +other inhabitants of the caves, disuse by itself seems to have done its work. + +It is difficult to imagine conditions of life more similar than deep limestone +caverns under a nearly similar climate; so that on the common view of the blind +animals having been separately created for the American and European caverns, +close similarity in their organisation and affinities might have been expected; +but, as Schiödte and others have remarked, this is not the case, and the +cave-insects of the two continents are not more closely allied than might have +been anticipated from the general resemblance of the other inhabitants of North +America and Europe. On my view we must suppose that American animals, having +ordinary powers of vision, slowly migrated by successive generations from the +outer world into the deeper and deeper recesses of the Kentucky caves, as did +European animals into the caves of Europe. We have some evidence of this +gradation of habit; for, as Schiödte remarks, “animals not far remote from +ordinary forms, prepare the transition from light to darkness. Next follow +those that are constructed for twilight; and, last of all, those destined for +total darkness.” By the time that an animal had reached, after numberless +generations, the deepest recesses, disuse will on this view have more or less +perfectly obliterated its eyes, and natural selection will often have effected +other changes, such as an increase in the length of the antennæ or palpi, as a +compensation for blindness. Notwithstanding such modifications, we might expect +still to see in the cave-animals of America, affinities to the other +inhabitants of that continent, and in those of Europe, to the inhabitants of +the European continent. And this is the case with some of the American +cave-animals, as I hear from Professor Dana; and some of the European +cave-insects are very closely allied to those of the surrounding country. It +would be most difficult to give any rational explanation of the affinities of +the blind cave-animals to the other inhabitants of the two continents on the +ordinary view of their independent creation. That several of the inhabitants of +the caves of the Old and New Worlds should be closely related, we might expect +from the well-known relationship of most of their other productions. Far from +feeling any surprise that some of the cave-animals should be very anomalous, as +Agassiz has remarked in regard to the blind fish, the Amblyopsis, and as is the +case with the blind Proteus with reference to the reptiles of Europe, I am only +surprised that more wrecks of ancient life have not been preserved, owing to +the less severe competition to which the inhabitants of these dark abodes will +probably have been exposed. + +Acclimatisation.—Habit is hereditary with plants, as in the period of +flowering, in the amount of rain requisite for seeds to germinate, in the time +of sleep, etc., and this leads me to say a few words on acclimatisation. As it +is extremely common for species of the same genus to inhabit very hot and very +cold countries, and as I believe that all the species of the same genus have +descended from a single parent, if this view be correct, acclimatisation must +be readily effected during long-continued descent. It is notorious that each +species is adapted to the climate of its own home: species from an arctic or +even from a temperate region cannot endure a tropical climate, or conversely. +So again, many succulent plants cannot endure a damp climate. But the degree of +adaptation of species to the climates under which they live is often overrated. +We may infer this from our frequent inability to predict whether or not an +imported plant will endure our climate, and from the number of plants and +animals brought from warmer countries which here enjoy good health. We have +reason to believe that species in a state of nature are limited in their ranges +by the competition of other organic beings quite as much as, or more than, by +adaptation to particular climates. But whether or not the adaptation be +generally very close, we have evidence, in the case of some few plants, of +their becoming, to a certain extent, naturally habituated to different +temperatures, or becoming acclimatised: thus the pines and rhododendrons, +raised from seed collected by Dr. Hooker from trees growing at different +heights on the Himalaya, were found in this country to possess different +constitutional powers of resisting cold. Mr. Thwaites informs me that he has +observed similar facts in Ceylon, and analogous observations have been made by +Mr. H. C. Watson on European species of plants brought from the Azores to +England. In regard to animals, several authentic cases could be given of +species within historical times having largely extended their range from warmer +to cooler latitudes, and conversely; but we do not positively know that these +animals were strictly adapted to their native climate, but in all ordinary +cases we assume such to be the case; nor do we know that they have subsequently +become acclimatised to their new homes. + +As I believe that our domestic animals were originally chosen by uncivilised +man because they were useful and bred readily under confinement, and not +because they were subsequently found capable of far-extended transportation, I +think the common and extraordinary capacity in our domestic animals of not only +withstanding the most different climates but of being perfectly fertile (a far +severer test) under them, may be used as an argument that a large proportion of +other animals, now in a state of nature, could easily be brought to bear widely +different climates. We must not, however, push the foregoing argument too far, +on account of the probable origin of some of our domestic animals from several +wild stocks: the blood, for instance, of a tropical and arctic wolf or wild dog +may perhaps be mingled in our domestic breeds. The rat and mouse cannot be +considered as domestic animals, but they have been transported by man to many +parts of the world, and now have a far wider range than any other rodent, +living free under the cold climate of Faroe in the north and of the Falklands +in the south, and on many islands in the torrid zones. Hence I am inclined to +look at adaptation to any special climate as a quality readily grafted on an +innate wide flexibility of constitution, which is common to most animals. On +this view, the capacity of enduring the most different climates by man himself +and by his domestic animals, and such facts as that former species of the +elephant and rhinoceros were capable of enduring a glacial climate, whereas the +living species are now all tropical or sub-tropical in their habits, ought not +to be looked at as anomalies, but merely as examples of a very common +flexibility of constitution, brought, under peculiar circumstances, into play. + +How much of the acclimatisation of species to any peculiar climate is due to +mere habit, and how much to the natural selection of varieties having different +innate constitutions, and how much to both means combined, is a very obscure +question. That habit or custom has some influence I must believe, both from +analogy, and from the incessant advice given in agricultural works, even in the +ancient Encyclopædias of China, to be very cautious in transposing animals from +one district to another; for it is not likely that man should have succeeded in +selecting so many breeds and sub-breeds with constitutions specially fitted for +their own districts: the result must, I think, be due to habit. On the other +hand, I can see no reason to doubt that natural selection will continually tend +to preserve those individuals which are born with constitutions best adapted to +their native countries. In treatises on many kinds of cultivated plants, +certain varieties are said to withstand certain climates better than others: +this is very strikingly shown in works on fruit trees published in the United +States, in which certain varieties are habitually recommended for the northern, +and others for the southern States; and as most of these varieties are of +recent origin, they cannot owe their constitutional differences to habit. The +case of the Jerusalem artichoke, which is never propagated by seed, and of +which consequently new varieties have not been produced, has even been +advanced—for it is now as tender as ever it was—as proving that acclimatisation +cannot be effected! The case, also, of the kidney-bean has been often cited for +a similar purpose, and with much greater weight; but until some one will sow, +during a score of generations, his kidney-beans so early that a very large +proportion are destroyed by frost, and then collect seed from the few +survivors, with care to prevent accidental crosses, and then again get seed +from these seedlings, with the same precautions, the experiment cannot be said +to have been even tried. Nor let it be supposed that no differences in the +constitution of seedling kidney-beans ever appear, for an account has been +published how much more hardy some seedlings appeared to be than others. + +On the whole, I think we may conclude that habit, use, and disuse, have, in +some cases, played a considerable part in the modification of the constitution, +and of the structure of various organs; but that the effects of use and disuse +have often been largely combined with, and sometimes overmastered by, the +natural selection of innate differences. + +Correlation of Growth.—I mean by this expression that the whole organisation is +so tied together during its growth and development, that when slight variations +in any one part occur, and are accumulated through natural selection, other +parts become modified. This is a very important subject, most imperfectly +understood. The most obvious case is, that modifications accumulated solely for +the good of the young or larva, will, it may safely be concluded, affect the +structure of the adult; in the same manner as any malconformation affecting the +early embryo, seriously affects the whole organisation of the adult. The +several parts of the body which are homologous, and which, at an early +embryonic period, are alike, seem liable to vary in an allied manner: we see +this in the right and left sides of the body varying in the same manner; in the +front and hind legs, and even in the jaws and limbs, varying together, for the +lower jaw is believed to be homologous with the limbs. These tendencies, I do +not doubt, may be mastered more or less completely by natural selection: thus a +family of stags once existed with an antler only on one side; and if this had +been of any great use to the breed it might probably have been rendered +permanent by natural selection. + +Homologous parts, as has been remarked by some authors, tend to cohere; this is +often seen in monstrous plants; and nothing is more common than the union of +homologous parts in normal structures, as the union of the petals of the +corolla into a tube. Hard parts seem to affect the form of adjoining soft +parts; it is believed by some authors that the diversity in the shape of the +pelvis in birds causes the remarkable diversity in the shape of their kidneys. +Others believe that the shape of the pelvis in the human mother influences by +pressure the shape of the head of the child. In snakes, according to Schlegel, +the shape of the body and the manner of swallowing determine the position of +several of the most important viscera. + +The nature of the bond of correlation is very frequently quite obscure. M. Is. +Geoffroy St. Hilaire has forcibly remarked, that certain malconformations very +frequently, and that others rarely coexist, without our being able to assign +any reason. What can be more singular than the relation between blue eyes and +deafness in cats, and the tortoise-shell colour with the female sex; the +feathered feet and skin between the outer toes in pigeons, and the presence of +more or less down on the young birds when first hatched, with the future colour +of their plumage; or, again, the relation between the hair and teeth in the +naked Turkish dog, though here probably homology comes into play? With respect +to this latter case of correlation, I think it can hardly be accidental, that +if we pick out the two orders of mammalia which are most abnormal in their +dermal coverings, viz. Cetacea (whales) and Edentata (armadilloes, scaly +ant-eaters, etc.), that these are likewise the most abnormal in their teeth. + +I know of no case better adapted to show the importance of the laws of +correlation in modifying important structures, independently of utility and, +therefore, of natural selection, than that of the difference between the outer +and inner flowers in some Compositous and Umbelliferous plants. Every one knows +the difference in the ray and central florets of, for instance, the daisy, and +this difference is often accompanied with the abortion of parts of the flower. +But, in some Compositous plants, the seeds also differ in shape and sculpture; +and even the ovary itself, with its accessory parts, differs, as has been +described by Cassini. These differences have been attributed by some authors to +pressure, and the shape of the seeds in the ray-florets in some Compositæ +countenances this idea; but, in the case of the corolla of the Umbelliferæ, it +is by no means, as Dr. Hooker informs me, in species with the densest heads +that the inner and outer flowers most frequently differ. It might have been +thought that the development of the ray-petals by drawing nourishment from +certain other parts of the flower had caused their abortion; but in some +Compositæ there is a difference in the seeds of the outer and inner florets +without any difference in the corolla. Possibly, these several differences may +be connected with some difference in the flow of nutriment towards the central +and external flowers: we know, at least, that in irregular flowers, those +nearest to the axis are oftenest subject to peloria, and become regular. I may +add, as an instance of this, and of a striking case of correlation, that I have +recently observed in some garden pelargoniums, that the central flower of the +truss often loses the patches of darker colour in the two upper petals; and +that when this occurs, the adherent nectary is quite aborted; when the colour +is absent from only one of the two upper petals, the nectary is only much +shortened. + +With respect to the difference in the corolla of the central and exterior +flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel’s +idea that the ray-florets serve to attract insects, whose agency is highly +advantageous in the fertilisation of plants of these two orders, is so +far-fetched, as it may at first appear: and if it be advantageous, natural +selection may have come into play. But in regard to the differences both in the +internal and external structure of the seeds, which are not always correlated +with any differences in the flowers, it seems impossible that they can be in +any way advantageous to the plant: yet in the Umbelliferæ these differences are +of such apparent importance—the seeds being in some cases, according to Tausch, +orthospermous in the exterior flowers and coelospermous in the central +flowers,—that the elder De Candolle founded his main divisions of the order on +analogous differences. Hence we see that modifications of structure, viewed by +systematists as of high value, may be wholly due to unknown laws of correlated +growth, and without being, as far as we can see, of the slightest service to +the species. + +We may often falsely attribute to correlation of growth, structures which are +common to whole groups of species, and which in truth are simply due to +inheritance; for an ancient progenitor may have acquired through natural +selection some one modification in structure, and, after thousands of +generations, some other and independent modification; and these two +modifications, having been transmitted to a whole group of descendants with +diverse habits, would naturally be thought to be correlated in some necessary +manner. So, again, I do not doubt that some apparent correlations, occurring +throughout whole orders, are entirely due to the manner alone in which natural +selection can act. For instance, Alph. De Candolle has remarked that winged +seeds are never found in fruits which do not open: I should explain the rule by +the fact that seeds could not gradually become winged through natural +selection, except in fruits which opened; so that the individual plants +producing seeds which were a little better fitted to be wafted further, might +get an advantage over those producing seed less fitted for dispersal; and this +process could not possibly go on in fruit which did not open. + +The elder Geoffroy and Goethe propounded, at about the same period, their law +of compensation or balancement of growth; or, as Goethe expressed it, “in order +to spend on one side, nature is forced to economise on the other side.” I think +this holds true to a certain extent with our domestic productions: if +nourishment flows to one part or organ in excess, it rarely flows, at least in +excess, to another part; thus it is difficult to get a cow to give much milk +and to fatten readily. The same varieties of the cabbage do not yield abundant +and nutritious foliage and a copious supply of oil-bearing seeds. When the +seeds in our fruits become atrophied, the fruit itself gains largely in size +and quality. In our poultry, a large tuft of feathers on the head is generally +accompanied by a diminished comb, and a large beard by diminished wattles. With +species in a state of nature it can hardly be maintained that the law is of +universal application; but many good observers, more especially botanists, +believe in its truth. I will not, however, here give any instances, for I see +hardly any way of distinguishing between the effects, on the one hand, of a +part being largely developed through natural selection and another and +adjoining part being reduced by this same process or by disuse, and, on the +other hand, the actual withdrawal of nutriment from one part owing to the +excess of growth in another and adjoining part. + +I suspect, also, that some of the cases of compensation which have been +advanced, and likewise some other facts, may be merged under a more general +principle, namely, that natural selection is continually trying to economise in +every part of the organisation. If under changed conditions of life a structure +before useful becomes less useful, any diminution, however slight, in its +development, will be seized on by natural selection, for it will profit the +individual not to have its nutriment wasted in building up an useless +structure. I can thus only understand a fact with which I was much struck when +examining cirripedes, and of which many other instances could be given: namely, +that when a cirripede is parasitic within another and is thus protected, it +loses more or less completely its own shell or carapace. This is the case with +the male Ibla, and in a truly extraordinary manner with the Proteolepas: for +the carapace in all other cirripedes consists of the three highly-important +anterior segments of the head enormously developed, and furnished with great +nerves and muscles; but in the parasitic and protected Proteolepas, the whole +anterior part of the head is reduced to the merest rudiment attached to the +bases of the prehensile antennæ. Now the saving of a large and complex +structure, when rendered superfluous by the parasitic habits of the +Proteolepas, though effected by slow steps, would be a decided advantage to +each successive individual of the species; for in the struggle for life to +which every animal is exposed, each individual Proteolepas would have a better +chance of supporting itself, by less nutriment being wasted in developing a +structure now become useless. + +Thus, as I believe, natural selection will always succeed in the long run in +reducing and saving every part of the organisation, as soon as it is rendered +superfluous, without by any means causing some other part to be largely +developed in a corresponding degree. And, conversely, that natural selection +may perfectly well succeed in largely developing any organ, without requiring +as a necessary compensation the reduction of some adjoining part. + +It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in +varieties and in species, that when any part or organ is repeated many times in +the structure of the same individual (as the vertebræ in snakes, and the +stamens in polyandrous flowers) the number is variable; whereas the number of +the same part or organ, when it occurs in lesser numbers, is constant. The same +author and some botanists have further remarked that multiple parts are also +very liable to variation in structure. Inasmuch as this “vegetative +repetition,” to use Professor Owen’s expression, seems to be a sign of low +organisation; the foregoing remark seems connected with the very general +opinion of naturalists, that beings low in the scale of nature are more +variable than those which are higher. I presume that lowness in this case means +that the several parts of the organisation have been but little specialised for +particular functions; and as long as the same part has to perform diversified +work, we can perhaps see why it should remain variable, that is, why natural +selection should have preserved or rejected each little deviation of form less +carefully than when the part has to serve for one special purpose alone. In the +same way that a knife which has to cut all sorts of things may be of almost any +shape; whilst a tool for some particular object had better be of some +particular shape. Natural selection, it should never be forgotten, can act on +each part of each being, solely through and for its advantage. + +Rudimentary parts, it has been stated by some authors, and I believe with +truth, are apt to be highly variable. We shall have to recur to the general +subject of rudimentary and aborted organs; and I will here only add that their +variability seems to be owing to their uselessness, and therefore to natural +selection having no power to check deviations in their structure. Thus +rudimentary parts are left to the free play of the various laws of growth, to +the effects of long-continued disuse, and to the tendency to reversion. + +A part developed in any species in an extraordinary degree or manner, in +comparison with the same part in allied species, tends to be highly variable +.—Several years ago I was much struck with a remark, nearly to the above +effect, published by Mr. Waterhouse. I infer also from an observation made by +Professor Owen, with respect to the length of the arms of the ourang-outang, +that he has come to a nearly similar conclusion. It is hopeless to attempt to +convince any one of the truth of this proposition without giving the long array +of facts which I have collected, and which cannot possibly be here introduced. +I can only state my conviction that it is a rule of high generality. I am aware +of several causes of error, but I hope that I have made due allowance for them. +It should be understood that the rule by no means applies to any part, however +unusually developed, unless it be unusually developed in comparison with the +same part in closely allied species. Thus, the bat’s wing is a most abnormal +structure in the class mammalia; but the rule would not here apply, because +there is a whole group of bats having wings; it would apply only if some one +species of bat had its wings developed in some remarkable manner in comparison +with the other species of the same genus. The rule applies very strongly in the +case of secondary sexual characters, when displayed in any unusual manner. The +term, secondary sexual characters, used by Hunter, applies to characters which +are attached to one sex, but are not directly connected with the act of +reproduction. The rule applies to males and females; but as females more rarely +offer remarkable secondary sexual characters, it applies more rarely to them. +The rule being so plainly applicable in the case of secondary sexual +characters, may be due to the great variability of these characters, whether or +not displayed in any unusual manner—of which fact I think there can be little +doubt. But that our rule is not confined to secondary sexual characters is +clearly shown in the case of hermaphrodite cirripedes; and I may here add, that +I particularly attended to Mr. Waterhouse’s remark, whilst investigating this +Order, and I am fully convinced that the rule almost invariably holds good with +cirripedes. I shall, in my future work, give a list of the more remarkable +cases; I will here only briefly give one, as it illustrates the rule in its +largest application. The opercular valves of sessile cirripedes (rock +barnacles) are, in every sense of the word, very important structures, and they +differ extremely little even in different genera; but in the several species of +one genus, Pyrgoma, these valves present a marvellous amount of +diversification: the homologous valves in the different species being sometimes +wholly unlike in shape; and the amount of variation in the individuals of +several of the species is so great, that it is no exaggeration to state that +the varieties differ more from each other in the characters of these important +valves than do other species of distinct genera. + +As birds within the same country vary in a remarkably small degree, I have +particularly attended to them, and the rule seems to me certainly to hold good +in this class. I cannot make out that it applies to plants, and this would +seriously have shaken my belief in its truth, had not the great variability in +plants made it particularly difficult to compare their relative degrees of +variability. + +When we see any part or organ developed in a remarkable degree or manner in any +species, the fair presumption is that it is of high importance to that species; +nevertheless the part in this case is eminently liable to variation. Why should +this be so? On the view that each species has been independently created, with +all its parts as we now see them, I can see no explanation. But on the view +that groups of species have descended from other species, and have been +modified through natural selection, I think we can obtain some light. In our +domestic animals, if any part, or the whole animal, be neglected and no +selection be applied, that part (for instance, the comb in the Dorking fowl) or +the whole breed will cease to have a nearly uniform character. The breed will +then be said to have degenerated. In rudimentary organs, and in those which +have been but little specialised for any particular purpose, and perhaps in +polymorphic groups, we see a nearly parallel natural case; for in such cases +natural selection either has not or cannot come into full play, and thus the +organisation is left in a fluctuating condition. But what here more especially +concerns us is, that in our domestic animals those points, which at the present +time are undergoing rapid change by continued selection, are also eminently +liable to variation. Look at the breeds of the pigeon; see what a prodigious +amount of difference there is in the beak of the different tumblers, in the +beak and wattle of the different carriers, in the carriage and tail of our +fantails, etc., these being the points now mainly attended to by English +fanciers. Even in the sub-breeds, as in the short-faced tumbler, it is +notoriously difficult to breed them nearly to perfection, and frequently +individuals are born which depart widely from the standard. There may be truly +said to be a constant struggle going on between, on the one hand, the tendency +to reversion to a less modified state, as well as an innate tendency to further +variability of all kinds, and, on the other hand, the power of steady selection +to keep the breed true. In the long run selection gains the day, and we do not +expect to fail so far as to breed a bird as coarse as a common tumbler from a +good short-faced strain. But as long as selection is rapidly going on, there +may always be expected to be much variability in the structure undergoing +modification. It further deserves notice that these variable characters, +produced by man’s selection, sometimes become attached, from causes quite +unknown to us, more to one sex than to the other, generally to the male sex, as +with the wattle of carriers and the enlarged crop of pouters. + +Now let us turn to nature. When a part has been developed in an extraordinary +manner in any one species, compared with the other species of the same genus, +we may conclude that this part has undergone an extraordinary amount of +modification, since the period when the species branched off from the common +progenitor of the genus. This period will seldom be remote in any extreme +degree, as species very rarely endure for more than one geological period. An +extraordinary amount of modification implies an unusually large and +long-continued amount of variability, which has continually been accumulated by +natural selection for the benefit of the species. But as the variability of the +extraordinarily-developed part or organ has been so great and long-continued +within a period not excessively remote, we might, as a general rule, expect +still to find more variability in such parts than in other parts of the +organisation, which have remained for a much longer period nearly constant. And +this, I am convinced, is the case. That the struggle between natural selection +on the one hand, and the tendency to reversion and variability on the other +hand, will in the course of time cease; and that the most abnormally developed +organs may be made constant, I can see no reason to doubt. Hence when an organ, +however abnormal it may be, has been transmitted in approximately the same +condition to many modified descendants, as in the case of the wing of the bat, +it must have existed, according to my theory, for an immense period in nearly +the same state; and thus it comes to be no more variable than any other +structure. It is only in those cases in which the modification has been +comparatively recent and extraordinarily great that we ought to find the +generative variability, as it may be called, still present in a high degree. +For in this case the variability will seldom as yet have been fixed by the +continued selection of the individuals varying in the required manner and +degree, and by the continued rejection of those tending to revert to a former +and less modified condition. + +The principle included in these remarks may be extended. It is notorious that +specific characters are more variable than generic. To explain by a simple +example what is meant. If some species in a large genus of plants had blue +flowers and some had red, the colour would be only a specific character, and no +one would be surprised at one of the blue species varying into red, or +conversely; but if all the species had blue flowers, the colour would become a +generic character, and its variation would be a more unusual circumstance. I +have chosen this example because an explanation is not in this case applicable, +which most naturalists would advance, namely, that specific characters are more +variable than generic, because they are taken from parts of less physiological +importance than those commonly used for classing genera. I believe this +explanation is partly, yet only indirectly, true; I shall, however, have to +return to this subject in our chapter on Classification. It would be almost +superfluous to adduce evidence in support of the above statement, that specific +characters are more variable than generic; but I have repeatedly noticed in +works on natural history, that when an author has remarked with surprise that +some important organ or part, which is generally very constant throughout large +groups of species, has differed considerably in closely-allied species, that it +has, also, been variable in the individuals of some of the species. And this +fact shows that a character, which is generally of generic value, when it sinks +in value and becomes only of specific value, often becomes variable, though its +physiological importance may remain the same. Something of the same kind +applies to monstrosities: at least Is. Geoffroy St. Hilaire seems to entertain +no doubt, that the more an organ normally differs in the different species of +the same group, the more subject it is to individual anomalies. + +On the ordinary view of each species having been independently created, why +should that part of the structure, which differs from the same part in other +independently-created species of the same genus, be more variable than those +parts which are closely alike in the several species? I do not see that any +explanation can be given. But on the view of species being only strongly marked +and fixed varieties, we might surely expect to find them still often continuing +to vary in those parts of their structure which have varied within a moderately +recent period, and which have thus come to differ. Or to state the case in +another manner:—the points in which all the species of a genus resemble each +other, and in which they differ from the species of some other genus, are +called generic characters; and these characters in common I attribute to +inheritance from a common progenitor, for it can rarely have happened that +natural selection will have modified several species, fitted to more or less +widely-different habits, in exactly the same manner: and as these so-called +generic characters have been inherited from a remote period, since that period +when the species first branched off from their common progenitor, and +subsequently have not varied or come to differ in any degree, or only in a +slight degree, it is not probable that they should vary at the present day. On +the other hand, the points in which species differ from other species of the +same genus, are called specific characters; and as these specific characters +have varied and come to differ within the period of the branching off of the +species from a common progenitor, it is probable that they should still often +be in some degree variable,—at least more variable than those parts of the +organisation which have for a very long period remained constant. + +In connexion with the present subject, I will make only two other remarks. I +think it will be admitted, without my entering on details, that secondary +sexual characters are very variable; I think it also will be admitted that +species of the same group differ from each other more widely in their secondary +sexual characters, than in other parts of their organisation; compare, for +instance, the amount of difference between the males of gallinaceous birds, in +which secondary sexual characters are strongly displayed, with the amount of +difference between their females; and the truth of this proposition will be +granted. The cause of the original variability of secondary sexual characters +is not manifest; but we can see why these characters should not have been +rendered as constant and uniform as other parts of the organisation; for +secondary sexual characters have been accumulated by sexual selection, which is +less rigid in its action than ordinary selection, as it does not entail death, +but only gives fewer offspring to the less favoured males. Whatever the cause +may be of the variability of secondary sexual characters, as they are highly +variable, sexual selection will have had a wide scope for action, and may thus +readily have succeeded in giving to the species of the same group a greater +amount of difference in their sexual characters, than in other parts of their +structure. + +It is a remarkable fact, that the secondary sexual differences between the two +sexes of the same species are generally displayed in the very same parts of the +organisation in which the different species of the same genus differ from each +other. Of this fact I will give in illustration two instances, the first which +happen to stand on my list; and as the differences in these cases are of a very +unusual nature, the relation can hardly be accidental. The same number of +joints in the tarsi is a character generally common to very large groups of +beetles, but in the Engidæ, as Westwood has remarked, the number varies +greatly; and the number likewise differs in the two sexes of the same species: +again in fossorial hymenoptera, the manner of neuration of the wings is a +character of the highest importance, because common to large groups; but in +certain genera the neuration differs in the different species, and likewise in +the two sexes of the same species. This relation has a clear meaning on my view +of the subject: I look at all the species of the same genus as having as +certainly descended from the same progenitor, as have the two sexes of any one +of the species. Consequently, whatever part of the structure of the common +progenitor, or of its early descendants, became variable; variations of this +part would it is highly probable, be taken advantage of by natural and sexual +selection, in order to fit the several species to their several places in the +economy of nature, and likewise to fit the two sexes of the same species to +each other, or to fit the males and females to different habits of life, or the +males to struggle with other males for the possession of the females. + +Finally, then, I conclude that the greater variability of specific characters, +or those which distinguish species from species, than of generic characters, or +those which the species possess in common;—that the frequent extreme +variability of any part which is developed in a species in an extraordinary +manner in comparison with the same part in its congeners; and the not great +degree of variability in a part, however extraordinarily it may be developed, +if it be common to a whole group of species;—that the great variability of +secondary sexual characters, and the great amount of difference in these same +characters between closely allied species;—that secondary sexual and ordinary +specific differences are generally displayed in the same parts of the +organisation,—are all principles closely connected together. All being mainly +due to the species of the same group having descended from a common progenitor, +from whom they have inherited much in common,—to parts which have recently and +largely varied being more likely still to go on varying than parts which have +long been inherited and have not varied,—to natural selection having more or +less completely, according to the lapse of time, overmastered the tendency to +reversion and to further variability,—to sexual selection being less rigid than +ordinary selection,—and to variations in the same parts having been accumulated +by natural and sexual selection, and thus adapted for secondary sexual, and for +ordinary specific purposes. + +Distinct species present analogous variations; and a variety of one species +often assumes some of the characters of an allied species, or reverts to some +of the characters of an early progenitor.—These propositions will be most +readily understood by looking to our domestic races. The most distinct breeds +of pigeons, in countries most widely apart, present sub-varieties with reversed +feathers on the head and feathers on the feet,—characters not possessed by the +aboriginal rock-pigeon; these then are analogous variations in two or more +distinct races. The frequent presence of fourteen or even sixteen tail-feathers +in the pouter, may be considered as a variation representing the normal +structure of another race, the fantail. I presume that no one will doubt that +all such analogous variations are due to the several races of the pigeon having +inherited from a common parent the same constitution and tendency to variation, +when acted on by similar unknown influences. In the vegetable kingdom we have a +case of analogous variation, in the enlarged stems, or roots as commonly +called, of the Swedish turnip and Ruta baga, plants which several botanists +rank as varieties produced by cultivation from a common parent: if this be not +so, the case will then be one of analogous variation in two so-called distinct +species; and to these a third may be added, namely, the common turnip. +According to the ordinary view of each species having been independently +created, we should have to attribute this similarity in the enlarged stems of +these three plants, not to the vera causa of community of descent, and a +consequent tendency to vary in a like manner, but to three separate yet closely +related acts of creation. + +With pigeons, however, we have another case, namely, the occasional appearance +in all the breeds, of slaty-blue birds with two black bars on the wings, a +white rump, a bar at the end of the tail, with the outer feathers externally +edged near their bases with white. As all these marks are characteristic of the +parent rock-pigeon, I presume that no one will doubt that this is a case of +reversion, and not of a new yet analogous variation appearing in the several +breeds. We may I think confidently come to this conclusion, because, as we have +seen, these coloured marks are eminently liable to appear in the crossed +offspring of two distinct and differently coloured breeds; and in this case +there is nothing in the external conditions of life to cause the reappearance +of the slaty-blue, with the several marks, beyond the influence of the mere act +of crossing on the laws of inheritance. + +No doubt it is a very surprising fact that characters should reappear after +having been lost for many, perhaps for hundreds of generations. But when a +breed has been crossed only once by some other breed, the offspring +occasionally show a tendency to revert in character to the foreign breed for +many generations—some say, for a dozen or even a score of generations. After +twelve generations, the proportion of blood, to use a common expression, of any +one ancestor, is only 1 in 2048; and yet, as we see, it is generally believed +that a tendency to reversion is retained by this very small proportion of +foreign blood. In a breed which has not been crossed, but in which both parents +have lost some character which their progenitor possessed, the tendency, +whether strong or weak, to reproduce the lost character might be, as was +formerly remarked, for all that we can see to the contrary, transmitted for +almost any number of generations. When a character which has been lost in a +breed, reappears after a great number of generations, the most probable +hypothesis is, not that the offspring suddenly takes after an ancestor some +hundred generations distant, but that in each successive generation there has +been a tendency to reproduce the character in question, which at last, under +unknown favourable conditions, gains an ascendancy. For instance, it is +probable that in each generation of the barb-pigeon, which produces most rarely +a blue and black-barred bird, there has been a tendency in each generation in +the plumage to assume this colour. This view is hypothetical, but could be +supported by some facts; and I can see no more abstract improbability in a +tendency to produce any character being inherited for an endless number of +generations, than in quite useless or rudimentary organs being, as we all know +them to be, thus inherited. Indeed, we may sometimes observe a mere tendency to +produce a rudiment inherited: for instance, in the common snapdragon +(Antirrhinum) a rudiment of a fifth stamen so often appears, that this plant +must have an inherited tendency to produce it. + +As all the species of the same genus are supposed, on my theory, to have +descended from a common parent, it might be expected that they would +occasionally vary in an analogous manner; so that a variety of one species +would resemble in some of its characters another species; this other species +being on my view only a well-marked and permanent variety. But characters thus +gained would probably be of an unimportant nature, for the presence of all +important characters will be governed by natural selection, in accordance with +the diverse habits of the species, and will not be left to the mutual action of +the conditions of life and of a similar inherited constitution. It might +further be expected that the species of the same genus would occasionally +exhibit reversions to lost ancestral characters. As, however, we never know the +exact character of the common ancestor of a group, we could not distinguish +these two cases: if, for instance, we did not know that the rock-pigeon was not +feather-footed or turn-crowned, we could not have told, whether these +characters in our domestic breeds were reversions or only analogous variations; +but we might have inferred that the blueness was a case of reversion, from the +number of the markings, which are correlated with the blue tint, and which it +does not appear probable would all appear together from simple variation. More +especially we might have inferred this, from the blue colour and marks so often +appearing when distinct breeds of diverse colours are crossed. Hence, though +under nature it must generally be left doubtful, what cases are reversions to +an anciently existing character, and what are new but analogous variations, yet +we ought, on my theory, sometimes to find the varying offspring of a species +assuming characters (either from reversion or from analogous variation) which +already occur in some other members of the same group. And this undoubtedly is +the case in nature. + +A considerable part of the difficulty in recognising a variable species in our +systematic works, is due to its varieties mocking, as it were, some of the +other species of the same genus. A considerable catalogue, also, could be given +of forms intermediate between two other forms, which themselves must be +doubtfully ranked as either varieties or species; and this shows, unless all +these forms be considered as independently created species, that the one in +varying has assumed some of the characters of the other, so as to produce the +intermediate form. But the best evidence is afforded by parts or organs of an +important and uniform nature occasionally varying so as to acquire, in some +degree, the character of the same part or organ in an allied species. I have +collected a long list of such cases; but here, as before, I lie under a great +disadvantage in not being able to give them. I can only repeat that such cases +certainly do occur, and seem to me very remarkable. + +I will, however, give one curious and complex case, not indeed as affecting any +important character, but from occurring in several species of the same genus, +partly under domestication and partly under nature. It is a case apparently of +reversion. The ass not rarely has very distinct transverse bars on its legs, +like those on the legs of a zebra: it has been asserted that these are plainest +in the foal, and from inquiries which I have made, I believe this to be true. +It has also been asserted that the stripe on each shoulder is sometimes double. +The shoulder stripe is certainly very variable in length and outline. A white +ass, but not an albino, has been described without either spinal or +shoulder-stripe; and these stripes are sometimes very obscure, or actually +quite lost, in dark-coloured asses. The koulan of Pallas is said to have been +seen with a double shoulder-stripe. The hemionus has no shoulder-stripe; but +traces of it, as stated by Mr. Blyth and others, occasionally appear: and I +have been informed by Colonel Poole that the foals of this species are +generally striped on the legs, and faintly on the shoulder. The quagga, though +so plainly barred like a zebra over the body, is without bars on the legs; but +Dr. Gray has figured one specimen with very distinct zebra-like bars on the +hocks. + +With respect to the horse, I have collected cases in England of the spinal +stripe in horses of the most distinct breeds, and of all colours; transverse +bars on the legs are not rare in duns, mouse-duns, and in one instance in a +chestnut: a faint shoulder-stripe may sometimes be seen in duns, and I have +seen a trace in a bay horse. My son made a careful examination and sketch for +me of a dun Belgian cart-horse with a double stripe on each shoulder and with +leg-stripes; and a man, whom I can implicitly trust, has examined for me a +small dun Welch pony with three short parallel stripes on each shoulder. + +In the north-west part of India the Kattywar breed of horses is so generally +striped, that, as I hear from Colonel Poole, who examined the breed for the +Indian Government, a horse without stripes is not considered as purely-bred. +The spine is always striped; the legs are generally barred; and the +shoulder-stripe, which is sometimes double and sometimes treble, is common; the +side of the face, moreover, is sometimes striped. The stripes are plainest in +the foal; and sometimes quite disappear in old horses. Colonel Poole has seen +both gray and bay Kattywar horses striped when first foaled. I have, also, +reason to suspect, from information given me by Mr. W. W. Edwards, that with +the English race-horse the spinal stripe is much commoner in the foal than in +the full-grown animal. Without here entering on further details, I may state +that I have collected cases of leg and shoulder stripes in horses of very +different breeds, in various countries from Britain to Eastern China; and from +Norway in the north to the Malay Archipelago in the south. In all parts of the +world these stripes occur far oftenest in duns and mouse-duns; by the term dun +a large range of colour is included, from one between brown and black to a +close approach to cream-colour. + +I am aware that Colonel Hamilton Smith, who has written on this subject, +believes that the several breeds of the horse have descended from several +aboriginal species—one of which, the dun, was striped; and that the +above-described appearances are all due to ancient crosses with the dun stock. +But I am not at all satisfied with this theory, and should be loth to apply it +to breeds so distinct as the heavy Belgian cart-horse, Welch ponies, cobs, the +lanky Kattywar race, etc., inhabiting the most distant parts of the world. + +Now let us turn to the effects of crossing the several species of the +horse-genus. Rollin asserts, that the common mule from the ass and horse is +particularly apt to have bars on its legs. I once saw a mule with its legs so +much striped that any one at first would have thought that it must have been +the product of a zebra; and Mr. W. C. Martin, in his excellent treatise on the +horse, has given a figure of a similar mule. In four coloured drawings, which I +have seen, of hybrids between the ass and zebra, the legs were much more +plainly barred than the rest of the body; and in one of them there was a double +shoulder-stripe. In Lord Moreton’s famous hybrid from a chestnut mare and male +quagga, the hybrid, and even the pure offspring subsequently produced from the +mare by a black Arabian sire, were much more plainly barred across the legs +than is even the pure quagga. Lastly, and this is another most remarkable case, +a hybrid has been figured by Dr. Gray (and he informs me that he knows of a +second case) from the ass and the hemionus; and this hybrid, though the ass +seldom has stripes on its legs and the hemionus has none and has not even a +shoulder-stripe, nevertheless had all four legs barred, and had three short +shoulder-stripes, like those on the dun Welch pony, and even had some +zebra-like stripes on the sides of its face. With respect to this last fact, I +was so convinced that not even a stripe of colour appears from what would +commonly be called an accident, that I was led solely from the occurrence of +the face-stripes on this hybrid from the ass and hemionus, to ask Colonel Poole +whether such face-stripes ever occur in the eminently striped Kattywar breed of +horses, and was, as we have seen, answered in the affirmative. + +What now are we to say to these several facts? We see several very distinct +species of the horse-genus becoming, by simple variation, striped on the legs +like a zebra, or striped on the shoulders like an ass. In the horse we see this +tendency strong whenever a dun tint appears—a tint which approaches to that of +the general colouring of the other species of the genus. The appearance of the +stripes is not accompanied by any change of form or by any other new character. +We see this tendency to become striped most strongly displayed in hybrids from +between several of the most distinct species. Now observe the case of the +several breeds of pigeons: they are descended from a pigeon (including two or +three sub-species or geographical races) of a bluish colour, with certain bars +and other marks; and when any breed assumes by simple variation a bluish tint, +these bars and other marks invariably reappear; but without any other change of +form or character. When the oldest and truest breeds of various colours are +crossed, we see a strong tendency for the blue tint and bars and marks to +reappear in the mongrels. I have stated that the most probable hypothesis to +account for the reappearance of very ancient characters, is—that there is a +tendency in the young of each successive generation to produce the long-lost +character, and that this tendency, from unknown causes, sometimes prevails. And +we have just seen that in several species of the horse-genus the stripes are +either plainer or appear more commonly in the young than in the old. Call the +breeds of pigeons, some of which have bred true for centuries, species; and how +exactly parallel is the case with that of the species of the horse-genus! For +myself, I venture confidently to look back thousands on thousands of +generations, and I see an animal striped like a zebra, but perhaps otherwise +very differently constructed, the common parent of our domestic horse, whether +or not it be descended from one or more wild stocks, of the ass, the hemionus, +quagga, and zebra. + +He who believes that each equine species was independently created, will, I +presume, assert that each species has been created with a tendency to vary, +both under nature and under domestication, in this particular manner, so as +often to become striped like other species of the genus; and that each has been +created with a strong tendency, when crossed with species inhabiting distant +quarters of the world, to produce hybrids resembling in their stripes, not +their own parents, but other species of the genus. To admit this view is, as it +seems to me, to reject a real for an unreal, or at least for an unknown, cause. +It makes the works of God a mere mockery and deception; I would almost as soon +believe with the old and ignorant cosmogonists, that fossil shells had never +lived, but had been created in stone so as to mock the shells now living on the +sea-shore. + +Summary.—Our ignorance of the laws of variation is profound. Not in one case +out of a hundred can we pretend to assign any reason why this or that part +differs, more or less, from the same part in the parents. But whenever we have +the means of instituting a comparison, the same laws appear to have acted in +producing the lesser differences between varieties of the same species, and the +greater differences between species of the same genus. The external conditions +of life, as climate and food, etc., seem to have induced some slight +modifications. Habit in producing constitutional differences, and use in +strengthening, and disuse in weakening and diminishing organs, seem to have +been more potent in their effects. Homologous parts tend to vary in the same +way, and homologous parts tend to cohere. Modifications in hard parts and in +external parts sometimes affect softer and internal parts. When one part is +largely developed, perhaps it tends to draw nourishment from the adjoining +parts; and every part of the structure which can be saved without detriment to +the individual, will be saved. Changes of structure at an early age will +generally affect parts subsequently developed; and there are very many other +correlations of growth, the nature of which we are utterly unable to +understand. Multiple parts are variable in number and in structure, perhaps +arising from such parts not having been closely specialised to any particular +function, so that their modifications have not been closely checked by natural +selection. It is probably from this same cause that organic beings low in the +scale of nature are more variable than those which have their whole +organisation more specialised, and are higher in the scale. Rudimentary organs, +from being useless, will be disregarded by natural selection, and hence +probably are variable. Specific characters—that is, the characters which have +come to differ since the several species of the same genus branched off from a +common parent—are more variable than generic characters, or those which have +long been inherited, and have not differed within this same period. In these +remarks we have referred to special parts or organs being still variable, +because they have recently varied and thus come to differ; but we have also +seen in the second Chapter that the same principle applies to the whole +individual; for in a district where many species of any genus are found—that +is, where there has been much former variation and differentiation, or where +the manufactory of new specific forms has been actively at work—there, on an +average, we now find most varieties or incipient species. Secondary sexual +characters are highly variable, and such characters differ much in the species +of the same group. Variability in the same parts of the organisation has +generally been taken advantage of in giving secondary sexual differences to the +sexes of the same species, and specific differences to the several species of +the same genus. Any part or organ developed to an extraordinary size or in an +extraordinary manner, in comparison with the same part or organ in the allied +species, must have gone through an extraordinary amount of modification since +the genus arose; and thus we can understand why it should often still be +variable in a much higher degree than other parts; for variation is a +long-continued and slow process, and natural selection will in such cases not +as yet have had time to overcome the tendency to further variability and to +reversion to a less modified state. But when a species with any +extraordinarily-developed organ has become the parent of many modified +descendants—which on my view must be a very slow process, requiring a long +lapse of time—in this case, natural selection may readily have succeeded in +giving a fixed character to the organ, in however extraordinary a manner it may +be developed. Species inheriting nearly the same constitution from a common +parent and exposed to similar influences will naturally tend to present +analogous variations, and these same species may occasionally revert to some of +the characters of their ancient progenitors. Although new and important +modifications may not arise from reversion and analogous variation, such +modifications will add to the beautiful and harmonious diversity of nature. + +Whatever the cause may be of each slight difference in the offspring from their +parents—and a cause for each must exist—it is the steady accumulation, through +natural selection, of such differences, when beneficial to the individual, that +gives rise to all the more important modifications of structure, by which the +innumerable beings on the face of this earth are enabled to struggle with each +other, and the best adapted to survive. + +CHAPTER VI. +DIFFICULTIES ON THEORY. + +Difficulties on the theory of descent with modification. Transitions. Absence +or rarity of transitional varieties. Transitions in habits of life. Diversified +habits in the same species. Species with habits widely different from those of +their allies. Organs of extreme perfection. Means of transition. Cases of +difficulty. Natura non facit saltum. Organs of small importance. Organs not in +all cases absolutely perfect. The law of Unity of Type and of the Conditions of +Existence embraced by the theory of Natural Selection. + +Long before having arrived at this part of my work, a crowd of difficulties +will have occurred to the reader. Some of them are so grave that to this day I +can never reflect on them without being staggered; but, to the best of my +judgment, the greater number are only apparent, and those that are real are +not, I think, fatal to my theory. + +These difficulties and objections may be classed under the following heads:— + +Firstly, why, if species have descended from other species by insensibly fine +gradations, do we not everywhere see innumerable transitional forms? Why is not +all nature in confusion instead of the species being, as we see them, well +defined? + +Secondly, is it possible that an animal having, for instance, the structure and +habits of a bat, could have been formed by the modification of some animal with +wholly different habits? Can we believe that natural selection could produce, +on the one hand, organs of trifling importance, such as the tail of a giraffe, +which serves as a fly-flapper, and, on the other hand, organs of such wonderful +structure, as the eye, of which we hardly as yet fully understand the +inimitable perfection? + +Thirdly, can instincts be acquired and modified through natural selection? What +shall we say to so marvellous an instinct as that which leads the bee to make +cells, which have practically anticipated the discoveries of profound +mathematicians? + +Fourthly, how can we account for species, when crossed, being sterile and +producing sterile offspring, whereas, when varieties are crossed, their +fertility is unimpaired? + +The two first heads shall be here discussed—Instinct and Hybridism in separate +chapters. + +On the absence or rarity of transitional varieties.—As natural selection acts +solely by the preservation of profitable modifications, each new form will tend +in a fully-stocked country to take the place of, and finally to exterminate, +its own less improved parent or other less-favoured forms with which it comes +into competition. Thus extinction and natural selection will, as we have seen, +go hand in hand. Hence, if we look at each species as descended from some other +unknown form, both the parent and all the transitional varieties will generally +have been exterminated by the very process of formation and perfection of the +new form. + +But, as by this theory innumerable transitional forms must have existed, why do +we not find them embedded in countless numbers in the crust of the earth? It +will be much more convenient to discuss this question in the chapter on the +Imperfection of the geological record; and I will here only state that I +believe the answer mainly lies in the record being incomparably less perfect +than is generally supposed; the imperfection of the record being chiefly due to +organic beings not inhabiting profound depths of the sea, and to their remains +being embedded and preserved to a future age only in masses of sediment +sufficiently thick and extensive to withstand an enormous amount of future +degradation; and such fossiliferous masses can be accumulated only where much +sediment is deposited on the shallow bed of the sea, whilst it slowly subsides. +These contingencies will concur only rarely, and after enormously long +intervals. Whilst the bed of the sea is stationary or is rising, or when very +little sediment is being deposited, there will be blanks in our geological +history. The crust of the earth is a vast museum; but the natural collections +have been made only at intervals of time immensely remote. + +But it may be urged that when several closely-allied species inhabit the same +territory we surely ought to find at the present time many transitional forms. +Let us take a simple case: in travelling from north to south over a continent, +we generally meet at successive intervals with closely allied or representative +species, evidently filling nearly the same place in the natural economy of the +land. These representative species often meet and interlock; and as the one +becomes rarer and rarer, the other becomes more and more frequent, till the one +replaces the other. But if we compare these species where they intermingle, +they are generally as absolutely distinct from each other in every detail of +structure as are specimens taken from the metropolis inhabited by each. By my +theory these allied species have descended from a common parent; and during the +process of modification, each has become adapted to the conditions of life of +its own region, and has supplanted and exterminated its original parent and all +the transitional varieties between its past and present states. Hence we ought +not to expect at the present time to meet with numerous transitional varieties +in each region, though they must have existed there, and may be embedded there +in a fossil condition. But in the intermediate region, having intermediate +conditions of life, why do we not now find closely-linking intermediate +varieties? This difficulty for a long time quite confounded me. But I think it +can be in large part explained. + +In the first place we should be extremely cautious in inferring, because an +area is now continuous, that it has been continuous during a long period. +Geology would lead us to believe that almost every continent has been broken up +into islands even during the later tertiary periods; and in such islands +distinct species might have been separately formed without the possibility of +intermediate varieties existing in the intermediate zones. By changes in the +form of the land and of climate, marine areas now continuous must often have +existed within recent times in a far less continuous and uniform condition than +at present. But I will pass over this way of escaping from the difficulty; for +I believe that many perfectly defined species have been formed on strictly +continuous areas; though I do not doubt that the formerly broken condition of +areas now continuous has played an important part in the formation of new +species, more especially with freely-crossing and wandering animals. + +In looking at species as they are now distributed over a wide area, we +generally find them tolerably numerous over a large territory, then becoming +somewhat abruptly rarer and rarer on the confines, and finally disappearing. +Hence the neutral territory between two representative species is generally +narrow in comparison with the territory proper to each. We see the same fact in +ascending mountains, and sometimes it is quite remarkable how abruptly, as +Alph. De Candolle has observed, a common alpine species disappears. The same +fact has been noticed by Forbes in sounding the depths of the sea with the +dredge. To those who look at climate and the physical conditions of life as the +all-important elements of distribution, these facts ought to cause surprise, as +climate and height or depth graduate away insensibly. But when we bear in mind +that almost every species, even in its metropolis, would increase immensely in +numbers, were it not for other competing species; that nearly all either prey +on or serve as prey for others; in short, that each organic being is either +directly or indirectly related in the most important manner to other organic +beings, we must see that the range of the inhabitants of any country by no +means exclusively depends on insensibly changing physical conditions, but in +large part on the presence of other species, on which it depends, or by which +it is destroyed, or with which it comes into competition; and as these species +are already defined objects (however they may have become so), not blending one +into another by insensible gradations, the range of any one species, depending +as it does on the range of others, will tend to be sharply defined. Moreover, +each species on the confines of its range, where it exists in lessened numbers, +will, during fluctuations in the number of its enemies or of its prey, or in +the seasons, be extremely liable to utter extermination; and thus its +geographical range will come to be still more sharply defined. + +If I am right in believing that allied or representative species, when +inhabiting a continuous area, are generally so distributed that each has a wide +range, with a comparatively narrow neutral territory between them, in which +they become rather suddenly rarer and rarer; then, as varieties do not +essentially differ from species, the same rule will probably apply to both; and +if we in imagination adapt a varying species to a very large area, we shall +have to adapt two varieties to two large areas, and a third variety to a narrow +intermediate zone. The intermediate variety, consequently, will exist in lesser +numbers from inhabiting a narrow and lesser area; and practically, as far as I +can make out, this rule holds good with varieties in a state of nature. I have +met with striking instances of the rule in the case of varieties intermediate +between well-marked varieties in the genus Balanus. And it would appear from +information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that +generally when varieties intermediate between two other forms occur, they are +much rarer numerically than the forms which they connect. Now, if we may trust +these facts and inferences, and therefore conclude that varieties linking two +other varieties together have generally existed in lesser numbers than the +forms which they connect, then, I think, we can understand why intermediate +varieties should not endure for very long periods;—why as a general rule they +should be exterminated and disappear, sooner than the forms which they +originally linked together. + +For any form existing in lesser numbers would, as already remarked, run a +greater chance of being exterminated than one existing in large numbers; and in +this particular case the intermediate form would be eminently liable to the +inroads of closely allied forms existing on both sides of it. But a far more +important consideration, as I believe, is that, during the process of further +modification, by which two varieties are supposed on my theory to be converted +and perfected into two distinct species, the two which exist in larger numbers +from inhabiting larger areas, will have a great advantage over the intermediate +variety, which exists in smaller numbers in a narrow and intermediate zone. For +forms existing in larger numbers will always have a better chance, within any +given period, of presenting further favourable variations for natural selection +to seize on, than will the rarer forms which exist in lesser numbers. Hence, +the more common forms, in the race for life, will tend to beat and supplant the +less common forms, for these will be more slowly modified and improved. It is +the same principle which, as I believe, accounts for the common species in each +country, as shown in the second chapter, presenting on an average a greater +number of well-marked varieties than do the rarer species. I may illustrate +what I mean by supposing three varieties of sheep to be kept, one adapted to an +extensive mountainous region; a second to a comparatively narrow, hilly tract; +and a third to wide plains at the base; and that the inhabitants are all trying +with equal steadiness and skill to improve their stocks by selection; the +chances in this case will be strongly in favour of the great holders on the +mountains or on the plains improving their breeds more quickly than the small +holders on the intermediate narrow, hilly tract; and consequently the improved +mountain or plain breed will soon take the place of the less improved hill +breed; and thus the two breeds, which originally existed in greater numbers, +will come into close contact with each other, without the interposition of the +supplanted, intermediate hill-variety. + +To sum up, I believe that species come to be tolerably well-defined objects, +and do not at any one period present an inextricable chaos of varying and +intermediate links: firstly, because new varieties are very slowly formed, for +variation is a very slow process, and natural selection can do nothing until +favourable variations chance to occur, and until a place in the natural polity +of the country can be better filled by some modification of some one or more of +its inhabitants. And such new places will depend on slow changes of climate, or +on the occasional immigration of new inhabitants, and, probably, in a still +more important degree, on some of the old inhabitants becoming slowly modified, +with the new forms thus produced and the old ones acting and reacting on each +other. So that, in any one region and at any one time, we ought only to see a +few species presenting slight modifications of structure in some degree +permanent; and this assuredly we do see. + +Secondly, areas now continuous must often have existed within the recent period +in isolated portions, in which many forms, more especially amongst the classes +which unite for each birth and wander much, may have separately been rendered +sufficiently distinct to rank as representative species. In this case, +intermediate varieties between the several representative species and their +common parent, must formerly have existed in each broken portion of the land, +but these links will have been supplanted and exterminated during the process +of natural selection, so that they will no longer exist in a living state. + +Thirdly, when two or more varieties have been formed in different portions of a +strictly continuous area, intermediate varieties will, it is probable, at first +have been formed in the intermediate zones, but they will generally have had a +short duration. For these intermediate varieties will, from reasons already +assigned (namely from what we know of the actual distribution of closely allied +or representative species, and likewise of acknowledged varieties), exist in +the intermediate zones in lesser numbers than the varieties which they tend to +connect. From this cause alone the intermediate varieties will be liable to +accidental extermination; and during the process of further modification +through natural selection, they will almost certainly be beaten and supplanted +by the forms which they connect; for these from existing in greater numbers +will, in the aggregate, present more variation, and thus be further improved +through natural selection and gain further advantages. + +Lastly, looking not to any one time, but to all time, if my theory be true, +numberless intermediate varieties, linking most closely all the species of the +same group together, must assuredly have existed; but the very process of +natural selection constantly tends, as has been so often remarked, to +exterminate the parent forms and the intermediate links. Consequently evidence +of their former existence could be found only amongst fossil remains, which are +preserved, as we shall in a future chapter attempt to show, in an extremely +imperfect and intermittent record. + +On the origin and transitions of organic beings with peculiar habits and +structure.—It has been asked by the opponents of such views as I hold, how, for +instance, a land carnivorous animal could have been converted into one with +aquatic habits; for how could the animal in its transitional state have +subsisted? It would be easy to show that within the same group carnivorous +animals exist having every intermediate grade between truly aquatic and +strictly terrestrial habits; and as each exists by a struggle for life, it is +clear that each is well adapted in its habits to its place in nature. Look at +the Mustela vison of North America, which has webbed feet and which resembles +an otter in its fur, short legs, and form of tail; during summer this animal +dives for and preys on fish, but during the long winter it leaves the frozen +waters, and preys like other polecats on mice and land animals. If a different +case had been taken, and it had been asked how an insectivorous quadruped could +possibly have been converted into a flying bat, the question would have been +far more difficult, and I could have given no answer. Yet I think such +difficulties have very little weight. + +Here, as on other occasions, I lie under a heavy disadvantage, for out of the +many striking cases which I have collected, I can give only one or two +instances of transitional habits and structures in closely allied species of +the same genus; and of diversified habits, either constant or occasional, in +the same species. And it seems to me that nothing less than a long list of such +cases is sufficient to lessen the difficulty in any particular case like that +of the bat. + +Look at the family of squirrels; here we have the finest gradation from animals +with their tails only slightly flattened, and from others, as Sir J. Richardson +has remarked, with the posterior part of their bodies rather wide and with the +skin on their flanks rather full, to the so-called flying squirrels; and flying +squirrels have their limbs and even the base of the tail united by a broad +expanse of skin, which serves as a parachute and allows them to glide through +the air to an astonishing distance from tree to tree. We cannot doubt that each +structure is of use to each kind of squirrel in its own country, by enabling it +to escape birds or beasts of prey, or to collect food more quickly, or, as +there is reason to believe, by lessening the danger from occasional falls. But +it does not follow from this fact that the structure of each squirrel is the +best that it is possible to conceive under all natural conditions. Let the +climate and vegetation change, let other competing rodents or new beasts of +prey immigrate, or old ones become modified, and all analogy would lead us to +believe that some at least of the squirrels would decrease in numbers or become +exterminated, unless they also became modified and improved in structure in a +corresponding manner. Therefore, I can see no difficulty, more especially under +changing conditions of life, in the continued preservation of individuals with +fuller and fuller flank-membranes, each modification being useful, each being +propagated, until by the accumulated effects of this process of natural +selection, a perfect so-called flying squirrel was produced. + +Now look at the Galeopithecus or flying lemur, which formerly was falsely +ranked amongst bats. It has an extremely wide flank-membrane, stretching from +the corners of the jaw to the tail, and including the limbs and the elongated +fingers: the flank membrane is, also, furnished with an extensor muscle. +Although no graduated links of structure, fitted for gliding through the air, +now connect the Galeopithecus with the other Lemuridæ, yet I can see no +difficulty in supposing that such links formerly existed, and that each had +been formed by the same steps as in the case of the less perfectly gliding +squirrels; and that each grade of structure had been useful to its possessor. +Nor can I see any insuperable difficulty in further believing it possible that +the membrane-connected fingers and fore-arm of the Galeopithecus might be +greatly lengthened by natural selection; and this, as far as the organs of +flight are concerned, would convert it into a bat. In bats which have the +wing-membrane extended from the top of the shoulder to the tail, including the +hind-legs, we perhaps see traces of an apparatus originally constructed for +gliding through the air rather than for flight. + +If about a dozen genera of birds had become extinct or were unknown, who would +have ventured to have surmised that birds might have existed which used their +wings solely as flappers, like the logger-headed duck (Micropterus of Eyton); +as fins in the water and front legs on the land, like the penguin; as sails, +like the ostrich; and functionally for no purpose, like the Apteryx. Yet the +structure of each of these birds is good for it, under the conditions of life +to which it is exposed, for each has to live by a struggle; but it is not +necessarily the best possible under all possible conditions. It must not be +inferred from these remarks that any of the grades of wing-structure here +alluded to, which perhaps may all have resulted from disuse, indicate the +natural steps by which birds have acquired their perfect power of flight; but +they serve, at least, to show what diversified means of transition are +possible. + +Seeing that a few members of such water-breathing classes as the Crustacea and +Mollusca are adapted to live on the land, and seeing that we have flying birds +and mammals, flying insects of the most diversified types, and formerly had +flying reptiles, it is conceivable that flying-fish, which now glide far +through the air, slightly rising and turning by the aid of their fluttering +fins, might have been modified into perfectly winged animals. If this had been +effected, who would have ever imagined that in an early transitional state they +had been inhabitants of the open ocean, and had used their incipient organs of +flight exclusively, as far as we know, to escape being devoured by other fish? + +When we see any structure highly perfected for any particular habit, as the +wings of a bird for flight, we should bear in mind that animals displaying +early transitional grades of the structure will seldom continue to exist to the +present day, for they will have been supplanted by the very process of +perfection through natural selection. Furthermore, we may conclude that +transitional grades between structures fitted for very different habits of life +will rarely have been developed at an early period in great numbers and under +many subordinate forms. Thus, to return to our imaginary illustration of the +flying-fish, it does not seem probable that fishes capable of true flight would +have been developed under many subordinate forms, for taking prey of many kinds +in many ways, on the land and in the water, until their organs of flight had +come to a high stage of perfection, so as to have given them a decided +advantage over other animals in the battle for life. Hence the chance of +discovering species with transitional grades of structure in a fossil condition +will always be less, from their having existed in lesser numbers, than in the +case of species with fully developed structures. + +I will now give two or three instances of diversified and of changed habits in +the individuals of the same species. When either case occurs, it would be easy +for natural selection to fit the animal, by some modification of its structure, +for its changed habits, or exclusively for one of its several different habits. +But it is difficult to tell, and immaterial for us, whether habits generally +change first and structure afterwards; or whether slight modifications of +structure lead to changed habits; both probably often change almost +simultaneously. Of cases of changed habits it will suffice merely to allude to +that of the many British insects which now feed on exotic plants, or +exclusively on artificial substances. Of diversified habits innumerable +instances could be given: I have often watched a tyrant flycatcher (Saurophagus +sulphuratus) in South America, hovering over one spot and then proceeding to +another, like a kestrel, and at other times standing stationary on the margin +of water, and then dashing like a kingfisher at a fish. In our own country the +larger titmouse (Parus major) may be seen climbing branches, almost like a +creeper; it often, like a shrike, kills small birds by blows on the head; and I +have many times seen and heard it hammering the seeds of the yew on a branch, +and thus breaking them like a nuthatch. In North America the black bear was +seen by Hearne swimming for hours with widely open mouth, thus catching, like a +whale, insects in the water. Even in so extreme a case as this, if the supply +of insects were constant, and if better adapted competitors did not already +exist in the country, I can see no difficulty in a race of bears being +rendered, by natural selection, more and more aquatic in their structure and +habits, with larger and larger mouths, till a creature was produced as +monstrous as a whale. + +As we sometimes see individuals of a species following habits widely different +from those both of their own species and of the other species of the same +genus, we might expect, on my theory, that such individuals would occasionally +have given rise to new species, having anomalous habits, and with their +structure either slightly or considerably modified from that of their proper +type. And such instances do occur in nature. Can a more striking instance of +adaptation be given than that of a woodpecker for climbing trees and for +seizing insects in the chinks of the bark? Yet in North America there are +woodpeckers which feed largely on fruit, and others with elongated wings which +chase insects on the wing; and on the plains of La Plata, where not a tree +grows, there is a woodpecker, which in every essential part of its +organisation, even in its colouring, in the harsh tone of its voice, and +undulatory flight, told me plainly of its close blood-relationship to our +common species; yet it is a woodpecker which never climbs a tree! + +Petrels are the most aërial and oceanic of birds, yet in the quiet Sounds of +Tierra del Fuego, the Puffinuria berardi, in its general habits, in its +astonishing power of diving, its manner of swimming, and of flying when +unwillingly it takes flight, would be mistaken by any one for an auk or grebe; +nevertheless, it is essentially a petrel, but with many parts of its +organisation profoundly modified. On the other hand, the acutest observer by +examining the dead body of the water-ouzel would never have suspected its +sub-aquatic habits; yet this anomalous member of the strictly terrestrial +thrush family wholly subsists by diving,—grasping the stones with its feet and +using its wings under water. + +He who believes that each being has been created as we now see it, must +occasionally have felt surprise when he has met with an animal having habits +and structure not at all in agreement. What can be plainer than that the webbed +feet of ducks and geese are formed for swimming? yet there are upland geese +with webbed feet which rarely or never go near the water; and no one except +Audubon has seen the frigate-bird, which has all its four toes webbed, alight +on the surface of the sea. On the other hand, grebes and coots are eminently +aquatic, although their toes are only bordered by membrane. What seems plainer +than that the long toes of grallatores are formed for walking over swamps and +floating plants, yet the water-hen is nearly as aquatic as the coot; and the +landrail nearly as terrestrial as the quail or partridge. In such cases, and +many others could be given, habits have changed without a corresponding change +of structure. The webbed feet of the upland goose may be said to have become +rudimentary in function, though not in structure. In the frigate-bird, the +deeply-scooped membrane between the toes shows that structure has begun to +change. + +He who believes in separate and innumerable acts of creation will say, that in +these cases it has pleased the Creator to cause a being of one type to take the +place of one of another type; but this seems to me only restating the fact in +dignified language. He who believes in the struggle for existence and in the +principle of natural selection, will acknowledge that every organic being is +constantly endeavouring to increase in numbers; and that if any one being vary +ever so little, either in habits or structure, and thus gain an advantage over +some other inhabitant of the country, it will seize on the place of that +inhabitant, however different it may be from its own place. Hence it will cause +him no surprise that there should be geese and frigate-birds with webbed feet, +either living on the dry land or most rarely alighting on the water; that there +should be long-toed corncrakes living in meadows instead of in swamps; that +there should be woodpeckers where not a tree grows; that there should be diving +thrushes, and petrels with the habits of auks. + +Organs of extreme perfection and complication.—To suppose that the eye, with +all its inimitable contrivances for adjusting the focus to different distances, +for admitting different amounts of light, and for the correction of spherical +and chromatic aberration, could have been formed by natural selection, seems, I +freely confess, absurd in the highest possible degree. Yet reason tells me, +that if numerous gradations from a perfect and complex eye to one very +imperfect and simple, each grade being useful to its possessor, can be shown to +exist; if further, the eye does vary ever so slightly, and the variations be +inherited, which is certainly the case; and if any variation or modification in +the organ be ever useful to an animal under changing conditions of life, then +the difficulty of believing that a perfect and complex eye could be formed by +natural selection, though insuperable by our imagination, can hardly be +considered real. How a nerve comes to be sensitive to light, hardly concerns us +more than how life itself first originated; but I may remark that several facts +make me suspect that any sensitive nerve may be rendered sensitive to light, +and likewise to those coarser vibrations of the air which produce sound. + +In looking for the gradations by which an organ in any species has been +perfected, we ought to look exclusively to its lineal ancestors; but this is +scarcely ever possible, and we are forced in each case to look to species of +the same group, that is to the collateral descendants from the same original +parent-form, in order to see what gradations are possible, and for the chance +of some gradations having been transmitted from the earlier stages of descent, +in an unaltered or little altered condition. Amongst existing Vertebrata, we +find but a small amount of gradation in the structure of the eye, and from +fossil species we can learn nothing on this head. In this great class we should +probably have to descend far beneath the lowest known fossiliferous stratum to +discover the earlier stages, by which the eye has been perfected. + +In the Articulata we can commence a series with an optic nerve merely coated +with pigment, and without any other mechanism; and from this low stage, +numerous gradations of structure, branching off in two fundamentally different +lines, can be shown to exist, until we reach a moderately high stage of +perfection. In certain crustaceans, for instance, there is a double cornea, the +inner one divided into facets, within each of which there is a lens-shaped +swelling. In other crustaceans the transparent cones which are coated by +pigment, and which properly act only by excluding lateral pencils of light, are +convex at their upper ends and must act by convergence; and at their lower ends +there seems to be an imperfect vitreous substance. With these facts, here far +too briefly and imperfectly given, which show that there is much graduated +diversity in the eyes of living crustaceans, and bearing in mind how small the +number of living animals is in proportion to those which have become extinct, I +can see no very great difficulty (not more than in the case of many other +structures) in believing that natural selection has converted the simple +apparatus of an optic nerve merely coated with pigment and invested by +transparent membrane, into an optical instrument as perfect as is possessed by +any member of the great Articulate class. + +He who will go thus far, if he find on finishing this treatise that large +bodies of facts, otherwise inexplicable, can be explained by the theory of +descent, ought not to hesitate to go further, and to admit that a structure +even as perfect as the eye of an eagle might be formed by natural selection, +although in this case he does not know any of the transitional grades. His +reason ought to conquer his imagination; though I have felt the difficulty far +too keenly to be surprised at any degree of hesitation in extending the +principle of natural selection to such startling lengths. + +It is scarcely possible to avoid comparing the eye to a telescope. We know that +this instrument has been perfected by the long-continued efforts of the highest +human intellects; and we naturally infer that the eye has been formed by a +somewhat analogous process. But may not this inference be presumptuous? Have we +any right to assume that the Creator works by intellectual powers like those of +man? If we must compare the eye to an optical instrument, we ought in +imagination to take a thick layer of transparent tissue, with a nerve sensitive +to light beneath, and then suppose every part of this layer to be continually +changing slowly in density, so as to separate into layers of different +densities and thicknesses, placed at different distances from each other, and +with the surfaces of each layer slowly changing in form. Further we must +suppose that there is a power always intently watching each slight accidental +alteration in the transparent layers; and carefully selecting each alteration +which, under varied circumstances, may in any way, or in any degree, tend to +produce a distincter image. We must suppose each new state of the instrument to +be multiplied by the million; and each to be preserved till a better be +produced, and then the old ones to be destroyed. In living bodies, variation +will cause the slight alterations, generation will multiply them almost +infinitely, and natural selection will pick out with unerring skill each +improvement. Let this process go on for millions on millions of years; and +during each year on millions of individuals of many kinds; and may we not +believe that a living optical instrument might thus be formed as superior to +one of glass, as the works of the Creator are to those of man? + +If it could be demonstrated that any complex organ existed, which could not +possibly have been formed by numerous, successive, slight modifications, my +theory would absolutely break down. But I can find out no such case. No doubt +many organs exist of which we do not know the transitional grades, more +especially if we look to much-isolated species, round which, according to my +theory, there has been much extinction. Or again, if we look to an organ common +to all the members of a large class, for in this latter case the organ must +have been first formed at an extremely remote period, since which all the many +members of the class have been developed; and in order to discover the early +transitional grades through which the organ has passed, we should have to look +to very ancient ancestral forms, long since become extinct. + +We should be extremely cautious in concluding that an organ could not have been +formed by transitional gradations of some kind. Numerous cases could be given +amongst the lower animals of the same organ performing at the same time wholly +distinct functions; thus the alimentary canal respires, digests, and excretes +in the larva of the dragon-fly and in the fish Cobites. In the Hydra, the +animal may be turned inside out, and the exterior surface will then digest and +the stomach respire. In such cases natural selection might easily specialise, +if any advantage were thus gained, a part or organ, which had performed two +functions, for one function alone, and thus wholly change its nature by +insensible steps. Two distinct organs sometimes perform simultaneously the same +function in the same individual; to give one instance, there are fish with +gills or branchiæ that breathe the air dissolved in the water, at the same time +that they breathe free air in their swimbladders, this latter organ having a +ductus pneumaticus for its supply, and being divided by highly vascular +partitions. In these cases, one of the two organs might with ease be modified +and perfected so as to perform all the work by itself, being aided during the +process of modification by the other organ; and then this other organ might be +modified for some other and quite distinct purpose, or be quite obliterated. + +The illustration of the swimbladder in fishes is a good one, because it shows +us clearly the highly important fact that an organ originally constructed for +one purpose, namely flotation, may be converted into one for a wholly different +purpose, namely respiration. The swimbladder has, also, been worked in as an +accessory to the auditory organs of certain fish, or, for I do not know which +view is now generally held, a part of the auditory apparatus has been worked in +as a complement to the swimbladder. All physiologists admit that the +swimbladder is homologous, or “ideally similar,” in position and structure with +the lungs of the higher vertebrate animals: hence there seems to me to be no +great difficulty in believing that natural selection has actually converted a +swimbladder into a lung, or organ used exclusively for respiration. + +I can, indeed, hardly doubt that all vertebrate animals having true lungs have +descended by ordinary generation from an ancient prototype, of which we know +nothing, furnished with a floating apparatus or swimbladder. We can thus, as I +infer from Professor Owen’s interesting description of these parts, understand +the strange fact that every particle of food and drink which we swallow has to +pass over the orifice of the trachea, with some risk of falling into the lungs, +notwithstanding the beautiful contrivance by which the glottis is closed. In +the higher Vertebrata the branchiæ have wholly disappeared—the slits on the +sides of the neck and the loop-like course of the arteries still marking in the +embryo their former position. But it is conceivable that the now utterly lost +branchiæ might have been gradually worked in by natural selection for some +quite distinct purpose: in the same manner as, on the view entertained by some +naturalists that the branchiæ and dorsal scales of Annelids are homologous with +the wings and wing-covers of insects, it is probable that organs which at a +very ancient period served for respiration have been actually converted into +organs of flight. + +In considering transitions of organs, it is so important to bear in mind the +probability of conversion from one function to another, that I will give one +more instance. Pedunculated cirripedes have two minute folds of skin, called by +me the ovigerous frena, which serve, through the means of a sticky secretion, +to retain the eggs until they are hatched within the sack. These cirripedes +have no branchiæ, the whole surface of the body and sack, including the small +frena, serving for respiration. The Balanidæ or sessile cirripedes, on the +other hand, have no ovigerous frena, the eggs lying loose at the bottom of the +sack, in the well-enclosed shell; but they have large folded branchiæ. Now I +think no one will dispute that the ovigerous frena in the one family are +strictly homologous with the branchiæ of the other family; indeed, they +graduate into each other. Therefore I do not doubt that little folds of skin, +which originally served as ovigerous frena, but which, likewise, very slightly +aided the act of respiration, have been gradually converted by natural +selection into branchiæ, simply through an increase in their size and the +obliteration of their adhesive glands. If all pedunculated cirripedes had +become extinct, and they have already suffered far more extinction than have +sessile cirripedes, who would ever have imagined that the branchiæ in this +latter family had originally existed as organs for preventing the ova from +being washed out of the sack? + +Although we must be extremely cautious in concluding that any organ could not +possibly have been produced by successive transitional gradations, yet, +undoubtedly, grave cases of difficulty occur, some of which will be discussed +in my future work. + +One of the gravest is that of neuter insects, which are often very differently +constructed from either the males or fertile females; but this case will be +treated of in the next chapter. The electric organs of fishes offer another +case of special difficulty; it is impossible to conceive by what steps these +wondrous organs have been produced; but, as Owen and others have remarked, +their intimate structure closely resembles that of common muscle; and as it has +lately been shown that Rays have an organ closely analogous to the electric +apparatus, and yet do not, as Matteuchi asserts, discharge any electricity, we +must own that we are far too ignorant to argue that no transition of any kind +is possible. + +The electric organs offer another and even more serious difficulty; for they +occur in only about a dozen fishes, of which several are widely remote in their +affinities. Generally when the same organ appears in several members of the +same class, especially if in members having very different habits of life, we +may attribute its presence to inheritance from a common ancestor; and its +absence in some of the members to its loss through disuse or natural selection. +But if the electric organs had been inherited from one ancient progenitor thus +provided, we might have expected that all electric fishes would have been +specially related to each other. Nor does geology at all lead to the belief +that formerly most fishes had electric organs, which most of their modified +descendants have lost. The presence of luminous organs in a few insects, +belonging to different families and orders, offers a parallel case of +difficulty. Other cases could be given; for instance in plants, the very +curious contrivance of a mass of pollen-grains, borne on a foot-stalk with a +sticky gland at the end, is the same in Orchis and Asclepias,—genera almost as +remote as possible amongst flowering plants. In all these cases of two very +distinct species furnished with apparently the same anomalous organ, it should +be observed that, although the general appearance and function of the organ may +be the same, yet some fundamental difference can generally be detected. I am +inclined to believe that in nearly the same way as two men have sometimes +independently hit on the very same invention, so natural selection, working for +the good of each being and taking advantage of analogous variations, has +sometimes modified in very nearly the same manner two parts in two organic +beings, which owe but little of their structure in common to inheritance from +the same ancestor. + +Although in many cases it is most difficult to conjecture by what transitions +an organ could have arrived at its present state; yet, considering that the +proportion of living and known forms to the extinct and unknown is very small, +I have been astonished how rarely an organ can be named, towards which no +transitional grade is known to lead. The truth of this remark is indeed shown +by that old canon in natural history of “Natura non facit saltum.” We meet with +this admission in the writings of almost every experienced naturalist; or, as +Milne Edwards has well expressed it, nature is prodigal in variety, but niggard +in innovation. Why, on the theory of Creation, should this be so? Why should +all the parts and organs of many independent beings, each supposed to have been +separately created for its proper place in nature, be so invariably linked +together by graduated steps? Why should not Nature have taken a leap from +structure to structure? On the theory of natural selection, we can clearly +understand why she should not; for natural selection can act only by taking +advantage of slight successive variations; she can never take a leap, but must +advance by the shortest and slowest steps. + +Organs of little apparent importance.—As natural selection acts by life and +death,—by the preservation of individuals with any favourable variation, and by +the destruction of those with any unfavourable deviation of structure,—I have +sometimes felt much difficulty in understanding the origin of simple parts, of +which the importance does not seem sufficient to cause the preservation of +successively varying individuals. I have sometimes felt as much difficulty, +though of a very different kind, on this head, as in the case of an organ as +perfect and complex as the eye. + +In the first place, we are much too ignorant in regard to the whole economy of +any one organic being, to say what slight modifications would be of importance +or not. In a former chapter I have given instances of most trifling characters, +such as the down on fruit and the colour of the flesh, which, from determining +the attacks of insects or from being correlated with constitutional +differences, might assuredly be acted on by natural selection. The tail of the +giraffe looks like an artificially constructed fly-flapper; and it seems at +first incredible that this could have been adapted for its present purpose by +successive slight modifications, each better and better, for so trifling an +object as driving away flies; yet we should pause before being too positive +even in this case, for we know that the distribution and existence of cattle +and other animals in South America absolutely depends on their power of +resisting the attacks of insects: so that individuals which could by any means +defend themselves from these small enemies, would be able to range into new +pastures and thus gain a great advantage. It is not that the larger quadrupeds +are actually destroyed (except in some rare cases) by the flies, but they are +incessantly harassed and their strength reduced, so that they are more subject +to disease, or not so well enabled in a coming dearth to search for food, or to +escape from beasts of prey. + +Organs now of trifling importance have probably in some cases been of high +importance to an early progenitor, and, after having been slowly perfected at a +former period, have been transmitted in nearly the same state, although now +become of very slight use; and any actually injurious deviations in their +structure will always have been checked by natural selection. Seeing how +important an organ of locomotion the tail is in most aquatic animals, its +general presence and use for many purposes in so many land animals, which in +their lungs or modified swim-bladders betray their aquatic origin, may perhaps +be thus accounted for. A well-developed tail having been formed in an aquatic +animal, it might subsequently come to be worked in for all sorts of purposes, +as a fly-flapper, an organ of prehension, or as an aid in turning, as with the +dog, though the aid must be slight, for the hare, with hardly any tail, can +double quickly enough. + +In the second place, we may sometimes attribute importance to characters which +are really of very little importance, and which have originated from quite +secondary causes, independently of natural selection. We should remember that +climate, food, etc., probably have some little direct influence on the +organisation; that characters reappear from the law of reversion; that +correlation of growth will have had a most important influence in modifying +various structures; and finally, that sexual selection will often have largely +modified the external characters of animals having a will, to give one male an +advantage in fighting with another or in charming the females. Moreover when a +modification of structure has primarily arisen from the above or other unknown +causes, it may at first have been of no advantage to the species, but may +subsequently have been taken advantage of by the descendants of the species +under new conditions of life and with newly acquired habits. + +To give a few instances to illustrate these latter remarks. If green +woodpeckers alone had existed, and we did not know that there were many black +and pied kinds, I dare say that we should have thought that the green colour +was a beautiful adaptation to hide this tree-frequenting bird from its enemies; +and consequently that it was a character of importance and might have been +acquired through natural selection; as it is, I have no doubt that the colour +is due to some quite distinct cause, probably to sexual selection. A trailing +bamboo in the Malay Archipelago climbs the loftiest trees by the aid of +exquisitely constructed hooks clustered around the ends of the branches, and +this contrivance, no doubt, is of the highest service to the plant; but as we +see nearly similar hooks on many trees which are not climbers, the hooks on the +bamboo may have arisen from unknown laws of growth, and have been subsequently +taken advantage of by the plant undergoing further modification and becoming a +climber. The naked skin on the head of a vulture is generally looked at as a +direct adaptation for wallowing in putridity; and so it may be, or it may +possibly be due to the direct action of putrid matter; but we should be very +cautious in drawing any such inference, when we see that the skin on the head +of the clean-feeding male turkey is likewise naked. The sutures in the skulls +of young mammals have been advanced as a beautiful adaptation for aiding +parturition, and no doubt they facilitate, or may be indispensable for this +act; but as sutures occur in the skulls of young birds and reptiles, which have +only to escape from a broken egg, we may infer that this structure has arisen +from the laws of growth, and has been taken advantage of in the parturition of +the higher animals. + +We are profoundly ignorant of the causes producing slight and unimportant +variations; and we are immediately made conscious of this by reflecting on the +differences in the breeds of our domesticated animals in different +countries,—more especially in the less civilized countries where there has been +but little artificial selection. Careful observers are convinced that a damp +climate affects the growth of the hair, and that with the hair the horns are +correlated. Mountain breeds always differ from lowland breeds; and a +mountainous country would probably affect the hind limbs from exercising them +more, and possibly even the form of the pelvis; and then by the law of +homologous variation, the front limbs and even the head would probably be +affected. The shape, also, of the pelvis might affect by pressure the shape of +the head of the young in the womb. The laborious breathing necessary in high +regions would, we have some reason to believe, increase the size of the chest; +and again correlation would come into play. Animals kept by savages in +different countries often have to struggle for their own subsistence, and would +be exposed to a certain extent to natural selection, and individuals with +slightly different constitutions would succeed best under different climates; +and there is reason to believe that constitution and colour are correlated. A +good observer, also, states that in cattle susceptibility to the attacks of +flies is correlated with colour, as is the liability to be poisoned by certain +plants; so that colour would be thus subjected to the action of natural +selection. But we are far too ignorant to speculate on the relative importance +of the several known and unknown laws of variation; and I have here alluded to +them only to show that, if we are unable to account for the characteristic +differences of our domestic breeds, which nevertheless we generally admit to +have arisen through ordinary generation, we ought not to lay too much stress on +our ignorance of the precise cause of the slight analogous differences between +species. I might have adduced for this same purpose the differences between the +races of man, which are so strongly marked; I may add that some little light +can apparently be thrown on the origin of these differences, chiefly through +sexual selection of a particular kind, but without here entering on copious +details my reasoning would appear frivolous. + +The foregoing remarks lead me to say a few words on the protest lately made by +some naturalists, against the utilitarian doctrine that every detail of +structure has been produced for the good of its possessor. They believe that +very many structures have been created for beauty in the eyes of man, or for +mere variety. This doctrine, if true, would be absolutely fatal to my theory. +Yet I fully admit that many structures are of no direct use to their +possessors. Physical conditions probably have had some little effect on +structure, quite independently of any good thus gained. Correlation of growth +has no doubt played a most important part, and a useful modification of one +part will often have entailed on other parts diversified changes of no direct +use. So again characters which formerly were useful, or which formerly had +arisen from correlation of growth, or from other unknown cause, may reappear +from the law of reversion, though now of no direct use. The effects of sexual +selection, when displayed in beauty to charm the females, can be called useful +only in rather a forced sense. But by far the most important consideration is +that the chief part of the organisation of every being is simply due to +inheritance; and consequently, though each being assuredly is well fitted for +its place in nature, many structures now have no direct relation to the habits +of life of each species. Thus, we can hardly believe that the webbed feet of +the upland goose or of the frigate-bird are of special use to these birds; we +cannot believe that the same bones in the arm of the monkey, in the fore leg of +the horse, in the wing of the bat, and in the flipper of the seal, are of +special use to these animals. We may safely attribute these structures to +inheritance. But to the progenitor of the upland goose and of the frigate-bird, +webbed feet no doubt were as useful as they now are to the most aquatic of +existing birds. So we may believe that the progenitor of the seal had not a +flipper, but a foot with five toes fitted for walking or grasping; and we may +further venture to believe that the several bones in the limbs of the monkey, +horse, and bat, which have been inherited from a common progenitor, were +formerly of more special use to that progenitor, or its progenitors, than they +now are to these animals having such widely diversified habits. Therefore we +may infer that these several bones might have been acquired through natural +selection, subjected formerly, as now, to the several laws of inheritance, +reversion, correlation of growth, etc. Hence every detail of structure in every +living creature (making some little allowance for the direct action of physical +conditions) may be viewed, either as having been of special use to some +ancestral form, or as being now of special use to the descendants of this +form—either directly, or indirectly through the complex laws of growth. + +Natural selection cannot possibly produce any modification in any one species +exclusively for the good of another species; though throughout nature one +species incessantly takes advantage of, and profits by, the structure of +another. But natural selection can and does often produce structures for the +direct injury of other species, as we see in the fang of the adder, and in the +ovipositor of the ichneumon, by which its eggs are deposited in the living +bodies of other insects. If it could be proved that any part of the structure +of any one species had been formed for the exclusive good of another species, +it would annihilate my theory, for such could not have been produced through +natural selection. Although many statements may be found in works on natural +history to this effect, I cannot find even one which seems to me of any weight. +It is admitted that the rattlesnake has a poison-fang for its own defence and +for the destruction of its prey; but some authors suppose that at the same time +this snake is furnished with a rattle for its own injury, namely, to warn its +prey to escape. I would almost as soon believe that the cat curls the end of +its tail when preparing to spring, in order to warn the doomed mouse. But I +have not space here to enter on this and other such cases. + +Natural selection will never produce in a being anything injurious to itself, +for natural selection acts solely by and for the good of each. No organ will be +formed, as Paley has remarked, for the purpose of causing pain or for doing an +injury to its possessor. If a fair balance be struck between the good and evil +caused by each part, each will be found on the whole advantageous. After the +lapse of time, under changing conditions of life, if any part comes to be +injurious, it will be modified; or if it be not so, the being will become +extinct, as myriads have become extinct. + +Natural selection tends only to make each organic being as perfect as, or +slightly more perfect than, the other inhabitants of the same country with +which it has to struggle for existence. And we see that this is the degree of +perfection attained under nature. The endemic productions of New Zealand, for +instance, are perfect one compared with another; but they are now rapidly +yielding before the advancing legions of plants and animals introduced from +Europe. Natural selection will not produce absolute perfection, nor do we +always meet, as far as we can judge, with this high standard under nature. The +correction for the aberration of light is said, on high authority, not to be +perfect even in that most perfect organ, the eye. If our reason leads us to +admire with enthusiasm a multitude of inimitable contrivances in nature, this +same reason tells us, though we may easily err on both sides, that some other +contrivances are less perfect. Can we consider the sting of the wasp or of the +bee as perfect, which, when used against many attacking animals, cannot be +withdrawn, owing to the backward serratures, and so inevitably causes the death +of the insect by tearing out its viscera? + +If we look at the sting of the bee, as having originally existed in a remote +progenitor as a boring and serrated instrument, like that in so many members of +the same great order, and which has been modified but not perfected for its +present purpose, with the poison originally adapted to cause galls subsequently +intensified, we can perhaps understand how it is that the use of the sting +should so often cause the insect’s own death: for if on the whole the power of +stinging be useful to the community, it will fulfil all the requirements of +natural selection, though it may cause the death of some few members. If we +admire the truly wonderful power of scent by which the males of many insects +find their females, can we admire the production for this single purpose of +thousands of drones, which are utterly useless to the community for any other +end, and which are ultimately slaughtered by their industrious and sterile +sisters? It may be difficult, but we ought to admire the savage instinctive +hatred of the queen-bee, which urges her instantly to destroy the young queens +her daughters as soon as born, or to perish herself in the combat; for +undoubtedly this is for the good of the community; and maternal love or +maternal hatred, though the latter fortunately is most rare, is all the same to +the inexorable principle of natural selection. If we admire the several +ingenious contrivances, by which the flowers of the orchis and of many other +plants are fertilised through insect agency, can we consider as equally perfect +the elaboration by our fir-trees of dense clouds of pollen, in order that a few +granules may be wafted by a chance breeze on to the ovules? + +Summary of Chapter.—We have in this chapter discussed some of the difficulties +and objections which may be urged against my theory. Many of them are very +grave; but I think that in the discussion light has been thrown on several +facts, which on the theory of independent acts of creation are utterly obscure. +We have seen that species at any one period are not indefinitely variable, and +are not linked together by a multitude of intermediate gradations, partly +because the process of natural selection will always be very slow, and will +act, at any one time, only on a very few forms; and partly because the very +process of natural selection almost implies the continual supplanting and +extinction of preceding and intermediate gradations. Closely allied species, +now living on a continuous area, must often have been formed when the area was +not continuous, and when the conditions of life did not insensibly graduate +away from one part to another. When two varieties are formed in two districts +of a continuous area, an intermediate variety will often be formed, fitted for +an intermediate zone; but from reasons assigned, the intermediate variety will +usually exist in lesser numbers than the two forms which it connects; +consequently the two latter, during the course of further modification, from +existing in greater numbers, will have a great advantage over the less numerous +intermediate variety, and will thus generally succeed in supplanting and +exterminating it. + +We have seen in this chapter how cautious we should be in concluding that the +most different habits of life could not graduate into each other; that a bat, +for instance, could not have been formed by natural selection from an animal +which at first could only glide through the air. + +We have seen that a species may under new conditions of life change its habits, +or have diversified habits, with some habits very unlike those of its nearest +congeners. Hence we can understand, bearing in mind that each organic being is +trying to live wherever it can live, how it has arisen that there are upland +geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with +the habits of auks. + +Although the belief that an organ so perfect as the eye could have been formed +by natural selection, is more than enough to stagger any one; yet in the case +of any organ, if we know of a long series of gradations in complexity, each +good for its possessor, then, under changing conditions of life, there is no +logical impossibility in the acquirement of any conceivable degree of +perfection through natural selection. In the cases in which we know of no +intermediate or transitional states, we should be very cautious in concluding +that none could have existed, for the homologies of many organs and their +intermediate states show that wonderful metamorphoses in function are at least +possible. For instance, a swim-bladder has apparently been converted into an +air-breathing lung. The same organ having performed simultaneously very +different functions, and then having been specialised for one function; and two +very distinct organs having performed at the same time the same function, the +one having been perfected whilst aided by the other, must often have largely +facilitated transitions. + +We are far too ignorant, in almost every case, to be enabled to assert that any +part or organ is so unimportant for the welfare of a species, that +modifications in its structure could not have been slowly accumulated by means +of natural selection. But we may confidently believe that many modifications, +wholly due to the laws of growth, and at first in no way advantageous to a +species, have been subsequently taken advantage of by the still further +modified descendants of this species. We may, also, believe that a part +formerly of high importance has often been retained (as the tail of an aquatic +animal by its terrestrial descendants), though it has become of such small +importance that it could not, in its present state, have been acquired by +natural selection,—a power which acts solely by the preservation of profitable +variations in the struggle for life. + +Natural selection will produce nothing in one species for the exclusive good or +injury of another; though it may well produce parts, organs, and excretions +highly useful or even indispensable, or highly injurious to another species, +but in all cases at the same time useful to the owner. Natural selection in +each well-stocked country, must act chiefly through the competition of the +inhabitants one with another, and consequently will produce perfection, or +strength in the battle for life, only according to the standard of that +country. Hence the inhabitants of one country, generally the smaller one, will +often yield, as we see they do yield, to the inhabitants of another and +generally larger country. For in the larger country there will have existed +more individuals, and more diversified forms, and the competition will have +been severer, and thus the standard of perfection will have been rendered +higher. Natural selection will not necessarily produce absolute perfection; +nor, as far as we can judge by our limited faculties, can absolute perfection +be everywhere found. + +On the theory of natural selection we can clearly understand the full meaning +of that old canon in natural history, “Natura non facit saltum.” This canon, if +we look only to the present inhabitants of the world, is not strictly correct, +but if we include all those of past times, it must by my theory be strictly +true. + +It is generally acknowledged that all organic beings have been formed on two +great laws—Unity of Type, and the Conditions of Existence. By unity of type is +meant that fundamental agreement in structure, which we see in organic beings +of the same class, and which is quite independent of their habits of life. On +my theory, unity of type is explained by unity of descent. The expression of +conditions of existence, so often insisted on by the illustrious Cuvier, is +fully embraced by the principle of natural selection. For natural selection +acts by either now adapting the varying parts of each being to its organic and +inorganic conditions of life; or by having adapted them during long-past +periods of time: the adaptations being aided in some cases by use and disuse, +being slightly affected by the direct action of the external conditions of +life, and being in all cases subjected to the several laws of growth. Hence, in +fact, the law of the Conditions of Existence is the higher law; as it includes, +through the inheritance of former adaptations, that of Unity of Type. + +CHAPTER VII. +INSTINCT. + +Instincts comparable with habits, but different in their origin. Instincts +graduated. Aphides and ants. Instincts variable. Domestic instincts, their +origin. Natural instincts of the cuckoo, ostrich, and parasitic bees. +Slave-making ants. Hive-bee, its cell-making instinct. Difficulties on the +theory of the Natural Selection of instincts. Neuter or sterile insects. +Summary. + +The subject of instinct might have been worked into the previous chapters; but +I have thought that it would be more convenient to treat the subject +separately, especially as so wonderful an instinct as that of the hive-bee +making its cells will probably have occurred to many readers, as a difficulty +sufficient to overthrow my whole theory. I must premise, that I have nothing to +do with the origin of the primary mental powers, any more than I have with that +of life itself. We are concerned only with the diversities of instinct and of +the other mental qualities of animals within the same class. + +I will not attempt any definition of instinct. It would be easy to show that +several distinct mental actions are commonly embraced by this term; but every +one understands what is meant, when it is said that instinct impels the cuckoo +to migrate and to lay her eggs in other birds’ nests. An action, which we +ourselves should require experience to enable us to perform, when performed by +an animal, more especially by a very young one, without any experience, and +when performed by many individuals in the same way, without their knowing for +what purpose it is performed, is usually said to be instinctive. But I could +show that none of these characters of instinct are universal. A little dose, as +Pierre Huber expresses it, of judgment or reason, often comes into play, even +in animals very low in the scale of nature. + +Frederick Cuvier and several of the older metaphysicians have compared instinct +with habit. This comparison gives, I think, a remarkably accurate notion of the +frame of mind under which an instinctive action is performed, but not of its +origin. How unconsciously many habitual actions are performed, indeed not +rarely in direct opposition to our conscious will! yet they may be modified by +the will or reason. Habits easily become associated with other habits, and with +certain periods of time and states of the body. When once acquired, they often +remain constant throughout life. Several other points of resemblance between +instincts and habits could be pointed out. As in repeating a well-known song, +so in instincts, one action follows another by a sort of rhythm; if a person be +interrupted in a song, or in repeating anything by rote, he is generally forced +to go back to recover the habitual train of thought: so P. Huber found it was +with a caterpillar, which makes a very complicated hammock; for if he took a +caterpillar which had completed its hammock up to, say, the sixth stage of +construction, and put it into a hammock completed up only to the third stage, +the caterpillar simply re-performed the fourth, fifth, and sixth stages of +construction. If, however, a caterpillar were taken out of a hammock made up, +for instance, to the third stage, and were put into one finished up to the +sixth stage, so that much of its work was already done for it, far from feeling +the benefit of this, it was much embarrassed, and, in order to complete its +hammock, seemed forced to start from the third stage, where it had left off, +and thus tried to complete the already finished work. + +If we suppose any habitual action to become inherited—and I think it can be +shown that this does sometimes happen—then the resemblance between what +originally was a habit and an instinct becomes so close as not to be +distinguished. If Mozart, instead of playing the pianoforte at three years old +with wonderfully little practice, had played a tune with no practice at all, he +might truly be said to have done so instinctively. But it would be the most +serious error to suppose that the greater number of instincts have been +acquired by habit in one generation, and then transmitted by inheritance to +succeeding generations. It can be clearly shown that the most wonderful +instincts with which we are acquainted, namely, those of the hive-bee and of +many ants, could not possibly have been thus acquired. + +It will be universally admitted that instincts are as important as corporeal +structure for the welfare of each species, under its present conditions of +life. Under changed conditions of life, it is at least possible that slight +modifications of instinct might be profitable to a species; and if it can be +shown that instincts do vary ever so little, then I can see no difficulty in +natural selection preserving and continually accumulating variations of +instinct to any extent that may be profitable. It is thus, as I believe, that +all the most complex and wonderful instincts have originated. As modifications +of corporeal structure arise from, and are increased by, use or habit, and are +diminished or lost by disuse, so I do not doubt it has been with instincts. But +I believe that the effects of habit are of quite subordinate importance to the +effects of the natural selection of what may be called accidental variations of +instincts;—that is of variations produced by the same unknown causes which +produce slight deviations of bodily structure. + +No complex instinct can possibly be produced through natural selection, except +by the slow and gradual accumulation of numerous, slight, yet profitable, +variations. Hence, as in the case of corporeal structures, we ought to find in +nature, not the actual transitional gradations by which each complex instinct +has been acquired—for these could be found only in the lineal ancestors of each +species—but we ought to find in the collateral lines of descent some evidence +of such gradations; or we ought at least to be able to show that gradations of +some kind are possible; and this we certainly can do. I have been surprised to +find, making allowance for the instincts of animals having been but little +observed except in Europe and North America, and for no instinct being known +amongst extinct species, how very generally gradations, leading to the most +complex instincts, can be discovered. The canon of “Natura non facit saltum” +applies with almost equal force to instincts as to bodily organs. Changes of +instinct may sometimes be facilitated by the same species having different +instincts at different periods of life, or at different seasons of the year, or +when placed under different circumstances, etc.; in which case either one or +the other instinct might be preserved by natural selection. And such instances +of diversity of instinct in the same species can be shown to occur in nature. + +Again as in the case of corporeal structure, and conformably with my theory, +the instinct of each species is good for itself, but has never, as far as we +can judge, been produced for the exclusive good of others. One of the strongest +instances of an animal apparently performing an action for the sole good of +another, with which I am acquainted, is that of aphides voluntarily yielding +their sweet excretion to ants: that they do so voluntarily, the following facts +show. I removed all the ants from a group of about a dozen aphides on a +dock-plant, and prevented their attendance during several hours. After this +interval, I felt sure that the aphides would want to excrete. I watched them +for some time through a lens, but not one excreted; I then tickled and stroked +them with a hair in the same manner, as well as I could, as the ants do with +their antennæ; but not one excreted. Afterwards I allowed an ant to visit them, +and it immediately seemed, by its eager way of running about, to be well aware +what a rich flock it had discovered; it then began to play with its antennæ on +the abdomen first of one aphis and then of another; and each aphis, as soon as +it felt the antennæ, immediately lifted up its abdomen and excreted a limpid +drop of sweet juice, which was eagerly devoured by the ant. Even the quite +young aphides behaved in this manner, showing that the action was instinctive, +and not the result of experience. But as the excretion is extremely viscid, it +is probably a convenience to the aphides to have it removed; and therefore +probably the aphides do not instinctively excrete for the sole good of the +ants. Although I do not believe that any animal in the world performs an action +for the exclusive good of another of a distinct species, yet each species tries +to take advantage of the instincts of others, as each takes advantage of the +weaker bodily structure of others. So again, in some few cases, certain +instincts cannot be considered as absolutely perfect; but as details on this +and other such points are not indispensable, they may be here passed over. + +As some degree of variation in instincts under a state of nature, and the +inheritance of such variations, are indispensable for the action of natural +selection, as many instances as possible ought to have been here given; but +want of space prevents me. I can only assert, that instincts certainly do +vary—for instance, the migratory instinct, both in extent and direction, and in +its total loss. So it is with the nests of birds, which vary partly in +dependence on the situations chosen, and on the nature and temperature of the +country inhabited, but often from causes wholly unknown to us: Audubon has +given several remarkable cases of differences in nests of the same species in +the northern and southern United States. Fear of any particular enemy is +certainly an instinctive quality, as may be seen in nestling birds, though it +is strengthened by experience, and by the sight of fear of the same enemy in +other animals. But fear of man is slowly acquired, as I have elsewhere shown, +by various animals inhabiting desert islands; and we may see an instance of +this, even in England, in the greater wildness of all our large birds than of +our small birds; for the large birds have been most persecuted by man. We may +safely attribute the greater wildness of our large birds to this cause; for in +uninhabited islands large birds are not more fearful than small; and the +magpie, so wary in England, is tame in Norway, as is the hooded crow in Egypt. + +That the general disposition of individuals of the same species, born in a +state of nature, is extremely diversified, can be shown by a multitude of +facts. Several cases also, could be given, of occasional and strange habits in +certain species, which might, if advantageous to the species, give rise, +through natural selection, to quite new instincts. But I am well aware that +these general statements, without facts given in detail, can produce but a +feeble effect on the reader’s mind. I can only repeat my assurance, that I do +not speak without good evidence. + +The possibility, or even probability, of inherited variations of instinct in a +state of nature will be strengthened by briefly considering a few cases under +domestication. We shall thus also be enabled to see the respective parts which +habit and the selection of so-called accidental variations have played in +modifying the mental qualities of our domestic animals. A number of curious and +authentic instances could be given of the inheritance of all shades of +disposition and tastes, and likewise of the oddest tricks, associated with +certain frames of mind or periods of time. But let us look to the familiar case +of the several breeds of dogs: it cannot be doubted that young pointers (I have +myself seen a striking instance) will sometimes point and even back other dogs +the very first time that they are taken out; retrieving is certainly in some +degree inherited by retrievers; and a tendency to run round, instead of at, a +flock of sheep, by shepherd-dogs. I cannot see that these actions, performed +without experience by the young, and in nearly the same manner by each +individual, performed with eager delight by each breed, and without the end +being known,—for the young pointer can no more know that he points to aid his +master, than the white butterfly knows why she lays her eggs on the leaf of the +cabbage,—I cannot see that these actions differ essentially from true +instincts. If we were to see one kind of wolf, when young and without any +training, as soon as it scented its prey, stand motionless like a statue, and +then slowly crawl forward with a peculiar gait; and another kind of wolf +rushing round, instead of at, a herd of deer, and driving them to a distant +point, we should assuredly call these actions instinctive. Domestic instincts, +as they may be called, are certainly far less fixed or invariable than natural +instincts; but they have been acted on by far less rigorous selection, and have +been transmitted for an incomparably shorter period, under less fixed +conditions of life. + +How strongly these domestic instincts, habits, and dispositions are inherited, +and how curiously they become mingled, is well shown when different breeds of +dogs are crossed. Thus it is known that a cross with a bull-dog has affected +for many generations the courage and obstinacy of greyhounds; and a cross with +a greyhound has given to a whole family of shepherd-dogs a tendency to hunt +hares. These domestic instincts, when thus tested by crossing, resemble natural +instincts, which in a like manner become curiously blended together, and for a +long period exhibit traces of the instincts of either parent: for example, Le +Roy describes a dog, whose great-grandfather was a wolf, and this dog showed a +trace of its wild parentage only in one way, by not coming in a straight line +to his master when called. + +Domestic instincts are sometimes spoken of as actions which have become +inherited solely from long-continued and compulsory habit, but this, I think, +is not true. No one would ever have thought of teaching, or probably could have +taught, the tumbler-pigeon to tumble,—an action which, as I have witnessed, is +performed by young birds, that have never seen a pigeon tumble. We may believe +that some one pigeon showed a slight tendency to this strange habit, and that +the long-continued selection of the best individuals in successive generations +made tumblers what they now are; and near Glasgow there are house-tumblers, as +I hear from Mr. Brent, which cannot fly eighteen inches high without going head +over heels. It may be doubted whether any one would have thought of training a +dog to point, had not some one dog naturally shown a tendency in this line; and +this is known occasionally to happen, as I once saw in a pure terrier. When the +first tendency was once displayed, methodical selection and the inherited +effects of compulsory training in each successive generation would soon +complete the work; and unconscious selection is still at work, as each man +tries to procure, without intending to improve the breed, dogs which will stand +and hunt best. On the other hand, habit alone in some cases has sufficed; no +animal is more difficult to tame than the young of the wild rabbit; scarcely +any animal is tamer than the young of the tame rabbit; but I do not suppose +that domestic rabbits have ever been selected for tameness; and I presume that +we must attribute the whole of the inherited change from extreme wildness to +extreme tameness, simply to habit and long-continued close confinement. + +Natural instincts are lost under domestication: a remarkable instance of this +is seen in those breeds of fowls which very rarely or never become “broody,” +that is, never wish to sit on their eggs. Familiarity alone prevents our seeing +how universally and largely the minds of our domestic animals have been +modified by domestication. It is scarcely possible to doubt that the love of +man has become instinctive in the dog. All wolves, foxes, jackals, and species +of the cat genus, when kept tame, are most eager to attack poultry, sheep, and +pigs; and this tendency has been found incurable in dogs which have been +brought home as puppies from countries, such as Tierra del Fuego and Australia, +where the savages do not keep these domestic animals. How rarely, on the other +hand, do our civilised dogs, even when quite young, require to be taught not to +attack poultry, sheep, and pigs! No doubt they occasionally do make an attack, +and are then beaten; and if not cured, they are destroyed; so that habit, with +some degree of selection, has probably concurred in civilising by inheritance +our dogs. On the other hand, young chickens have lost, wholly by habit, that +fear of the dog and cat which no doubt was originally instinctive in them, in +the same way as it is so plainly instinctive in young pheasants, though reared +under a hen. It is not that chickens have lost all fear, but fear only of dogs +and cats, for if the hen gives the danger-chuckle, they will run (more +especially young turkeys) from under her, and conceal themselves in the +surrounding grass or thickets; and this is evidently done for the instinctive +purpose of allowing, as we see in wild ground-birds, their mother to fly away. +But this instinct retained by our chickens has become useless under +domestication, for the mother-hen has almost lost by disuse the power of +flight. + +Hence, we may conclude, that domestic instincts have been acquired and natural +instincts have been lost partly by habit, and partly by man selecting and +accumulating during successive generations, peculiar mental habits and actions, +which at first appeared from what we must in our ignorance call an accident. In +some cases compulsory habit alone has sufficed to produce such inherited mental +changes; in other cases compulsory habit has done nothing, and all has been the +result of selection, pursued both methodically and unconsciously; but in most +cases, probably, habit and selection have acted together. + +We shall, perhaps, best understand how instincts in a state of nature have +become modified by selection, by considering a few cases. I will select only +three, out of the several which I shall have to discuss in my future +work,—namely, the instinct which leads the cuckoo to lay her eggs in other +birds’ nests; the slave-making instinct of certain ants; and the comb-making +power of the hive-bee: these two latter instincts have generally, and most +justly, been ranked by naturalists as the most wonderful of all known +instincts. + +It is now commonly admitted that the more immediate and final cause of the +cuckoo’s instinct is, that she lays her eggs, not daily, but at intervals of +two or three days; so that, if she were to make her own nest and sit on her own +eggs, those first laid would have to be left for some time unincubated, or +there would be eggs and young birds of different ages in the same nest. If this +were the case, the process of laying and hatching might be inconveniently long, +more especially as she has to migrate at a very early period; and the first +hatched young would probably have to be fed by the male alone. But the American +cuckoo is in this predicament; for she makes her own nest and has eggs and +young successively hatched, all at the same time. It has been asserted that the +American cuckoo occasionally lays her eggs in other birds’ nests; but I hear on +the high authority of Dr. Brewer, that this is a mistake. Nevertheless, I could +give several instances of various birds which have been known occasionally to +lay their eggs in other birds’ nests. Now let us suppose that the ancient +progenitor of our European cuckoo had the habits of the American cuckoo; but +that occasionally she laid an egg in another bird’s nest. If the old bird +profited by this occasional habit, or if the young were made more vigorous by +advantage having been taken of the mistaken maternal instinct of another bird, +than by their own mother’s care, encumbered as she can hardly fail to be by +having eggs and young of different ages at the same time; then the old birds or +the fostered young would gain an advantage. And analogy would lead me to +believe, that the young thus reared would be apt to follow by inheritance the +occasional and aberrant habit of their mother, and in their turn would be apt +to lay their eggs in other birds’ nests, and thus be successful in rearing +their young. By a continued process of this nature, I believe that the strange +instinct of our cuckoo could be, and has been, generated. I may add that, +according to Dr. Gray and to some other observers, the European cuckoo has not +utterly lost all maternal love and care for her own offspring. + +The occasional habit of birds laying their eggs in other birds’ nests, either +of the same or of a distinct species, is not very uncommon with the Gallinaceæ; +and this perhaps explains the origin of a singular instinct in the allied group +of ostriches. For several hen ostriches, at least in the case of the American +species, unite and lay first a few eggs in one nest and then in another; and +these are hatched by the males. This instinct may probably be accounted for by +the fact of the hens laying a large number of eggs; but, as in the case of the +cuckoo, at intervals of two or three days. This instinct, however, of the +American ostrich has not as yet been perfected; for a surprising number of eggs +lie strewed over the plains, so that in one day’s hunting I picked up no less +than twenty lost and wasted eggs. + +Many bees are parasitic, and always lay their eggs in the nests of bees of +other kinds. This case is more remarkable than that of the cuckoo; for these +bees have not only their instincts but their structure modified in accordance +with their parasitic habits; for they do not possess the pollen-collecting +apparatus which would be necessary if they had to store food for their own +young. Some species, likewise, of Sphegidæ (wasp-like insects) are parasitic on +other species; and M. Fabre has lately shown good reason for believing that +although the Tachytes nigra generally makes its own burrow and stores it with +paralysed prey for its own larvæ to feed on, yet that when this insect finds a +burrow already made and stored by another sphex, it takes advantage of the +prize, and becomes for the occasion parasitic. In this case, as with the +supposed case of the cuckoo, I can see no difficulty in natural selection +making an occasional habit permanent, if of advantage to the species, and if +the insect whose nest and stored food are thus feloniously appropriated, be not +thus exterminated. + +Slave-making instinct.—This remarkable instinct was first discovered in the +Formica (Polyerges) rufescens by Pierre Huber, a better observer even than his +celebrated father. This ant is absolutely dependent on its slaves; without +their aid, the species would certainly become extinct in a single year. The +males and fertile females do no work. The workers or sterile females, though +most energetic and courageous in capturing slaves, do no other work. They are +incapable of making their own nests, or of feeding their own larvæ. When the +old nest is found inconvenient, and they have to migrate, it is the slaves +which determine the migration, and actually carry their masters in their jaws. +So utterly helpless are the masters, that when Huber shut up thirty of them +without a slave, but with plenty of the food which they like best, and with +their larvæ and pupæ to stimulate them to work, they did nothing; they could +not even feed themselves, and many perished of hunger. Huber then introduced a +single slave (F. fusca), and she instantly set to work, fed and saved the +survivors; made some cells and tended the larvæ, and put all to rights. What +can be more extraordinary than these well-ascertained facts? If we had not +known of any other slave-making ant, it would have been hopeless to have +speculated how so wonderful an instinct could have been perfected. + +Formica sanguinea was likewise first discovered by P. Huber to be a +slave-making ant. This species is found in the southern parts of England, and +its habits have been attended to by Mr. F. Smith, of the British Museum, to +whom I am much indebted for information on this and other subjects. Although +fully trusting to the statements of Huber and Mr. Smith, I tried to approach +the subject in a sceptical frame of mind, as any one may well be excused for +doubting the truth of so extraordinary and odious an instinct as that of making +slaves. Hence I will give the observations which I have myself made, in some +little detail. I opened fourteen nests of F. sanguinea, and found a few slaves +in all. Males and fertile females of the slave-species are found only in their +own proper communities, and have never been observed in the nests of F. +sanguinea. The slaves are black and not above half the size of their red +masters, so that the contrast in their appearance is very great. When the nest +is slightly disturbed, the slaves occasionally come out, and like their masters +are much agitated and defend the nest: when the nest is much disturbed and the +larvæ and pupæ are exposed, the slaves work energetically with their masters in +carrying them away to a place of safety. Hence, it is clear, that the slaves +feel quite at home. During the months of June and July, on three successive +years, I have watched for many hours several nests in Surrey and Sussex, and +never saw a slave either leave or enter a nest. As, during these months, the +slaves are very few in number, I thought that they might behave differently +when more numerous; but Mr. Smith informs me that he has watched the nests at +various hours during May, June and August, both in Surrey and Hampshire, and +has never seen the slaves, though present in large numbers in August, either +leave or enter the nest. Hence he considers them as strictly household slaves. +The masters, on the other hand, may be constantly seen bringing in materials +for the nest, and food of all kinds. During the present year, however, in the +month of July, I came across a community with an unusually large stock of +slaves, and I observed a few slaves mingled with their masters leaving the +nest, and marching along the same road to a tall Scotch-fir-tree, twenty-five +yards distant, which they ascended together, probably in search of aphides or +cocci. According to Huber, who had ample opportunities for observation, in +Switzerland the slaves habitually work with their masters in making the nest, +and they alone open and close the doors in the morning and evening; and, as +Huber expressly states, their principal office is to search for aphides. This +difference in the usual habits of the masters and slaves in the two countries, +probably depends merely on the slaves being captured in greater numbers in +Switzerland than in England. + +One day I fortunately chanced to witness a migration from one nest to another, +and it was a most interesting spectacle to behold the masters carefully +carrying, as Huber has described, their slaves in their jaws. Another day my +attention was struck by about a score of the slave-makers haunting the same +spot, and evidently not in search of food; they approached and were vigorously +repulsed by an independent community of the slave species (F. fusca); sometimes +as many as three of these ants clinging to the legs of the slave-making F. +sanguinea. The latter ruthlessly killed their small opponents, and carried +their dead bodies as food to their nest, twenty-nine yards distant; but they +were prevented from getting any pupæ to rear as slaves. I then dug up a small +parcel of the pupæ of F. fusca from another nest, and put them down on a bare +spot near the place of combat; they were eagerly seized, and carried off by the +tyrants, who perhaps fancied that, after all, they had been victorious in their +late combat. + +At the same time I laid on the same place a small parcel of the pupæ of another +species, F. flava, with a few of these little yellow ants still clinging to the +fragments of the nest. This species is sometimes, though rarely, made into +slaves, as has been described by Mr. Smith. Although so small a species, it is +very courageous, and I have seen it ferociously attack other ants. In one +instance I found to my surprise an independent community of F. flava under a +stone beneath a nest of the slave-making F. sanguinea; and when I had +accidentally disturbed both nests, the little ants attacked their big +neighbours with surprising courage. Now I was curious to ascertain whether F. +sanguinea could distinguish the pupæ of F. fusca, which they habitually make +into slaves, from those of the little and furious F. flava, which they rarely +capture, and it was evident that they did at once distinguish them: for we have +seen that they eagerly and instantly seized the pupæ of F. fusca, whereas they +were much terrified when they came across the pupæ, or even the earth from the +nest of F. flava, and quickly ran away; but in about a quarter of an hour, +shortly after all the little yellow ants had crawled away, they took heart and +carried off the pupæ. + +One evening I visited another community of F. sanguinea, and found a number of +these ants entering their nest, carrying the dead bodies of F. fusca (showing +that it was not a migration) and numerous pupæ. I traced the returning file +burthened with booty, for about forty yards, to a very thick clump of heath, +whence I saw the last individual of F. sanguinea emerge, carrying a pupa; but I +was not able to find the desolated nest in the thick heath. The nest, however, +must have been close at hand, for two or three individuals of F. fusca were +rushing about in the greatest agitation, and one was perched motionless with +its own pupa in its mouth on the top of a spray of heath over its ravaged home. + +Such are the facts, though they did not need confirmation by me, in regard to +the wonderful instinct of making slaves. Let it be observed what a contrast the +instinctive habits of F. sanguinea present with those of the F. rufescens. The +latter does not build its own nest, does not determine its own migrations, does +not collect food for itself or its young, and cannot even feed itself: it is +absolutely dependent on its numerous slaves. Formica sanguinea, on the other +hand, possesses much fewer slaves, and in the early part of the summer +extremely few. The masters determine when and where a new nest shall be formed, +and when they migrate, the masters carry the slaves. Both in Switzerland and +England the slaves seem to have the exclusive care of the larvæ, and the +masters alone go on slave-making expeditions. In Switzerland the slaves and +masters work together, making and bringing materials for the nest: both, but +chiefly the slaves, tend, and milk as it may be called, their aphides; and thus +both collect food for the community. In England the masters alone usually leave +the nest to collect building materials and food for themselves, their slaves +and larvæ. So that the masters in this country receive much less service from +their slaves than they do in Switzerland. + +By what steps the instinct of F. sanguinea originated I will not pretend to +conjecture. But as ants, which are not slave-makers, will, as I have seen, +carry off pupæ of other species, if scattered near their nests, it is possible +that pupæ originally stored as food might become developed; and the ants thus +unintentionally reared would then follow their proper instincts, and do what +work they could. If their presence proved useful to the species which had +seized them—if it were more advantageous to this species to capture workers +than to procreate them—the habit of collecting pupæ originally for food might +by natural selection be strengthened and rendered permanent for the very +different purpose of raising slaves. When the instinct was once acquired, if +carried out to a much less extent even than in our British F. sanguinea, which, +as we have seen, is less aided by its slaves than the same species in +Switzerland, I can see no difficulty in natural selection increasing and +modifying the instinct—always supposing each modification to be of use to the +species—until an ant was formed as abjectly dependent on its slaves as is the +Formica rufescens. + +Cell-making instinct of the Hive-Bee.—I will not here enter on minute details +on this subject, but will merely give an outline of the conclusions at which I +have arrived. He must be a dull man who can examine the exquisite structure of +a comb, so beautifully adapted to its end, without enthusiastic admiration. We +hear from mathematicians that bees have practically solved a recondite problem, +and have made their cells of the proper shape to hold the greatest possible +amount of honey, with the least possible consumption of precious wax in their +construction. It has been remarked that a skilful workman, with fitting tools +and measures, would find it very difficult to make cells of wax of the true +form, though this is perfectly effected by a crowd of bees working in a dark +hive. Grant whatever instincts you please, and it seems at first quite +inconceivable how they can make all the necessary angles and planes, or even +perceive when they are correctly made. But the difficulty is not nearly so +great as it at first appears: all this beautiful work can be shown, I think, to +follow from a few very simple instincts. + +I was led to investigate this subject by Mr. Waterhouse, who has shown that the +form of the cell stands in close relation to the presence of adjoining cells; +and the following view may, perhaps, be considered only as a modification of +his theory. Let us look to the great principle of gradation, and see whether +Nature does not reveal to us her method of work. At one end of a short series +we have humble-bees, which use their old cocoons to hold honey, sometimes +adding to them short tubes of wax, and likewise making separate and very +irregular rounded cells of wax. At the other end of the series we have the +cells of the hive-bee, placed in a double layer: each cell, as is well known, +is an hexagonal prism, with the basal edges of its six sides bevelled so as to +join on to a pyramid, formed of three rhombs. These rhombs have certain angles, +and the three which form the pyramidal base of a single cell on one side of the +comb, enter into the composition of the bases of three adjoining cells on the +opposite side. In the series between the extreme perfection of the cells of the +hive-bee and the simplicity of those of the humble-bee, we have the cells of +the Mexican Melipona domestica, carefully described and figured by Pierre +Huber. The Melipona itself is intermediate in structure between the hive and +humble bee, but more nearly related to the latter: it forms a nearly regular +waxen comb of cylindrical cells, in which the young are hatched, and, in +addition, some large cells of wax for holding honey. These latter cells are +nearly spherical and of nearly equal sizes, and are aggregated into an +irregular mass. But the important point to notice, is that these cells are +always made at that degree of nearness to each other, that they would have +intersected or broken into each other, if the spheres had been completed; but +this is never permitted, the bees building perfectly flat walls of wax between +the spheres which thus tend to intersect. Hence each cell consists of an outer +spherical portion and of two, three, or more perfectly flat surfaces, according +as the cell adjoins two, three or more other cells. When one cell comes into +contact with three other cells, which, from the spheres being nearly of the +same size, is very frequently and necessarily the case, the three flat surfaces +are united into a pyramid; and this pyramid, as Huber has remarked, is +manifestly a gross imitation of the three-sided pyramidal basis of the cell of +the hive-bee. As in the cells of the hive-bee, so here, the three plane +surfaces in any one cell necessarily enter into the construction of three +adjoining cells. It is obvious that the Melipona saves wax by this manner of +building; for the flat walls between the adjoining cells are not double, but +are of the same thickness as the outer spherical portions, and yet each flat +portion forms a part of two cells. + +Reflecting on this case, it occurred to me that if the Melipona had made its +spheres at some given distance from each other, and had made them of equal +sizes and had arranged them symmetrically in a double layer, the resulting +structure would probably have been as perfect as the comb of the hive-bee. +Accordingly I wrote to Professor Miller, of Cambridge, and this geometer has +kindly read over the following statement, drawn up from his information, and +tells me that it is strictly correct:— + +If a number of equal spheres be described with their centres placed in two +parallel layers; with the centre of each sphere at the distance of radius x the +square root of 2 or radius x 1.41421 (or at some lesser distance), from the +centres of the six surrounding spheres in the same layer; and at the same +distance from the centres of the adjoining spheres in the other and parallel +layer; then, if planes of intersection between the several spheres in both +layers be formed, there will result a double layer of hexagonal prisms united +together by pyramidal bases formed of three rhombs; and the rhombs and the +sides of the hexagonal prisms will have every angle identically the same with +the best measurements which have been made of the cells of the hive-bee. + +Hence we may safely conclude that if we could slightly modify the instincts +already possessed by the Melipona, and in themselves not very wonderful, this +bee would make a structure as wonderfully perfect as that of the hive-bee. We +must suppose the Melipona to make her cells truly spherical, and of equal +sizes; and this would not be very surprising, seeing that she already does so +to a certain extent, and seeing what perfectly cylindrical burrows in wood many +insects can make, apparently by turning round on a fixed point. We must suppose +the Melipona to arrange her cells in level layers, as she already does her +cylindrical cells; and we must further suppose, and this is the greatest +difficulty, that she can somehow judge accurately at what distance to stand +from her fellow-labourers when several are making their spheres; but she is +already so far enabled to judge of distance, that she always describes her +spheres so as to intersect largely; and then she unites the points of +intersection by perfectly flat surfaces. We have further to suppose, but this +is no difficulty, that after hexagonal prisms have been formed by the +intersection of adjoining spheres in the same layer, she can prolong the +hexagon to any length requisite to hold the stock of honey; in the same way as +the rude humble-bee adds cylinders of wax to the circular mouths of her old +cocoons. By such modifications of instincts in themselves not very +wonderful,—hardly more wonderful than those which guide a bird to make its +nest,—I believe that the hive-bee has acquired, through natural selection, her +inimitable architectural powers. + +But this theory can be tested by experiment. Following the example of Mr. +Tegetmeier, I separated two combs, and put between them a long, thick, square +strip of wax: the bees instantly began to excavate minute circular pits in it; +and as they deepened these little pits, they made them wider and wider until +they were converted into shallow basins, appearing to the eye perfectly true or +parts of a sphere, and of about the diameter of a cell. It was most interesting +to me to observe that wherever several bees had begun to excavate these basins +near together, they had begun their work at such a distance from each other, +that by the time the basins had acquired the above stated width (i.e. about the +width of an ordinary cell), and were in depth about one sixth of the diameter +of the sphere of which they formed a part, the rims of the basins intersected +or broke into each other. As soon as this occurred, the bees ceased to +excavate, and began to build up flat walls of wax on the lines of intersection +between the basins, so that each hexagonal prism was built upon the festooned +edge of a smooth basin, instead of on the straight edges of a three-sided +pyramid as in the case of ordinary cells. + +I then put into the hive, instead of a thick, square piece of wax, a thin and +narrow, knife-edged ridge, coloured with vermilion. The bees instantly began on +both sides to excavate little basins near to each other, in the same way as +before; but the ridge of wax was so thin, that the bottoms of the basins, if +they had been excavated to the same depth as in the former experiment, would +have broken into each other from the opposite sides. The bees, however, did not +suffer this to happen, and they stopped their excavations in due time; so that +the basins, as soon as they had been a little deepened, came to have flat +bottoms; and these flat bottoms, formed by thin little plates of the vermilion +wax having been left ungnawed, were situated, as far as the eye could judge, +exactly along the planes of imaginary intersection between the basins on the +opposite sides of the ridge of wax. In parts, only little bits, in other parts, +large portions of a rhombic plate had been left between the opposed basins, but +the work, from the unnatural state of things, had not been neatly performed. +The bees must have worked at very nearly the same rate on the opposite sides of +the ridge of vermilion wax, as they circularly gnawed away and deepened the +basins on both sides, in order to have succeeded in thus leaving flat plates +between the basins, by stopping work along the intermediate planes or planes of +intersection. + +Considering how flexible thin wax is, I do not see that there is any difficulty +in the bees, whilst at work on the two sides of a strip of wax, perceiving when +they have gnawed the wax away to the proper thinness, and then stopping their +work. In ordinary combs it has appeared to me that the bees do not always +succeed in working at exactly the same rate from the opposite sides; for I have +noticed half-completed rhombs at the base of a just-commenced cell, which were +slightly concave on one side, where I suppose that the bees had excavated too +quickly, and convex on the opposed side, where the bees had worked less +quickly. In one well-marked instance, I put the comb back into the hive, and +allowed the bees to go on working for a short time, and again examined the +cell, and I found that the rhombic plate had been completed, and had become +perfectly flat: it was absolutely impossible, from the extreme thinness of the +little rhombic plate, that they could have effected this by gnawing away the +convex side; and I suspect that the bees in such cases stand in the opposed +cells and push and bend the ductile and warm wax (which as I have tried is +easily done) into its proper intermediate plane, and thus flatten it. + +From the experiment of the ridge of vermilion wax, we can clearly see that if +the bees were to build for themselves a thin wall of wax, they could make their +cells of the proper shape, by standing at the proper distance from each other, +by excavating at the same rate, and by endeavouring to make equal spherical +hollows, but never allowing the spheres to break into each other. Now bees, as +may be clearly seen by examining the edge of a growing comb, do make a rough, +circumferential wall or rim all round the comb; and they gnaw into this from +the opposite sides, always working circularly as they deepen each cell. They do +not make the whole three-sided pyramidal base of any one cell at the same time, +but only the one rhombic plate which stands on the extreme growing margin, or +the two plates, as the case may be; and they never complete the upper edges of +the rhombic plates, until the hexagonal walls are commenced. Some of these +statements differ from those made by the justly celebrated elder Huber, but I +am convinced of their accuracy; and if I had space, I could show that they are +conformable with my theory. + +Huber’s statement that the very first cell is excavated out of a little +parallel-sided wall of wax, is not, as far as I have seen, strictly correct; +the first commencement having always been a little hood of wax; but I will not +here enter on these details. We see how important a part excavation plays in +the construction of the cells; but it would be a great error to suppose that +the bees cannot build up a rough wall of wax in the proper position—that is, +along the plane of intersection between two adjoining spheres. I have several +specimens showing clearly that they can do this. Even in the rude +circumferential rim or wall of wax round a growing comb, flexures may sometimes +be observed, corresponding in position to the planes of the rhombic basal +plates of future cells. But the rough wall of wax has in every case to be +finished off, by being largely gnawed away on both sides. The manner in which +the bees build is curious; they always make the first rough wall from ten to +twenty times thicker than the excessively thin finished wall of the cell, which +will ultimately be left. We shall understand how they work, by supposing masons +first to pile up a broad ridge of cement, and then to begin cutting it away +equally on both sides near the ground, till a smooth, very thin wall is left in +the middle; the masons always piling up the cut-away cement, and adding fresh +cement, on the summit of the ridge. We shall thus have a thin wall steadily +growing upward; but always crowned by a gigantic coping. From all the cells, +both those just commenced and those completed, being thus crowned by a strong +coping of wax, the bees can cluster and crawl over the comb without injuring +the delicate hexagonal walls, which are only about one four-hundredth of an +inch in thickness; the plates of the pyramidal basis being about twice as +thick. By this singular manner of building, strength is continually given to +the comb, with the utmost ultimate economy of wax. + +It seems at first to add to the difficulty of understanding how the cells are +made, that a multitude of bees all work together; one bee after working a short +time at one cell going to another, so that, as Huber has stated, a score of +individuals work even at the commencement of the first cell. I was able +practically to show this fact, by covering the edges of the hexagonal walls of +a single cell, or the extreme margin of the circumferential rim of a growing +comb, with an extremely thin layer of melted vermilion wax; and I invariably +found that the colour was most delicately diffused by the bees—as delicately as +a painter could have done with his brush—by atoms of the coloured wax having +been taken from the spot on which it had been placed, and worked into the +growing edges of the cells all round. The work of construction seems to be a +sort of balance struck between many bees, all instinctively standing at the +same relative distance from each other, all trying to sweep equal spheres, and +then building up, or leaving ungnawed, the planes of intersection between these +spheres. It was really curious to note in cases of difficulty, as when two +pieces of comb met at an angle, how often the bees would entirely pull down and +rebuild in different ways the same cell, sometimes recurring to a shape which +they had at first rejected. + +When bees have a place on which they can stand in their proper positions for +working,—for instance, on a slip of wood, placed directly under the middle of a +comb growing downwards so that the comb has to be built over one face of the +slip—in this case the bees can lay the foundations of one wall of a new +hexagon, in its strictly proper place, projecting beyond the other completed +cells. It suffices that the bees should be enabled to stand at their proper +relative distances from each other and from the walls of the last completed +cells, and then, by striking imaginary spheres, they can build up a wall +intermediate between two adjoining spheres; but, as far as I have seen, they +never gnaw away and finish off the angles of a cell till a large part both of +that cell and of the adjoining cells has been built. This capacity in bees of +laying down under certain circumstances a rough wall in its proper place +between two just-commenced cells, is important, as it bears on a fact, which +seems at first quite subversive of the foregoing theory; namely, that the cells +on the extreme margin of wasp-combs are sometimes strictly hexagonal; but I +have not space here to enter on this subject. Nor does there seem to me any +great difficulty in a single insect (as in the case of a queen-wasp) making +hexagonal cells, if she work alternately on the inside and outside of two or +three cells commenced at the same time, always standing at the proper relative +distance from the parts of the cells just begun, sweeping spheres or cylinders, +and building up intermediate planes. It is even conceivable that an insect +might, by fixing on a point at which to commence a cell, and then moving +outside, first to one point, and then to five other points, at the proper +relative distances from the central point and from each other, strike the +planes of intersection, and so make an isolated hexagon: but I am not aware +that any such case has been observed; nor would any good be derived from a +single hexagon being built, as in its construction more materials would be +required than for a cylinder. + +As natural selection acts only by the accumulation of slight modifications of +structure or instinct, each profitable to the individual under its conditions +of life, it may reasonably be asked, how a long and graduated succession of +modified architectural instincts, all tending towards the present perfect plan +of construction, could have profited the progenitors of the hive-bee? I think +the answer is not difficult: it is known that bees are often hard pressed to +get sufficient nectar; and I am informed by Mr. Tegetmeier that it has been +experimentally found that no less than from twelve to fifteen pounds of dry +sugar are consumed by a hive of bees for the secretion of each pound of wax; so +that a prodigious quantity of fluid nectar must be collected and consumed by +the bees in a hive for the secretion of the wax necessary for the construction +of their combs. Moreover, many bees have to remain idle for many days during +the process of secretion. A large store of honey is indispensable to support a +large stock of bees during the winter; and the security of the hive is known +mainly to depend on a large number of bees being supported. Hence the saving of +wax by largely saving honey must be a most important element of success in any +family of bees. Of course the success of any species of bee may be dependent on +the number of its parasites or other enemies, or on quite distinct causes, and +so be altogether independent of the quantity of honey which the bees could +collect. But let us suppose that this latter circumstance determined, as it +probably often does determine, the numbers of a humble-bee which could exist in +a country; and let us further suppose that the community lived throughout the +winter, and consequently required a store of honey: there can in this case be +no doubt that it would be an advantage to our humble-bee, if a slight +modification of her instinct led her to make her waxen cells near together, so +as to intersect a little; for a wall in common even to two adjoining cells, +would save some little wax. Hence it would continually be more and more +advantageous to our humble-bee, if she were to make her cells more and more +regular, nearer together, and aggregated into a mass, like the cells of the +Melipona; for in this case a large part of the bounding surface of each cell +would serve to bound other cells, and much wax would be saved. Again, from the +same cause, it would be advantageous to the Melipona, if she were to make her +cells closer together, and more regular in every way than at present; for then, +as we have seen, the spherical surfaces would wholly disappear, and would all +be replaced by plane surfaces; and the Melipona would make a comb as perfect as +that of the hive-bee. Beyond this stage of perfection in architecture, natural +selection could not lead; for the comb of the hive-bee, as far as we can see, +is absolutely perfect in economising wax. + +Thus, as I believe, the most wonderful of all known instincts, that of the +hive-bee, can be explained by natural selection having taken advantage of +numerous, successive, slight modifications of simpler instincts; natural +selection having by slow degrees, more and more perfectly, led the bees to +sweep equal spheres at a given distance from each other in a double layer, and +to build up and excavate the wax along the planes of intersection. The bees, of +course, no more knowing that they swept their spheres at one particular +distance from each other, than they know what are the several angles of the +hexagonal prisms and of the basal rhombic plates. The motive power of the +process of natural selection having been economy of wax; that individual swarm +which wasted least honey in the secretion of wax, having succeeded best, and +having transmitted by inheritance its newly acquired economical instinct to new +swarms, which in their turn will have had the best chance of succeeding in the +struggle for existence. + +No doubt many instincts of very difficult explanation could be opposed to the +theory of natural selection,—cases, in which we cannot see how an instinct +could possibly have originated; cases, in which no intermediate gradations are +known to exist; cases of instinct of apparently such trifling importance, that +they could hardly have been acted on by natural selection; cases of instincts +almost identically the same in animals so remote in the scale of nature, that +we cannot account for their similarity by inheritance from a common parent, and +must therefore believe that they have been acquired by independent acts of +natural selection. I will not here enter on these several cases, but will +confine myself to one special difficulty, which at first appeared to me +insuperable, and actually fatal to my whole theory. I allude to the neuters or +sterile females in insect-communities: for these neuters often differ widely in +instinct and in structure from both the males and fertile females, and yet, +from being sterile, they cannot propagate their kind. + +The subject well deserves to be discussed at great length, but I will here take +only a single case, that of working or sterile ants. How the workers have been +rendered sterile is a difficulty; but not much greater than that of any other +striking modification of structure; for it can be shown that some insects and +other articulate animals in a state of nature occasionally become sterile; and +if such insects had been social, and it had been profitable to the community +that a number should have been annually born capable of work, but incapable of +procreation, I can see no very great difficulty in this being effected by +natural selection. But I must pass over this preliminary difficulty. The great +difficulty lies in the working ants differing widely from both the males and +the fertile females in structure, as in the shape of the thorax and in being +destitute of wings and sometimes of eyes, and in instinct. As far as instinct +alone is concerned, the prodigious difference in this respect between the +workers and the perfect females, would have been far better exemplified by the +hive-bee. If a working ant or other neuter insect had been an animal in the +ordinary state, I should have unhesitatingly assumed that all its characters +had been slowly acquired through natural selection; namely, by an individual +having been born with some slight profitable modification of structure, this +being inherited by its offspring, which again varied and were again selected, +and so onwards. But with the working ant we have an insect differing greatly +from its parents, yet absolutely sterile; so that it could never have +transmitted successively acquired modifications of structure or instinct to its +progeny. It may well be asked how is it possible to reconcile this case with +the theory of natural selection? + +First, let it be remembered that we have innumerable instances, both in our +domestic productions and in those in a state of nature, of all sorts of +differences of structure which have become correlated to certain ages, and to +either sex. We have differences correlated not only to one sex, but to that +short period alone when the reproductive system is active, as in the nuptial +plumage of many birds, and in the hooked jaws of the male salmon. We have even +slight differences in the horns of different breeds of cattle in relation to an +artificially imperfect state of the male sex; for oxen of certain breeds have +longer horns than in other breeds, in comparison with the horns of the bulls or +cows of these same breeds. Hence I can see no real difficulty in any character +having become correlated with the sterile condition of certain members of +insect-communities: the difficulty lies in understanding how such correlated +modifications of structure could have been slowly accumulated by natural +selection. + +This difficulty, though appearing insuperable, is lessened, or, as I believe, +disappears, when it is remembered that selection may be applied to the family, +as well as to the individual, and may thus gain the desired end. Thus, a +well-flavoured vegetable is cooked, and the individual is destroyed; but the +horticulturist sows seeds of the same stock, and confidently expects to get +nearly the same variety; breeders of cattle wish the flesh and fat to be well +marbled together; the animal has been slaughtered, but the breeder goes with +confidence to the same family. I have such faith in the powers of selection, +that I do not doubt that a breed of cattle, always yielding oxen with +extraordinarily long horns, could be slowly formed by carefully watching which +individual bulls and cows, when matched, produced oxen with the longest horns; +and yet no one ox could ever have propagated its kind. Thus I believe it has +been with social insects: a slight modification of structure, or instinct, +correlated with the sterile condition of certain members of the community, has +been advantageous to the community: consequently the fertile males and females +of the same community flourished, and transmitted to their fertile offspring a +tendency to produce sterile members having the same modification. And I believe +that this process has been repeated, until that prodigious amount of difference +between the fertile and sterile females of the same species has been produced, +which we see in many social insects. + +But we have not as yet touched on the climax of the difficulty; namely, the +fact that the neuters of several ants differ, not only from the fertile females +and males, but from each other, sometimes to an almost incredible degree, and +are thus divided into two or even three castes. The castes, moreover, do not +generally graduate into each other, but are perfectly well defined; being as +distinct from each other, as are any two species of the same genus, or rather +as any two genera of the same family. Thus in Eciton, there are working and +soldier neuters, with jaws and instincts extraordinarily different: in +Cryptocerus, the workers of one caste alone carry a wonderful sort of shield on +their heads, the use of which is quite unknown: in the Mexican Myrmecocystus, +the workers of one caste never leave the nest; they are fed by the workers of +another caste, and they have an enormously developed abdomen which secretes a +sort of honey, supplying the place of that excreted by the aphides, or the +domestic cattle as they may be called, which our European ants guard or +imprison. + +It will indeed be thought that I have an overweening confidence in the +principle of natural selection, when I do not admit that such wonderful and +well-established facts at once annihilate my theory. In the simpler case of +neuter insects all of one caste or of the same kind, which have been rendered +by natural selection, as I believe to be quite possible, different from the +fertile males and females,—in this case, we may safely conclude from the +analogy of ordinary variations, that each successive, slight, profitable +modification did not probably at first appear in all the individual neuters in +the same nest, but in a few alone; and that by the long-continued selection of +the fertile parents which produced most neuters with the profitable +modification, all the neuters ultimately came to have the desired character. On +this view we ought occasionally to find neuter-insects of the same species, in +the same nest, presenting gradations of structure; and this we do find, even +often, considering how few neuter-insects out of Europe have been carefully +examined. Mr. F. Smith has shown how surprisingly the neuters of several +British ants differ from each other in size and sometimes in colour; and that +the extreme forms can sometimes be perfectly linked together by individuals +taken out of the same nest: I have myself compared perfect gradations of this +kind. It often happens that the larger or the smaller sized workers are the +most numerous; or that both large and small are numerous, with those of an +intermediate size scanty in numbers. Formica flava has larger and smaller +workers, with some of intermediate size; and, in this species, as Mr. F. Smith +has observed, the larger workers have simple eyes (ocelli), which though small +can be plainly distinguished, whereas the smaller workers have their ocelli +rudimentary. Having carefully dissected several specimens of these workers, I +can affirm that the eyes are far more rudimentary in the smaller workers than +can be accounted for merely by their proportionally lesser size; and I fully +believe, though I dare not assert so positively, that the workers of +intermediate size have their ocelli in an exactly intermediate condition. So +that we here have two bodies of sterile workers in the same nest, differing not +only in size, but in their organs of vision, yet connected by some few members +in an intermediate condition. I may digress by adding, that if the smaller +workers had been the most useful to the community, and those males and females +had been continually selected, which produced more and more of the smaller +workers, until all the workers had come to be in this condition; we should then +have had a species of ant with neuters very nearly in the same condition with +those of Myrmica. For the workers of Myrmica have not even rudiments of ocelli, +though the male and female ants of this genus have well-developed ocelli. + +I may give one other case: so confidently did I expect to find gradations in +important points of structure between the different castes of neuters in the +same species, that I gladly availed myself of Mr. F. Smith’s offer of numerous +specimens from the same nest of the driver ant (Anomma) of West Africa. The +reader will perhaps best appreciate the amount of difference in these workers, +by my giving not the actual measurements, but a strictly accurate illustration: +the difference was the same as if we were to see a set of workmen building a +house of whom many were five feet four inches high, and many sixteen feet high; +but we must suppose that the larger workmen had heads four instead of three +times as big as those of the smaller men, and jaws nearly five times as big. +The jaws, moreover, of the working ants of the several sizes differed +wonderfully in shape, and in the form and number of the teeth. But the +important fact for us is, that though the workers can be grouped into castes of +different sizes, yet they graduate insensibly into each other, as does the +widely-different structure of their jaws. I speak confidently on this latter +point, as Mr. Lubbock made drawings for me with the camera lucida of the jaws +which I had dissected from the workers of the several sizes. + +With these facts before me, I believe that natural selection, by acting on the +fertile parents, could form a species which should regularly produce neuters, +either all of large size with one form of jaw, or all of small size with jaws +having a widely different structure; or lastly, and this is our climax of +difficulty, one set of workers of one size and structure, and simultaneously +another set of workers of a different size and structure;—a graduated series +having been first formed, as in the case of the driver ant, and then the +extreme forms, from being the most useful to the community, having been +produced in greater and greater numbers through the natural selection of the +parents which generated them; until none with an intermediate structure were +produced. + +Thus, as I believe, the wonderful fact of two distinctly defined castes of +sterile workers existing in the same nest, both widely different from each +other and from their parents, has originated. We can see how useful their +production may have been to a social community of insects, on the same +principle that the division of labour is useful to civilised man. As ants work +by inherited instincts and by inherited tools or weapons, and not by acquired +knowledge and manufactured instruments, a perfect division of labour could be +effected with them only by the workers being sterile; for had they been +fertile, they would have intercrossed, and their instincts and structure would +have become blended. And nature has, as I believe, effected this admirable +division of labour in the communities of ants, by the means of natural +selection. But I am bound to confess, that, with all my faith in this +principle, I should never have anticipated that natural selection could have +been efficient in so high a degree, had not the case of these neuter insects +convinced me of the fact. I have, therefore, discussed this case, at some +little but wholly insufficient length, in order to show the power of natural +selection, and likewise because this is by far the most serious special +difficulty, which my theory has encountered. The case, also, is very +interesting, as it proves that with animals, as with plants, any amount of +modification in structure can be effected by the accumulation of numerous, +slight, and as we must call them accidental, variations, which are in any +manner profitable, without exercise or habit having come into play. For no +amount of exercise, or habit, or volition, in the utterly sterile members of a +community could possibly have affected the structure or instincts of the +fertile members, which alone leave descendants. I am surprised that no one has +advanced this demonstrative case of neuter insects, against the well-known +doctrine of Lamarck. + +Summary.—I have endeavoured briefly in this chapter to show that the mental +qualities of our domestic animals vary, and that the variations are inherited. +Still more briefly I have attempted to show that instincts vary slightly in a +state of nature. No one will dispute that instincts are of the highest +importance to each animal. Therefore I can see no difficulty, under changing +conditions of life, in natural selection accumulating slight modifications of +instinct to any extent, in any useful direction. In some cases habit or use and +disuse have probably come into play. I do not pretend that the facts given in +this chapter strengthen in any great degree my theory; but none of the cases of +difficulty, to the best of my judgment, annihilate it. On the other hand, the +fact that instincts are not always absolutely perfect and are liable to +mistakes;—that no instinct has been produced for the exclusive good of other +animals, but that each animal takes advantage of the instincts of others;—that +the canon in natural history, of “natura non facit saltum” is applicable to +instincts as well as to corporeal structure, and is plainly explicable on the +foregoing views, but is otherwise inexplicable,—all tend to corroborate the +theory of natural selection. + +This theory is, also, strengthened by some few other facts in regard to +instincts; as by that common case of closely allied, but certainly distinct, +species, when inhabiting distant parts of the world and living under +considerably different conditions of life, yet often retaining nearly the same +instincts. For instance, we can understand on the principle of inheritance, how +it is that the thrush of South America lines its nest with mud, in the same +peculiar manner as does our British thrush: how it is that the male wrens +(Troglodytes) of North America, build “cock-nests,” to roost in, like the males +of our distinct Kitty-wrens,—a habit wholly unlike that of any other known +bird. Finally, it may not be a logical deduction, but to my imagination it is +far more satisfactory to look at such instincts as the young cuckoo ejecting +its foster-brothers,—ants making slaves,—the larvæ of ichneumonidæ feeding +within the live bodies of caterpillars,—not as specially endowed or created +instincts, but as small consequences of one general law, leading to the +advancement of all organic beings, namely, multiply, vary, let the strongest +live and the weakest die. + +CHAPTER VIII. +HYBRIDISM. + +Distinction between the sterility of first crosses and of hybrids. Sterility +various in degree, not universal, affected by close interbreeding, removed by +domestication. Laws governing the sterility of hybrids. Sterility not a special +endowment, but incidental on other differences. Causes of the sterility of +first crosses and of hybrids. Parallelism between the effects of changed +conditions of life and crossing. Fertility of varieties when crossed and of +their mongrel offspring not universal. Hybrids and mongrels compared +independently of their fertility. Summary. + +The view generally entertained by naturalists is that species, when +intercrossed, have been specially endowed with the quality of sterility, in +order to prevent the confusion of all organic forms. This view certainly seems +at first probable, for species within the same country could hardly have kept +distinct had they been capable of crossing freely. The importance of the fact +that hybrids are very generally sterile, has, I think, been much underrated by +some late writers. On the theory of natural selection the case is especially +important, inasmuch as the sterility of hybrids could not possibly be of any +advantage to them, and therefore could not have been acquired by the continued +preservation of successive profitable degrees of sterility. I hope, however, to +be able to show that sterility is not a specially acquired or endowed quality, +but is incidental on other acquired differences. + +In treating this subject, two classes of facts, to a large extent fundamentally +different, have generally been confounded together; namely, the sterility of +two species when first crossed, and the sterility of the hybrids produced from +them. + +Pure species have of course their organs of reproduction in a perfect +condition, yet when intercrossed they produce either few or no offspring. +Hybrids, on the other hand, have their reproductive organs functionally +impotent, as may be clearly seen in the state of the male element in both +plants and animals; though the organs themselves are perfect in structure, as +far as the microscope reveals. In the first case the two sexual elements which +go to form the embryo are perfect; in the second case they are either not at +all developed, or are imperfectly developed. This distinction is important, +when the cause of the sterility, which is common to the two cases, has to be +considered. The distinction has probably been slurred over, owing to the +sterility in both cases being looked on as a special endowment, beyond the +province of our reasoning powers. + +The fertility of varieties, that is of the forms known or believed to have +descended from common parents, when intercrossed, and likewise the fertility of +their mongrel offspring, is, on my theory, of equal importance with the +sterility of species; for it seems to make a broad and clear distinction +between varieties and species. + +First, for the sterility of species when crossed and of their hybrid offspring. +It is impossible to study the several memoirs and works of those two +conscientious and admirable observers, Kölreuter and Gärtner, who almost +devoted their lives to this subject, without being deeply impressed with the +high generality of some degree of sterility. Kölreuter makes the rule +universal; but then he cuts the knot, for in ten cases in which he found two +forms, considered by most authors as distinct species, quite fertile together, +he unhesitatingly ranks them as varieties. Gärtner, also, makes the rule +equally universal; and he disputes the entire fertility of Kölreuter’s ten +cases. But in these and in many other cases, Gärtner is obliged carefully to +count the seeds, in order to show that there is any degree of sterility. He +always compares the maximum number of seeds produced by two species when +crossed and by their hybrid offspring, with the average number produced by both +pure parent-species in a state of nature. But a serious cause of error seems to +me to be here introduced: a plant to be hybridised must be castrated, and, what +is often more important, must be secluded in order to prevent pollen being +brought to it by insects from other plants. Nearly all the plants +experimentised on by Gärtner were potted, and apparently were kept in a chamber +in his house. That these processes are often injurious to the fertility of a +plant cannot be doubted; for Gärtner gives in his table about a score of cases +of plants which he castrated, and artificially fertilised with their own +pollen, and (excluding all cases such as the Leguminosæ, in which there is an +acknowledged difficulty in the manipulation) half of these twenty plants had +their fertility in some degree impaired. Moreover, as Gärtner during several +years repeatedly crossed the primrose and cowslip, which we have such good +reason to believe to be varieties, and only once or twice succeeded in getting +fertile seed; as he found the common red and blue pimpernels (Anagallis +arvensis and coerulea), which the best botanists rank as varieties, absolutely +sterile together; and as he came to the same conclusion in several other +analogous cases; it seems to me that we may well be permitted to doubt whether +many other species are really so sterile, when intercrossed, as Gärtner +believes. + +It is certain, on the one hand, that the sterility of various species when +crossed is so different in degree and graduates away so insensibly, and, on the +other hand, that the fertility of pure species is so easily affected by various +circumstances, that for all practical purposes it is most difficult to say +where perfect fertility ends and sterility begins. I think no better evidence +of this can be required than that the two most experienced observers who have +ever lived, namely, Kölreuter and Gärtner, should have arrived at diametrically +opposite conclusions in regard to the very same species. It is also most +instructive to compare—but I have not space here to enter on details—the +evidence advanced by our best botanists on the question whether certain +doubtful forms should be ranked as species or varieties, with the evidence from +fertility adduced by different hybridisers, or by the same author, from +experiments made during different years. It can thus be shown that neither +sterility nor fertility affords any clear distinction between species and +varieties; but that the evidence from this source graduates away, and is +doubtful in the same degree as is the evidence derived from other +constitutional and structural differences. + +In regard to the sterility of hybrids in successive generations; though Gärtner +was enabled to rear some hybrids, carefully guarding them from a cross with +either pure parent, for six or seven, and in one case for ten generations, yet +he asserts positively that their fertility never increased, but generally +greatly decreased. I do not doubt that this is usually the case, and that the +fertility often suddenly decreases in the first few generations. Nevertheless I +believe that in all these experiments the fertility has been diminished by an +independent cause, namely, from close interbreeding. I have collected so large +a body of facts, showing that close interbreeding lessens fertility, and, on +the other hand, that an occasional cross with a distinct individual or variety +increases fertility, that I cannot doubt the correctness of this almost +universal belief amongst breeders. Hybrids are seldom raised by +experimentalists in great numbers; and as the parent-species, or other allied +hybrids, generally grow in the same garden, the visits of insects must be +carefully prevented during the flowering season: hence hybrids will generally +be fertilised during each generation by their own individual pollen; and I am +convinced that this would be injurious to their fertility, already lessened by +their hybrid origin. I am strengthened in this conviction by a remarkable +statement repeatedly made by Gärtner, namely, that if even the less fertile +hybrids be artificially fertilised with hybrid pollen of the same kind, their +fertility, notwithstanding the frequent ill effects of manipulation, sometimes +decidedly increases, and goes on increasing. Now, in artificial fertilisation +pollen is as often taken by chance (as I know from my own experience) from the +anthers of another flower, as from the anthers of the flower itself which is to +be fertilised; so that a cross between two flowers, though probably on the same +plant, would be thus effected. Moreover, whenever complicated experiments are +in progress, so careful an observer as Gärtner would have castrated his +hybrids, and this would have insured in each generation a cross with the pollen +from a distinct flower, either from the same plant or from another plant of the +same hybrid nature. And thus, the strange fact of the increase of fertility in +the successive generations of artificially fertilised hybrids may, I believe, +be accounted for by close interbreeding having been avoided. + +Now let us turn to the results arrived at by the third most experienced +hybridiser, namely, the Honourable and Reverend W. Herbert. He is as emphatic +in his conclusion that some hybrids are perfectly fertile—as fertile as the +pure parent-species—as are Kölreuter and Gärtner that some degree of sterility +between distinct species is a universal law of nature. He experimentised on +some of the very same species as did Gärtner. The difference in their results +may, I think, be in part accounted for by Herbert’s great horticultural skill, +and by his having hothouses at his command. Of his many important statements I +will here give only a single one as an example, namely, that “every ovule in a +pod of Crinum capense fertilised by C. revolutum produced a plant, which (he +says) I never saw to occur in a case of its natural fecundation.” So that we +here have perfect, or even more than commonly perfect, fertility in a first +cross between two distinct species. + +This case of the Crinum leads me to refer to a most singular fact, namely, that +there are individual plants, as with certain species of Lobelia, and with all +the species of the genus Hippeastrum, which can be far more easily fertilised +by the pollen of another and distinct species, than by their own pollen. For +these plants have been found to yield seed to the pollen of a distinct species, +though quite sterile with their own pollen, notwithstanding that their own +pollen was found to be perfectly good, for it fertilised distinct species. So +that certain individual plants and all the individuals of certain species can +actually be hybridised much more readily than they can be self-fertilised! For +instance, a bulb of Hippeastrum aulicum produced four flowers; three were +fertilised by Herbert with their own pollen, and the fourth was subsequently +fertilised by the pollen of a compound hybrid descended from three other and +distinct species: the result was that “the ovaries of the three first flowers +soon ceased to grow, and after a few days perished entirely, whereas the pod +impregnated by the pollen of the hybrid made vigorous growth and rapid progress +to maturity, and bore good seed, which vegetated freely.” In a letter to me, in +1839, Mr. Herbert told me that he had then tried the experiment during five +years, and he continued to try it during several subsequent years, and always +with the same result. This result has, also, been confirmed by other observers +in the case of Hippeastrum with its sub-genera, and in the case of some other +genera, as Lobelia, Passiflora and Verbascum. Although the plants in these +experiments appeared perfectly healthy, and although both the ovules and pollen +of the same flower were perfectly good with respect to other species, yet as +they were functionally imperfect in their mutual self-action, we must infer +that the plants were in an unnatural state. Nevertheless these facts show on +what slight and mysterious causes the lesser or greater fertility of species +when crossed, in comparison with the same species when self-fertilised, +sometimes depends. + +The practical experiments of horticulturists, though not made with scientific +precision, deserve some notice. It is notorious in how complicated a manner the +species of Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, etc., have +been crossed, yet many of these hybrids seed freely. For instance, Herbert +asserts that a hybrid from Calceolaria integrifolia and plantaginea, species +most widely dissimilar in general habit, “reproduced itself as perfectly as if +it had been a natural species from the mountains of Chile.” I have taken some +pains to ascertain the degree of fertility of some of the complex crosses of +Rhododendrons, and I am assured that many of them are perfectly fertile. Mr. C. +Noble, for instance, informs me that he raises stocks for grafting from a +hybrid between Rhododendron Ponticum and Catawbiense, and that this hybrid +“seeds as freely as it is possible to imagine.” Had hybrids, when fairly +treated, gone on decreasing in fertility in each successive generation, as +Gärtner believes to be the case, the fact would have been notorious to +nurserymen. Horticulturists raise large beds of the same hybrids, and such +alone are fairly treated, for by insect agency the several individuals of the +same hybrid variety are allowed to freely cross with each other, and the +injurious influence of close interbreeding is thus prevented. Any one may +readily convince himself of the efficiency of insect-agency by examining the +flowers of the more sterile kinds of hybrid rhododendrons, which produce no +pollen, for he will find on their stigmas plenty of pollen brought from other +flowers. + +In regard to animals, much fewer experiments have been carefully tried than +with plants. If our systematic arrangements can be trusted, that is if the +genera of animals are as distinct from each other, as are the genera of plants, +then we may infer that animals more widely separated in the scale of nature can +be more easily crossed than in the case of plants; but the hybrids themselves +are, I think, more sterile. I doubt whether any case of a perfectly fertile +hybrid animal can be considered as thoroughly well authenticated. It should, +however, be borne in mind that, owing to few animals breeding freely under +confinement, few experiments have been fairly tried: for instance, the +canary-bird has been crossed with nine other finches, but as not one of these +nine species breeds freely in confinement, we have no right to expect that the +first crosses between them and the canary, or that their hybrids, should be +perfectly fertile. Again, with respect to the fertility in successive +generations of the more fertile hybrid animals, I hardly know of an instance in +which two families of the same hybrid have been raised at the same time from +different parents, so as to avoid the ill effects of close interbreeding. On +the contrary, brothers and sisters have usually been crossed in each successive +generation, in opposition to the constantly repeated admonition of every +breeder. And in this case, it is not at all surprising that the inherent +sterility in the hybrids should have gone on increasing. If we were to act +thus, and pair brothers and sisters in the case of any pure animal, which from +any cause had the least tendency to sterility, the breed would assuredly be +lost in a very few generations. + +Although I do not know of any thoroughly well-authenticated cases of perfectly +fertile hybrid animals, I have some reason to believe that the hybrids from +Cervulus vaginalis and Reevesii, and from Phasianus colchicus with P. torquatus +and with P. versicolor are perfectly fertile. The hybrids from the common and +Chinese geese (A. cygnoides), species which are so different that they are +generally ranked in distinct genera, have often bred in this country with +either pure parent, and in one single instance they have bred inter se. This +was effected by Mr. Eyton, who raised two hybrids from the same parents but +from different hatches; and from these two birds he raised no less than eight +hybrids (grandchildren of the pure geese) from one nest. In India, however, +these cross-bred geese must be far more fertile; for I am assured by two +eminently capable judges, namely Mr. Blyth and Capt. Hutton, that whole flocks +of these crossed geese are kept in various parts of the country; and as they +are kept for profit, where neither pure parent-species exists, they must +certainly be highly fertile. + +A doctrine which originated with Pallas, has been largely accepted by modern +naturalists; namely, that most of our domestic animals have descended from two +or more aboriginal species, since commingled by intercrossing. On this view, +the aboriginal species must either at first have produced quite fertile +hybrids, or the hybrids must have become in subsequent generations quite +fertile under domestication. This latter alternative seems to me the most +probable, and I am inclined to believe in its truth, although it rests on no +direct evidence. I believe, for instance, that our dogs have descended from +several wild stocks; yet, with perhaps the exception of certain indigenous +domestic dogs of South America, all are quite fertile together; and analogy +makes me greatly doubt, whether the several aboriginal species would at first +have freely bred together and have produced quite fertile hybrids. So again +there is reason to believe that our European and the humped Indian cattle are +quite fertile together; but from facts communicated to me by Mr. Blyth, I think +they must be considered as distinct species. On this view of the origin of many +of our domestic animals, we must either give up the belief of the almost +universal sterility of distinct species of animals when crossed; or we must +look at sterility, not as an indelible characteristic, but as one capable of +being removed by domestication. + +Finally, looking to all the ascertained facts on the intercrossing of plants +and animals, it may be concluded that some degree of sterility, both in first +crosses and in hybrids,is an extremely general result; but that it cannot, +under our present state of knowledge, be considered as absolutely universal. + +Laws governing the Sterility of first Crosses and of Hybrids.—We will now +consider a little more in detail the circumstances and rules governing the +sterility of first crosses and of hybrids. Our chief object will be to see +whether or not the rules indicate that species have specially been endowed with +this quality, in order to prevent their crossing and blending together in utter +confusion. The following rules and conclusions are chiefly drawn up from +Gärtner’s admirable work on the hybridisation of plants. I have taken much +pains to ascertain how far the rules apply to animals, and considering how +scanty our knowledge is in regard to hybrid animals, I have been surprised to +find how generally the same rules apply to both kingdoms. + +It has been already remarked, that the degree of fertility, both of first +crosses and of hybrids, graduates from zero to perfect fertility. It is +surprising in how many curious ways this gradation can be shown to exist; but +only the barest outline of the facts can here be given. When pollen from a +plant of one family is placed on the stigma of a plant of a distinct family, it +exerts no more influence than so much inorganic dust. From this absolute zero +of fertility, the pollen of different species of the same genus applied to the +stigma of some one species, yields a perfect gradation in the number of seeds +produced, up to nearly complete or even quite complete fertility; and, as we +have seen, in certain abnormal cases, even to an excess of fertility, beyond +that which the plant’s own pollen will produce. So in hybrids themselves, there +are some which never have produced, and probably never would produce, even with +the pollen of either pure parent, a single fertile seed: but in some of these +cases a first trace of fertility may be detected, by the pollen of one of the +pure parent-species causing the flower of the hybrid to wither earlier than it +otherwise would have done; and the early withering of the flower is well known +to be a sign of incipient fertilisation. From this extreme degree of sterility +we have self-fertilised hybrids producing a greater and greater number of seeds +up to perfect fertility. + +Hybrids from two species which are very difficult to cross, and which rarely +produce any offspring, are generally very sterile; but the parallelism between +the difficulty of making a first cross, and the sterility of the hybrids thus +produced—two classes of facts which are generally confounded together—is by no +means strict. There are many cases, in which two pure species can be united +with unusual facility, and produce numerous hybrid-offspring, yet these hybrids +are remarkably sterile. On the other hand, there are species which can be +crossed very rarely, or with extreme difficulty, but the hybrids, when at last +produced, are very fertile. Even within the limits of the same genus, for +instance in Dianthus, these two opposite cases occur. + +The fertility, both of first crosses and of hybrids, is more easily affected by +unfavourable conditions, than is the fertility of pure species. But the degree +of fertility is likewise innately variable; for it is not always the same when +the same two species are crossed under the same circumstances, but depends in +part upon the constitution of the individuals which happen to have been chosen +for the experiment. So it is with hybrids, for their degree of fertility is +often found to differ greatly in the several individuals raised from seed out +of the same capsule and exposed to exactly the same conditions. + +By the term systematic affinity is meant, the resemblance between species in +structure and in constitution, more especially in the structure of parts which +are of high physiological importance and which differ little in the allied +species. Now the fertility of first crosses between species, and of the hybrids +produced from them, is largely governed by their systematic affinity. This is +clearly shown by hybrids never having been raised between species ranked by +systematists in distinct families; and on the other hand, by very closely +allied species generally uniting with facility. But the correspondence between +systematic affinity and the facility of crossing is by no means strict. A +multitude of cases could be given of very closely allied species which will not +unite, or only with extreme difficulty; and on the other hand of very distinct +species which unite with the utmost facility. In the same family there may be a +genus, as Dianthus, in which very many species can most readily be crossed; and +another genus, as Silene, in which the most persevering efforts have failed to +produce between extremely close species a single hybrid. Even within the limits +of the same genus, we meet with this same difference; for instance, the many +species of Nicotiana have been more largely crossed than the species of almost +any other genus; but Gärtner found that N. acuminata, which is not a +particularly distinct species, obstinately failed to fertilise, or to be +fertilised by, no less than eight other species of Nicotiana. Very many +analogous facts could be given. + +No one has been able to point out what kind, or what amount, of difference in +any recognisable character is sufficient to prevent two species crossing. It +can be shown that plants most widely different in habit and general appearance, +and having strongly marked differences in every part of the flower, even in the +pollen, in the fruit, and in the cotyledons, can be crossed. Annual and +perennial plants, deciduous and evergreen trees, plants inhabiting different +stations and fitted for extremely different climates, can often be crossed with +ease. + +By a reciprocal cross between two species, I mean the case, for instance, of a +stallion-horse being first crossed with a female-ass, and then a male-ass with +a mare: these two species may then be said to have been reciprocally crossed. +There is often the widest possible difference in the facility of making +reciprocal crosses. Such cases are highly important, for they prove that the +capacity in any two species to cross is often completely independent of their +systematic affinity, or of any recognisable difference in their whole +organisation. On the other hand, these cases clearly show that the capacity for +crossing is connected with constitutional differences imperceptible by us, and +confined to the reproductive system. This difference in the result of +reciprocal crosses between the same two species was long ago observed by +Kölreuter. To give an instance: Mirabilis jalappa can easily be fertilised by +the pollen of M. longiflora, and the hybrids thus produced are sufficiently +fertile; but Kölreuter tried more than two hundred times, during eight +following years, to fertilise reciprocally M. longiflora with the pollen of M. +jalappa, and utterly failed. Several other equally striking cases could be +given. Thuret has observed the same fact with certain sea-weeds or Fuci. +Gärtner, moreover, found that this difference of facility in making reciprocal +crosses is extremely common in a lesser degree. He has observed it even between +forms so closely related (as Matthiola annua and glabra) that many botanists +rank them only as varieties. It is also a remarkable fact, that hybrids raised +from reciprocal crosses, though of course compounded of the very same two +species, the one species having first been used as the father and then as the +mother, generally differ in fertility in a small, and occasionally in a high +degree. + +Several other singular rules could be given from Gärtner: for instance, some +species have a remarkable power of crossing with other species; other species +of the same genus have a remarkable power of impressing their likeness on their +hybrid offspring; but these two powers do not at all necessarily go together. +There are certain hybrids which instead of having, as is usual, an intermediate +character between their two parents, always closely resemble one of them; and +such hybrids, though externally so like one of their pure parent-species, are +with rare exceptions extremely sterile. So again amongst hybrids which are +usually intermediate in structure between their parents, exceptional and +abnormal individuals sometimes are born, which closely resemble one of their +pure parents; and these hybrids are almost always utterly sterile, even when +the other hybrids raised from seed from the same capsule have a considerable +degree of fertility. These facts show how completely fertility in the hybrid is +independent of its external resemblance to either pure parent. + +Considering the several rules now given, which govern the fertility of first +crosses and of hybrids, we see that when forms, which must be considered as +good and distinct species, are united, their fertility graduates from zero to +perfect fertility, or even to fertility under certain conditions in excess. +That their fertility, besides being eminently susceptible to favourable and +unfavourable conditions, is innately variable. That it is by no means always +the same in degree in the first cross and in the hybrids produced from this +cross. That the fertility of hybrids is not related to the degree in which they +resemble in external appearance either parent. And lastly, that the facility of +making a first cross between any two species is not always governed by their +systematic affinity or degree of resemblance to each other. This latter +statement is clearly proved by reciprocal crosses between the same two species, +for according as the one species or the other is used as the father or the +mother, there is generally some difference, and occasionally the widest +possible difference, in the facility of effecting an union. The hybrids, +moreover, produced from reciprocal crosses often differ in fertility. + +Now do these complex and singular rules indicate that species have been endowed +with sterility simply to prevent their becoming confounded in nature? I think +not. For why should the sterility be so extremely different in degree, when +various species are crossed, all of which we must suppose it would be equally +important to keep from blending together? Why should the degree of sterility be +innately variable in the individuals of the same species? Why should some +species cross with facility, and yet produce very sterile hybrids; and other +species cross with extreme difficulty, and yet produce fairly fertile hybrids? +Why should there often be so great a difference in the result of a reciprocal +cross between the same two species? Why, it may even be asked, has the +production of hybrids been permitted? to grant to species the special power of +producing hybrids, and then to stop their further propagation by different +degrees of sterility, not strictly related to the facility of the first union +between their parents, seems to be a strange arrangement. + +The foregoing rules and facts, on the other hand, appear to me clearly to +indicate that the sterility both of first crosses and of hybrids is simply +incidental or dependent on unknown differences, chiefly in the reproductive +systems, of the species which are crossed. The differences being of so peculiar +and limited a nature, that, in reciprocal crosses between two species the male +sexual element of the one will often freely act on the female sexual element of +the other, but not in a reversed direction. It will be advisable to explain a +little more fully by an example what I mean by sterility being incidental on +other differences, and not a specially endowed quality. As the capacity of one +plant to be grafted or budded on another is so entirely unimportant for its +welfare in a state of nature, I presume that no one will suppose that this +capacity is a specially endowed quality, but will admit that it is incidental +on differences in the laws of growth of the two plants. We can sometimes see +the reason why one tree will not take on another, from differences in their +rate of growth, in the hardness of their wood, in the period of the flow or +nature of their sap, etc.; but in a multitude of cases we can assign no reason +whatever. Great diversity in the size of two plants, one being woody and the +other herbaceous, one being evergreen and the other deciduous, and adaptation +to widely different climates, does not always prevent the two grafting +together. As in hybridisation, so with grafting, the capacity is limited by +systematic affinity, for no one has been able to graft trees together belonging +to quite distinct families; and, on the other hand, closely allied species, and +varieties of the same species, can usually, but not invariably, be grafted with +ease. But this capacity, as in hybridisation, is by no means absolutely +governed by systematic affinity. Although many distinct genera within the same +family have been grafted together, in other cases species of the same genus +will not take on each other. The pear can be grafted far more readily on the +quince, which is ranked as a distinct genus, than on the apple, which is a +member of the same genus. Even different varieties of the pear take with +different degrees of facility on the quince; so do different varieties of the +apricot and peach on certain varieties of the plum. + +As Gärtner found that there was sometimes an innate difference in different +individuals of the same two species in crossing; so Sagaret believes this to be +the case with different individuals of the same two species in being grafted +together. As in reciprocal crosses, the facility of effecting an union is often +very far from equal, so it sometimes is in grafting; the common gooseberry, for +instance, cannot be grafted on the currant, whereas the currant will take, +though with difficulty, on the gooseberry. + +We have seen that the sterility of hybrids, which have their reproductive +organs in an imperfect condition, is a very different case from the difficulty +of uniting two pure species, which have their reproductive organs perfect; yet +these two distinct cases run to a certain extent parallel. Something analogous +occurs in grafting; for Thouin found that three species of Robinia, which +seeded freely on their own roots, and which could be grafted with no great +difficulty on another species, when thus grafted were rendered barren. On the +other hand, certain species of Sorbus, when grafted on other species, yielded +twice as much fruit as when on their own roots. We are reminded by this latter +fact of the extraordinary case of Hippeastrum, Lobelia, etc., which seeded much +more freely when fertilised with the pollen of distinct species, than when +self-fertilised with their own pollen. + +We thus see, that although there is a clear and fundamental difference between +the mere adhesion of grafted stocks, and the union of the male and female +elements in the act of reproduction, yet that there is a rude degree of +parallelism in the results of grafting and of crossing distinct species. And as +we must look at the curious and complex laws governing the facility with which +trees can be grafted on each other as incidental on unknown differences in +their vegetative systems, so I believe that the still more complex laws +governing the facility of first crosses, are incidental on unknown differences, +chiefly in their reproductive systems. These differences, in both cases, follow +to a certain extent, as might have been expected, systematic affinity, by which +every kind of resemblance and dissimilarity between organic beings is attempted +to be expressed. The facts by no means seem to me to indicate that the greater +or lesser difficulty of either grafting or crossing together various species +has been a special endowment; although in the case of crossing, the difficulty +is as important for the endurance and stability of specific forms, as in the +case of grafting it is unimportant for their welfare. + +Causes of the Sterility of first Crosses and of Hybrids.—We may now look a +little closer at the probable causes of the sterility of first crosses and of +hybrids. These two cases are fundamentally different, for, as just remarked, in +the union of two pure species the male and female sexual elements are perfect, +whereas in hybrids they are imperfect. Even in first crosses, the greater or +lesser difficulty in effecting a union apparently depends on several distinct +causes. There must sometimes be a physical impossibility in the male element +reaching the ovule, as would be the case with a plant having a pistil too long +for the pollen-tubes to reach the ovarium. It has also been observed that when +pollen of one species is placed on the stigma of a distantly allied species, +though the pollen-tubes protrude, they do not penetrate the stigmatic surface. +Again, the male element may reach the female element, but be incapable of +causing an embryo to be developed, as seems to have been the case with some of +Thuret’s experiments on Fuci. No explanation can be given of these facts, any +more than why certain trees cannot be grafted on others. Lastly, an embryo may +be developed, and then perish at an early period. This latter alternative has +not been sufficiently attended to; but I believe, from observations +communicated to me by Mr. Hewitt, who has had great experience in hybridising +gallinaceous birds, that the early death of the embryo is a very frequent cause +of sterility in first crosses. I was at first very unwilling to believe in this +view; as hybrids, when once born, are generally healthy and long-lived, as we +see in the case of the common mule. Hybrids, however, are differently +circumstanced before and after birth: when born and living in a country where +their two parents can live, they are generally placed under suitable conditions +of life. But a hybrid partakes of only half of the nature and constitution of +its mother, and therefore before birth, as long as it is nourished within its +mother’s womb or within the egg or seed produced by the mother, it may be +exposed to conditions in some degree unsuitable, and consequently be liable to +perish at an early period; more especially as all very young beings seem +eminently sensitive to injurious or unnatural conditions of life. + +In regard to the sterility of hybrids, in which the sexual elements are +imperfectly developed, the case is very different. I have more than once +alluded to a large body of facts, which I have collected, showing that when +animals and plants are removed from their natural conditions, they are +extremely liable to have their reproductive systems seriously affected. This, +in fact, is the great bar to the domestication of animals. Between the +sterility thus superinduced and that of hybrids, there are many points of +similarity. In both cases the sterility is independent of general health, and +is often accompanied by excess of size or great luxuriance. In both cases, the +sterility occurs in various degrees; in both, the male element is the most +liable to be affected; but sometimes the female more than the male. In both, +the tendency goes to a certain extent with systematic affinity, for whole +groups of animals and plants are rendered impotent by the same unnatural +conditions; and whole groups of species tend to produce sterile hybrids. On the +other hand, one species in a group will sometimes resist great changes of +conditions with unimpaired fertility; and certain species in a group will +produce unusually fertile hybrids. No one can tell, till he tries, whether any +particular animal will breed under confinement or any plant seed freely under +culture; nor can he tell, till he tries, whether any two species of a genus +will produce more or less sterile hybrids. Lastly, when organic beings are +placed during several generations under conditions not natural to them, they +are extremely liable to vary, which is due, as I believe, to their reproductive +systems having been specially affected, though in a lesser degree than when +sterility ensues. So it is with hybrids, for hybrids in successive generations +are eminently liable to vary, as every experimentalist has observed. + +Thus we see that when organic beings are placed under new and unnatural +conditions, and when hybrids are produced by the unnatural crossing of two +species, the reproductive system, independently of the general state of health, +is affected by sterility in a very similar manner. In the one case, the +conditions of life have been disturbed, though often in so slight a degree as +to be inappreciable by us; in the other case, or that of hybrids, the external +conditions have remained the same, but the organisation has been disturbed by +two different structures and constitutions having been blended into one. For it +is scarcely possible that two organisations should be compounded into one, +without some disturbance occurring in the development, or periodical action, or +mutual relation of the different parts and organs one to another, or to the +conditions of life. When hybrids are able to breed inter se, they transmit to +their offspring from generation to generation the same compounded organisation, +and hence we need not be surprised that their sterility, though in some degree +variable, rarely diminishes. + +It must, however, be confessed that we cannot understand, excepting on vague +hypotheses, several facts with respect to the sterility of hybrids; for +instance, the unequal fertility of hybrids produced from reciprocal crosses; or +the increased sterility in those hybrids which occasionally and exceptionally +resemble closely either pure parent. Nor do I pretend that the foregoing +remarks go to the root of the matter: no explanation is offered why an +organism, when placed under unnatural conditions, is rendered sterile. All that +I have attempted to show, is that in two cases, in some respects allied, +sterility is the common result,—in the one case from the conditions of life +having been disturbed, in the other case from the organisation having been +disturbed by two organisations having been compounded into one. + +It may seem fanciful, but I suspect that a similar parallelism extends to an +allied yet very different class of facts. It is an old and almost universal +belief, founded, I think, on a considerable body of evidence, that slight +changes in the conditions of life are beneficial to all living things. We see +this acted on by farmers and gardeners in their frequent exchanges of seed, +tubers, etc., from one soil or climate to another, and back again. During the +convalescence of animals, we plainly see that great benefit is derived from +almost any change in the habits of life. Again, both with plants and animals, +there is abundant evidence, that a cross between very distinct individuals of +the same species, that is between members of different strains or sub-breeds, +gives vigour and fertility to the offspring. I believe, indeed, from the facts +alluded to in our fourth chapter, that a certain amount of crossing is +indispensable even with hermaphrodites; and that close interbreeding continued +during several generations between the nearest relations, especially if these +be kept under the same conditions of life, always induces weakness and +sterility in the progeny. + +Hence it seems that, on the one hand, slight changes in the conditions of life +benefit all organic beings, and on the other hand, that slight crosses, that is +crosses between the males and females of the same species which have varied and +become slightly different, give vigour and fertility to the offspring. But we +have seen that greater changes, or changes of a particular nature, often render +organic beings in some degree sterile; and that greater crosses, that is +crosses between males and females which have become widely or specifically +different, produce hybrids which are generally sterile in some degree. I cannot +persuade myself that this parallelism is an accident or an illusion. Both +series of facts seem to be connected together by some common but unknown bond, +which is essentially related to the principle of life. + +Fertility of Varieties when crossed, and of their Mongrel offspring.—It may be +urged, as a most forcible argument, that there must be some essential +distinction between species and varieties, and that there must be some error in +all the foregoing remarks, inasmuch as varieties, however much they may differ +from each other in external appearance, cross with perfect facility, and yield +perfectly fertile offspring. I fully admit that this is almost invariably the +case. But if we look to varieties produced under nature, we are immediately +involved in hopeless difficulties; for if two hitherto reputed varieties be +found in any degree sterile together, they are at once ranked by most +naturalists as species. For instance, the blue and red pimpernel, the primrose +and cowslip, which are considered by many of our best botanists as varieties, +are said by Gärtner not to be quite fertile when crossed, and he consequently +ranks them as undoubted species. If we thus argue in a circle, the fertility of +all varieties produced under nature will assuredly have to be granted. + +If we turn to varieties, produced, or supposed to have been produced, under +domestication, we are still involved in doubt. For when it is stated, for +instance, that the German Spitz dog unites more easily than other dogs with +foxes, or that certain South American indigenous domestic dogs do not readily +cross with European dogs, the explanation which will occur to everyone, and +probably the true one, is that these dogs have descended from several +aboriginally distinct species. Nevertheless the perfect fertility of so many +domestic varieties, differing widely from each other in appearance, for +instance of the pigeon or of the cabbage, is a remarkable fact; more especially +when we reflect how many species there are, which, though resembling each other +most closely, are utterly sterile when intercrossed. Several considerations, +however, render the fertility of domestic varieties less remarkable than at +first appears. It can, in the first place, be clearly shown that mere external +dissimilarity between two species does not determine their greater or lesser +degree of sterility when crossed; and we may apply the same rule to domestic +varieties. In the second place, some eminent naturalists believe that a long +course of domestication tends to eliminate sterility in the successive +generations of hybrids, which were at first only slightly sterile; and if this +be so, we surely ought not to expect to find sterility both appearing and +disappearing under nearly the same conditions of life. Lastly, and this seems +to me by far the most important consideration, new races of animals and plants +are produced under domestication by man’s methodical and unconscious power of +selection, for his own use and pleasure: he neither wishes to select, nor could +select, slight differences in the reproductive system, or other constitutional +differences correlated with the reproductive system. He supplies his several +varieties with the same food; treats them in nearly the same manner, and does +not wish to alter their general habits of life. Nature acts uniformly and +slowly during vast periods of time on the whole organisation, in any way which +may be for each creature’s own good; and thus she may, either directly, or more +probably indirectly, through correlation, modify the reproductive system in the +several descendants from any one species. Seeing this difference in the process +of selection, as carried on by man and nature, we need not be surprised at some +difference in the result. + +I have as yet spoken as if the varieties of the same species were invariably +fertile when intercrossed. But it seems to me impossible to resist the evidence +of the existence of a certain amount of sterility in the few following cases, +which I will briefly abstract. The evidence is at least as good as that from +which we believe in the sterility of a multitude of species. The evidence is, +also, derived from hostile witnesses, who in all other cases consider fertility +and sterility as safe criterions of specific distinction. Gärtner kept during +several years a dwarf kind of maize with yellow seeds, and a tall variety with +red seeds, growing near each other in his garden; and although these plants +have separated sexes, they never naturally crossed. He then fertilised thirteen +flowers of the one with the pollen of the other; but only a single head +produced any seed, and this one head produced only five grains. Manipulation in +this case could not have been injurious, as the plants have separated sexes. No +one, I believe, has suspected that these varieties of maize are distinct +species; and it is important to notice that the hybrid plants thus raised were +themselves perfectly fertile; so that even Gärtner did not venture to consider +the two varieties as specifically distinct. + +Girou de Buzareingues crossed three varieties of gourd, which like the maize +has separated sexes, and he asserts that their mutual fertilisation is by so +much the less easy as their differences are greater. How far these experiments +may be trusted, I know not; but the forms experimentised on, are ranked by +Sagaret, who mainly founds his classification by the test of infertility, as +varieties. + +The following case is far more remarkable, and seems at first quite incredible; +but it is the result of an astonishing number of experiments made during many +years on nine species of Verbascum, by so good an observer and so hostile a +witness, as Gärtner: namely, that yellow and white varieties of the same +species of Verbascum when intercrossed produce less seed, than do either +coloured varieties when fertilised with pollen from their own coloured flowers. +Moreover, he asserts that when yellow and white varieties of one species are +crossed with yellow and white varieties of a distinct species, more seed is +produced by the crosses between the same coloured flowers, than between those +which are differently coloured. Yet these varieties of Verbascum present no +other difference besides the mere colour of the flower; and one variety can +sometimes be raised from the seed of the other. + +From observations which I have made on certain varieties of hollyhock, I am +inclined to suspect that they present analogous facts. + +Kölreuter, whose accuracy has been confirmed by every subsequent observer, has +proved the remarkable fact, that one variety of the common tobacco is more +fertile, when crossed with a widely distinct species, than are the other +varieties. He experimentised on five forms, which are commonly reputed to be +varieties, and which he tested by the severest trial, namely, by reciprocal +crosses, and he found their mongrel offspring perfectly fertile. But one of +these five varieties, when used either as father or mother, and crossed with +the Nicotiana glutinosa, always yielded hybrids not so sterile as those which +were produced from the four other varieties when crossed with N. glutinosa. +Hence the reproductive system of this one variety must have been in some manner +and in some degree modified. + +From these facts; from the great difficulty of ascertaining the infertility of +varieties in a state of nature, for a supposed variety if infertile in any +degree would generally be ranked as species; from man selecting only external +characters in the production of the most distinct domestic varieties, and from +not wishing or being able to produce recondite and functional differences in +the reproductive system; from these several considerations and facts, I do not +think that the very general fertility of varieties can be proved to be of +universal occurrence, or to form a fundamental distinction between varieties +and species. The general fertility of varieties does not seem to me sufficient +to overthrow the view which I have taken with respect to the very general, but +not invariable, sterility of first crosses and of hybrids, namely, that it is +not a special endowment, but is incidental on slowly acquired modifications, +more especially in the reproductive systems of the forms which are crossed. + +Hybrids and Mongrels compared, independently of their fertility.—Independently +of the question of fertility, the offspring of species when crossed and of +varieties when crossed may be compared in several other respects. Gärtner, +whose strong wish was to draw a marked line of distinction between species and +varieties, could find very few and, as it seems to me, quite unimportant +differences between the so-called hybrid offspring of species, and the +so-called mongrel offspring of varieties. And, on the other hand, they agree +most closely in very many important respects. + +I shall here discuss this subject with extreme brevity. The most important +distinction is, that in the first generation mongrels are more variable than +hybrids; but Gärtner admits that hybrids from species which have long been +cultivated are often variable in the first generation; and I have myself seen +striking instances of this fact. Gärtner further admits that hybrids between +very closely allied species are more variable than those from very distinct +species; and this shows that the difference in the degree of variability +graduates away. When mongrels and the more fertile hybrids are propagated for +several generations an extreme amount of variability in their offspring is +notorious; but some few cases both of hybrids and mongrels long retaining +uniformity of character could be given. The variability, however, in the +successive generations of mongrels is, perhaps, greater than in hybrids. + +This greater variability of mongrels than of hybrids does not seem to me at all +surprising. For the parents of mongrels are varieties, and mostly domestic +varieties (very few experiments having been tried on natural varieties), and +this implies in most cases that there has been recent variability; and +therefore we might expect that such variability would often continue and be +super-added to that arising from the mere act of crossing. The slight degree of +variability in hybrids from the first cross or in the first generation, in +contrast with their extreme variability in the succeeding generations, is a +curious fact and deserves attention. For it bears on and corroborates the view +which I have taken on the cause of ordinary variability; namely, that it is due +to the reproductive system being eminently sensitive to any change in the +conditions of life, being thus often rendered either impotent or at least +incapable of its proper function of producing offspring identical with the +parent-form. Now hybrids in the first generation are descended from species +(excluding those long cultivated) which have not had their reproductive systems +in any way affected, and they are not variable; but hybrids themselves have +their reproductive systems seriously affected, and their descendants are highly +variable. + +But to return to our comparison of mongrels and hybrids: Gärtner states that +mongrels are more liable than hybrids to revert to either parent-form; but +this, if it be true, is certainly only a difference in degree. Gärtner further +insists that when any two species, although most closely allied to each other, +are crossed with a third species, the hybrids are widely different from each +other; whereas if two very distinct varieties of one species are crossed with +another species, the hybrids do not differ much. But this conclusion, as far as +I can make out, is founded on a single experiment; and seems directly opposed +to the results of several experiments made by Kölreuter. + +These alone are the unimportant differences, which Gärtner is able to point +out, between hybrid and mongrel plants. On the other hand, the resemblance in +mongrels and in hybrids to their respective parents, more especially in hybrids +produced from nearly related species, follows according to Gärtner the same +laws. When two species are crossed, one has sometimes a prepotent power of +impressing its likeness on the hybrid; and so I believe it to be with varieties +of plants. With animals one variety certainly often has this prepotent power +over another variety. Hybrid plants produced from a reciprocal cross, generally +resemble each other closely; and so it is with mongrels from a reciprocal +cross. Both hybrids and mongrels can be reduced to either pure parent-form, by +repeated crosses in successive generations with either parent. + +These several remarks are apparently applicable to animals; but the subject is +here excessively complicated, partly owing to the existence of secondary sexual +characters; but more especially owing to prepotency in transmitting likeness +running more strongly in one sex than in the other, both when one species is +crossed with another, and when one variety is crossed with another variety. For +instance, I think those authors are right, who maintain that the ass has a +prepotent power over the horse, so that both the mule and the hinny more +resemble the ass than the horse; but that the prepotency runs more strongly in +the male-ass than in the female, so that the mule, which is the offspring of +the male-ass and mare, is more like an ass, than is the hinny, which is the +offspring of the female-ass and stallion. + +Much stress has been laid by some authors on the supposed fact, that mongrel +animals alone are born closely like one of their parents; but it can be shown +that this does sometimes occur with hybrids; yet I grant much less frequently +with hybrids than with mongrels. Looking to the cases which I have collected of +cross-bred animals closely resembling one parent, the resemblances seem chiefly +confined to characters almost monstrous in their nature, and which have +suddenly appeared—such as albinism, melanism, deficiency of tail or horns, or +additional fingers and toes; and do not relate to characters which have been +slowly acquired by selection. Consequently, sudden reversions to the perfect +character of either parent would be more likely to occur with mongrels, which +are descended from varieties often suddenly produced and semi-monstrous in +character, than with hybrids, which are descended from species slowly and +naturally produced. On the whole I entirely agree with Dr. Prosper Lucas, who, +after arranging an enormous body of facts with respect to animals, comes to the +conclusion, that the laws of resemblance of the child to its parents are the +same, whether the two parents differ much or little from each other, namely in +the union of individuals of the same variety, or of different varieties, or of +distinct species. + +Laying aside the question of fertility and sterility, in all other respects +there seems to be a general and close similarity in the offspring of crossed +species, and of crossed varieties. If we look at species as having been +specially created, and at varieties as having been produced by secondary laws, +this similarity would be an astonishing fact. But it harmonises perfectly with +the view that there is no essential distinction between species and varieties. + +Summary of Chapter.—First crosses between forms sufficiently distinct to be +ranked as species, and their hybrids, are very generally, but not universally, +sterile. The sterility is of all degrees, and is often so slight that the two +most careful experimentalists who have ever lived, have come to diametrically +opposite conclusions in ranking forms by this test. The sterility is innately +variable in individuals of the same species, and is eminently susceptible of +favourable and unfavourable conditions. The degree of sterility does not +strictly follow systematic affinity, but is governed by several curious and +complex laws. It is generally different, and sometimes widely different, in +reciprocal crosses between the same two species. It is not always equal in +degree in a first cross and in the hybrid produced from this cross. + +In the same manner as in grafting trees, the capacity of one species or variety +to take on another, is incidental on generally unknown differences in their +vegetative systems, so in crossing, the greater or less facility of one species +to unite with another, is incidental on unknown differences in their +reproductive systems. There is no more reason to think that species have been +specially endowed with various degrees of sterility to prevent them crossing +and blending in nature, than to think that trees have been specially endowed +with various and somewhat analogous degrees of difficulty in being grafted +together in order to prevent them becoming inarched in our forests. + +The sterility of first crosses between pure species, which have their +reproductive systems perfect, seems to depend on several circumstances; in some +cases largely on the early death of the embryo. The sterility of hybrids, which +have their reproductive systems imperfect, and which have had this system and +their whole organisation disturbed by being compounded of two distinct species, +seems closely allied to that sterility which so frequently affects pure +species, when their natural conditions of life have been disturbed. This view +is supported by a parallelism of another kind;—namely, that the crossing of +forms only slightly different is favourable to the vigour and fertility of +their offspring; and that slight changes in the conditions of life are +apparently favourable to the vigour and fertility of all organic beings. It is +not surprising that the degree of difficulty in uniting two species, and the +degree of sterility of their hybrid-offspring should generally correspond, +though due to distinct causes; for both depend on the amount of difference of +some kind between the species which are crossed. Nor is it surprising that the +facility of effecting a first cross, the fertility of the hybrids produced, and +the capacity of being grafted together—though this latter capacity evidently +depends on widely different circumstances—should all run, to a certain extent, +parallel with the systematic affinity of the forms which are subjected to +experiment; for systematic affinity attempts to express all kinds of +resemblance between all species. + +First crosses between forms known to be varieties, or sufficiently alike to be +considered as varieties, and their mongrel offspring, are very generally, but +not quite universally, fertile. Nor is this nearly general and perfect +fertility surprising, when we remember how liable we are to argue in a circle +with respect to varieties in a state of nature; and when we remember that the +greater number of varieties have been produced under domestication by the +selection of mere external differences, and not of differences in the +reproductive system. In all other respects, excluding fertility, there is a +close general resemblance between hybrids and mongrels. Finally, then, the +facts briefly given in this chapter do not seem to me opposed to, but even +rather to support the view, that there is no fundamental distinction between +species and varieties. + +CHAPTER IX. +ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + +On the absence of intermediate varieties at the present day. On the nature of +extinct intermediate varieties; on their number. On the vast lapse of time, as +inferred from the rate of deposition and of denudation. On the poorness of our +palæontological collections. On the intermittence of geological formations. On +the absence of intermediate varieties in any one formation. On the sudden +appearance of groups of species. On their sudden appearance in the lowest known +fossiliferous strata. + +In the sixth chapter I enumerated the chief objections which might be justly +urged against the views maintained in this volume. Most of them have now been +discussed. One, namely the distinctness of specific forms, and their not being +blended together by innumerable transitional links, is a very obvious +difficulty. I assigned reasons why such links do not commonly occur at the +present day, under the circumstances apparently most favourable for their +presence, namely on an extensive and continuous area with graduated physical +conditions. I endeavoured to show, that the life of each species depends in a +more important manner on the presence of other already defined organic forms, +than on climate; and, therefore, that the really governing conditions of life +do not graduate away quite insensibly like heat or moisture. I endeavoured, +also, to show that intermediate varieties, from existing in lesser numbers than +the forms which they connect, will generally be beaten out and exterminated +during the course of further modification and improvement. The main cause, +however, of innumerable intermediate links not now occurring everywhere +throughout nature depends on the very process of natural selection, through +which new varieties continually take the places of and exterminate their +parent-forms. But just in proportion as this process of extermination has acted +on an enormous scale, so must the number of intermediate varieties, which have +formerly existed on the earth, be truly enormous. Why then is not every +geological formation and every stratum full of such intermediate links? Geology +assuredly does not reveal any such finely graduated organic chain; and this, +perhaps, is the most obvious and gravest objection which can be urged against +my theory. The explanation lies, as I believe, in the extreme imperfection of +the geological record. + +In the first place it should always be borne in mind what sort of intermediate +forms must, on my theory, have formerly existed. I have found it difficult, +when looking at any two species, to avoid picturing to myself, forms directly +intermediate between them. But this is a wholly false view; we should always +look for forms intermediate between each species and a common but unknown +progenitor; and the progenitor will generally have differed in some respects +from all its modified descendants. To give a simple illustration: the fantail +and pouter pigeons have both descended from the rock-pigeon; if we possessed +all the intermediate varieties which have ever existed, we should have an +extremely close series between both and the rock-pigeon; but we should have no +varieties directly intermediate between the fantail and pouter; none, for +instance, combining a tail somewhat expanded with a crop somewhat enlarged, the +characteristic features of these two breeds. These two breeds, moreover, have +become so much modified, that if we had no historical or indirect evidence +regarding their origin, it would not have been possible to have determined from +a mere comparison of their structure with that of the rock-pigeon, whether they +had descended from this species or from some other allied species, such as C. +oenas. + +So with natural species, if we look to forms very distinct, for instance to the +horse and tapir, we have no reason to suppose that links ever existed directly +intermediate between them, but between each and an unknown common parent. The +common parent will have had in its whole organisation much general resemblance +to the tapir and to the horse; but in some points of structure may have +differed considerably from both, even perhaps more than they differ from each +other. Hence in all such cases, we should be unable to recognise the +parent-form of any two or more species, even if we closely compared the +structure of the parent with that of its modified descendants, unless at the +same time we had a nearly perfect chain of the intermediate links. + +It is just possible by my theory, that one of two living forms might have +descended from the other; for instance, a horse from a tapir; and in this case +direct intermediate links will have existed between them. But such a case would +imply that one form had remained for a very long period unaltered, whilst its +descendants had undergone a vast amount of change; and the principle of +competition between organism and organism, between child and parent, will +render this a very rare event; for in all cases the new and improved forms of +life will tend to supplant the old and unimproved forms. + +By the theory of natural selection all living species have been connected with +the parent-species of each genus, by differences not greater than we see +between the varieties of the same species at the present day; and these +parent-species, now generally extinct, have in their turn been similarly +connected with more ancient species; and so on backwards, always converging to +the common ancestor of each great class. So that the number of intermediate and +transitional links, between all living and extinct species, must have been +inconceivably great. But assuredly, if this theory be true, such have lived +upon this earth. + +On the lapse of Time.—Independently of our not finding fossil remains of such +infinitely numerous connecting links, it may be objected, that time will not +have sufficed for so great an amount of organic change, all changes having been +effected very slowly through natural selection. It is hardly possible for me +even to recall to the reader, who may not be a practical geologist, the facts +leading the mind feebly to comprehend the lapse of time. He who can read Sir +Charles Lyell’s grand work on the Principles of Geology, which the future +historian will recognise as having produced a revolution in natural science, +yet does not admit how incomprehensibly vast have been the past periods of +time, may at once close this volume. Not that it suffices to study the +Principles of Geology, or to read special treatises by different observers on +separate formations, and to mark how each author attempts to give an inadequate +idea of the duration of each formation or even each stratum. A man must for +years examine for himself great piles of superimposed strata, and watch the sea +at work grinding down old rocks and making fresh sediment, before he can hope +to comprehend anything of the lapse of time, the monuments of which we see +around us. + +It is good to wander along lines of sea-coast, when formed of moderately hard +rocks, and mark the process of degradation. The tides in most cases reach the +cliffs only for a short time twice a day, and the waves eat into them only when +they are charged with sand or pebbles; for there is reason to believe that pure +water can effect little or nothing in wearing away rock. At last the base of +the cliff is undermined, huge fragments fall down, and these remaining fixed, +have to be worn away, atom by atom, until reduced in size they can be rolled +about by the waves, and then are more quickly ground into pebbles, sand, or +mud. But how often do we see along the bases of retreating cliffs rounded +boulders, all thickly clothed by marine productions, showing how little they +are abraded and how seldom they are rolled about! Moreover, if we follow for a +few miles any line of rocky cliff, which is undergoing degradation, we find +that it is only here and there, along a short length or round a promontory, +that the cliffs are at the present time suffering. The appearance of the +surface and the vegetation show that elsewhere years have elapsed since the +waters washed their base. + +He who most closely studies the action of the sea on our shores, will, I +believe, be most deeply impressed with the slowness with which rocky coasts are +worn away. The observations on this head by Hugh Miller, and by that excellent +observer Mr. Smith of Jordan Hill, are most impressive. With the mind thus +impressed, let any one examine beds of conglomerate many thousand feet in +thickness, which, though probably formed at a quicker rate than many other +deposits, yet, from being formed of worn and rounded pebbles, each of which +bears the stamp of time, are good to show how slowly the mass has been +accumulated. Let him remember Lyell’s profound remark, that the thickness and +extent of sedimentary formations are the result and measure of the degradation +which the earth’s crust has elsewhere suffered. And what an amount of +degradation is implied by the sedimentary deposits of many countries! Professor +Ramsay has given me the maximum thickness, in most cases from actual +measurement, in a few cases from estimate, of each formation in different parts +of Great Britain; and this is the result:— + + Feet + + Palæozoic strata (not including igneous beds)...57,154. + Secondary strata................................13,190. + Tertiary strata..................................2,240. + +—making altogether 72,584 feet; that is, very nearly thirteen and +three-quarters British miles. Some of these formations, which are represented +in England by thin beds, are thousands of feet in thickness on the Continent. +Moreover, between each successive formation, we have, in the opinion of most +geologists, enormously long blank periods. So that the lofty pile of +sedimentary rocks in Britain, gives but an inadequate idea of the time which +has elapsed during their accumulation; yet what time this must have consumed! +Good observers have estimated that sediment is deposited by the great +Mississippi river at the rate of only 600 feet in a hundred thousand years. +This estimate may be quite erroneous; yet, considering over what wide spaces +very fine sediment is transported by the currents of the sea, the process of +accumulation in any one area must be extremely slow. + +But the amount of denudation which the strata have in many places suffered, +independently of the rate of accumulation of the degraded matter, probably +offers the best evidence of the lapse of time. I remember having been much +struck with the evidence of denudation, when viewing volcanic islands, which +have been worn by the waves and pared all round into perpendicular cliffs of +one or two thousand feet in height; for the gentle slope of the lava-streams, +due to their formerly liquid state, showed at a glance how far the hard, rocky +beds had once extended into the open ocean. The same story is still more +plainly told by faults,—those great cracks along which the strata have been +upheaved on one side, or thrown down on the other, to the height or depth of +thousands of feet; for since the crust cracked, the surface of the land has +been so completely planed down by the action of the sea, that no trace of these +vast dislocations is externally visible. + +The Craven fault, for instance, extends for upwards of 30 miles, and along this +line the vertical displacement of the strata has varied from 600 to 3000 feet. +Professor Ramsay has published an account of a downthrow in Anglesea of 2300 +feet; and he informs me that he fully believes there is one in Merionethshire +of 12,000 feet; yet in these cases there is nothing on the surface to show such +prodigious movements; the pile of rocks on the one or other side having been +smoothly swept away. The consideration of these facts impresses my mind almost +in the same manner as does the vain endeavour to grapple with the idea of +eternity. + +I am tempted to give one other case, the well-known one of the denudation of +the Weald. Though it must be admitted that the denudation of the Weald has been +a mere trifle, in comparison with that which has removed masses of our +palæozoic strata, in parts ten thousand feet in thickness, as shown in +Professor Ramsay’s masterly memoir on this subject. Yet it is an admirable +lesson to stand on the North Downs and to look at the distant South Downs; for, +remembering that at no great distance to the west the northern and southern +escarpments meet and close, one can safely picture to oneself the great dome of +rocks which must have covered up the Weald within so limited a period as since +the latter part of the Chalk formation. The distance from the northern to the +southern Downs is about 22 miles, and the thickness of the several formations +is on an average about 1100 feet, as I am informed by Professor Ramsay. But if, +as some geologists suppose, a range of older rocks underlies the Weald, on the +flanks of which the overlying sedimentary deposits might have accumulated in +thinner masses than elsewhere, the above estimate would be erroneous; but this +source of doubt probably would not greatly affect the estimate as applied to +the western extremity of the district. If, then, we knew the rate at which the +sea commonly wears away a line of cliff of any given height, we could measure +the time requisite to have denuded the Weald. This, of course, cannot be done; +but we may, in order to form some crude notion on the subject, assume that the +sea would eat into cliffs 500 feet in height at the rate of one inch in a +century. This will at first appear much too small an allowance; but it is the +same as if we were to assume a cliff one yard in height to be eaten back along +a whole line of coast at the rate of one yard in nearly every twenty-two years. +I doubt whether any rock, even as soft as chalk, would yield at this rate +excepting on the most exposed coasts; though no doubt the degradation of a +lofty cliff would be more rapid from the breakage of the fallen fragments. On +the other hand, I do not believe that any line of coast, ten or twenty miles in +length, ever suffers degradation at the same time along its whole indented +length; and we must remember that almost all strata contain harder layers or +nodules, which from long resisting attrition form a breakwater at the base. +Hence, under ordinary circumstances, I conclude that for a cliff 500 feet in +height, a denudation of one inch per century for the whole length would be an +ample allowance. At this rate, on the above data, the denudation of the Weald +must have required 306,662,400 years; or say three hundred million years. + +The action of fresh water on the gently inclined Wealden district, when +upraised, could hardly have been great, but it would somewhat reduce the above +estimate. On the other hand, during oscillations of level, which we know this +area has undergone, the surface may have existed for millions of years as land, +and thus have escaped the action of the sea: when deeply submerged for perhaps +equally long periods, it would, likewise, have escaped the action of the +coast-waves. So that in all probability a far longer period than 300 million +years has elapsed since the latter part of the Secondary period. + +I have made these few remarks because it is highly important for us to gain +some notion, however imperfect, of the lapse of years. During each of these +years, over the whole world, the land and the water has been peopled by hosts +of living forms. What an infinite number of generations, which the mind cannot +grasp, must have succeeded each other in the long roll of years! Now turn to +our richest geological museums, and what a paltry display we behold! + +On the poorness of our Palæontological collections.—That our palæontological +collections are very imperfect, is admitted by every one. The remark of that +admirable palæontologist, the late Edward Forbes, should not be forgotten, +namely, that numbers of our fossil species are known and named from single and +often broken specimens, or from a few specimens collected on some one spot. +Only a small portion of the surface of the earth has been geologically +explored, and no part with sufficient care, as the important discoveries made +every year in Europe prove. No organism wholly soft can be preserved. Shells +and bones will decay and disappear when left on the bottom of the sea, where +sediment is not accumulating. I believe we are continually taking a most +erroneous view, when we tacitly admit to ourselves that sediment is being +deposited over nearly the whole bed of the sea, at a rate sufficiently quick to +embed and preserve fossil remains. Throughout an enormously large proportion of +the ocean, the bright blue tint of the water bespeaks its purity. The many +cases on record of a formation conformably covered, after an enormous interval +of time, by another and later formation, without the underlying bed having +suffered in the interval any wear and tear, seem explicable only on the view of +the bottom of the sea not rarely lying for ages in an unaltered condition. The +remains which do become embedded, if in sand or gravel, will when the beds are +upraised generally be dissolved by the percolation of rain-water. I suspect +that but few of the very many animals which live on the beach between high and +low watermark are preserved. For instance, the several species of the +Chthamalinæ (a sub-family of sessile cirripedes) coat the rocks all over the +world in infinite numbers: they are all strictly littoral, with the exception +of a single Mediterranean species, which inhabits deep water and has been found +fossil in Sicily, whereas not one other species has hitherto been found in any +tertiary formation: yet it is now known that the genus Chthamalus existed +during the chalk period. The molluscan genus Chiton offers a partially +analogous case. + +With respect to the terrestrial productions which lived during the Secondary +and Palæozoic periods, it is superfluous to state that our evidence from fossil +remains is fragmentary in an extreme degree. For instance, not a land shell is +known belonging to either of these vast periods, with one exception discovered +by Sir C. Lyell in the carboniferous strata of North America. In regard to +mammiferous remains, a single glance at the historical table published in the +Supplement to Lyell’s Manual, will bring home the truth, how accidental and +rare is their preservation, far better than pages of detail. Nor is their +rarity surprising, when we remember how large a proportion of the bones of +tertiary mammals have been discovered either in caves or in lacustrine +deposits; and that not a cave or true lacustrine bed is known belonging to the +age of our secondary or palæozoic formations. + +But the imperfection in the geological record mainly results from another and +more important cause than any of the foregoing; namely, from the several +formations being separated from each other by wide intervals of time. When we +see the formations tabulated in written works, or when we follow them in +nature, it is difficult to avoid believing that they are closely consecutive. +But we know, for instance, from Sir R. Murchison’s great work on Russia, what +wide gaps there are in that country between the superimposed formations; so it +is in North America, and in many other parts of the world. The most skilful +geologist, if his attention had been exclusively confined to these large +territories, would never have suspected that during the periods which were +blank and barren in his own country, great piles of sediment, charged with new +and peculiar forms of life, had elsewhere been accumulated. And if in each +separate territory, hardly any idea can be formed of the length of time which +has elapsed between the consecutive formations, we may infer that this could +nowhere be ascertained. The frequent and great changes in the mineralogical +composition of consecutive formations, generally implying great changes in the +geography of the surrounding lands, whence the sediment has been derived, +accords with the belief of vast intervals of time having elapsed between each +formation. + +But we can, I think, see why the geological formations of each region are +almost invariably intermittent; that is, have not followed each other in close +sequence. Scarcely any fact struck me more when examining many hundred miles of +the South American coasts, which have been upraised several hundred feet within +the recent period, than the absence of any recent deposits sufficiently +extensive to last for even a short geological period. Along the whole west +coast, which is inhabited by a peculiar marine fauna, tertiary beds are so +scantily developed, that no record of several successive and peculiar marine +faunas will probably be preserved to a distant age. A little reflection will +explain why along the rising coast of the western side of South America, no +extensive formations with recent or tertiary remains can anywhere be found, +though the supply of sediment must for ages have been great, from the enormous +degradation of the coast-rocks and from muddy streams entering the sea. The +explanation, no doubt, is, that the littoral and sub-littoral deposits are +continually worn away, as soon as they are brought up by the slow and gradual +rising of the land within the grinding action of the coast-waves. + +We may, I think, safely conclude that sediment must be accumulated in extremely +thick, solid, or extensive masses, in order to withstand the incessant action +of the waves, when first upraised and during subsequent oscillations of level. +Such thick and extensive accumulations of sediment may be formed in two ways; +either, in profound depths of the sea, in which case, judging from the +researches of E. Forbes, we may conclude that the bottom will be inhabited by +extremely few animals, and the mass when upraised will give a most imperfect +record of the forms of life which then existed; or, sediment may be accumulated +to any thickness and extent over a shallow bottom, if it continue slowly to +subside. In this latter case, as long as the rate of subsidence and supply of +sediment nearly balance each other, the sea will remain shallow and favourable +for life, and thus a fossiliferous formation thick enough, when upraised, to +resist any amount of degradation, may be formed. + +I am convinced that all our ancient formations, which are rich in fossils, have +thus been formed during subsidence. Since publishing my views on this subject +in 1845, I have watched the progress of Geology, and have been surprised to +note how author after author, in treating of this or that great formation, has +come to the conclusion that it was accumulated during subsidence. I may add, +that the only ancient tertiary formation on the west coast of South America, +which has been bulky enough to resist such degradation as it has as yet +suffered, but which will hardly last to a distant geological age, was certainly +deposited during a downward oscillation of level, and thus gained considerable +thickness. + +All geological facts tell us plainly that each area has undergone numerous slow +oscillations of level, and apparently these oscillations have affected wide +spaces. Consequently formations rich in fossils and sufficiently thick and +extensive to resist subsequent degradation, may have been formed over wide +spaces during periods of subsidence, but only where the supply of sediment was +sufficient to keep the sea shallow and to embed and preserve the remains before +they had time to decay. On the other hand, as long as the bed of the sea +remained stationary, thick deposits could not have been accumulated in the +shallow parts, which are the most favourable to life. Still less could this +have happened during the alternate periods of elevation; or, to speak more +accurately, the beds which were then accumulated will have been destroyed by +being upraised and brought within the limits of the coast-action. + +Thus the geological record will almost necessarily be rendered intermittent. I +feel much confidence in the truth of these views, for they are in strict +accordance with the general principles inculcated by Sir C. Lyell; and E. +Forbes independently arrived at a similar conclusion. + +One remark is here worth a passing notice. During periods of elevation the area +of the land and of the adjoining shoal parts of the sea will be increased, and +new stations will often be formed;—all circumstances most favourable, as +previously explained, for the formation of new varieties and species; but +during such periods there will generally be a blank in the geological record. +On the other hand, during subsidence, the inhabited area and number of +inhabitants will decrease (excepting the productions on the shores of a +continent when first broken up into an archipelago), and consequently during +subsidence, though there will be much extinction, fewer new varieties or +species will be formed; and it is during these very periods of subsidence, that +our great deposits rich in fossils have been accumulated. Nature may almost be +said to have guarded against the frequent discovery of her transitional or +linking forms. + +From the foregoing considerations it cannot be doubted that the geological +record, viewed as a whole, is extremely imperfect; but if we confine our +attention to any one formation, it becomes more difficult to understand, why we +do not therein find closely graduated varieties between the allied species +which lived at its commencement and at its close. Some cases are on record of +the same species presenting distinct varieties in the upper and lower parts of +the same formation, but, as they are rare, they may be here passed over. +Although each formation has indisputably required a vast number of years for +its deposition, I can see several reasons why each should not include a +graduated series of links between the species which then lived; but I can by no +means pretend to assign due proportional weight to the following +considerations. + +Although each formation may mark a very long lapse of years, each perhaps is +short compared with the period requisite to change one species into another. I +am aware that two palæontologists, whose opinions are worthy of much deference, +namely Bronn and Woodward, have concluded that the average duration of each +formation is twice or thrice as long as the average duration of specific forms. +But insuperable difficulties, as it seems to me, prevent us coming to any just +conclusion on this head. When we see a species first appearing in the middle of +any formation, it would be rash in the extreme to infer that it had not +elsewhere previously existed. So again when we find a species disappearing +before the uppermost layers have been deposited, it would be equally rash to +suppose that it then became wholly extinct. We forget how small the area of +Europe is compared with the rest of the world; nor have the several stages of +the same formation throughout Europe been correlated with perfect accuracy. + +With marine animals of all kinds, we may safely infer a large amount of +migration during climatal and other changes; and when we see a species first +appearing in any formation, the probability is that it only then first +immigrated into that area. It is well known, for instance, that several species +appeared somewhat earlier in the palæozoic beds of North America than in those +of Europe; time having apparently been required for their migration from the +American to the European seas. In examining the latest deposits of various +quarters of the world, it has everywhere been noted, that some few still +existing species are common in the deposit, but have become extinct in the +immediately surrounding sea; or, conversely, that some are now abundant in the +neighbouring sea, but are rare or absent in this particular deposit. It is an +excellent lesson to reflect on the ascertained amount of migration of the +inhabitants of Europe during the Glacial period, which forms only a part of one +whole geological period; and likewise to reflect on the great changes of level, +on the inordinately great change of climate, on the prodigious lapse of time, +all included within this same glacial period. Yet it may be doubted whether in +any quarter of the world, sedimentary deposits, including fossil remains, have +gone on accumulating within the same area during the whole of this period. It +is not, for instance, probable that sediment was deposited during the whole of +the glacial period near the mouth of the Mississippi, within that limit of +depth at which marine animals can flourish; for we know what vast geographical +changes occurred in other parts of America during this space of time. When such +beds as were deposited in shallow water near the mouth of the Mississippi +during some part of the glacial period shall have been upraised, organic +remains will probably first appear and disappear at different levels, owing to +the migration of species and to geographical changes. And in the distant +future, a geologist examining these beds, might be tempted to conclude that the +average duration of life of the embedded fossils had been less than that of the +glacial period, instead of having been really far greater, that is extending +from before the glacial epoch to the present day. + +In order to get a perfect gradation between two forms in the upper and lower +parts of the same formation, the deposit must have gone on accumulating for a +very long period, in order to have given sufficient time for the slow process +of variation; hence the deposit will generally have to be a very thick one; and +the species undergoing modification will have had to live on the same area +throughout this whole time. But we have seen that a thick fossiliferous +formation can only be accumulated during a period of subsidence; and to keep +the depth approximately the same, which is necessary in order to enable the +same species to live on the same space, the supply of sediment must nearly have +counterbalanced the amount of subsidence. But this same movement of subsidence +will often tend to sink the area whence the sediment is derived, and thus +diminish the supply whilst the downward movement continues. In fact, this +nearly exact balancing between the supply of sediment and the amount of +subsidence is probably a rare contingency; for it has been observed by more +than one palæontologist, that very thick deposits are usually barren of organic +remains, except near their upper or lower limits. + +It would seem that each separate formation, like the whole pile of formations +in any country, has generally been intermittent in its accumulation. When we +see, as is so often the case, a formation composed of beds of different +mineralogical composition, we may reasonably suspect that the process of +deposition has been much interrupted, as a change in the currents of the sea +and a supply of sediment of a different nature will generally have been due to +geographical changes requiring much time. Nor will the closest inspection of a +formation give any idea of the time which its deposition has consumed. Many +instances could be given of beds only a few feet in thickness, representing +formations, elsewhere thousands of feet in thickness, and which must have +required an enormous period for their accumulation; yet no one ignorant of this +fact would have suspected the vast lapse of time represented by the thinner +formation. Many cases could be given of the lower beds of a formation having +been upraised, denuded, submerged, and then re-covered by the upper beds of the +same formation,—facts, showing what wide, yet easily overlooked, intervals have +occurred in its accumulation. In other cases we have the plainest evidence in +great fossilised trees, still standing upright as they grew, of many long +intervals of time and changes of level during the process of deposition, which +would never even have been suspected, had not the trees chanced to have been +preserved: thus, Messrs. Lyell and Dawson found carboniferous beds 1400 feet +thick in Nova Scotia, with ancient root-bearing strata, one above the other, at +no less than sixty-eight different levels. Hence, when the same species occur +at the bottom, middle, and top of a formation, the probability is that they +have not lived on the same spot during the whole period of deposition, but have +disappeared and reappeared, perhaps many times, during the same geological +period. So that if such species were to undergo a considerable amount of +modification during any one geological period, a section would not probably +include all the fine intermediate gradations which must on my theory have +existed between them, but abrupt, though perhaps very slight, changes of form. + +It is all-important to remember that naturalists have no golden rule by which +to distinguish species and varieties; they grant some little variability to +each species, but when they meet with a somewhat greater amount of difference +between any two forms, they rank both as species, unless they are enabled to +connect them together by close intermediate gradations. And this from the +reasons just assigned we can seldom hope to effect in any one geological +section. Supposing B and C to be two species, and a third, A, to be found in an +underlying bed; even if A were strictly intermediate between B and C, it would +simply be ranked as a third and distinct species, unless at the same time it +could be most closely connected with either one or both forms by intermediate +varieties. Nor should it be forgotten, as before explained, that A might be the +actual progenitor of B and C, and yet might not at all necessarily be strictly +intermediate between them in all points of structure. So that we might obtain +the parent-species and its several modified descendants from the lower and +upper beds of a formation, and unless we obtained numerous transitional +gradations, we should not recognise their relationship, and should consequently +be compelled to rank them all as distinct species. + +It is notorious on what excessively slight differences many palæontologists +have founded their species; and they do this the more readily if the specimens +come from different sub-stages of the same formation. Some experienced +conchologists are now sinking many of the very fine species of D’Orbigny and +others into the rank of varieties; and on this view we do find the kind of +evidence of change which on my theory we ought to find. Moreover, if we look to +rather wider intervals, namely, to distinct but consecutive stages of the same +great formation, we find that the embedded fossils, though almost universally +ranked as specifically different, yet are far more closely allied to each other +than are the species found in more widely separated formations; but to this +subject I shall have to return in the following chapter. + +One other consideration is worth notice: with animals and plants that can +propagate rapidly and are not highly locomotive, there is reason to suspect, as +we have formerly seen, that their varieties are generally at first local; and +that such local varieties do not spread widely and supplant their parent-forms +until they have been modified and perfected in some considerable degree. +According to this view, the chance of discovering in a formation in any one +country all the early stages of transition between any two forms, is small, for +the successive changes are supposed to have been local or confined to some one +spot. Most marine animals have a wide range; and we have seen that with plants +it is those which have the widest range, that oftenest present varieties; so +that with shells and other marine animals, it is probably those which have had +the widest range, far exceeding the limits of the known geological formations +of Europe, which have oftenest given rise, first to local varieties and +ultimately to new species; and this again would greatly lessen the chance of +our being able to trace the stages of transition in any one geological +formation. + +It should not be forgotten, that at the present day, with perfect specimens for +examination, two forms can seldom be connected by intermediate varieties and +thus proved to be the same species, until many specimens have been collected +from many places; and in the case of fossil species this could rarely be +effected by palæontologists. We shall, perhaps, best perceive the improbability +of our being enabled to connect species by numerous, fine, intermediate, fossil +links, by asking ourselves whether, for instance, geologists at some future +period will be able to prove, that our different breeds of cattle, sheep, +horses, and dogs have descended from a single stock or from several aboriginal +stocks; or, again, whether certain sea-shells inhabiting the shores of North +America, which are ranked by some conchologists as distinct species from their +European representatives, and by other conchologists as only varieties, are +really varieties or are, as it is called, specifically distinct. This could be +effected only by the future geologist discovering in a fossil state numerous +intermediate gradations; and such success seems to me improbable in the highest +degree. + +Geological research, though it has added numerous species to existing and +extinct genera, and has made the intervals between some few groups less wide +than they otherwise would have been, yet has done scarcely anything in breaking +down the distinction between species, by connecting them together by numerous, +fine, intermediate varieties; and this not having been effected, is probably +the gravest and most obvious of all the many objections which may be urged +against my views. Hence it will be worth while to sum up the foregoing remarks, +under an imaginary illustration. The Malay Archipelago is of about the size of +Europe from the North Cape to the Mediterranean, and from Britain to Russia; +and therefore equals all the geological formations which have been examined +with any accuracy, excepting those of the United States of America. I fully +agree with Mr. Godwin-Austen, that the present condition of the Malay +Archipelago, with its numerous large islands separated by wide and shallow +seas, probably represents the former state of Europe, when most of our +formations were accumulating. The Malay Archipelago is one of the richest +regions of the whole world in organic beings; yet if all the species were to be +collected which have ever lived there, how imperfectly would they represent the +natural history of the world! + +But we have every reason to believe that the terrestrial productions of the +archipelago would be preserved in an excessively imperfect manner in the +formations which we suppose to be there accumulating. I suspect that not many +of the strictly littoral animals, or of those which lived on naked submarine +rocks, would be embedded; and those embedded in gravel or sand, would not +endure to a distant epoch. Wherever sediment did not accumulate on the bed of +the sea, or where it did not accumulate at a sufficient rate to protect organic +bodies from decay, no remains could be preserved. + +In our archipelago, I believe that fossiliferous formations could be formed of +sufficient thickness to last to an age, as distant in futurity as the secondary +formations lie in the past, only during periods of subsidence. These periods of +subsidence would be separated from each other by enormous intervals, during +which the area would be either stationary or rising; whilst rising, each +fossiliferous formation would be destroyed, almost as soon as accumulated, by +the incessant coast-action, as we now see on the shores of South America. +During the periods of subsidence there would probably be much extinction of +life; during the periods of elevation, there would be much variation, but the +geological record would then be least perfect. + +It may be doubted whether the duration of any one great period of subsidence +over the whole or part of the archipelago, together with a contemporaneous +accumulation of sediment, would exceed the average duration of the same +specific forms; and these contingencies are indispensable for the preservation +of all the transitional gradations between any two or more species. If such +gradations were not fully preserved, transitional varieties would merely appear +as so many distinct species. It is, also, probable that each great period of +subsidence would be interrupted by oscillations of level, and that slight +climatal changes would intervene during such lengthy periods; and in these +cases the inhabitants of the archipelago would have to migrate, and no closely +consecutive record of their modifications could be preserved in any one +formation. + +Very many of the marine inhabitants of the archipelago now range thousands of +miles beyond its confines; and analogy leads me to believe that it would be +chiefly these far-ranging species which would oftenest produce new varieties; +and the varieties would at first generally be local or confined to one place, +but if possessed of any decided advantage, or when further modified and +improved, they would slowly spread and supplant their parent-forms. When such +varieties returned to their ancient homes, as they would differ from their +former state, in a nearly uniform, though perhaps extremely slight degree, they +would, according to the principles followed by many palæontologists, be ranked +as new and distinct species. + +If then, there be some degree of truth in these remarks, we have no right to +expect to find in our geological formations, an infinite number of those fine +transitional forms, which on my theory assuredly have connected all the past +and present species of the same group into one long and branching chain of +life. We ought only to look for a few links, some more closely, some more +distantly related to each other; and these links, let them be ever so close, if +found in different stages of the same formation, would, by most +palæontologists, be ranked as distinct species. But I do not pretend that I +should ever have suspected how poor a record of the mutations of life, the best +preserved geological section presented, had not the difficulty of our not +discovering innumerable transitional links between the species which appeared +at the commencement and close of each formation, pressed so hardly on my +theory. + +On the sudden appearance of whole groups of Allied Species.—The abrupt manner +in which whole groups of species suddenly appear in certain formations, has +been urged by several palæontologists, for instance, by Agassiz, Pictet, and by +none more forcibly than by Professor Sedgwick, as a fatal objection to the +belief in the transmutation of species. If numerous species, belonging to the +same genera or families, have really started into life all at once, the fact +would be fatal to the theory of descent with slow modification through natural +selection. For the development of a group of forms, all of which have descended +from some one progenitor, must have been an extremely slow process; and the +progenitors must have lived long ages before their modified descendants. But we +continually over-rate the perfection of the geological record, and falsely +infer, because certain genera or families have not been found beneath a certain +stage, that they did not exist before that stage. We continually forget how +large the world is, compared with the area over which our geological formations +have been carefully examined; we forget that groups of species may elsewhere +have long existed and have slowly multiplied before they invaded the ancient +archipelagoes of Europe and of the United States. We do not make due allowance +for the enormous intervals of time, which have probably elapsed between our +consecutive formations,—longer perhaps in some cases than the time required for +the accumulation of each formation. These intervals will have given time for +the multiplication of species from some one or some few parent-forms; and in +the succeeding formation such species will appear as if suddenly created. + +I may here recall a remark formerly made, namely that it might require a long +succession of ages to adapt an organism to some new and peculiar line of life, +for instance to fly through the air; but that when this had been effected, and +a few species had thus acquired a great advantage over other organisms, a +comparatively short time would be necessary to produce many divergent forms, +which would be able to spread rapidly and widely throughout the world. + +I will now give a few examples to illustrate these remarks; and to show how +liable we are to error in supposing that whole groups of species have suddenly +been produced. I may recall the well-known fact that in geological treatises, +published not many years ago, the great class of mammals was always spoken of +as having abruptly come in at the commencement of the tertiary series. And now +one of the richest known accumulations of fossil mammals belongs to the middle +of the secondary series; and one true mammal has been discovered in the new red +sandstone at nearly the commencement of this great series. Cuvier used to urge +that no monkey occurred in any tertiary stratum; but now extinct species have +been discovered in India, South America, and in Europe even as far back as the +eocene stage. The most striking case, however, is that of the Whale family; as +these animals have huge bones, are marine, and range over the world, the fact +of not a single bone of a whale having been discovered in any secondary +formation, seemed fully to justify the belief that this great and distinct +order had been suddenly produced in the interval between the latest secondary +and earliest tertiary formation. But now we may read in the Supplement to +Lyell’s ‘Manual,’ published in 1858, clear evidence of the existence of whales +in the upper greensand, some time before the close of the secondary period. + +I may give another instance, which from having passed under my own eyes has +much struck me. In a memoir on Fossil Sessile Cirripedes, I have stated that, +from the number of existing and extinct tertiary species; from the +extraordinary abundance of the individuals of many species all over the world, +from the Arctic regions to the equator, inhabiting various zones of depths from +the upper tidal limits to 50 fathoms; from the perfect manner in which +specimens are preserved in the oldest tertiary beds; from the ease with which +even a fragment of a valve can be recognised; from all these circumstances, I +inferred that had sessile cirripedes existed during the secondary periods, they +would certainly have been preserved and discovered; and as not one species had +been discovered in beds of this age, I concluded that this great group had been +suddenly developed at the commencement of the tertiary series. This was a sore +trouble to me, adding as I thought one more instance of the abrupt appearance +of a great group of species. But my work had hardly been published, when a +skilful palæontologist, M. Bosquet, sent me a drawing of a perfect specimen of +an unmistakeable sessile cirripede, which he had himself extracted from the +chalk of Belgium. And, as if to make the case as striking as possible, this +sessile cirripede was a Chthamalus, a very common, large, and ubiquitous genus, +of which not one specimen has as yet been found even in any tertiary stratum. +Hence we now positively know that sessile cirripedes existed during the +secondary period; and these cirripedes might have been the progenitors of our +many tertiary and existing species. + +The case most frequently insisted on by palæontologists of the apparently +sudden appearance of a whole group of species, is that of the teleostean +fishes, low down in the Chalk period. This group includes the large majority of +existing species. Lately, Professor Pictet has carried their existence one +sub-stage further back; and some palæontologists believe that certain much +older fishes, of which the affinities are as yet imperfectly known, are really +teleostean. Assuming, however, that the whole of them did appear, as Agassiz +believes, at the commencement of the chalk formation, the fact would certainly +be highly remarkable; but I cannot see that it would be an insuperable +difficulty on my theory, unless it could likewise be shown that the species of +this group appeared suddenly and simultaneously throughout the world at this +same period. It is almost superfluous to remark that hardly any fossil-fish are +known from south of the equator; and by running through Pictet’s Palæontology +it will be seen that very few species are known from several formations in +Europe. Some few families of fish now have a confined range; the teleostean +fish might formerly have had a similarly confined range, and after having been +largely developed in some one sea, might have spread widely. Nor have we any +right to suppose that the seas of the world have always been so freely open +from south to north as they are at present. Even at this day, if the Malay +Archipelago were converted into land, the tropical parts of the Indian Ocean +would form a large and perfectly enclosed basin, in which any great group of +marine animals might be multiplied; and here they would remain confined, until +some of the species became adapted to a cooler climate, and were enabled to +double the southern capes of Africa or Australia, and thus reach other and +distant seas. + +From these and similar considerations, but chiefly from our ignorance of the +geology of other countries beyond the confines of Europe and the United States; +and from the revolution in our palæontological ideas on many points, which the +discoveries of even the last dozen years have effected, it seems to me to be +about as rash in us to dogmatize on the succession of organic beings throughout +the world, as it would be for a naturalist to land for five minutes on some one +barren point in Australia, and then to discuss the number and range of its +productions. + +On the sudden appearance of groups of Allied Species in the lowest known +fossiliferous strata.—There is another and allied difficulty, which is much +graver. I allude to the manner in which numbers of species of the same group, +suddenly appear in the lowest known fossiliferous rocks. Most of the arguments +which have convinced me that all the existing species of the same group have +descended from one progenitor, apply with nearly equal force to the earliest +known species. For instance, I cannot doubt that all the Silurian trilobites +have descended from some one crustacean, which must have lived long before the +Silurian age, and which probably differed greatly from any known animal. Some +of the most ancient Silurian animals, as the Nautilus, Lingula, etc., do not +differ much from living species; and it cannot on my theory be supposed, that +these old species were the progenitors of all the species of the orders to +which they belong, for they do not present characters in any degree +intermediate between them. If, moreover, they had been the progenitors of these +orders, they would almost certainly have been long ago supplanted and +exterminated by their numerous and improved descendants. + +Consequently, if my theory be true, it is indisputable that before the lowest +Silurian stratum was deposited, long periods elapsed, as long as, or probably +far longer than, the whole interval from the Silurian age to the present day; +and that during these vast, yet quite unknown, periods of time, the world +swarmed with living creatures. + +To the question why we do not find records of these vast primordial periods, I +can give no satisfactory answer. Several of the most eminent geologists, with +Sir R. Murchison at their head, are convinced that we see in the organic +remains of the lowest Silurian stratum the dawn of life on this planet. Other +highly competent judges, as Lyell and the late E. Forbes, dispute this +conclusion. We should not forget that only a small portion of the world is +known with accuracy. M. Barrande has lately added another and lower stage to +the Silurian system, abounding with new and peculiar species. Traces of life +have been detected in the Longmynd beds beneath Barrande’s so-called primordial +zone. The presence of phosphatic nodules and bituminous matter in some of the +lowest azoic rocks, probably indicates the former existence of life at these +periods. But the difficulty of understanding the absence of vast piles of +fossiliferous strata, which on my theory no doubt were somewhere accumulated +before the Silurian epoch, is very great. If these most ancient beds had been +wholly worn away by denudation, or obliterated by metamorphic action, we ought +to find only small remnants of the formations next succeeding them in age, and +these ought to be very generally in a metamorphosed condition. But the +descriptions which we now possess of the Silurian deposits over immense +territories in Russia and in North America, do not support the view, that the +older a formation is, the more it has suffered the extremity of denudation and +metamorphism. + +The case at present must remain inexplicable; and may be truly urged as a valid +argument against the views here entertained. To show that it may hereafter +receive some explanation, I will give the following hypothesis. From the nature +of the organic remains, which do not appear to have inhabited profound depths, +in the several formations of Europe and of the United States; and from the +amount of sediment, miles in thickness, of which the formations are composed, +we may infer that from first to last large islands or tracts of land, whence +the sediment was derived, occurred in the neighbourhood of the existing +continents of Europe and North America. But we do not know what was the state +of things in the intervals between the successive formations; whether Europe +and the United States during these intervals existed as dry land, or as a +submarine surface near land, on which sediment was not deposited, or again as +the bed of an open and unfathomable sea. + +Looking to the existing oceans, which are thrice as extensive as the land, we +see them studded with many islands; but not one oceanic island is as yet known +to afford even a remnant of any palæozoic or secondary formation. Hence we may +perhaps infer, that during the palæozoic and secondary periods, neither +continents nor continental islands existed where our oceans now extend; for had +they existed there, palæozoic and secondary formations would in all probability +have been accumulated from sediment derived from their wear and tear; and would +have been at least partially upheaved by the oscillations of level, which we +may fairly conclude must have intervened during these enormously long periods. +If then we may infer anything from these facts, we may infer that where our +oceans now extend, oceans have extended from the remotest period of which we +have any record; and on the other hand, that where continents now exist, large +tracts of land have existed, subjected no doubt to great oscillations of level, +since the earliest silurian period. The coloured map appended to my volume on +Coral Reefs, led me to conclude that the great oceans are still mainly areas of +subsidence, the great archipelagoes still areas of oscillations of level, and +the continents areas of elevation. But have we any right to assume that things +have thus remained from eternity? Our continents seem to have been formed by a +preponderance, during many oscillations of level, of the force of elevation; +but may not the areas of preponderant movement have changed in the lapse of +ages? At a period immeasurably antecedent to the silurian epoch, continents may +have existed where oceans are now spread out; and clear and open oceans may +have existed where our continents now stand. Nor should we be justified in +assuming that if, for instance, the bed of the Pacific Ocean were now converted +into a continent, we should there find formations older than the silurian +strata, supposing such to have been formerly deposited; for it might well +happen that strata which had subsided some miles nearer to the centre of the +earth, and which had been pressed on by an enormous weight of superincumbent +water, might have undergone far more metamorphic action than strata which have +always remained nearer to the surface. The immense areas in some parts of the +world, for instance in South America, of bare metamorphic rocks, which must +have been heated under great pressure, have always seemed to me to require some +special explanation; and we may perhaps believe that we see in these large +areas, the many formations long anterior to the silurian epoch in a completely +metamorphosed condition. + +The several difficulties here discussed, namely our not finding in the +successive formations infinitely numerous transitional links between the many +species which now exist or have existed; the sudden manner in which whole +groups of species appear in our European formations; the almost entire absence, +as at present known, of fossiliferous formations beneath the Silurian strata, +are all undoubtedly of the gravest nature. We see this in the plainest manner +by the fact that all the most eminent palæontologists, namely Cuvier, Owen, +Agassiz, Barrande, Falconer, E. Forbes, etc., and all our greatest geologists, +as Lyell, Murchison, Sedgwick, etc., have unanimously, often vehemently, +maintained the immutability of species. But I have reason to believe that one +great authority, Sir Charles Lyell, from further reflexion entertains grave +doubts on this subject. I feel how rash it is to differ from these great +authorities, to whom, with others, we owe all our knowledge. Those who think +the natural geological record in any degree perfect, and who do not attach much +weight to the facts and arguments of other kinds given in this volume, will +undoubtedly at once reject my theory. For my part, following out Lyell’s +metaphor, I look at the natural geological record, as a history of the world +imperfectly kept, and written in a changing dialect; of this history we possess +the last volume alone, relating only to two or three countries. Of this volume, +only here and there a short chapter has been preserved; and of each page, only +here and there a few lines. Each word of the slowly-changing language, in which +the history is supposed to be written, being more or less different in the +interrupted succession of chapters, may represent the apparently abruptly +changed forms of life, entombed in our consecutive, but widely separated +formations. On this view, the difficulties above discussed are greatly +diminished, or even disappear. + +CHAPTER X. +ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + +On the slow and successive appearance of new species. On their different rates +of change. Species once lost do not reappear. Groups of species follow the same +general rules in their appearance and disappearance as do single species. On +Extinction. On simultaneous changes in the forms of life throughout the world. +On the affinities of extinct species to each other and to living species. On +the state of development of ancient forms. On the succession of the same types +within the same areas. Summary of preceding and present chapters. + +Let us now see whether the several facts and rules relating to the geological +succession of organic beings, better accord with the common view of the +immutability of species, or with that of their slow and gradual modification, +through descent and natural selection. + +New species have appeared very slowly, one after another, both on the land and +in the waters. Lyell has shown that it is hardly possible to resist the +evidence on this head in the case of the several tertiary stages; and every +year tends to fill up the blanks between them, and to make the percentage +system of lost and new forms more gradual. In some of the most recent beds, +though undoubtedly of high antiquity if measured by years, only one or two +species are lost forms, and only one or two are new forms, having here appeared +for the first time, either locally, or, as far as we know, on the face of the +earth. If we may trust the observations of Philippi in Sicily, the successive +changes in the marine inhabitants of that island have been many and most +gradual. The secondary formations are more broken; but, as Bronn has remarked, +neither the appearance nor disappearance of their many now extinct species has +been simultaneous in each separate formation. + +Species of different genera and classes have not changed at the same rate, or +in the same degree. In the oldest tertiary beds a few living shells may still +be found in the midst of a multitude of extinct forms. Falconer has given a +striking instance of a similar fact, in an existing crocodile associated with +many strange and lost mammals and reptiles in the sub-Himalayan deposits. The +Silurian Lingula differs but little from the living species of this genus; +whereas most of the other Silurian Molluscs and all the Crustaceans have +changed greatly. The productions of the land seem to change at a quicker rate +than those of the sea, of which a striking instance has lately been observed in +Switzerland. There is some reason to believe that organisms, considered high in +the scale of nature, change more quickly than those that are low: though there +are exceptions to this rule. The amount of organic change, as Pictet has +remarked, does not strictly correspond with the succession of our geological +formations; so that between each two consecutive formations, the forms of life +have seldom changed in exactly the same degree. Yet if we compare any but the +most closely related formations, all the species will be found to have +undergone some change. When a species has once disappeared from the face of the +earth, we have reason to believe that the same identical form never reappears. +The strongest apparent exception to this latter rule, is that of the so-called +“colonies” of M. Barrande, which intrude for a period in the midst of an older +formation, and then allow the pre-existing fauna to reappear; but Lyell’s +explanation, namely, that it is a case of temporary migration from a distinct +geographical province, seems to me satisfactory. + +These several facts accord well with my theory. I believe in no fixed law of +development, causing all the inhabitants of a country to change abruptly, or +simultaneously, or to an equal degree. The process of modification must be +extremely slow. The variability of each species is quite independent of that of +all others. Whether such variability be taken advantage of by natural +selection, and whether the variations be accumulated to a greater or lesser +amount, thus causing a greater or lesser amount of modification in the varying +species, depends on many complex contingencies,—on the variability being of a +beneficial nature, on the power of intercrossing, on the rate of breeding, on +the slowly changing physical conditions of the country, and more especially on +the nature of the other inhabitants with which the varying species comes into +competition. Hence it is by no means surprising that one species should retain +the same identical form much longer than others; or, if changing, that it +should change less. We see the same fact in geographical distribution; for +instance, in the land-shells and coleopterous insects of Madeira having come to +differ considerably from their nearest allies on the continent of Europe, +whereas the marine shells and birds have remained unaltered. We can perhaps +understand the apparently quicker rate of change in terrestrial and in more +highly organised productions compared with marine and lower productions, by the +more complex relations of the higher beings to their organic and inorganic +conditions of life, as explained in a former chapter. When many of the +inhabitants of a country have become modified and improved, we can understand, +on the principle of competition, and on that of the many all-important +relations of organism to organism, that any form which does not become in some +degree modified and improved, will be liable to be exterminated. Hence we can +see why all the species in the same region do at last, if we look to wide +enough intervals of time, become modified; for those which do not change will +become extinct. + +In members of the same class the average amount of change, during long and +equal periods of time, may, perhaps, be nearly the same; but as the +accumulation of long-enduring fossiliferous formations depends on great masses +of sediment having been deposited on areas whilst subsiding, our formations +have been almost necessarily accumulated at wide and irregularly intermittent +intervals; consequently the amount of organic change exhibited by the fossils +embedded in consecutive formations is not equal. Each formation, on this view, +does not mark a new and complete act of creation, but only an occasional scene, +taken almost at hazard, in a slowly changing drama. + +We can clearly understand why a species when once lost should never reappear, +even if the very same conditions of life, organic and inorganic, should recur. +For though the offspring of one species might be adapted (and no doubt this has +occurred in innumerable instances) to fill the exact place of another species +in the economy of nature, and thus supplant it; yet the two forms—the old and +the new—would not be identically the same; for both would almost certainly +inherit different characters from their distinct progenitors. For instance, it +is just possible, if our fantail-pigeons were all destroyed, that fanciers, by +striving during long ages for the same object, might make a new breed hardly +distinguishable from our present fantail; but if the parent rock-pigeon were +also destroyed, and in nature we have every reason to believe that the +parent-form will generally be supplanted and exterminated by its improved +offspring, it is quite incredible that a fantail, identical with the existing +breed, could be raised from any other species of pigeon, or even from the other +well-established races of the domestic pigeon, for the newly-formed fantail +would be almost sure to inherit from its new progenitor some slight +characteristic differences. + +Groups of species, that is, genera and families, follow the same general rules +in their appearance and disappearance as do single species, changing more or +less quickly, and in a greater or lesser degree. A group does not reappear +after it has once disappeared; or its existence, as long as it lasts, is +continuous. I am aware that there are some apparent exceptions to this rule, +but the exceptions are surprisingly few, so few, that E. Forbes, Pictet, and +Woodward (though all strongly opposed to such views as I maintain) admit its +truth; and the rule strictly accords with my theory. For as all the species of +the same group have descended from some one species, it is clear that as long +as any species of the group have appeared in the long succession of ages, so +long must its members have continuously existed, in order to have generated +either new and modified or the same old and unmodified forms. Species of the +genus Lingula, for instance, must have continuously existed by an unbroken +succession of generations, from the lowest Silurian stratum to the present day. + +We have seen in the last chapter that the species of a group sometimes falsely +appear to have come in abruptly; and I have attempted to give an explanation of +this fact, which if true would have been fatal to my views. But such cases are +certainly exceptional; the general rule being a gradual increase in number, +till the group reaches its maximum, and then, sooner or later, it gradually +decreases. If the number of the species of a genus, or the number of the genera +of a family, be represented by a vertical line of varying thickness, crossing +the successive geological formations in which the species are found, the line +will sometimes falsely appear to begin at its lower end, not in a sharp point, +but abruptly; it then gradually thickens upwards, sometimes keeping for a space +of equal thickness, and ultimately thins out in the upper beds, marking the +decrease and final extinction of the species. This gradual increase in number +of the species of a group is strictly conformable with my theory; as the +species of the same genus, and the genera of the same family, can increase only +slowly and progressively; for the process of modification and the production of +a number of allied forms must be slow and gradual,—one species giving rise +first to two or three varieties, these being slowly converted into species, +which in their turn produce by equally slow steps other species, and so on, +like the branching of a great tree from a single stem, till the group becomes +large. + +On Extinction.—We have as yet spoken only incidentally of the disappearance of +species and of groups of species. On the theory of natural selection the +extinction of old forms and the production of new and improved forms are +intimately connected together. The old notion of all the inhabitants of the +earth having been swept away at successive periods by catastrophes, is very +generally given up, even by those geologists, as Elie de Beaumont, Murchison, +Barrande, etc., whose general views would naturally lead them to this +conclusion. On the contrary, we have every reason to believe, from the study of +the tertiary formations, that species and groups of species gradually +disappear, one after another, first from one spot, then from another, and +finally from the world. Both single species and whole groups of species last +for very unequal periods; some groups, as we have seen, having endured from the +earliest known dawn of life to the present day; some having disappeared before +the close of the palæozoic period. No fixed law seems to determine the length +of time during which any single species or any single genus endures. There is +reason to believe that the complete extinction of the species of a group is +generally a slower process than their production: if the appearance and +disappearance of a group of species be represented, as before, by a vertical +line of varying thickness, the line is found to taper more gradually at its +upper end, which marks the progress of extermination, than at its lower end, +which marks the first appearance and increase in numbers of the species. In +some cases, however, the extermination of whole groups of beings, as of +ammonites towards the close of the secondary period, has been wonderfully +sudden. + +The whole subject of the extinction of species has been involved in the most +gratuitous mystery. Some authors have even supposed that as the individual has +a definite length of life, so have species a definite duration. No one I think +can have marvelled more at the extinction of species, than I have done. When I +found in La Plata the tooth of a horse embedded with the remains of Mastodon, +Megatherium, Toxodon, and other extinct monsters, which all co-existed with +still living shells at a very late geological period, I was filled with +astonishment; for seeing that the horse, since its introduction by the +Spaniards into South America, has run wild over the whole country and has +increased in numbers at an unparalleled rate, I asked myself what could so +recently have exterminated the former horse under conditions of life apparently +so favourable. But how utterly groundless was my astonishment! Professor Owen +soon perceived that the tooth, though so like that of the existing horse, +belonged to an extinct species. Had this horse been still living, but in some +degree rare, no naturalist would have felt the least surprise at its rarity; +for rarity is the attribute of a vast number of species of all classes, in all +countries. If we ask ourselves why this or that species is rare, we answer that +something is unfavourable in its conditions of life; but what that something +is, we can hardly ever tell. On the supposition of the fossil horse still +existing as a rare species, we might have felt certain from the analogy of all +other mammals, even of the slow-breeding elephant, and from the history of the +naturalisation of the domestic horse in South America, that under more +favourable conditions it would in a very few years have stocked the whole +continent. But we could not have told what the unfavourable conditions were +which checked its increase, whether some one or several contingencies, and at +what period of the horse’s life, and in what degree, they severally acted. If +the conditions had gone on, however slowly, becoming less and less favourable, +we assuredly should not have perceived the fact, yet the fossil horse would +certainly have become rarer and rarer, and finally extinct;—its place being +seized on by some more successful competitor. + +It is most difficult always to remember that the increase of every living being +is constantly being checked by unperceived injurious agencies; and that these +same unperceived agencies are amply sufficient to cause rarity, and finally +extinction. We see in many cases in the more recent tertiary formations, that +rarity precedes extinction; and we know that this has been the progress of +events with those animals which have been exterminated, either locally or +wholly, through man’s agency. I may repeat what I published in 1845, namely, +that to admit that species generally become rare before they become extinct—to +feel no surprise at the rarity of a species, and yet to marvel greatly when it +ceases to exist, is much the same as to admit that sickness in the individual +is the forerunner of death—to feel no surprise at sickness, but when the sick +man dies, to wonder and to suspect that he died by some unknown deed of +violence. + +The theory of natural selection is grounded on the belief that each new +variety, and ultimately each new species, is produced and maintained by having +some advantage over those with which it comes into competition; and the +consequent extinction of less-favoured forms almost inevitably follows. It is +the same with our domestic productions: when a new and slightly improved +variety has been raised, it at first supplants the less improved varieties in +the same neighbourhood; when much improved it is transported far and near, like +our short-horn cattle, and takes the place of other breeds in other countries. +Thus the appearance of new forms and the disappearance of old forms, both +natural and artificial, are bound together. In certain flourishing groups, the +number of new specific forms which have been produced within a given time is +probably greater than that of the old forms which have been exterminated; but +we know that the number of species has not gone on indefinitely increasing, at +least during the later geological periods, so that looking to later times we +may believe that the production of new forms has caused the extinction of about +the same number of old forms. + +The competition will generally be most severe, as formerly explained and +illustrated by examples, between the forms which are most like each other in +all respects. Hence the improved and modified descendants of a species will +generally cause the extermination of the parent-species; and if many new forms +have been developed from any one species, the nearest allies of that species, +i.e. the species of the same genus, will be the most liable to extermination. +Thus, as I believe, a number of new species descended from one species, that is +a new genus, comes to supplant an old genus, belonging to the same family. But +it must often have happened that a new species belonging to some one group will +have seized on the place occupied by a species belonging to a distinct group, +and thus caused its extermination; and if many allied forms be developed from +the successful intruder, many will have to yield their places; and it will +generally be allied forms, which will suffer from some inherited inferiority in +common. But whether it be species belonging to the same or to a distinct class, +which yield their places to other species which have been modified and +improved, a few of the sufferers may often long be preserved, from being fitted +to some peculiar line of life, or from inhabiting some distant and isolated +station, where they have escaped severe competition. For instance, a single +species of Trigonia, a great genus of shells in the secondary formations, +survives in the Australian seas; and a few members of the great and almost +extinct group of Ganoid fishes still inhabit our fresh waters. Therefore the +utter extinction of a group is generally, as we have seen, a slower process +than its production. + +With respect to the apparently sudden extermination of whole families or +orders, as of Trilobites at the close of the palæozoic period and of Ammonites +at the close of the secondary period, we must remember what has been already +said on the probable wide intervals of time between our consecutive formations; +and in these intervals there may have been much slow extermination. Moreover, +when by sudden immigration or by unusually rapid development, many species of a +new group have taken possession of a new area, they will have exterminated in a +correspondingly rapid manner many of the old inhabitants; and the forms which +thus yield their places will commonly be allied, for they will partake of some +inferiority in common. + +Thus, as it seems to me, the manner in which single species and whole groups of +species become extinct, accords well with the theory of natural selection. We +need not marvel at extinction; if we must marvel, let it be at our presumption +in imagining for a moment that we understand the many complex contingencies, on +which the existence of each species depends. If we forget for an instant, that +each species tends to increase inordinately, and that some check is always in +action, yet seldom perceived by us, the whole economy of nature will be utterly +obscured. Whenever we can precisely say why this species is more abundant in +individuals than that; why this species and not another can be naturalised in a +given country; then, and not till then, we may justly feel surprise why we +cannot account for the extinction of this particular species or group of +species. + +On the Forms of Life changing almost simultaneously throughout the World +.—Scarcely any palæontological discovery is more striking than the fact, that +the forms of life change almost simultaneously throughout the world. Thus our +European Chalk formation can be recognised in many distant parts of the world, +under the most different climates, where not a fragment of the mineral chalk +itself can be found; namely, in North America, in equatorial South America, in +Tierra del Fuego, at the Cape of Good Hope, and in the peninsula of India. For +at these distant points, the organic remains in certain beds present an +unmistakeable degree of resemblance to those of the Chalk. It is not that the +same species are met with; for in some cases not one species is identically the +same, but they belong to the same families, genera, and sections of genera, and +sometimes are similarly characterised in such trifling points as mere +superficial sculpture. Moreover other forms, which are not found in the Chalk +of Europe, but which occur in the formations either above or below, are +similarly absent at these distant points of the world. In the several +successive palæozoic formations of Russia, Western Europe and North America, a +similar parallelism in the forms of life has been observed by several authors: +so it is, according to Lyell, with the several European and North American +tertiary deposits. Even if the few fossil species which are common to the Old +and New Worlds be kept wholly out of view, the general parallelism in the +successive forms of life, in the stages of the widely separated palæozoic and +tertiary periods, would still be manifest, and the several formations could be +easily correlated. + +These observations, however, relate to the marine inhabitants of distant parts +of the world: we have not sufficient data to judge whether the productions of +the land and of fresh water change at distant points in the same parallel +manner. We may doubt whether they have thus changed: if the Megatherium, +Mylodon, Macrauchenia, and Toxodon had been brought to Europe from La Plata, +without any information in regard to their geological position, no one would +have suspected that they had coexisted with still living sea-shells; but as +these anomalous monsters coexisted with the Mastodon and Horse, it might at +least have been inferred that they had lived during one of the latter tertiary +stages. + +When the marine forms of life are spoken of as having changed simultaneously +throughout the world, it must not be supposed that this expression relates to +the same thousandth or hundred-thousandth year, or even that it has a very +strict geological sense; for if all the marine animals which live at the +present day in Europe, and all those that lived in Europe during the +pleistocene period (an enormously remote period as measured by years, including +the whole glacial epoch), were to be compared with those now living in South +America or in Australia, the most skilful naturalist would hardly be able to +say whether the existing or the pleistocene inhabitants of Europe resembled +most closely those of the southern hemisphere. So, again, several highly +competent observers believe that the existing productions of the United States +are more closely related to those which lived in Europe during certain later +tertiary stages, than to those which now live here; and if this be so, it is +evident that fossiliferous beds deposited at the present day on the shores of +North America would hereafter be liable to be classed with somewhat older +European beds. Nevertheless, looking to a remotely future epoch, there can, I +think, be little doubt that all the more modern marine formations, namely, the +upper pliocene, the pleistocene and strictly modern beds, of Europe, North and +South America, and Australia, from containing fossil remains in some degree +allied, and from not including those forms which are only found in the older +underlying deposits, would be correctly ranked as simultaneous in a geological +sense. + +The fact of the forms of life changing simultaneously, in the above large +sense, at distant parts of the world, has greatly struck those admirable +observers, MM. de Verneuil and d’Archiac. After referring to the parallelism of +the palæozoic forms of life in various parts of Europe, they add, “If struck by +this strange sequence, we turn our attention to North America, and there +discover a series of analogous phenomena, it will appear certain that all these +modifications of species, their extinction, and the introduction of new ones, +cannot be owing to mere changes in marine currents or other causes more or less +local and temporary, but depend on general laws which govern the whole animal +kingdom.” M. Barrande has made forcible remarks to precisely the same effect. +It is, indeed, quite futile to look to changes of currents, climate, or other +physical conditions, as the cause of these great mutations in the forms of life +throughout the world, under the most different climates. We must, as Barrande +has remarked, look to some special law. We shall see this more clearly when we +treat of the present distribution of organic beings, and find how slight is the +relation between the physical conditions of various countries, and the nature +of their inhabitants. + +This great fact of the parallel succession of the forms of life throughout the +world, is explicable on the theory of natural selection. New species are formed +by new varieties arising, which have some advantage over older forms; and those +forms, which are already dominant, or have some advantage over the other forms +in their own country, would naturally oftenest give rise to new varieties or +incipient species; for these latter must be victorious in a still higher degree +in order to be preserved and to survive. We have distinct evidence on this +head, in the plants which are dominant, that is, which are commonest in their +own homes, and are most widely diffused, having produced the greatest number of +new varieties. It is also natural that the dominant, varying, and far-spreading +species, which already have invaded to a certain extent the territories of +other species, should be those which would have the best chance of spreading +still further, and of giving rise in new countries to new varieties and +species. The process of diffusion may often be very slow, being dependent on +climatal and geographical changes, or on strange accidents, but in the long run +the dominant forms will generally succeed in spreading. The diffusion would, it +is probable, be slower with the terrestrial inhabitants of distinct continents +than with the marine inhabitants of the continuous sea. We might therefore +expect to find, as we apparently do find, a less strict degree of parallel +succession in the productions of the land than of the sea. + +Dominant species spreading from any region might encounter still more dominant +species, and then their triumphant course, or even their existence, would +cease. We know not at all precisely what are all the conditions most favourable +for the multiplication of new and dominant species; but we can, I think, +clearly see that a number of individuals, from giving a better chance of the +appearance of favourable variations, and that severe competition with many +already existing forms, would be highly favourable, as would be the power of +spreading into new territories. A certain amount of isolation, recurring at +long intervals of time, would probably be also favourable, as before explained. +One quarter of the world may have been most favourable for the production of +new and dominant species on the land, and another for those in the waters of +the sea. If two great regions had been for a long period favourably +circumstanced in an equal degree, whenever their inhabitants met, the battle +would be prolonged and severe; and some from one birthplace and some from the +other might be victorious. But in the course of time, the forms dominant in the +highest degree, wherever produced, would tend everywhere to prevail. As they +prevailed, they would cause the extinction of other and inferior forms; and as +these inferior forms would be allied in groups by inheritance, whole groups +would tend slowly to disappear; though here and there a single member might +long be enabled to survive. + +Thus, as it seems to me, the parallel, and, taken in a large sense, +simultaneous, succession of the same forms of life throughout the world, +accords well with the principle of new species having been formed by dominant +species spreading widely and varying; the new species thus produced being +themselves dominant owing to inheritance, and to having already had some +advantage over their parents or over other species; these again spreading, +varying, and producing new species. The forms which are beaten and which yield +their places to the new and victorious forms, will generally be allied in +groups, from inheriting some inferiority in common; and therefore as new and +improved groups spread throughout the world, old groups will disappear from the +world; and the succession of forms in both ways will everywhere tend to +correspond. + +There is one other remark connected with this subject worth making. I have +given my reasons for believing that all our greater fossiliferous formations +were deposited during periods of subsidence; and that blank intervals of vast +duration occurred during the periods when the bed of the sea was either +stationary or rising, and likewise when sediment was not thrown down quickly +enough to embed and preserve organic remains. During these long and blank +intervals I suppose that the inhabitants of each region underwent a +considerable amount of modification and extinction, and that there was much +migration from other parts of the world. As we have reason to believe that +large areas are affected by the same movement, it is probable that strictly +contemporaneous formations have often been accumulated over very wide spaces in +the same quarter of the world; but we are far from having any right to conclude +that this has invariably been the case, and that large areas have invariably +been affected by the same movements. When two formations have been deposited in +two regions during nearly, but not exactly the same period, we should find in +both, from the causes explained in the foregoing paragraphs, the same general +succession in the forms of life; but the species would not exactly correspond; +for there will have been a little more time in the one region than in the other +for modification, extinction, and immigration. + +I suspect that cases of this nature have occurred in Europe. Mr. Prestwich, in +his admirable Memoirs on the eocene deposits of England and France, is able to +draw a close general parallelism between the successive stages in the two +countries; but when he compares certain stages in England with those in France, +although he finds in both a curious accordance in the numbers of the species +belonging to the same genera, yet the species themselves differ in a manner +very difficult to account for, considering the proximity of the two +areas,—unless, indeed, it be assumed that an isthmus separated two seas +inhabited by distinct, but contemporaneous, faunas. Lyell has made similar +observations on some of the later tertiary formations. Barrande, also, shows +that there is a striking general parallelism in the successive Silurian +deposits of Bohemia and Scandinavia; nevertheless he finds a surprising amount +of difference in the species. If the several formations in these regions have +not been deposited during the same exact periods,—a formation in one region +often corresponding with a blank interval in the other,—and if in both regions +the species have gone on slowly changing during the accumulation of the several +formations and during the long intervals of time between them; in this case, +the several formations in the two regions could be arranged in the same order, +in accordance with the general succession of the form of life, and the order +would falsely appear to be strictly parallel; nevertheless the species would +not all be the same in the apparently corresponding stages in the two regions. + +On the Affinities of extinct Species to each other, and to living forms.—Let us +now look to the mutual affinities of extinct and living species. They all fall +into one grand natural system; and this fact is at once explained on the +principle of descent. The more ancient any form is, the more, as a general +rule, it differs from living forms. But, as Buckland long ago remarked, all +fossils can be classed either in still existing groups, or between them. That +the extinct forms of life help to fill up the wide intervals between existing +genera, families, and orders, cannot be disputed. For if we confine our +attention either to the living or to the extinct alone, the series is far less +perfect than if we combine both into one general system. With respect to the +Vertebrata, whole pages could be filled with striking illustrations from our +great palæontologist, Owen, showing how extinct animals fall in between +existing groups. Cuvier ranked the Ruminants and Pachyderms, as the two most +distinct orders of mammals; but Owen has discovered so many fossil links, that +he has had to alter the whole classification of these two orders; and has +placed certain pachyderms in the same sub-order with ruminants: for example, he +dissolves by fine gradations the apparently wide difference between the pig and +the camel. In regard to the Invertebrata, Barrande, and a higher authority +could not be named, asserts that he is every day taught that palæozoic animals, +though belonging to the same orders, families, or genera with those living at +the present day, were not at this early epoch limited in such distinct groups +as they now are. + +Some writers have objected to any extinct species or group of species being +considered as intermediate between living species or groups. If by this term it +is meant that an extinct form is directly intermediate in all its characters +between two living forms, the objection is probably valid. But I apprehend that +in a perfectly natural classification many fossil species would have to stand +between living species, and some extinct genera between living genera, even +between genera belonging to distinct families. The most common case, especially +with respect to very distinct groups, such as fish and reptiles, seems to be, +that supposing them to be distinguished at the present day from each other by a +dozen characters, the ancient members of the same two groups would be +distinguished by a somewhat lesser number of characters, so that the two +groups, though formerly quite distinct, at that period made some small approach +to each other. + +It is a common belief that the more ancient a form is, by so much the more it +tends to connect by some of its characters groups now widely separated from +each other. This remark no doubt must be restricted to those groups which have +undergone much change in the course of geological ages; and it would be +difficult to prove the truth of the proposition, for every now and then even a +living animal, as the Lepidosiren, is discovered having affinities directed +towards very distinct groups. Yet if we compare the older Reptiles and +Batrachians, the older Fish, the older Cephalopods, and the eocene Mammals, +with the more recent members of the same classes, we must admit that there is +some truth in the remark. + +Let us see how far these several facts and inferences accord with the theory of +descent with modification. As the subject is somewhat complex, I must request +the reader to turn to the diagram in the fourth chapter. We may suppose that +the numbered letters represent genera, and the dotted lines diverging from them +the species in each genus. The diagram is much too simple, too few genera and +too few species being given, but this is unimportant for us. The horizontal +lines may represent successive geological formations, and all the forms beneath +the uppermost line may be considered as extinct. The three existing genera, a^ +14, q^14, p^14, will form a small family; b^14 and f^14 a closely allied family +or sub-family; and o^14, e^14, m^14, a third family. These three families, +together with the many extinct genera on the several lines of descent diverging +from the parent-form A, will form an order; for all will have inherited +something in common from their ancient and common progenitor. On the principle +of the continued tendency to divergence of character, which was formerly +illustrated by this diagram, the more recent any form is, the more it will +generally differ from its ancient progenitor. Hence we can understand the rule +that the most ancient fossils differ most from existing forms. We must not, +however, assume that divergence of character is a necessary contingency; it +depends solely on the descendants from a species being thus enabled to seize on +many and different places in the economy of nature. Therefore it is quite +possible, as we have seen in the case of some Silurian forms, that a species +might go on being slightly modified in relation to its slightly altered +conditions of life, and yet retain throughout a vast period the same general +characteristics. This is represented in the diagram by the letter F^14. + +All the many forms, extinct and recent, descended from A, make, as before +remarked, one order; and this order, from the continued effects of extinction +and divergence of character, has become divided into several sub-families and +families, some of which are supposed to have perished at different periods, and +some to have endured to the present day. + +By looking at the diagram we can see that if many of the extinct forms, +supposed to be embedded in the successive formations, were discovered at +several points low down in the series, the three existing families on the +uppermost line would be rendered less distinct from each other. If, for +instance, the genera a^1, a^5, a^10, f^8, m^3, m^6, m^9 were disinterred, these +three families would be so closely linked together that they probably would +have to be united into one great family, in nearly the same manner as has +occurred with ruminants and pachyderms. Yet he who objected to call the extinct +genera, which thus linked the living genera of three families together, +intermediate in character, would be justified, as they are intermediate, not +directly, but only by a long and circuitous course through many widely +different forms. If many extinct forms were to be discovered above one of the +middle horizontal lines or geological formations—for instance, above Number +VI.—but none from beneath this line, then only the two families on the left +hand (namely, a^14, etc., and b^14, etc.) would have to be united into one +family; and the two other families (namely, a^14 to f^14 now including five +genera, and o^14 to m^14) would yet remain distinct. These two families, +however, would be less distinct from each other than they were before the +discovery of the fossils. If, for instance, we suppose the existing genera of +the two families to differ from each other by a dozen characters, in this case +the genera, at the early period marked VI., would differ by a lesser number of +characters; for at this early stage of descent they have not diverged in +character from the common progenitor of the order, nearly so much as they +subsequently diverged. Thus it comes that ancient and extinct genera are often +in some slight degree intermediate in character between their modified +descendants, or between their collateral relations. + +In nature the case will be far more complicated than is represented in the +diagram; for the groups will have been more numerous, they will have endured +for extremely unequal lengths of time, and will have been modified in various +degrees. As we possess only the last volume of the geological record, and that +in a very broken condition, we have no right to expect, except in very rare +cases, to fill up wide intervals in the natural system, and thus unite distinct +families or orders. All that we have a right to expect, is that those groups, +which have within known geological periods undergone much modification, should +in the older formations make some slight approach to each other; so that the +older members should differ less from each other in some of their characters +than do the existing members of the same groups; and this by the concurrent +evidence of our best palæontologists seems frequently to be the case. + +Thus, on the theory of descent with modification, the main facts with respect +to the mutual affinities of the extinct forms of life to each other and to +living forms, seem to me explained in a satisfactory manner. And they are +wholly inexplicable on any other view. + +On this same theory, it is evident that the fauna of any great period in the +earth’s history will be intermediate in general character between that which +preceded and that which succeeded it. Thus, the species which lived at the +sixth great stage of descent in the diagram are the modified offspring of those +which lived at the fifth stage, and are the parents of those which became still +more modified at the seventh stage; hence they could hardly fail to be nearly +intermediate in character between the forms of life above and below. We must, +however, allow for the entire extinction of some preceding forms, and for the +coming in of quite new forms by immigration, and for a large amount of +modification, during the long and blank intervals between the successive +formations. Subject to these allowances, the fauna of each geological period +undoubtedly is intermediate in character, between the preceding and succeeding +faunas. I need give only one instance, namely, the manner in which the fossils +of the Devonian system, when this system was first discovered, were at once +recognised by palæontologists as intermediate in character between those of the +overlying carboniferous, and underlying Silurian system. But each fauna is not +necessarily exactly intermediate, as unequal intervals of time have elapsed +between consecutive formations. + +It is no real objection to the truth of the statement, that the fauna of each +period as a whole is nearly intermediate in character between the preceding and +succeeding faunas, that certain genera offer exceptions to the rule. For +instance, mastodons and elephants, when arranged by Dr. Falconer in two series, +first according to their mutual affinities and then according to their periods +of existence, do not accord in arrangement. The species extreme in character +are not the oldest, or the most recent; nor are those which are intermediate in +character, intermediate in age. But supposing for an instant, in this and other +such cases, that the record of the first appearance and disappearance of the +species was perfect, we have no reason to believe that forms successively +produced necessarily endure for corresponding lengths of time: a very ancient +form might occasionally last much longer than a form elsewhere subsequently +produced, especially in the case of terrestrial productions inhabiting +separated districts. To compare small things with great: if the principal +living and extinct races of the domestic pigeon were arranged as well as they +could be in serial affinity, this arrangement would not closely accord with the +order in time of their production, and still less with the order of their +disappearance; for the parent rock-pigeon now lives; and many varieties between +the rock-pigeon and the carrier have become extinct; and carriers which are +extreme in the important character of length of beak originated earlier than +short-beaked tumblers, which are at the opposite end of the series in this same +respect. + +Closely connected with the statement, that the organic remains from an +intermediate formation are in some degree intermediate in character, is the +fact, insisted on by all palæontologists, that fossils from two consecutive +formations are far more closely related to each other, than are the fossils +from two remote formations. Pictet gives as a well-known instance, the general +resemblance of the organic remains from the several stages of the chalk +formation, though the species are distinct in each stage. This fact alone, from +its generality, seems to have shaken Professor Pictet in his firm belief in the +immutability of species. He who is acquainted with the distribution of existing +species over the globe, will not attempt to account for the close resemblance +of the distinct species in closely consecutive formations, by the physical +conditions of the ancient areas having remained nearly the same. Let it be +remembered that the forms of life, at least those inhabiting the sea, have +changed almost simultaneously throughout the world, and therefore under the +most different climates and conditions. Consider the prodigious vicissitudes of +climate during the pleistocene period, which includes the whole glacial period, +and note how little the specific forms of the inhabitants of the sea have been +affected. + +On the theory of descent, the full meaning of the fact of fossil remains from +closely consecutive formations, though ranked as distinct species, being +closely related, is obvious. As the accumulation of each formation has often +been interrupted, and as long blank intervals have intervened between +successive formations, we ought not to expect to find, as I attempted to show +in the last chapter, in any one or two formations all the intermediate +varieties between the species which appeared at the commencement and close of +these periods; but we ought to find after intervals, very long as measured by +years, but only moderately long as measured geologically, closely allied forms, +or, as they have been called by some authors, representative species; and these +we assuredly do find. We find, in short, such evidence of the slow and scarcely +sensible mutation of specific forms, as we have a just right to expect to find. + +On the state of Development of Ancient Forms.—There has been much discussion +whether recent forms are more highly developed than ancient. I will not here +enter on this subject, for naturalists have not as yet defined to each other’s +satisfaction what is meant by high and low forms. But in one particular sense +the more recent forms must, on my theory, be higher than the more ancient; for +each new species is formed by having had some advantage in the struggle for +life over other and preceding forms. If under a nearly similar climate, the +eocene inhabitants of one quarter of the world were put into competition with +the existing inhabitants of the same or some other quarter, the eocene fauna or +flora would certainly be beaten and exterminated; as would a secondary fauna by +an eocene, and a palæozoic fauna by a secondary fauna. I do not doubt that this +process of improvement has affected in a marked and sensible manner the +organisation of the more recent and victorious forms of life, in comparison +with the ancient and beaten forms; but I can see no way of testing this sort of +progress. Crustaceans, for instance, not the highest in their own class, may +have beaten the highest molluscs. From the extraordinary manner in which +European productions have recently spread over New Zealand, and have seized on +places which must have been previously occupied, we may believe, if all the +animals and plants of Great Britain were set free in New Zealand, that in the +course of time a multitude of British forms would become thoroughly naturalized +there, and would exterminate many of the natives. On the other hand, from what +we see now occurring in New Zealand, and from hardly a single inhabitant of the +southern hemisphere having become wild in any part of Europe, we may doubt, if +all the productions of New Zealand were set free in Great Britain, whether any +considerable number would be enabled to seize on places now occupied by our +native plants and animals. Under this point of view, the productions of Great +Britain may be said to be higher than those of New Zealand. Yet the most +skilful naturalist from an examination of the species of the two countries +could not have foreseen this result. + +Agassiz insists that ancient animals resemble to a certain extent the embryos +of recent animals of the same classes; or that the geological succession of +extinct forms is in some degree parallel to the embryological development of +recent forms. I must follow Pictet and Huxley in thinking that the truth of +this doctrine is very far from proved. Yet I fully expect to see it hereafter +confirmed, at least in regard to subordinate groups, which have branched off +from each other within comparatively recent times. For this doctrine of Agassiz +accords well with the theory of natural selection. In a future chapter I shall +attempt to show that the adult differs from its embryo, owing to variations +supervening at a not early age, and being inherited at a corresponding age. +This process, whilst it leaves the embryo almost unaltered, continually adds, +in the course of successive generations, more and more difference to the adult. + +Thus the embryo comes to be left as a sort of picture, preserved by nature, of +the ancient and less modified condition of each animal. This view may be true, +and yet it may never be capable of full proof. Seeing, for instance, that the +oldest known mammals, reptiles, and fish strictly belong to their own proper +classes, though some of these old forms are in a slight degree less distinct +from each other than are the typical members of the same groups at the present +day, it would be vain to look for animals having the common embryological +character of the Vertebrata, until beds far beneath the lowest Silurian strata +are discovered—a discovery of which the chance is very small. + +On the Succession of the same Types within the same areas, during the later +tertiary periods.—Mr. Clift many years ago showed that the fossil mammals from +the Australian caves were closely allied to the living marsupials of that +continent. In South America, a similar relationship is manifest, even to an +uneducated eye, in the gigantic pieces of armour like those of the armadillo, +found in several parts of La Plata; and Professor Owen has shown in the most +striking manner that most of the fossil mammals, buried there in such numbers, +are related to South American types. This relationship is even more clearly +seen in the wonderful collection of fossil bones made by MM. Lund and Clausen +in the caves of Brazil. I was so much impressed with these facts that I +strongly insisted, in 1839 and 1845, on this “law of the succession of +types,”—on “this wonderful relationship in the same continent between the dead +and the living.” Professor Owen has subsequently extended the same +generalisation to the mammals of the Old World. We see the same law in this +author’s restorations of the extinct and gigantic birds of New Zealand. We see +it also in the birds of the caves of Brazil. Mr. Woodward has shown that the +same law holds good with sea-shells, but from the wide distribution of most +genera of molluscs, it is not well displayed by them. Other cases could be +added, as the relation between the extinct and living land-shells of Madeira; +and between the extinct and living brackish-water shells of the Aralo-Caspian +Sea. + +Now what does this remarkable law of the succession of the same types within +the same areas mean? He would be a bold man, who after comparing the present +climate of Australia and of parts of South America under the same latitude, +would attempt to account, on the one hand, by dissimilar physical conditions +for the dissimilarity of the inhabitants of these two continents, and, on the +other hand, by similarity of conditions, for the uniformity of the same types +in each during the later tertiary periods. Nor can it be pretended that it is +an immutable law that marsupials should have been chiefly or solely produced in +Australia; or that Edentata and other American types should have been solely +produced in South America. For we know that Europe in ancient times was peopled +by numerous marsupials; and I have shown in the publications above alluded to, +that in America the law of distribution of terrestrial mammals was formerly +different from what it now is. North America formerly partook strongly of the +present character of the southern half of the continent; and the southern half +was formerly more closely allied, than it is at present, to the northern half. +In a similar manner we know from Falconer and Cautley’s discoveries, that +northern India was formerly more closely related in its mammals to Africa than +it is at the present time. Analogous facts could be given in relation to the +distribution of marine animals. + +On the theory of descent with modification, the great law of the long enduring, +but not immutable, succession of the same types within the same areas, is at +once explained; for the inhabitants of each quarter of the world will obviously +tend to leave in that quarter, during the next succeeding period of time, +closely allied though in some degree modified descendants. If the inhabitants +of one continent formerly differed greatly from those of another continent, so +will their modified descendants still differ in nearly the same manner and +degree. But after very long intervals of time and after great geographical +changes, permitting much inter-migration, the feebler will yield to the more +dominant forms, and there will be nothing immutable in the laws of past and +present distribution. + +It may be asked in ridicule, whether I suppose that the megatherium and other +allied huge monsters have left behind them in South America the sloth, +armadillo, and anteater, as their degenerate descendants. This cannot for an +instant be admitted. These huge animals have become wholly extinct, and have +left no progeny. But in the caves of Brazil, there are many extinct species +which are closely allied in size and in other characters to the species still +living in South America; and some of these fossils may be the actual +progenitors of living species. It must not be forgotten that, on my theory, all +the species of the same genus have descended from some one species; so that if +six genera, each having eight species, be found in one geological formation, +and in the next succeeding formation there be six other allied or +representative genera with the same number of species, then we may conclude +that only one species of each of the six older genera has left modified +descendants, constituting the six new genera. The other seven species of the +old genera have all died out and have left no progeny. Or, which would probably +be a far commoner case, two or three species of two or three alone of the six +older genera will have been the parents of the six new genera; the other old +species and the other whole genera having become utterly extinct. In failing +orders, with the genera and species decreasing in numbers, as apparently is the +case of the Edentata of South America, still fewer genera and species will have +left modified blood-descendants. + +Summary of the preceding and present Chapters.—I have attempted to show that +the geological record is extremely imperfect; that only a small portion of the +globe has been geologically explored with care; that only certain classes of +organic beings have been largely preserved in a fossil state; that the number +both of specimens and of species, preserved in our museums, is absolutely as +nothing compared with the incalculable number of generations which must have +passed away even during a single formation; that, owing to subsidence being +necessary for the accumulation of fossiliferous deposits thick enough to resist +future degradation, enormous intervals of time have elapsed between the +successive formations; that there has probably been more extinction during the +periods of subsidence, and more variation during the periods of elevation, and +during the latter the record will have been least perfectly kept; that each +single formation has not been continuously deposited; that the duration of each +formation is, perhaps, short compared with the average duration of specific +forms; that migration has played an important part in the first appearance of +new forms in any one area and formation; that widely ranging species are those +which have varied most, and have oftenest given rise to new species; and that +varieties have at first often been local. All these causes taken conjointly, +must have tended to make the geological record extremely imperfect, and will to +a large extent explain why we do not find interminable varieties, connecting +together all the extinct and existing forms of life by the finest graduated +steps. + +He who rejects these views on the nature of the geological record, will rightly +reject my whole theory. For he may ask in vain where are the numberless +transitional links which must formerly have connected the closely allied or +representative species, found in the several stages of the same great +formation. He may disbelieve in the enormous intervals of time which have +elapsed between our consecutive formations; he may overlook how important a +part migration must have played, when the formations of any one great region +alone, as that of Europe, are considered; he may urge the apparent, but often +falsely apparent, sudden coming in of whole groups of species. He may ask where +are the remains of those infinitely numerous organisms which must have existed +long before the first bed of the Silurian system was deposited: I can answer +this latter question only hypothetically, by saying that as far as we can see, +where our oceans now extend they have for an enormous period extended, and +where our oscillating continents now stand they have stood ever since the +Silurian epoch; but that long before that period, the world may have presented +a wholly different aspect; and that the older continents, formed of formations +older than any known to us, may now all be in a metamorphosed condition, or may +lie buried under the ocean. + +Passing from these difficulties, all the other great leading facts in +palæontology seem to me simply to follow on the theory of descent with +modification through natural selection. We can thus understand how it is that +new species come in slowly and successively; how species of different classes +do not necessarily change together, or at the same rate, or in the same degree; +yet in the long run that all undergo modification to some extent. The +extinction of old forms is the almost inevitable consequence of the production +of new forms. We can understand why when a species has once disappeared it +never reappears. Groups of species increase in numbers slowly, and endure for +unequal periods of time; for the process of modification is necessarily slow, +and depends on many complex contingencies. The dominant species of the larger +dominant groups tend to leave many modified descendants, and thus new +sub-groups and groups are formed. As these are formed, the species of the less +vigorous groups, from their inferiority inherited from a common progenitor, +tend to become extinct together, and to leave no modified offspring on the face +of the earth. But the utter extinction of a whole group of species may often be +a very slow process, from the survival of a few descendants, lingering in +protected and isolated situations. When a group has once wholly disappeared, it +does not reappear; for the link of generation has been broken. + +We can understand how the spreading of the dominant forms of life, which are +those that oftenest vary, will in the long run tend to people the world with +allied, but modified, descendants; and these will generally succeed in taking +the places of those groups of species which are their inferiors in the struggle +for existence. Hence, after long intervals of time, the productions of the +world will appear to have changed simultaneously. + +We can understand how it is that all the forms of life, ancient and recent, +make together one grand system; for all are connected by generation. We can +understand, from the continued tendency to divergence of character, why the +more ancient a form is, the more it generally differs from those now living. +Why ancient and extinct forms often tend to fill up gaps between existing +forms, sometimes blending two groups previously classed as distinct into one; +but more commonly only bringing them a little closer together. The more ancient +a form is, the more often, apparently, it displays characters in some degree +intermediate between groups now distinct; for the more ancient a form is, the +more nearly it will be related to, and consequently resemble, the common +progenitor of groups, since become widely divergent. Extinct forms are seldom +directly intermediate between existing forms; but are intermediate only by a +long and circuitous course through many extinct and very different forms. We +can clearly see why the organic remains of closely consecutive formations are +more closely allied to each other, than are those of remote formations; for the +forms are more closely linked together by generation: we can clearly see why +the remains of an intermediate formation are intermediate in character. + +The inhabitants of each successive period in the world’s history have beaten +their predecessors in the race for life, and are, in so far, higher in the +scale of nature; and this may account for that vague yet ill-defined sentiment, +felt by many palæontologists, that organisation on the whole has progressed. If +it should hereafter be proved that ancient animals resemble to a certain extent +the embryos of more recent animals of the same class, the fact will be +intelligible. The succession of the same types of structure within the same +areas during the later geological periods ceases to be mysterious, and is +simply explained by inheritance. + +If then the geological record be as imperfect as I believe it to be, and it may +at least be asserted that the record cannot be proved to be much more perfect, +the main objections to the theory of natural selection are greatly diminished +or disappear. On the other hand, all the chief laws of palæontology plainly +proclaim, as it seems to me, that species have been produced by ordinary +generation: old forms having been supplanted by new and improved forms of life, +produced by the laws of variation still acting round us, and preserved by +Natural Selection. + +CHAPTER XI. +GEOGRAPHICAL DISTRIBUTION. + +Present distribution cannot be accounted for by differences in physical +conditions. Importance of barriers. Affinity of the productions of the same +continent. Centres of creation. Means of dispersal, by changes of climate and +of the level of the land, and by occasional means. Dispersal during the Glacial +period co-extensive with the world. + +In considering the distribution of organic beings over the face of the globe, +the first great fact which strikes us is, that neither the similarity nor the +dissimilarity of the inhabitants of various regions can be accounted for by +their climatal and other physical conditions. Of late, almost every author who +has studied the subject has come to this conclusion. The case of America alone +would almost suffice to prove its truth: for if we exclude the northern parts +where the circumpolar land is almost continuous, all authors agree that one of +the most fundamental divisions in geographical distribution is that between the +New and Old Worlds; yet if we travel over the vast American continent, from the +central parts of the United States to its extreme southern point, we meet with +the most diversified conditions; the most humid districts, arid deserts, lofty +mountains, grassy plains, forests, marshes, lakes, and great rivers, under +almost every temperature. There is hardly a climate or condition in the Old +World which cannot be paralleled in the New—at least as closely as the same +species generally require; for it is a most rare case to find a group of +organisms confined to any small spot, having conditions peculiar in only a +slight degree; for instance, small areas in the Old World could be pointed out +hotter than any in the New World, yet these are not inhabited by a peculiar +fauna or flora. Notwithstanding this parallelism in the conditions of the Old +and New Worlds, how widely different are their living productions! + +In the southern hemisphere, if we compare large tracts of land in Australia, +South Africa, and western South America, between latitudes 25° and 35°, we +shall find parts extremely similar in all their conditions, yet it would not be +possible to point out three faunas and floras more utterly dissimilar. Or again +we may compare the productions of South America south of lat. 35° with those +north of 25°, which consequently inhabit a considerably different climate, and +they will be found incomparably more closely related to each other, than they +are to the productions of Australia or Africa under nearly the same climate. +Analogous facts could be given with respect to the inhabitants of the sea. + +A second great fact which strikes us in our general review is, that barriers of +any kind, or obstacles to free migration, are related in a close and important +manner to the differences between the productions of various regions. We see +this in the great difference of nearly all the terrestrial productions of the +New and Old Worlds, excepting in the northern parts, where the land almost +joins, and where, under a slightly different climate, there might have been +free migration for the northern temperate forms, as there now is for the +strictly arctic productions. We see the same fact in the great difference +between the inhabitants of Australia, Africa, and South America under the same +latitude: for these countries are almost as much isolated from each other as is +possible. On each continent, also, we see the same fact; for on the opposite +sides of lofty and continuous mountain-ranges, and of great deserts, and +sometimes even of large rivers, we find different productions; though as +mountain chains, deserts, etc., are not as impassable, or likely to have +endured so long as the oceans separating continents, the differences are very +inferior in degree to those characteristic of distinct continents. + +Turning to the sea, we find the same law. No two marine faunas are more +distinct, with hardly a fish, shell, or crab in common, than those of the +eastern and western shores of South and Central America; yet these great faunas +are separated only by the narrow, but impassable, isthmus of Panama. Westward +of the shores of America, a wide space of open ocean extends, with not an +island as a halting-place for emigrants; here we have a barrier of another +kind, and as soon as this is passed we meet in the eastern islands of the +Pacific, with another and totally distinct fauna. So that here three marine +faunas range far northward and southward, in parallel lines not far from each +other, under corresponding climates; but from being separated from each other +by impassable barriers, either of land or open sea, they are wholly distinct. +On the other hand, proceeding still further westward from the eastern islands +of the tropical parts of the Pacific, we encounter no impassable barriers, and +we have innumerable islands as halting-places, until after travelling over a +hemisphere we come to the shores of Africa; and over this vast space we meet +with no well-defined and distinct marine faunas. Although hardly one shell, +crab or fish is common to the above-named three approximate faunas of Eastern +and Western America and the eastern Pacific islands, yet many fish range from +the Pacific into the Indian Ocean, and many shells are common to the eastern +islands of the Pacific and the eastern shores of Africa, on almost exactly +opposite meridians of longitude. + +A third great fact, partly included in the foregoing statements, is the +affinity of the productions of the same continent or sea, though the species +themselves are distinct at different points and stations. It is a law of the +widest generality, and every continent offers innumerable instances. +Nevertheless the naturalist in travelling, for instance, from north to south +never fails to be struck by the manner in which successive groups of beings, +specifically distinct, yet clearly related, replace each other. He hears from +closely allied, yet distinct kinds of birds, notes nearly similar, and sees +their nests similarly constructed, but not quite alike, with eggs coloured in +nearly the same manner. The plains near the Straits of Magellan are inhabited +by one species of Rhea (American ostrich), and northward the plains of La Plata +by another species of the same genus; and not by a true ostrich or emeu, like +those found in Africa and Australia under the same latitude. On these same +plains of La Plata, we see the agouti and bizcacha, animals having nearly the +same habits as our hares and rabbits and belonging to the same order of +Rodents, but they plainly display an American type of structure. We ascend the +lofty peaks of the Cordillera and we find an alpine species of bizcacha; we +look to the waters, and we do not find the beaver or musk-rat, but the coypu +and capybara, rodents of the American type. Innumerable other instances could +be given. If we look to the islands off the American shore, however much they +may differ in geological structure, the inhabitants, though they may be all +peculiar species, are essentially American. We may look back to past ages, as +shown in the last chapter, and we find American types then prevalent on the +American continent and in the American seas. We see in these facts some deep +organic bond, prevailing throughout space and time, over the same areas of land +and water, and independent of their physical conditions. The naturalist must +feel little curiosity, who is not led to inquire what this bond is. + +This bond, on my theory, is simply inheritance, that cause which alone, as far +as we positively know, produces organisms quite like, or, as we see in the case +of varieties nearly like each other. The dissimilarity of the inhabitants of +different regions may be attributed to modification through natural selection, +and in a quite subordinate degree to the direct influence of different physical +conditions. The degree of dissimilarity will depend on the migration of the +more dominant forms of life from one region into another having been effected +with more or less ease, at periods more or less remote;—on the nature and +number of the former immigrants;—and on their action and reaction, in their +mutual struggles for life;—the relation of organism to organism being, as I +have already often remarked, the most important of all relations. Thus the high +importance of barriers comes into play by checking migration; as does time for +the slow process of modification through natural selection. Widely-ranging +species, abounding in individuals, which have already triumphed over many +competitors in their own widely-extended homes will have the best chance of +seizing on new places, when they spread into new countries. In their new homes +they will be exposed to new conditions, and will frequently undergo further +modification and improvement; and thus they will become still further +victorious, and will produce groups of modified descendants. On this principle +of inheritance with modification, we can understand how it is that sections of +genera, whole genera, and even families are confined to the same areas, as is +so commonly and notoriously the case. + +I believe, as was remarked in the last chapter, in no law of necessary +development. As the variability of each species is an independent property, and +will be taken advantage of by natural selection, only so far as it profits the +individual in its complex struggle for life, so the degree of modification in +different species will be no uniform quantity. If, for instance, a number of +species, which stand in direct competition with each other, migrate in a body +into a new and afterwards isolated country, they will be little liable to +modification; for neither migration nor isolation in themselves can do +anything. These principles come into play only by bringing organisms into new +relations with each other, and in a lesser degree with the surrounding physical +conditions. As we have seen in the last chapter that some forms have retained +nearly the same character from an enormously remote geological period, so +certain species have migrated over vast spaces, and have not become greatly +modified. + +On these views, it is obvious, that the several species of the same genus, +though inhabiting the most distant quarters of the world, must originally have +proceeded from the same source, as they have descended from the same +progenitor. In the case of those species, which have undergone during whole +geological periods but little modification, there is not much difficulty in +believing that they may have migrated from the same region; for during the vast +geographical and climatal changes which will have supervened since ancient +times, almost any amount of migration is possible. But in many other cases, in +which we have reason to believe that the species of a genus have been produced +within comparatively recent times, there is great difficulty on this head. It +is also obvious that the individuals of the same species, though now inhabiting +distant and isolated regions, must have proceeded from one spot, where their +parents were first produced: for, as explained in the last chapter, it is +incredible that individuals identically the same should ever have been produced +through natural selection from parents specifically distinct. + +We are thus brought to the question which has been largely discussed by +naturalists, namely, whether species have been created at one or more points of +the earth’s surface. Undoubtedly there are very many cases of extreme +difficulty, in understanding how the same species could possibly have migrated +from some one point to the several distant and isolated points, where now +found. Nevertheless the simplicity of the view that each species was first +produced within a single region captivates the mind. He who rejects it, rejects +the vera causa of ordinary generation with subsequent migration, and calls in +the agency of a miracle. It is universally admitted, that in most cases the +area inhabited by a species is continuous; and when a plant or animal inhabits +two points so distant from each other, or with an interval of such a nature, +that the space could not be easily passed over by migration, the fact is given +as something remarkable and exceptional. The capacity of migrating across the +sea is more distinctly limited in terrestrial mammals, than perhaps in any +other organic beings; and, accordingly, we find no inexplicable cases of the +same mammal inhabiting distant points of the world. No geologist will feel any +difficulty in such cases as Great Britain having been formerly united to +Europe, and consequently possessing the same quadrupeds. But if the same +species can be produced at two separate points, why do we not find a single +mammal common to Europe and Australia or South America? The conditions of life +are nearly the same, so that a multitude of European animals and plants have +become naturalised in America and Australia; and some of the aboriginal plants +are identically the same at these distant points of the northern and southern +hemispheres? The answer, as I believe, is, that mammals have not been able to +migrate, whereas some plants, from their varied means of dispersal, have +migrated across the vast and broken interspace. The great and striking +influence which barriers of every kind have had on distribution, is +intelligible only on the view that the great majority of species have been +produced on one side alone, and have not been able to migrate to the other +side. Some few families, many sub-families, very many genera, and a still +greater number of sections of genera are confined to a single region; and it +has been observed by several naturalists, that the most natural genera, or +those genera in which the species are most closely related to each other, are +generally local, or confined to one area. What a strange anomaly it would be, +if, when coming one step lower in the series, to the individuals of the same +species, a directly opposite rule prevailed; and species were not local, but +had been produced in two or more distinct areas! + +Hence it seems to me, as it has to many other naturalists, that the view of +each species having been produced in one area alone, and having subsequently +migrated from that area as far as its powers of migration and subsistence under +past and present conditions permitted, is the most probable. Undoubtedly many +cases occur, in which we cannot explain how the same species could have passed +from one point to the other. But the geographical and climatal changes, which +have certainly occurred within recent geological times, must have interrupted +or rendered discontinuous the formerly continuous range of many species. So +that we are reduced to consider whether the exceptions to continuity of range +are so numerous and of so grave a nature, that we ought to give up the belief, +rendered probable by general considerations, that each species has been +produced within one area, and has migrated thence as far as it could. It would +be hopelessly tedious to discuss all the exceptional cases of the same species, +now living at distant and separated points; nor do I for a moment pretend that +any explanation could be offered of many such cases. But after some preliminary +remarks, I will discuss a few of the most striking classes of facts; namely, +the existence of the same species on the summits of distant mountain-ranges, +and at distant points in the arctic and antarctic regions; and secondly (in the +following chapter), the wide distribution of freshwater productions; and +thirdly, the occurrence of the same terrestrial species on islands and on the +mainland, though separated by hundreds of miles of open sea. If the existence +of the same species at distant and isolated points of the earth’s surface, can +in many instances be explained on the view of each species having migrated from +a single birthplace; then, considering our ignorance with respect to former +climatal and geographical changes and various occasional means of transport, +the belief that this has been the universal law, seems to me incomparably the +safest. + +In discussing this subject, we shall be enabled at the same time to consider a +point equally important for us, namely, whether the several distinct species of +a genus, which on my theory have all descended from a common progenitor, can +have migrated (undergoing modification during some part of their migration) +from the area inhabited by their progenitor. If it can be shown to be almost +invariably the case, that a region, of which most of its inhabitants are +closely related to, or belong to the same genera with the species of a second +region, has probably received at some former period immigrants from this other +region, my theory will be strengthened; for we can clearly understand, on the +principle of modification, why the inhabitants of a region should be related to +those of another region, whence it has been stocked. A volcanic island, for +instance, upheaved and formed at the distance of a few hundreds of miles from a +continent, would probably receive from it in the course of time a few +colonists, and their descendants, though modified, would still be plainly +related by inheritance to the inhabitants of the continent. Cases of this +nature are common, and are, as we shall hereafter more fully see, inexplicable +on the theory of independent creation. This view of the relation of species in +one region to those in another, does not differ much (by substituting the word +variety for species) from that lately advanced in an ingenious paper by Mr. +Wallace, in which he concludes, that “every species has come into existence +coincident both in space and time with a pre-existing closely allied species.” +And I now know from correspondence, that this coincidence he attributes to +generation with modification. + +The previous remarks on “single and multiple centres of creation” do not +directly bear on another allied question,—namely whether all the individuals of +the same species have descended from a single pair, or single hermaphrodite, or +whether, as some authors suppose, from many individuals simultaneously created. +With those organic beings which never intercross (if such exist), the species, +on my theory, must have descended from a succession of improved varieties, +which will never have blended with other individuals or varieties, but will +have supplanted each other; so that, at each successive stage of modification +and improvement, all the individuals of each variety will have descended from a +single parent. But in the majority of cases, namely, with all organisms which +habitually unite for each birth, or which often intercross, I believe that +during the slow process of modification the individuals of the species will +have been kept nearly uniform by intercrossing; so that many individuals will +have gone on simultaneously changing, and the whole amount of modification will +not have been due, at each stage, to descent from a single parent. To +illustrate what I mean: our English racehorses differ slightly from the horses +of every other breed; but they do not owe their difference and superiority to +descent from any single pair, but to continued care in selecting and training +many individuals during many generations. + +Before discussing the three classes of facts, which I have selected as +presenting the greatest amount of difficulty on the theory of “single centres +of creation,” I must say a few words on the means of dispersal. + +Means of Dispersal.—Sir C. Lyell and other authors have ably treated this +subject. I can give here only the briefest abstract of the more important +facts. Change of climate must have had a powerful influence on migration: a +region when its climate was different may have been a high road for migration, +but now be impassable; I shall, however, presently have to discuss this branch +of the subject in some detail. Changes of level in the land must also have been +highly influential: a narrow isthmus now separates two marine faunas; submerge +it, or let it formerly have been submerged, and the two faunas will now blend +or may formerly have blended: where the sea now extends, land may at a former +period have connected islands or possibly even continents together, and thus +have allowed terrestrial productions to pass from one to the other. No +geologist will dispute that great mutations of level have occurred within the +period of existing organisms. Edward Forbes insisted that all the islands in +the Atlantic must recently have been connected with Europe or Africa, and +Europe likewise with America. Other authors have thus hypothetically bridged +over every ocean, and have united almost every island to some mainland. If +indeed the arguments used by Forbes are to be trusted, it must be admitted that +scarcely a single island exists which has not recently been united to some +continent. This view cuts the Gordian knot of the dispersal of the same species +to the most distant points, and removes many a difficulty: but to the best of +my judgment we are not authorized in admitting such enormous geographical +changes within the period of existing species. It seems to me that we have +abundant evidence of great oscillations of level in our continents; but not of +such vast changes in their position and extension, as to have united them +within the recent period to each other and to the several intervening oceanic +islands. I freely admit the former existence of many islands, now buried +beneath the sea, which may have served as halting places for plants and for +many animals during their migration. In the coral-producing oceans such sunken +islands are now marked, as I believe, by rings of coral or atolls standing over +them. Whenever it is fully admitted, as I believe it will some day be, that +each species has proceeded from a single birthplace, and when in the course of +time we know something definite about the means of distribution, we shall be +enabled to speculate with security on the former extension of the land. But I +do not believe that it will ever be proved that within the recent period +continents which are now quite separate, have been continuously, or almost +continuously, united with each other, and with the many existing oceanic +islands. Several facts in distribution,—such as the great difference in the +marine faunas on the opposite sides of almost every continent,—the close +relation of the tertiary inhabitants of several lands and even seas to their +present inhabitants,—a certain degree of relation (as we shall hereafter see) +between the distribution of mammals and the depth of the sea,—these and other +such facts seem to me opposed to the admission of such prodigious geographical +revolutions within the recent period, as are necessitated on the view advanced +by Forbes and admitted by his many followers. The nature and relative +proportions of the inhabitants of oceanic islands likewise seem to me opposed +to the belief of their former continuity with continents. Nor does their almost +universally volcanic composition favour the admission that they are the wrecks +of sunken continents;—if they had originally existed as mountain-ranges on the +land, some at least of the islands would have been formed, like other +mountain-summits, of granite, metamorphic schists, old fossiliferous or other +such rocks, instead of consisting of mere piles of volcanic matter. + +I must now say a few words on what are called accidental means, but which more +properly might be called occasional means of distribution. I shall here confine +myself to plants. In botanical works, this or that plant is stated to be ill +adapted for wide dissemination; but for transport across the sea, the greater +or less facilities may be said to be almost wholly unknown. Until I tried, with +Mr. Berkeley’s aid, a few experiments, it was not even known how far seeds +could resist the injurious action of sea-water. To my surprise I found that out +of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an +immersion of 137 days. For convenience sake I chiefly tried small seeds, +without the capsule or fruit; and as all of these sank in a few days, they +could not be floated across wide spaces of the sea, whether or not they were +injured by the salt-water. Afterwards I tried some larger fruits, capsules, +etc., and some of these floated for a long time. It is well known what a +difference there is in the buoyancy of green and seasoned timber; and it +occurred to me that floods might wash down plants or branches, and that these +might be dried on the banks, and then by a fresh rise in the stream be washed +into the sea. Hence I was led to dry stems and branches of 94 plants with ripe +fruit, and to place them on sea water. The majority sank quickly, but some +which whilst green floated for a very short time, when dried floated much +longer; for instance, ripe hazel-nuts sank immediately, but when dried, they +floated for 90 days and afterwards when planted they germinated; an asparagus +plant with ripe berries floated for 23 days, when dried it floated for 85 days, +and the seeds afterwards germinated: the ripe seeds of Helosciadium sank in two +days, when dried they floated for above 90 days, and afterwards germinated. +Altogether out of the 94 dried plants, 18 floated for above 28 days, and some +of the 18 floated for a very much longer period. So that as 64/87 seeds +germinated after an immersion of 28 days; and as 18/94 plants with ripe fruit +(but not all the same species as in the foregoing experiment) floated, after +being dried, for above 28 days, as far as we may infer anything from these +scanty facts, we may conclude that the seeds of 14/100 plants of any country +might be floated by sea-currents during 28 days, and would retain their power +of germination. In Johnston’s Physical Atlas, the average rate of the several +Atlantic currents is 33 miles per diem (some currents running at the rate of 60 +miles per diem); on this average, the seeds of 14/100 plants belonging to one +country might be floated across 924 miles of sea to another country; and when +stranded, if blown to a favourable spot by an inland gale, they would +germinate. + +Subsequently to my experiments, M. Martens tried similar ones, but in a much +better manner, for he placed the seeds in a box in the actual sea, so that they +were alternately wet and exposed to the air like really floating plants. He +tried 98 seeds, mostly different from mine; but he chose many large fruits and +likewise seeds from plants which live near the sea; and this would have +favoured the average length of their flotation and of their resistance to the +injurious action of the salt-water. On the other hand he did not previously dry +the plants or branches with the fruit; and this, as we have seen, would have +caused some of them to have floated much longer. The result was that 18/98 of +his seeds floated for 42 days, and were then capable of germination. But I do +not doubt that plants exposed to the waves would float for a less time than +those protected from violent movement as in our experiments. Therefore it would +perhaps be safer to assume that the seeds of about 10/100 plants of a flora, +after having been dried, could be floated across a space of sea 900 miles in +width, and would then germinate. The fact of the larger fruits often floating +longer than the small, is interesting; as plants with large seeds or fruit +could hardly be transported by any other means; and Alph. de Candolle has shown +that such plants generally have restricted ranges. + +But seeds may be occasionally transported in another manner. Drift timber is +thrown up on most islands, even on those in the midst of the widest oceans; and +the natives of the coral-islands in the Pacific, procure stones for their +tools, solely from the roots of drifted trees, these stones being a valuable +royal tax. I find on examination, that when irregularly shaped stones are +embedded in the roots of trees, small parcels of earth are very frequently +enclosed in their interstices and behind them,—so perfectly that not a particle +could be washed away in the longest transport: out of one small portion of +earth thus completely enclosed by wood in an oak about 50 years old, three +dicotyledonous plants germinated: I am certain of the accuracy of this +observation. Again, I can show that the carcasses of birds, when floating on +the sea, sometimes escape being immediately devoured; and seeds of many kinds +in the crops of floating birds long retain their vitality: peas and vetches, +for instance, are killed by even a few days’ immersion in sea-water; but some +taken out of the crop of a pigeon, which had floated on artificial salt-water +for 30 days, to my surprise nearly all germinated. + +Living birds can hardly fail to be highly effective agents in the +transportation of seeds. I could give many facts showing how frequently birds +of many kinds are blown by gales to vast distances across the ocean. We may I +think safely assume that under such circumstances their rate of flight would +often be 35 miles an hour; and some authors have given a far higher estimate. I +have never seen an instance of nutritious seeds passing through the intestines +of a bird; but hard seeds of fruit will pass uninjured through even the +digestive organs of a turkey. In the course of two months, I picked up in my +garden 12 kinds of seeds, out of the excrement of small birds, and these seemed +perfect, and some of them, which I tried, germinated. But the following fact is +more important: the crops of birds do not secrete gastric juice, and do not in +the least injure, as I know by trial, the germination of seeds; now after a +bird has found and devoured a large supply of food, it is positively asserted +that all the grains do not pass into the gizzard for 12 or even 18 hours. A +bird in this interval might easily be blown to the distance of 500 miles, and +hawks are known to look out for tired birds, and the contents of their torn +crops might thus readily get scattered. Mr. Brent informs me that a friend of +his had to give up flying carrier-pigeons from France to England, as the hawks +on the English coast destroyed so many on their arrival. Some hawks and owls +bolt their prey whole, and after an interval of from twelve to twenty hours, +disgorge pellets, which, as I know from experiments made in the Zoological +Gardens, include seeds capable of germination. Some seeds of the oat, wheat, +millet, canary, hemp, clover, and beet germinated after having been from twelve +to twenty-one hours in the stomachs of different birds of prey; and two seeds +of beet grew after having been thus retained for two days and fourteen hours. +Freshwater fish, I find, eat seeds of many land and water plants: fish are +frequently devoured by birds, and thus the seeds might be transported from +place to place. I forced many kinds of seeds into the stomachs of dead fish, +and then gave their bodies to fishing-eagles, storks, and pelicans; these birds +after an interval of many hours, either rejected the seeds in pellets or passed +them in their excrement; and several of these seeds retained their power of +germination. Certain seeds, however, were always killed by this process. + +Although the beaks and feet of birds are generally quite clean, I can show that +earth sometimes adheres to them: in one instance I removed twenty-two grains of +dry argillaceous earth from one foot of a partridge, and in this earth there +was a pebble quite as large as the seed of a vetch. Thus seeds might +occasionally be transported to great distances; for many facts could be given +showing that soil almost everywhere is charged with seeds. Reflect for a moment +on the millions of quails which annually cross the Mediterranean; and can we +doubt that the earth adhering to their feet would sometimes include a few +minute seeds? But I shall presently have to recur to this subject. + +As icebergs are known to be sometimes loaded with earth and stones, and have +even carried brushwood, bones, and the nest of a land-bird, I can hardly doubt +that they must occasionally have transported seeds from one part to another of +the arctic and antarctic regions, as suggested by Lyell; and during the Glacial +period from one part of the now temperate regions to another. In the Azores, +from the large number of the species of plants common to Europe, in comparison +with the plants of other oceanic islands nearer to the mainland, and (as +remarked by Mr. H. C. Watson) from the somewhat northern character of the flora +in comparison with the latitude, I suspected that these islands had been partly +stocked by ice-borne seeds, during the Glacial epoch. At my request Sir C. +Lyell wrote to M. Hartung to inquire whether he had observed erratic boulders +on these islands, and he answered that he had found large fragments of granite +and other rocks, which do not occur in the archipelago. Hence we may safely +infer that icebergs formerly landed their rocky burthens on the shores of these +mid-ocean islands, and it is at least possible that they may have brought +thither the seeds of northern plants. + +Considering that the several above means of transport, and that several other +means, which without doubt remain to be discovered, have been in action year +after year, for centuries and tens of thousands of years, it would I think be a +marvellous fact if many plants had not thus become widely transported. These +means of transport are sometimes called accidental, but this is not strictly +correct: the currents of the sea are not accidental, nor is the direction of +prevalent gales of wind. It should be observed that scarcely any means of +transport would carry seeds for very great distances; for seeds do not retain +their vitality when exposed for a great length of time to the action of +seawater; nor could they be long carried in the crops or intestines of birds. +These means, however, would suffice for occasional transport across tracts of +sea some hundred miles in breadth, or from island to island, or from a +continent to a neighbouring island, but not from one distant continent to +another. The floras of distant continents would not by such means become +mingled in any great degree; but would remain as distinct as we now see them to +be. The currents, from their course, would never bring seeds from North America +to Britain, though they might and do bring seeds from the West Indies to our +western shores, where, if not killed by so long an immersion in salt-water, +they could not endure our climate. Almost every year, one or two land-birds are +blown across the whole Atlantic Ocean, from North America to the western shores +of Ireland and England; but seeds could be transported by these wanderers only +by one means, namely, in dirt sticking to their feet, which is in itself a rare +accident. Even in this case, how small would the chance be of a seed falling on +favourable soil, and coming to maturity! But it would be a great error to argue +that because a well-stocked island, like Great Britain, has not, as far as is +known (and it would be very difficult to prove this), received within the last +few centuries, through occasional means of transport, immigrants from Europe or +any other continent, that a poorly-stocked island, though standing more remote +from the mainland, would not receive colonists by similar means. I do not doubt +that out of twenty seeds or animals transported to an island, even if far less +well-stocked than Britain, scarcely more than one would be so well fitted to +its new home, as to become naturalised. But this, as it seems to me, is no +valid argument against what would be effected by occasional means of transport, +during the long lapse of geological time, whilst an island was being upheaved +and formed, and before it had become fully stocked with inhabitants. On almost +bare land, with few or no destructive insects or birds living there, nearly +every seed, which chanced to arrive, would be sure to germinate and survive. + +Dispersal during the Glacial period.—The identity of many plants and animals, +on mountain-summits, separated from each other by hundreds of miles of +lowlands, where the Alpine species could not possibly exist, is one of the most +striking cases known of the same species living at distant points, without the +apparent possibility of their having migrated from one to the other. It is +indeed a remarkable fact to see so many of the same plants living on the snowy +regions of the Alps or Pyrenees, and in the extreme northern parts of Europe; +but it is far more remarkable, that the plants on the White Mountains, in the +United States of America, are all the same with those of Labrador, and nearly +all the same, as we hear from Asa Gray, with those on the loftiest mountains of +Europe. Even as long ago as 1747, such facts led Gmelin to conclude that the +same species must have been independently created at several distinct points; +and we might have remained in this same belief, had not Agassiz and others +called vivid attention to the Glacial period, which, as we shall immediately +see, affords a simple explanation of these facts. We have evidence of almost +every conceivable kind, organic and inorganic, that within a very recent +geological period, central Europe and North America suffered under an Arctic +climate. The ruins of a house burnt by fire do not tell their tale more +plainly, than do the mountains of Scotland and Wales, with their scored flanks, +polished surfaces, and perched boulders, of the icy streams with which their +valleys were lately filled. So greatly has the climate of Europe changed, that +in Northern Italy, gigantic moraines, left by old glaciers, are now clothed by +the vine and maize. Throughout a large part of the United States, erratic +boulders, and rocks scored by drifted icebergs and coast-ice, plainly reveal a +former cold period. + +The former influence of the glacial climate on the distribution of the +inhabitants of Europe, as explained with remarkable clearness by Edward Forbes, +is substantially as follows. But we shall follow the changes more readily, by +supposing a new glacial period to come slowly on, and then pass away, as +formerly occurred. As the cold came on, and as each more southern zone became +fitted for arctic beings and ill-fitted for their former more temperate +inhabitants, the latter would be supplanted and arctic productions would take +their places. The inhabitants of the more temperate regions would at the same +time travel southward, unless they were stopped by barriers, in which case they +would perish. The mountains would become covered with snow and ice, and their +former Alpine inhabitants would descend to the plains. By the time that the +cold had reached its maximum, we should have a uniform arctic fauna and flora, +covering the central parts of Europe, as far south as the Alps and Pyrenees, +and even stretching into Spain. The now temperate regions of the United States +would likewise be covered by arctic plants and animals, and these would be +nearly the same with those of Europe; for the present circumpolar inhabitants, +which we suppose to have everywhere travelled southward, are remarkably uniform +round the world. We may suppose that the Glacial period came on a little +earlier or later in North America than in Europe, so will the southern +migration there have been a little earlier or later; but this will make no +difference in the final result. + +As the warmth returned, the arctic forms would retreat northward, closely +followed up in their retreat by the productions of the more temperate regions. +And as the snow melted from the bases of the mountains, the arctic forms would +seize on the cleared and thawed ground, always ascending higher and higher, as +the warmth increased, whilst their brethren were pursuing their northern +journey. Hence, when the warmth had fully returned, the same arctic species, +which had lately lived in a body together on the lowlands of the Old and New +Worlds, would be left isolated on distant mountain-summits (having been +exterminated on all lesser heights) and in the arctic regions of both +hemispheres. + +Thus we can understand the identity of many plants at points so immensely +remote as on the mountains of the United States and of Europe. We can thus also +understand the fact that the Alpine plants of each mountain-range are more +especially related to the arctic forms living due north or nearly due north of +them: for the migration as the cold came on, and the re-migration on the +returning warmth, will generally have been due south and north. The Alpine +plants, for example, of Scotland, as remarked by Mr. H. C. Watson, and those of +the Pyrenees, as remarked by Ramond, are more especially allied to the plants +of northern Scandinavia; those of the United States to Labrador; those of the +mountains of Siberia to the arctic regions of that country. These views, +grounded as they are on the perfectly well-ascertained occurrence of a former +Glacial period, seem to me to explain in so satisfactory a manner the present +distribution of the Alpine and Arctic productions of Europe and America, that +when in other regions we find the same species on distant mountain-summits, we +may almost conclude without other evidence, that a colder climate permitted +their former migration across the low intervening tracts, since become too warm +for their existence. + +If the climate, since the Glacial period, has ever been in any degree warmer +than at present (as some geologists in the United States believe to have been +the case, chiefly from the distribution of the fossil Gnathodon), then the +arctic and temperate productions will at a very late period have marched a +little further north, and subsequently have retreated to their present homes; +but I have met with no satisfactory evidence with respect to this intercalated +slightly warmer period, since the Glacial period. + +The arctic forms, during their long southern migration and re-migration +northward, will have been exposed to nearly the same climate, and, as is +especially to be noticed, they will have kept in a body together; consequently +their mutual relations will not have been much disturbed, and, in accordance +with the principles inculcated in this volume, they will not have been liable +to much modification. But with our Alpine productions, left isolated from the +moment of the returning warmth, first at the bases and ultimately on the +summits of the mountains, the case will have been somewhat different; for it is +not likely that all the same arctic species will have been left on mountain +ranges distant from each other, and have survived there ever since; they will, +also, in all probability have become mingled with ancient Alpine species, which +must have existed on the mountains before the commencement of the Glacial +epoch, and which during its coldest period will have been temporarily driven +down to the plains; they will, also, have been exposed to somewhat different +climatal influences. Their mutual relations will thus have been in some degree +disturbed; consequently they will have been liable to modification; and this we +find has been the case; for if we compare the present Alpine plants and animals +of the several great European mountain-ranges, though very many of the species +are identically the same, some present varieties, some are ranked as doubtful +forms, and some few are distinct yet closely allied or representative species. + +In illustrating what, as I believe, actually took place during the Glacial +period, I assumed that at its commencement the arctic productions were as +uniform round the polar regions as they are at the present day. But the +foregoing remarks on distribution apply not only to strictly arctic forms, but +also to many sub-arctic and to some few northern temperate forms, for some of +these are the same on the lower mountains and on the plains of North America +and Europe; and it may be reasonably asked how I account for the necessary +degree of uniformity of the sub-arctic and northern temperate forms round the +world, at the commencement of the Glacial period. At the present day, the +sub-arctic and northern temperate productions of the Old and New Worlds are +separated from each other by the Atlantic Ocean and by the extreme northern +part of the Pacific. During the Glacial period, when the inhabitants of the Old +and New Worlds lived further southwards than at present, they must have been +still more completely separated by wider spaces of ocean. I believe the above +difficulty may be surmounted by looking to still earlier changes of climate of +an opposite nature. We have good reason to believe that during the newer +Pliocene period, before the Glacial epoch, and whilst the majority of the +inhabitants of the world were specifically the same as now, the climate was +warmer than at the present day. Hence we may suppose that the organisms now +living under the climate of latitude 60°, during the Pliocene period lived +further north under the Polar Circle, in latitude 66°-67°; and that the +strictly arctic productions then lived on the broken land still nearer to the +pole. Now if we look at a globe, we shall see that under the Polar Circle there +is almost continuous land from western Europe, through Siberia, to eastern +America. And to this continuity of the circumpolar land, and to the consequent +freedom for intermigration under a more favourable climate, I attribute the +necessary amount of uniformity in the sub-arctic and northern temperate +productions of the Old and New Worlds, at a period anterior to the Glacial +epoch. + +Believing, from reasons before alluded to, that our continents have long +remained in nearly the same relative position, though subjected to large, but +partial oscillations of level, I am strongly inclined to extend the above view, +and to infer that during some earlier and still warmer period, such as the +older Pliocene period, a large number of the same plants and animals inhabited +the almost continuous circumpolar land; and that these plants and animals, both +in the Old and New Worlds, began slowly to migrate southwards as the climate +became less warm, long before the commencement of the Glacial period. We now +see, as I believe, their descendants, mostly in a modified condition, in the +central parts of Europe and the United States. On this view we can understand +the relationship, with very little identity, between the productions of North +America and Europe,—a relationship which is most remarkable, considering the +distance of the two areas, and their separation by the Atlantic Ocean. We can +further understand the singular fact remarked on by several observers, that the +productions of Europe and America during the later tertiary stages were more +closely related to each other than they are at the present time; for during +these warmer periods the northern parts of the Old and New Worlds will have +been almost continuously united by land, serving as a bridge, since rendered +impassable by cold, for the inter-migration of their inhabitants. + +During the slowly decreasing warmth of the Pliocene period, as soon as the +species in common, which inhabited the New and Old Worlds, migrated south of +the Polar Circle, they must have been completely cut off from each other. This +separation, as far as the more temperate productions are concerned, took place +long ages ago. And as the plants and animals migrated southward, they will have +become mingled in the one great region with the native American productions, +and have had to compete with them; and in the other great region, with those of +the Old World. Consequently we have here everything favourable for much +modification,—for far more modification than with the Alpine productions, left +isolated, within a much more recent period, on the several mountain-ranges and +on the arctic lands of the two Worlds. Hence it has come, that when we compare +the now living productions of the temperate regions of the New and Old Worlds, +we find very few identical species (though Asa Gray has lately shown that more +plants are identical than was formerly supposed), but we find in every great +class many forms, which some naturalists rank as geographical races, and others +as distinct species; and a host of closely allied or representative forms which +are ranked by all naturalists as specifically distinct. + +As on the land, so in the waters of the sea, a slow southern migration of a +marine fauna, which during the Pliocene or even a somewhat earlier period, was +nearly uniform along the continuous shores of the Polar Circle, will account, +on the theory of modification, for many closely allied forms now living in +areas completely sundered. Thus, I think, we can understand the presence of +many existing and tertiary representative forms on the eastern and western +shores of temperate North America; and the still more striking case of many +closely allied crustaceans (as described in Dana’s admirable work), of some +fish and other marine animals, in the Mediterranean and in the seas of +Japan,—areas now separated by a continent and by nearly a hemisphere of +equatorial ocean. + +These cases of relationship, without identity, of the inhabitants of seas now +disjoined, and likewise of the past and present inhabitants of the temperate +lands of North America and Europe, are inexplicable on the theory of creation. +We cannot say that they have been created alike, in correspondence with the +nearly similar physical conditions of the areas; for if we compare, for +instance, certain parts of South America with the southern continents of the +Old World, we see countries closely corresponding in all their physical +conditions, but with their inhabitants utterly dissimilar. + +But we must return to our more immediate subject, the Glacial period. I am +convinced that Forbes’s view may be largely extended. In Europe we have the +plainest evidence of the cold period, from the western shores of Britain to the +Oural range, and southward to the Pyrenees. We may infer, from the frozen +mammals and nature of the mountain vegetation, that Siberia was similarly +affected. Along the Himalaya, at points 900 miles apart, glaciers have left the +marks of their former low descent; and in Sikkim, Dr. Hooker saw maize growing +on gigantic ancient moraines. South of the equator, we have some direct +evidence of former glacial action in New Zealand; and the same plants, found on +widely separated mountains in this island, tell the same story. If one account +which has been published can be trusted, we have direct evidence of glacial +action in the south-eastern corner of Australia. + +Looking to America; in the northern half, ice-borne fragments of rock have been +observed on the eastern side as far south as lat. 36°-37°, and on the shores of +the Pacific, where the climate is now so different, as far south as lat. 46 +deg; erratic boulders have, also, been noticed on the Rocky Mountains. In the +Cordillera of Equatorial South America, glaciers once extended far below their +present level. In central Chile I was astonished at the structure of a vast +mound of detritus, about 800 feet in height, crossing a valley of the Andes; +and this I now feel convinced was a gigantic moraine, left far below any +existing glacier. Further south on both sides of the continent, from lat. 41° +to the southernmost extremity, we have the clearest evidence of former glacial +action, in huge boulders transported far from their parent source. + +We do not know that the Glacial epoch was strictly simultaneous at these +several far distant points on opposite sides of the world. But we have good +evidence in almost every case, that the epoch was included within the latest +geological period. We have, also, excellent evidence, that it endured for an +enormous time, as measured by years, at each point. The cold may have come on, +or have ceased, earlier at one point of the globe than at another, but seeing +that it endured for long at each, and that it was contemporaneous in a +geological sense, it seems to me probable that it was, during a part at least +of the period, actually simultaneous throughout the world. Without some +distinct evidence to the contrary, we may at least admit as probable that the +glacial action was simultaneous on the eastern and western sides of North +America, in the Cordillera under the equator and under the warmer temperate +zones, and on both sides of the southern extremity of the continent. If this be +admitted, it is difficult to avoid believing that the temperature of the whole +world was at this period simultaneously cooler. But it would suffice for my +purpose, if the temperature was at the same time lower along certain broad +belts of longitude. + +On this view of the whole world, or at least of broad longitudinal belts, +having been simultaneously colder from pole to pole, much light can be thrown +on the present distribution of identical and allied species. In America, Dr. +Hooker has shown that between forty and fifty of the flowering plants of Tierra +del Fuego, forming no inconsiderable part of its scanty flora, are common to +Europe, enormously remote as these two points are; and there are many closely +allied species. On the lofty mountains of equatorial America a host of peculiar +species belonging to European genera occur. On the highest mountains of Brazil, +some few European genera were found by Gardner, which do not exist in the wide +intervening hot countries. So on the Silla of Caraccas the illustrious Humboldt +long ago found species belonging to genera characteristic of the Cordillera. On +the mountains of Abyssinia, several European forms and some few representatives +of the peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope +a very few European species, believed not to have been introduced by man, and +on the mountains, some few representative European forms are found, which have +not been discovered in the intertropical parts of Africa. On the Himalaya, and +on the isolated mountain-ranges of the peninsula of India, on the heights of +Ceylon, and on the volcanic cones of Java, many plants occur, either +identically the same or representing each other, and at the same time +representing plants of Europe, not found in the intervening hot lowlands. A +list of the genera collected on the loftier peaks of Java raises a picture of a +collection made on a hill in Europe! Still more striking is the fact that +southern Australian forms are clearly represented by plants growing on the +summits of the mountains of Borneo. Some of these Australian forms, as I hear +from Dr. Hooker, extend along the heights of the peninsula of Malacca, and are +thinly scattered, on the one hand over India and on the other as far north as +Japan. + +On the southern mountains of Australia, Dr. F. Müller has discovered several +European species; other species, not introduced by man, occur on the lowlands; +and a long list can be given, as I am informed by Dr. Hooker, of European +genera, found in Australia, but not in the intermediate torrid regions. In the +admirable ‘Introduction to the Flora of New Zealand,’ by Dr. Hooker, analogous +and striking facts are given in regard to the plants of that large island. +Hence we see that throughout the world, the plants growing on the more lofty +mountains, and on the temperate lowlands of the northern and southern +hemispheres, are sometimes identically the same; but they are much oftener +specifically distinct, though related to each other in a most remarkable +manner. + +This brief abstract applies to plants alone: some strictly analogous facts +could be given on the distribution of terrestrial animals. In marine +productions, similar cases occur; as an example, I may quote a remark by the +highest authority, Professor Dana, that “it is certainly a wonderful fact that +New Zealand should have a closer resemblance in its crustacea to Great Britain, +its antipode, than to any other part of the world.” Sir J. Richardson, also, +speaks of the reappearance on the shores of New Zealand, Tasmania, etc., of +northern forms of fish. Dr. Hooker informs me that twenty-five species of Algæ +are common to New Zealand and to Europe, but have not been found in the +intermediate tropical seas. + +It should be observed that the northern species and forms found in the southern +parts of the southern hemisphere, and on the mountain-ranges of the +intertropical regions, are not arctic, but belong to the northern temperate +zones. As Mr. H. C. Watson has recently remarked, “In receding from polar +towards equatorial latitudes, the Alpine or mountain floras really become less +and less arctic.” Many of the forms living on the mountains of the warmer +regions of the earth and in the southern hemisphere are of doubtful value, +being ranked by some naturalists as specifically distinct, by others as +varieties; but some are certainly identical, and many, though closely related +to northern forms, must be ranked as distinct species. + +Now let us see what light can be thrown on the foregoing facts, on the belief, +supported as it is by a large body of geological evidence, that the whole +world, or a large part of it, was during the Glacial period simultaneously much +colder than at present. The Glacial period, as measured by years, must have +been very long; and when we remember over what vast spaces some naturalised +plants and animals have spread within a few centuries, this period will have +been ample for any amount of migration. As the cold came slowly on, all the +tropical plants and other productions will have retreated from both sides +towards the equator, followed in the rear by the temperate productions, and +these by the arctic; but with the latter we are not now concerned. The tropical +plants probably suffered much extinction; how much no one can say; perhaps +formerly the tropics supported as many species as we see at the present day +crowded together at the Cape of Good Hope, and in parts of temperate Australia. +As we know that many tropical plants and animals can withstand a considerable +amount of cold, many might have escaped extermination during a moderate fall of +temperature, more especially by escaping into the warmest spots. But the great +fact to bear in mind is, that all tropical productions will have suffered to a +certain extent. On the other hand, the temperate productions, after migrating +nearer to the equator, though they will have been placed under somewhat new +conditions, will have suffered less. And it is certain that many temperate +plants, if protected from the inroads of competitors, can withstand a much +warmer climate than their own. Hence, it seems to me possible, bearing in mind +that the tropical productions were in a suffering state and could not have +presented a firm front against intruders, that a certain number of the more +vigorous and dominant temperate forms might have penetrated the native ranks +and have reached or even crossed the equator. The invasion would, of course, +have been greatly favoured by high land, and perhaps by a dry climate; for Dr. +Falconer informs me that it is the damp with the heat of the tropics which is +so destructive to perennial plants from a temperate climate. On the other hand, +the most humid and hottest districts will have afforded an asylum to the +tropical natives. The mountain-ranges north-west of the Himalaya, and the long +line of the Cordillera, seem to have afforded two great lines of invasion: and +it is a striking fact, lately communicated to me by Dr. Hooker, that all the +flowering plants, about forty-six in number, common to Tierra del Fuego and to +Europe still exist in North America, which must have lain on the line of march. +But I do not doubt that some temperate productions entered and crossed even the +lowlands of the tropics at the period when the cold was most intense,—when +arctic forms had migrated some twenty-five degrees of latitude from their +native country and covered the land at the foot of the Pyrenees. At this period +of extreme cold, I believe that the climate under the equator at the level of +the sea was about the same with that now felt there at the height of six or +seven thousand feet. During this the coldest period, I suppose that large +spaces of the tropical lowlands were clothed with a mingled tropical and +temperate vegetation, like that now growing with strange luxuriance at the base +of the Himalaya, as graphically described by Hooker. + +Thus, as I believe, a considerable number of plants, a few terrestrial animals, +and some marine productions, migrated during the Glacial period from the +northern and southern temperate zones into the intertropical regions, and some +even crossed the equator. As the warmth returned, these temperate forms would +naturally ascend the higher mountains, being exterminated on the lowlands; +those which had not reached the equator, would re-migrate northward or +southward towards their former homes; but the forms, chiefly northern, which +had crossed the equator, would travel still further from their homes into the +more temperate latitudes of the opposite hemisphere. Although we have reason to +believe from geological evidence that the whole body of arctic shells underwent +scarcely any modification during their long southern migration and re-migration +northward, the case may have been wholly different with those intruding forms +which settled themselves on the intertropical mountains, and in the southern +hemisphere. These being surrounded by strangers will have had to compete with +many new forms of life; and it is probable that selected modifications in their +structure, habits, and constitutions will have profited them. Thus many of +these wanderers, though still plainly related by inheritance to their brethren +of the northern or southern hemispheres, now exist in their new homes as +well-marked varieties or as distinct species. + +It is a remarkable fact, strongly insisted on by Hooker in regard to America, +and by Alph. de Candolle in regard to Australia, that many more identical +plants and allied forms have apparently migrated from the north to the south, +than in a reversed direction. We see, however, a few southern vegetable forms +on the mountains of Borneo and Abyssinia. I suspect that this preponderant +migration from north to south is due to the greater extent of land in the +north, and to the northern forms having existed in their own homes in greater +numbers, and having consequently been advanced through natural selection and +competition to a higher stage of perfection or dominating power, than the +southern forms. And thus, when they became commingled during the Glacial +period, the northern forms were enabled to beat the less powerful southern +forms. Just in the same manner as we see at the present day, that very many +European productions cover the ground in La Plata, and in a lesser degree in +Australia, and have to a certain extent beaten the natives; whereas extremely +few southern forms have become naturalised in any part of Europe, though hides, +wool, and other objects likely to carry seeds have been largely imported into +Europe during the last two or three centuries from La Plata, and during the +last thirty or forty years from Australia. Something of the same kind must have +occurred on the intertropical mountains: no doubt before the Glacial period +they were stocked with endemic Alpine forms; but these have almost everywhere +largely yielded to the more dominant forms, generated in the larger areas and +more efficient workshops of the north. In many islands the native productions +are nearly equalled or even outnumbered by the naturalised; and if the natives +have not been actually exterminated, their numbers have been greatly reduced, +and this is the first stage towards extinction. A mountain is an island on the +land; and the intertropical mountains before the Glacial period must have been +completely isolated; and I believe that the productions of these islands on the +land yielded to those produced within the larger areas of the north, just in +the same way as the productions of real islands have everywhere lately yielded +to continental forms, naturalised by man’s agency. + +I am far from supposing that all difficulties are removed on the view here +given in regard to the range and affinities of the allied species which live in +the northern and southern temperate zones and on the mountains of the +intertropical regions. Very many difficulties remain to be solved. I do not +pretend to indicate the exact lines and means of migration, or the reason why +certain species and not others have migrated; why certain species have been +modified and have given rise to new groups of forms, and others have remained +unaltered. We cannot hope to explain such facts, until we can say why one +species and not another becomes naturalised by man’s agency in a foreign land; +why one ranges twice or thrice as far, and is twice or thrice as common, as +another species within their own homes. + +I have said that many difficulties remain to be solved: some of the most +remarkable are stated with admirable clearness by Dr. Hooker in his botanical +works on the antarctic regions. These cannot be here discussed. I will only say +that as far as regards the occurrence of identical species at points so +enormously remote as Kerguelen Land, New Zealand, and Fuegia, I believe that +towards the close of the Glacial period, icebergs, as suggested by Lyell, have +been largely concerned in their dispersal. But the existence of several quite +distinct species, belonging to genera exclusively confined to the south, at +these and other distant points of the southern hemisphere, is, on my theory of +descent with modification, a far more remarkable case of difficulty. For some +of these species are so distinct, that we cannot suppose that there has been +time since the commencement of the Glacial period for their migration, and for +their subsequent modification to the necessary degree. The facts seem to me to +indicate that peculiar and very distinct species have migrated in radiating +lines from some common centre; and I am inclined to look in the southern, as in +the northern hemisphere, to a former and warmer period, before the commencement +of the Glacial period, when the antarctic lands, now covered with ice, +supported a highly peculiar and isolated flora. I suspect that before this +flora was exterminated by the Glacial epoch, a few forms were widely dispersed +to various points of the southern hemisphere by occasional means of transport, +and by the aid, as halting-places, of existing and now sunken islands, and +perhaps at the commencement of the Glacial period, by icebergs. By these means, +as I believe, the southern shores of America, Australia, New Zealand have +become slightly tinted by the same peculiar forms of vegetable life. + +Sir C. Lyell in a striking passage has speculated, in language almost identical +with mine, on the effects of great alternations of climate on geographical +distribution. I believe that the world has recently felt one of his great +cycles of change; and that on this view, combined with modification through +natural selection, a multitude of facts in the present distribution both of the +same and of allied forms of life can be explained. The living waters may be +said to have flowed during one short period from the north and from the south, +and to have crossed at the equator; but to have flowed with greater force from +the north so as to have freely inundated the south. As the tide leaves its +drift in horizontal lines, though rising higher on the shores where the tide +rises highest, so have the living waters left their living drift on our +mountain-summits, in a line gently rising from the arctic lowlands to a great +height under the equator. The various beings thus left stranded may be compared +with savage races of man, driven up and surviving in the mountain-fastnesses of +almost every land, which serve as a record, full of interest to us, of the +former inhabitants of the surrounding lowlands. + +CHAPTER XII. +GEOGRAPHICAL DISTRIBUTION—continued. + +Distribution of fresh-water productions. On the inhabitants of oceanic islands. +Absence of Batrachians and of terrestrial Mammals. On the relation of the +inhabitants of islands to those of the nearest mainland. On colonisation from +the nearest source with subsequent modification. Summary of the last and +present chapters. + +As lakes and river-systems are separated from each other by barriers of land, +it might have been thought that fresh-water productions would not have ranged +widely within the same country, and as the sea is apparently a still more +impassable barrier, that they never would have extended to distant countries. +But the case is exactly the reverse. Not only have many fresh-water species, +belonging to quite different classes, an enormous range, but allied species +prevail in a remarkable manner throughout the world. I well remember, when +first collecting in the fresh waters of Brazil, feeling much surprise at the +similarity of the fresh-water insects, shells, etc., and at the dissimilarity +of the surrounding terrestrial beings, compared with those of Britain. + +But this power in fresh-water productions of ranging widely, though so +unexpected, can, I think, in most cases be explained by their having become +fitted, in a manner highly useful to them, for short and frequent migrations +from pond to pond, or from stream to stream; and liability to wide dispersal +would follow from this capacity as an almost necessary consequence. We can here +consider only a few cases. In regard to fish, I believe that the same species +never occur in the fresh waters of distant continents. But on the same +continent the species often range widely and almost capriciously; for two +river-systems will have some fish in common and some different. A few facts +seem to favour the possibility of their occasional transport by accidental +means; like that of the live fish not rarely dropped by whirlwinds in India, +and the vitality of their ova when removed from the water. But I am inclined to +attribute the dispersal of fresh-water fish mainly to slight changes within the +recent period in the level of the land, having caused rivers to flow into each +other. Instances, also, could be given of this having occurred during floods, +without any change of level. We have evidence in the loess of the Rhine of +considerable changes of level in the land within a very recent geological +period, and when the surface was peopled by existing land and fresh-water +shells. The wide difference of the fish on opposite sides of continuous +mountain-ranges, which from an early period must have parted river-systems and +completely prevented their inosculation, seems to lead to this same conclusion. +With respect to allied fresh-water fish occurring at very distant points of the +world, no doubt there are many cases which cannot at present be explained: but +some fresh-water fish belong to very ancient forms, and in such cases there +will have been ample time for great geographical changes, and consequently time +and means for much migration. In the second place, salt-water fish can with +care be slowly accustomed to live in fresh water; and, according to +Valenciennes, there is hardly a single group of fishes confined exclusively to +fresh water, so that we may imagine that a marine member of a fresh-water group +might travel far along the shores of the sea, and subsequently become modified +and adapted to the fresh waters of a distant land. + +Some species of fresh-water shells have a very wide range, and allied species, +which, on my theory, are descended from a common parent and must have proceeded +from a single source, prevail throughout the world. Their distribution at first +perplexed me much, as their ova are not likely to be transported by birds, and +they are immediately killed by sea water, as are the adults. I could not even +understand how some naturalised species have rapidly spread throughout the same +country. But two facts, which I have observed—and no doubt many others remain +to be observed—throw some light on this subject. When a duck suddenly emerges +from a pond covered with duck-weed, I have twice seen these little plants +adhering to its back; and it has happened to me, in removing a little duck-weed +from one aquarium to another, that I have quite unintentionally stocked the one +with fresh-water shells from the other. But another agency is perhaps more +effectual: I suspended a duck’s feet, which might represent those of a bird +sleeping in a natural pond, in an aquarium, where many ova of fresh-water +shells were hatching; and I found that numbers of the extremely minute and just +hatched shells crawled on the feet, and clung to them so firmly that when taken +out of the water they could not be jarred off, though at a somewhat more +advanced age they would voluntarily drop off. These just hatched molluscs, +though aquatic in their nature, survived on the duck’s feet, in damp air, from +twelve to twenty hours; and in this length of time a duck or heron might fly at +least six or seven hundred miles, and would be sure to alight on a pool or +rivulet, if blown across sea to an oceanic island or to any other distant +point. Sir Charles Lyell also informs me that a Dyticus has been caught with an +Ancylus (a fresh-water shell like a limpet) firmly adhering to it; and a +water-beetle of the same family, a Colymbetes, once flew on board the ‘Beagle,’ +when forty-five miles distant from the nearest land: how much farther it might +have flown with a favouring gale no one can tell. + +With respect to plants, it has long been known what enormous ranges many +fresh-water and even marsh-species have, both over continents and to the most +remote oceanic islands. This is strikingly shown, as remarked by Alph. de +Candolle, in large groups of terrestrial plants, which have only a very few +aquatic members; for these latter seem immediately to acquire, as if in +consequence, a very wide range. I think favourable means of dispersal explain +this fact. I have before mentioned that earth occasionally, though rarely, +adheres in some quantity to the feet and beaks of birds. Wading birds, which +frequent the muddy edges of ponds, if suddenly flushed, would be the most +likely to have muddy feet. Birds of this order I can show are the greatest +wanderers, and are occasionally found on the most remote and barren islands in +the open ocean; they would not be likely to alight on the surface of the sea, +so that the dirt would not be washed off their feet; when making land, they +would be sure to fly to their natural fresh-water haunts. I do not believe that +botanists are aware how charged the mud of ponds is with seeds: I have tried +several little experiments, but will here give only the most striking case: I +took in February three table-spoonfuls of mud from three different points, +beneath water, on the edge of a little pond; this mud when dry weighed only 6 3 +/4 ounces; I kept it covered up in my study for six months, pulling up and +counting each plant as it grew; the plants were of many kinds, and were +altogether 537 in number; and yet the viscid mud was all contained in a +breakfast cup! Considering these facts, I think it would be an inexplicable +circumstance if water-birds did not transport the seeds of fresh-water plants +to vast distances, and if consequently the range of these plants was not very +great. The same agency may have come into play with the eggs of some of the +smaller fresh-water animals. + +Other and unknown agencies probably have also played a part. I have stated that +fresh-water fish eat some kinds of seeds, though they reject many other kinds +after having swallowed them; even small fish swallow seeds of moderate size, as +of the yellow water-lily and Potamogeton. Herons and other birds, century after +century, have gone on daily devouring fish; they then take flight and go to +other waters, or are blown across the sea; and we have seen that seeds retain +their power of germination, when rejected in pellets or in excrement, many +hours afterwards. When I saw the great size of the seeds of that fine +water-lily, the Nelumbium, and remembered Alph. de Candolle’s remarks on this +plant, I thought that its distribution must remain quite inexplicable; but +Audubon states that he found the seeds of the great southern water-lily +(probably, according to Dr. Hooker, the Nelumbium luteum) in a heron’s stomach; +although I do not know the fact, yet analogy makes me believe that a heron +flying to another pond and getting a hearty meal of fish, would probably reject +from its stomach a pellet containing the seeds of the Nelumbium undigested; or +the seeds might be dropped by the bird whilst feeding its young, in the same +way as fish are known sometimes to be dropped. + +In considering these several means of distribution, it should be remembered +that when a pond or stream is first formed, for instance, on a rising islet, it +will be unoccupied; and a single seed or egg will have a good chance of +succeeding. Although there will always be a struggle for life between the +individuals of the species, however few, already occupying any pond, yet as the +number of kinds is small, compared with those on the land, the competition will +probably be less severe between aquatic than between terrestrial species; +consequently an intruder from the waters of a foreign country, would have a +better chance of seizing on a place, than in the case of terrestrial colonists. +We should, also, remember that some, perhaps many, fresh-water productions are +low in the scale of nature, and that we have reason to believe that such low +beings change or become modified less quickly than the high; and this will give +longer time than the average for the migration of the same aquatic species. We +should not forget the probability of many species having formerly ranged as +continuously as fresh-water productions ever can range, over immense areas, and +having subsequently become extinct in intermediate regions. But the wide +distribution of fresh-water plants and of the lower animals, whether retaining +the same identical form or in some degree modified, I believe mainly depends on +the wide dispersal of their seeds and eggs by animals, more especially by +fresh-water birds, which have large powers of flight, and naturally travel from +one to another and often distant piece of water. Nature, like a careful +gardener, thus takes her seeds from a bed of a particular nature, and drops +them in another equally well fitted for them. + +On the Inhabitants of Oceanic Islands.—We now come to the last of the three +classes of facts, which I have selected as presenting the greatest amount of +difficulty, on the view that all the individuals both of the same and of allied +species have descended from a single parent; and therefore have all proceeded +from a common birthplace, notwithstanding that in the course of time they have +come to inhabit distant points of the globe. I have already stated that I +cannot honestly admit Forbes’s view on continental extensions, which, if +legitimately followed out, would lead to the belief that within the recent +period all existing islands have been nearly or quite joined to some continent. +This view would remove many difficulties, but it would not, I think, explain +all the facts in regard to insular productions. In the following remarks I +shall not confine myself to the mere question of dispersal; but shall consider +some other facts, which bear on the truth of the two theories of independent +creation and of descent with modification. + +The species of all kinds which inhabit oceanic islands are few in number +compared with those on equal continental areas: Alph. de Candolle admits this +for plants, and Wollaston for insects. If we look to the large size and varied +stations of New Zealand, extending over 780 miles of latitude, and compare its +flowering plants, only 750 in number, with those on an equal area at the Cape +of Good Hope or in Australia, we must, I think, admit that something quite +independently of any difference in physical conditions has caused so great a +difference in number. Even the uniform county of Cambridge has 847 plants, and +the little island of Anglesea 764, but a few ferns and a few introduced plants +are included in these numbers, and the comparison in some other respects is not +quite fair. We have evidence that the barren island of Ascension aboriginally +possessed under half-a-dozen flowering plants; yet many have become naturalised +on it, as they have on New Zealand and on every other oceanic island which can +be named. In St. Helena there is reason to believe that the naturalised plants +and animals have nearly or quite exterminated many native productions. He who +admits the doctrine of the creation of each separate species, will have to +admit, that a sufficient number of the best adapted plants and animals have not +been created on oceanic islands; for man has unintentionally stocked them from +various sources far more fully and perfectly than has nature. + +Although in oceanic islands the number of kinds of inhabitants is scanty, the +proportion of endemic species (i.e. those found nowhere else in the world) is +often extremely large. If we compare, for instance, the number of the endemic +land-shells in Madeira, or of the endemic birds in the Galapagos Archipelago, +with the number found on any continent, and then compare the area of the +islands with that of the continent, we shall see that this is true. This fact +might have been expected on my theory, for, as already explained, species +occasionally arriving after long intervals in a new and isolated district, and +having to compete with new associates, will be eminently liable to +modification, and will often produce groups of modified descendants. But it by +no means follows, that, because in an island nearly all the species of one +class are peculiar, those of another class, or of another section of the same +class, are peculiar; and this difference seems to depend on the species which +do not become modified having immigrated with facility and in a body, so that +their mutual relations have not been much disturbed. Thus in the Galapagos +Islands nearly every land-bird, but only two out of the eleven marine birds, +are peculiar; and it is obvious that marine birds could arrive at these islands +more easily than land-birds. Bermuda, on the other hand, which lies at about +the same distance from North America as the Galapagos Islands do from South +America, and which has a very peculiar soil, does not possess one endemic land +bird; and we know from Mr. J. M. Jones’s admirable account of Bermuda, that +very many North American birds, during their great annual migrations, visit +either periodically or occasionally this island. Madeira does not possess one +peculiar bird, and many European and African birds are almost every year blown +there, as I am informed by Mr. E. V. Harcourt. So that these two islands of +Bermuda and Madeira have been stocked by birds, which for long ages have +struggled together in their former homes, and have become mutually adapted to +each other; and when settled in their new homes, each kind will have been kept +by the others to their proper places and habits, and will consequently have +been little liable to modification. Madeira, again, is inhabited by a wonderful +number of peculiar land-shells, whereas not one species of sea-shell is +confined to its shores: now, though we do not know how seashells are dispersed, +yet we can see that their eggs or larvæ, perhaps attached to seaweed or +floating timber, or to the feet of wading-birds, might be transported far more +easily than land-shells, across three or four hundred miles of open sea. The +different orders of insects in Madeira apparently present analogous facts. + +Oceanic islands are sometimes deficient in certain classes, and their places +are apparently occupied by the other inhabitants; in the Galapagos Islands +reptiles, and in New Zealand gigantic wingless birds, take the place of +mammals. In the plants of the Galapagos Islands, Dr. Hooker has shown that the +proportional numbers of the different orders are very different from what they +are elsewhere. Such cases are generally accounted for by the physical +conditions of the islands; but this explanation seems to me not a little +doubtful. Facility of immigration, I believe, has been at least as important as +the nature of the conditions. + +Many remarkable little facts could be given with respect to the inhabitants of +remote islands. For instance, in certain islands not tenanted by mammals, some +of the endemic plants have beautifully hooked seeds; yet few relations are more +striking than the adaptation of hooked seeds for transportal by the wool and +fur of quadrupeds. This case presents no difficulty on my view, for a hooked +seed might be transported to an island by some other means; and the plant then +becoming slightly modified, but still retaining its hooked seeds, would form an +endemic species, having as useless an appendage as any rudimentary organ,—for +instance, as the shrivelled wings under the soldered elytra of many insular +beetles. Again, islands often possess trees or bushes belonging to orders which +elsewhere include only herbaceous species; now trees, as Alph. de Candolle has +shown, generally have, whatever the cause may be, confined ranges. Hence trees +would be little likely to reach distant oceanic islands; and an herbaceous +plant, though it would have no chance of successfully competing in stature with +a fully developed tree, when established on an island and having to compete +with herbaceous plants alone, might readily gain an advantage by growing taller +and taller and overtopping the other plants. If so, natural selection would +often tend to add to the stature of herbaceous plants when growing on an +island, to whatever order they belonged, and thus convert them first into +bushes and ultimately into trees. + +With respect to the absence of whole orders on oceanic islands, Bory St. +Vincent long ago remarked that Batrachians (frogs, toads, newts) have never +been found on any of the many islands with which the great oceans are studded. +I have taken pains to verify this assertion, and I have found it strictly true. +I have, however, been assured that a frog exists on the mountains of the great +island of New Zealand; but I suspect that this exception (if the information be +correct) may be explained through glacial agency. This general absence of +frogs, toads, and newts on so many oceanic islands cannot be accounted for by +their physical conditions; indeed it seems that islands are peculiarly well +fitted for these animals; for frogs have been introduced into Madeira, the +Azores, and Mauritius, and have multiplied so as to become a nuisance. But as +these animals and their spawn are known to be immediately killed by sea-water, +on my view we can see that there would be great difficulty in their transportal +across the sea, and therefore why they do not exist on any oceanic island. But +why, on the theory of creation, they should not have been created there, it +would be very difficult to explain. + +Mammals offer another and similar case. I have carefully searched the oldest +voyages, but have not finished my search; as yet I have not found a single +instance, free from doubt, of a terrestrial mammal (excluding domesticated +animals kept by the natives) inhabiting an island situated above 300 miles from +a continent or great continental island; and many islands situated at a much +less distance are equally barren. The Falkland Islands, which are inhabited by +a wolf-like fox, come nearest to an exception; but this group cannot be +considered as oceanic, as it lies on a bank connected with the mainland; +moreover, icebergs formerly brought boulders to its western shores, and they +may have formerly transported foxes, as so frequently now happens in the arctic +regions. Yet it cannot be said that small islands will not support small +mammals, for they occur in many parts of the world on very small islands, if +close to a continent; and hardly an island can be named on which our smaller +quadrupeds have not become naturalised and greatly multiplied. It cannot be +said, on the ordinary view of creation, that there has not been time for the +creation of mammals; many volcanic islands are sufficiently ancient, as shown +by the stupendous degradation which they have suffered and by their tertiary +strata: there has also been time for the production of endemic species +belonging to other classes; and on continents it is thought that mammals appear +and disappear at a quicker rate than other and lower animals. Though +terrestrial mammals do not occur on oceanic islands, ærial mammals do occur on +almost every island. New Zealand possesses two bats found nowhere else in the +world: Norfolk Island, the Viti Archipelago, the Bonin Islands, the Caroline +and Marianne Archipelagoes, and Mauritius, all possess their peculiar bats. +Why, it may be asked, has the supposed creative force produced bats and no +other mammals on remote islands? On my view this question can easily be +answered; for no terrestrial mammal can be transported across a wide space of +sea, but bats can fly across. Bats have been seen wandering by day far over the +Atlantic Ocean; and two North American species either regularly or occasionally +visit Bermuda, at the distance of 600 miles from the mainland. I hear from Mr. +Tomes, who has specially studied this family, that many of the same species +have enormous ranges, and are found on continents and on far distant islands. +Hence we have only to suppose that such wandering species have been modified +through natural selection in their new homes in relation to their new position, +and we can understand the presence of endemic bats on islands, with the absence +of all terrestrial mammals. + +Besides the absence of terrestrial mammals in relation to the remoteness of +islands from continents, there is also a relation, to a certain extent +independent of distance, between the depth of the sea separating an island from +the neighbouring mainland, and the presence in both of the same mammiferous +species or of allied species in a more or less modified condition. Mr. Windsor +Earl has made some striking observations on this head in regard to the great +Malay Archipelago, which is traversed near Celebes by a space of deep ocean; +and this space separates two widely distinct mammalian faunas. On either side +the islands are situated on moderately deep submarine banks, and they are +inhabited by closely allied or identical quadrupeds. No doubt some few +anomalies occur in this great archipelago, and there is much difficulty in +forming a judgment in some cases owing to the probable naturalisation of +certain mammals through man’s agency; but we shall soon have much light thrown +on the natural history of this archipelago by the admirable zeal and researches +of Mr. Wallace. I have not as yet had time to follow up this subject in all +other quarters of the world; but as far as I have gone, the relation generally +holds good. We see Britain separated by a shallow channel from Europe, and the +mammals are the same on both sides; we meet with analogous facts on many +islands separated by similar channels from Australia. The West Indian Islands +stand on a deeply submerged bank, nearly 1000 fathoms in depth, and here we +find American forms, but the species and even the genera are distinct. As the +amount of modification in all cases depends to a certain degree on the lapse of +time, and as during changes of level it is obvious that islands separated by +shallow channels are more likely to have been continuously united within a +recent period to the mainland than islands separated by deeper channels, we can +understand the frequent relation between the depth of the sea and the degree of +affinity of the mammalian inhabitants of islands with those of a neighbouring +continent,—an inexplicable relation on the view of independent acts of +creation. + +All the foregoing remarks on the inhabitants of oceanic islands,—namely, the +scarcity of kinds—the richness in endemic forms in particular classes or +sections of classes,—the absence of whole groups, as of batrachians, and of +terrestrial mammals notwithstanding the presence of ærial bats,—the singular +proportions of certain orders of plants,—herbaceous forms having been developed +into trees, etc.,—seem to me to accord better with the view of occasional means +of transport having been largely efficient in the long course of time, than +with the view of all our oceanic islands having been formerly connected by +continuous land with the nearest continent; for on this latter view the +migration would probably have been more complete; and if modification be +admitted, all the forms of life would have been more equally modified, in +accordance with the paramount importance of the relation of organism to +organism. + +I do not deny that there are many and grave difficulties in understanding how +several of the inhabitants of the more remote islands, whether still retaining +the same specific form or modified since their arrival, could have reached +their present homes. But the probability of many islands having existed as +halting-places, of which not a wreck now remains, must not be overlooked. I +will here give a single instance of one of the cases of difficulty. Almost all +oceanic islands, even the most isolated and smallest, are inhabited by +land-shells, generally by endemic species, but sometimes by species found +elsewhere. Dr. Aug. A. Gould has given several interesting cases in regard to +the land-shells of the islands of the Pacific. Now it is notorious that +land-shells are very easily killed by salt; their eggs, at least such as I have +tried, sink in sea-water and are killed by it. Yet there must be, on my view, +some unknown, but highly efficient means for their transportal. Would the +just-hatched young occasionally crawl on and adhere to the feet of birds +roosting on the ground, and thus get transported? It occurred to me that +land-shells, when hybernating and having a membranous diaphragm over the mouth +of the shell, might be floated in chinks of drifted timber across moderately +wide arms of the sea. And I found that several species did in this state +withstand uninjured an immersion in sea-water during seven days: one of these +shells was the Helix pomatia, and after it had again hybernated I put it in +sea-water for twenty days, and it perfectly recovered. As this species has a +thick calcareous operculum, I removed it, and when it had formed a new +membranous one, I immersed it for fourteen days in sea-water, and it recovered +and crawled away: but more experiments are wanted on this head. + +The most striking and important fact for us in regard to the inhabitants of +islands, is their affinity to those of the nearest mainland, without being +actually the same species. Numerous instances could be given of this fact. I +will give only one, that of the Galapagos Archipelago, situated under the +equator, between 500 and 600 miles from the shores of South America. Here +almost every product of the land and water bears the unmistakeable stamp of the +American continent. There are twenty-six land birds, and twenty-five of these +are ranked by Mr. Gould as distinct species, supposed to have been created +here; yet the close affinity of most of these birds to American species in +every character, in their habits, gestures, and tones of voice, was manifest. +So it is with the other animals, and with nearly all the plants, as shown by +Dr. Hooker in his admirable memoir on the Flora of this archipelago. The +naturalist, looking at the inhabitants of these volcanic islands in the +Pacific, distant several hundred miles from the continent, yet feels that he is +standing on American land. Why should this be so? why should the species which +are supposed to have been created in the Galapagos Archipelago, and nowhere +else, bear so plain a stamp of affinity to those created in America? There is +nothing in the conditions of life, in the geological nature of the islands, in +their height or climate, or in the proportions in which the several classes are +associated together, which resembles closely the conditions of the South +American coast: in fact there is a considerable dissimilarity in all these +respects. On the other hand, there is a considerable degree of resemblance in +the volcanic nature of the soil, in climate, height, and size of the islands, +between the Galapagos and Cape de Verde Archipelagos: but what an entire and +absolute difference in their inhabitants! The inhabitants of the Cape de Verde +Islands are related to those of Africa, like those of the Galapagos to America. +I believe this grand fact can receive no sort of explanation on the ordinary +view of independent creation; whereas on the view here maintained, it is +obvious that the Galapagos Islands would be likely to receive colonists, +whether by occasional means of transport or by formerly continuous land, from +America; and the Cape de Verde Islands from Africa; and that such colonists +would be liable to modification;—the principle of inheritance still betraying +their original birthplace. + +Many analogous facts could be given: indeed it is an almost universal rule that +the endemic productions of islands are related to those of the nearest +continent, or of other near islands. The exceptions are few, and most of them +can be explained. Thus the plants of Kerguelen Land, though standing nearer to +Africa than to America, are related, and that very closely, as we know from Dr. +Hooker’s account, to those of America: but on the view that this island has +been mainly stocked by seeds brought with earth and stones on icebergs, drifted +by the prevailing currents, this anomaly disappears. New Zealand in its endemic +plants is much more closely related to Australia, the nearest mainland, than to +any other region: and this is what might have been expected; but it is also +plainly related to South America, which, although the next nearest continent, +is so enormously remote, that the fact becomes an anomaly. But this difficulty +almost disappears on the view that both New Zealand, South America, and other +southern lands were long ago partially stocked from a nearly intermediate +though distant point, namely from the antarctic islands, when they were clothed +with vegetation, before the commencement of the Glacial period. The affinity, +which, though feeble, I am assured by Dr. Hooker is real, between the flora of +the south-western corner of Australia and of the Cape of Good Hope, is a far +more remarkable case, and is at present inexplicable: but this affinity is +confined to the plants, and will, I do not doubt, be some day explained. + +The law which causes the inhabitants of an archipelago, though specifically +distinct, to be closely allied to those of the nearest continent, we sometimes +see displayed on a small scale, yet in a most interesting manner, within the +limits of the same archipelago. Thus the several islands of the Galapagos +Archipelago are tenanted, as I have elsewhere shown, in a quite marvellous +manner, by very closely related species; so that the inhabitants of each +separate island, though mostly distinct, are related in an incomparably closer +degree to each other than to the inhabitants of any other part of the world. +And this is just what might have been expected on my view, for the islands are +situated so near each other that they would almost certainly receive immigrants +from the same original source, or from each other. But this dissimilarity +between the endemic inhabitants of the islands may be used as an argument +against my views; for it may be asked, how has it happened in the several +islands situated within sight of each other, having the same geological nature, +the same height, climate, etc., that many of the immigrants should have been +differently modified, though only in a small degree. This long appeared to me a +great difficulty: but it arises in chief part from the deeply-seated error of +considering the physical conditions of a country as the most important for its +inhabitants; whereas it cannot, I think, be disputed that the nature of the +other inhabitants, with which each has to compete, is at least as important, +and generally a far more important element of success. Now if we look to those +inhabitants of the Galapagos Archipelago which are found in other parts of the +world (laying on one side for the moment the endemic species, which cannot be +here fairly included, as we are considering how they have come to be modified +since their arrival), we find a considerable amount of difference in the +several islands. This difference might indeed have been expected on the view of +the islands having been stocked by occasional means of transport—a seed, for +instance, of one plant having been brought to one island, and that of another +plant to another island. Hence when in former times an immigrant settled on any +one or more of the islands, or when it subsequently spread from one island to +another, it would undoubtedly be exposed to different conditions of life in the +different islands, for it would have to compete with different sets of +organisms: a plant, for instance, would find the best-fitted ground more +perfectly occupied by distinct plants in one island than in another, and it +would be exposed to the attacks of somewhat different enemies. If then it +varied, natural selection would probably favour different varieties in the +different islands. Some species, however, might spread and yet retain the same +character throughout the group, just as we see on continents some species +spreading widely and remaining the same. + +The really surprising fact in this case of the Galapagos Archipelago, and in a +lesser degree in some analogous instances, is that the new species formed in +the separate islands have not quickly spread to the other islands. But the +islands, though in sight of each other, are separated by deep arms of the sea, +in most cases wider than the British Channel, and there is no reason to suppose +that they have at any former period been continuously united. The currents of +the sea are rapid and sweep across the archipelago, and gales of wind are +extraordinarily rare; so that the islands are far more effectually separated +from each other than they appear to be on a map. Nevertheless a good many +species, both those found in other parts of the world and those confined to the +archipelago, are common to the several islands, and we may infer from certain +facts that these have probably spread from some one island to the others. But +we often take, I think, an erroneous view of the probability of closely allied +species invading each other’s territory, when put into free intercommunication. +Undoubtedly if one species has any advantage whatever over another, it will in +a very brief time wholly or in part supplant it; but if both are equally well +fitted for their own places in nature, both probably will hold their own places +and keep separate for almost any length of time. Being familiar with the fact +that many species, naturalised through man’s agency, have spread with +astonishing rapidity over new countries, we are apt to infer that most species +would thus spread; but we should remember that the forms which become +naturalised in new countries are not generally closely allied to the aboriginal +inhabitants, but are very distinct species, belonging in a large proportion of +cases, as shown by Alph. de Candolle, to distinct genera. In the Galapagos +Archipelago, many even of the birds, though so well adapted for flying from +island to island, are distinct on each; thus there are three closely-allied +species of mocking-thrush, each confined to its own island. Now let us suppose +the mocking-thrush of Chatham Island to be blown to Charles Island, which has +its own mocking-thrush: why should it succeed in establishing itself there? We +may safely infer that Charles Island is well stocked with its own species, for +annually more eggs are laid there than can possibly be reared; and we may infer +that the mocking-thrush peculiar to Charles Island is at least as well fitted +for its home as is the species peculiar to Chatham Island. Sir C. Lyell and Mr. +Wollaston have communicated to me a remarkable fact bearing on this subject; +namely, that Madeira and the adjoining islet of Porto Santo possess many +distinct but representative land-shells, some of which live in crevices of +stone; and although large quantities of stone are annually transported from +Porto Santo to Madeira, yet this latter island has not become colonised by the +Porto Santo species: nevertheless both islands have been colonised by some +European land-shells, which no doubt had some advantage over the indigenous +species. From these considerations I think we need not greatly marvel at the +endemic and representative species, which inhabit the several islands of the +Galapagos Archipelago, not having universally spread from island to island. In +many other instances, as in the several districts of the same continent, +pre-occupation has probably played an important part in checking the +commingling of species under the same conditions of life. Thus, the south-east +and south-west corners of Australia have nearly the same physical conditions, +and are united by continuous land, yet they are inhabited by a vast number of +distinct mammals, birds, and plants. + +The principle which determines the general character of the fauna and flora of +oceanic islands, namely, that the inhabitants, when not identically the same, +yet are plainly related to the inhabitants of that region whence colonists +could most readily have been derived,—the colonists having been subsequently +modified and better fitted to their new homes,—is of the widest application +throughout nature. We see this on every mountain, in every lake and marsh. For +Alpine species, excepting in so far as the same forms, chiefly of plants, have +spread widely throughout the world during the recent Glacial epoch, are related +to those of the surrounding lowlands;—thus we have in South America, Alpine +humming-birds, Alpine rodents, Alpine plants, etc., all of strictly American +forms, and it is obvious that a mountain, as it became slowly upheaved, would +naturally be colonised from the surrounding lowlands. So it is with the +inhabitants of lakes and marshes, excepting in so far as great facility of +transport has given the same general forms to the whole world. We see this same +principle in the blind animals inhabiting the caves of America and of Europe. +Other analogous facts could be given. And it will, I believe, be universally +found to be true, that wherever in two regions, let them be ever so distant, +many closely allied or representative species occur, there will likewise be +found some identical species, showing, in accordance with the foregoing view, +that at some former period there has been intercommunication or migration +between the two regions. And wherever many closely-allied species occur, there +will be found many forms which some naturalists rank as distinct species, and +some as varieties; these doubtful forms showing us the steps in the process of +modification. + +This relation between the power and extent of migration of a species, either at +the present time or at some former period under different physical conditions, +and the existence at remote points of the world of other species allied to it, +is shown in another and more general way. Mr. Gould remarked to me long ago, +that in those genera of birds which range over the world, many of the species +have very wide ranges. I can hardly doubt that this rule is generally true, +though it would be difficult to prove it. Amongst mammals, we see it strikingly +displayed in Bats, and in a lesser degree in the Felidæ and Canidæ. We see it, +if we compare the distribution of butterflies and beetles. So it is with most +fresh-water productions, in which so many genera range over the world, and many +individual species have enormous ranges. It is not meant that in world-ranging +genera all the species have a wide range, or even that they have on an average +a wide range; but only that some of the species range very widely; for the +facility with which widely-ranging species vary and give rise to new forms will +largely determine their average range. For instance, two varieties of the same +species inhabit America and Europe, and the species thus has an immense range; +but, if the variation had been a little greater, the two varieties would have +been ranked as distinct species, and the common range would have been greatly +reduced. Still less is it meant, that a species which apparently has the +capacity of crossing barriers and ranging widely, as in the case of certain +powerfully-winged birds, will necessarily range widely; for we should never +forget that to range widely implies not only the power of crossing barriers, +but the more important power of being victorious in distant lands in the +struggle for life with foreign associates. But on the view of all the species +of a genus having descended from a single parent, though now distributed to the +most remote points of the world, we ought to find, and I believe as a general +rule we do find, that some at least of the species range very widely; for it is +necessary that the unmodified parent should range widely, undergoing +modification during its diffusion, and should place itself under diverse +conditions favourable for the conversion of its offspring, firstly into new +varieties and ultimately into new species. + +In considering the wide distribution of certain genera, we should bear in mind +that some are extremely ancient, and must have branched off from a common +parent at a remote epoch; so that in such cases there will have been ample time +for great climatal and geographical changes and for accidents of transport; and +consequently for the migration of some of the species into all quarters of the +world, where they may have become slightly modified in relation to their new +conditions. There is, also, some reason to believe from geological evidence +that organisms low in the scale within each great class, generally change at a +slower rate than the higher forms; and consequently the lower forms will have +had a better chance of ranging widely and of still retaining the same specific +character. This fact, together with the seeds and eggs of many low forms being +very minute and better fitted for distant transportation, probably accounts for +a law which has long been observed, and which has lately been admirably +discussed by Alph. de Candolle in regard to plants, namely, that the lower any +group of organisms is, the more widely it is apt to range. + +The relations just discussed,—namely, low and slowly-changing organisms ranging +more widely than the high,—some of the species of widely-ranging genera +themselves ranging widely,—such facts, as alpine, lacustrine, and marsh +productions being related (with the exceptions before specified) to those on +the surrounding low lands and dry lands, though these stations are so +different—the very close relation of the distinct species which inhabit the +islets of the same archipelago,—and especially the striking relation of the +inhabitants of each whole archipelago or island to those of the nearest +mainland,—are, I think, utterly inexplicable on the ordinary view of the +independent creation of each species, but are explicable on the view of +colonisation from the nearest and readiest source, together with the subsequent +modification and better adaptation of the colonists to their new homes. + +Summary of last and present Chapters.—In these chapters I have endeavoured to +show, that if we make due allowance for our ignorance of the full effects of +all the changes of climate and of the level of the land, which have certainly +occurred within the recent period, and of other similar changes which may have +occurred within the same period; if we remember how profoundly ignorant we are +with respect to the many and curious means of occasional transport,—a subject +which has hardly ever been properly experimentised on; if we bear in mind how +often a species may have ranged continuously over a wide area, and then have +become extinct in the intermediate tracts, I think the difficulties in +believing that all the individuals of the same species, wherever located, have +descended from the same parents, are not insuperable. And we are led to this +conclusion, which has been arrived at by many naturalists under the designation +of single centres of creation, by some general considerations, more especially +from the importance of barriers and from the analogical distribution of +sub-genera, genera, and families. + +With respect to the distinct species of the same genus, which on my theory must +have spread from one parent-source; if we make the same allowances as before +for our ignorance, and remember that some forms of life change most slowly, +enormous periods of time being thus granted for their migration, I do not think +that the difficulties are insuperable; though they often are in this case, and +in that of the individuals of the same species, extremely grave. + +As exemplifying the effects of climatal changes on distribution, I have +attempted to show how important has been the influence of the modern Glacial +period, which I am fully convinced simultaneously affected the whole world, or +at least great meridional belts. As showing how diversified are the means of +occasional transport, I have discussed at some little length the means of +dispersal of fresh-water productions. + +If the difficulties be not insuperable in admitting that in the long course of +time the individuals of the same species, and likewise of allied species, have +proceeded from some one source; then I think all the grand leading facts of +geographical distribution are explicable on the theory of migration (generally +of the more dominant forms of life), together with subsequent modification and +the multiplication of new forms. We can thus understand the high importance of +barriers, whether of land or water, which separate our several zoological and +botanical provinces. We can thus understand the localisation of sub-genera, +genera, and families; and how it is that under different latitudes, for +instance in South America, the inhabitants of the plains and mountains, of the +forests, marshes, and deserts, are in so mysterious a manner linked together by +affinity, and are likewise linked to the extinct beings which formerly +inhabited the same continent. Bearing in mind that the mutual relations of +organism to organism are of the highest importance, we can see why two areas +having nearly the same physical conditions should often be inhabited by very +different forms of life; for according to the length of time which has elapsed +since new inhabitants entered one region; according to the nature of the +communication which allowed certain forms and not others to enter, either in +greater or lesser numbers; according or not, as those which entered happened to +come in more or less direct competition with each other and with the +aborigines; and according as the immigrants were capable of varying more or +less rapidly, there would ensue in different regions, independently of their +physical conditions, infinitely diversified conditions of life,—there would be +an almost endless amount of organic action and reaction,—and we should find, as +we do find, some groups of beings greatly, and some only slightly +modified,—some developed in great force, some existing in scanty numbers—in the +different great geographical provinces of the world. + +On these same principles, we can understand, as I have endeavoured to show, why +oceanic islands should have few inhabitants, but of these a great number should +be endemic or peculiar; and why, in relation to the means of migration, one +group of beings, even within the same class, should have all its species +endemic, and another group should have all its species common to other quarters +of the world. We can see why whole groups of organisms, as batrachians and +terrestrial mammals, should be absent from oceanic islands, whilst the most +isolated islands possess their own peculiar species of ærial mammals or bats. +We can see why there should be some relation between the presence of mammals, +in a more or less modified condition, and the depth of the sea between an +island and the mainland. We can clearly see why all the inhabitants of an +archipelago, though specifically distinct on the several islets, should be +closely related to each other, and likewise be related, but less closely, to +those of the nearest continent or other source whence immigrants were probably +derived. We can see why in two areas, however distant from each other, there +should be a correlation, in the presence of identical species, of varieties, of +doubtful species, and of distinct but representative species. + +As the late Edward Forbes often insisted, there is a striking parallelism in +the laws of life throughout time and space: the laws governing the succession +of forms in past times being nearly the same with those governing at the +present time the differences in different areas. We see this in many facts. The +endurance of each species and group of species is continuous in time; for the +exceptions to the rule are so few, that they may fairly be attributed to our +not having as yet discovered in an intermediate deposit the forms which are +therein absent, but which occur above and below: so in space, it certainly is +the general rule that the area inhabited by a single species, or by a group of +species, is continuous; and the exceptions, which are not rare, may, as I have +attempted to show, be accounted for by migration at some former period under +different conditions or by occasional means of transport, and by the species +having become extinct in the intermediate tracts. Both in time and space, +species and groups of species have their points of maximum development. Groups +of species, belonging either to a certain period of time, or to a certain area, +are often characterised by trifling characters in common, as of sculpture or +colour. In looking to the long succession of ages, as in now looking to distant +provinces throughout the world, we find that some organisms differ little, +whilst others belonging to a different class, or to a different order, or even +only to a different family of the same order, differ greatly. In both time and +space the lower members of each class generally change less than the higher; +but there are in both cases marked exceptions to the rule. On my theory these +several relations throughout time and space are intelligible; for whether we +look to the forms of life which have changed during successive ages within the +same quarter of the world, or to those which have changed after having migrated +into distant quarters, in both cases the forms within each class have been +connected by the same bond of ordinary generation; and the more nearly any two +forms are related in blood, the nearer they will generally stand to each other +in time and space; in both cases the laws of variation have been the same, and +modifications have been accumulated by the same power of natural selection. + +CHAPTER XIII. +MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY +ORGANS. + +CLASSIFICATION, groups subordinate to groups. Natural system. Rules and +difficulties in classification, explained on the theory of descent with +modification. Classification of varieties. Descent always used in +classification. Analogical or adaptive characters. Affinities, general, complex +and radiating. Extinction separates and defines groups. MORPHOLOGY, between +members of the same class, between parts of the same individual. EMBRYOLOGY, +laws of, explained by variations not supervening at an early age, and being +inherited at a corresponding age. RUDIMENTARY ORGANS; their origin explained. +Summary. + +From the first dawn of life, all organic beings are found to resemble each +other in descending degrees, so that they can be classed in groups under +groups. This classification is evidently not arbitrary like the grouping of the +stars in constellations. The existence of groups would have been of simple +signification, if one group had been exclusively fitted to inhabit the land, +and another the water; one to feed on flesh, another on vegetable matter, and +so on; but the case is widely different in nature; for it is notorious how +commonly members of even the same subgroup have different habits. In our second +and fourth chapters, on Variation and on Natural Selection, I have attempted to +show that it is the widely ranging, the much diffused and common, that is the +dominant species belonging to the larger genera, which vary most. The +varieties, or incipient species, thus produced ultimately become converted, as +I believe, into new and distinct species; and these, on the principle of +inheritance, tend to produce other new and dominant species. Consequently the +groups which are now large, and which generally include many dominant species, +tend to go on increasing indefinitely in size. I further attempted to show that +from the varying descendants of each species trying to occupy as many and as +different places as possible in the economy of nature, there is a constant +tendency in their characters to diverge. This conclusion was supported by +looking at the great diversity of the forms of life which, in any small area, +come into the closest competition, and by looking to certain facts in +naturalisation. + +I attempted also to show that there is a constant tendency in the forms which +are increasing in number and diverging in character, to supplant and +exterminate the less divergent, the less improved, and preceding forms. I +request the reader to turn to the diagram illustrating the action, as formerly +explained, of these several principles; and he will see that the inevitable +result is that the modified descendants proceeding from one progenitor become +broken up into groups subordinate to groups. In the diagram each letter on the +uppermost line may represent a genus including several species; and all the +genera on this line form together one class, for all have descended from one +ancient but unseen parent, and, consequently, have inherited something in +common. But the three genera on the left hand have, on this same principle, +much in common, and form a sub-family, distinct from that including the next +two genera on the right hand, which diverged from a common parent at the fifth +stage of descent. These five genera have also much, though less, in common; and +they form a family distinct from that including the three genera still further +to the right hand, which diverged at a still earlier period. And all these +genera, descended from (A), form an order distinct from the genera descended +from (I). So that we here have many species descended from a single progenitor +grouped into genera; and the genera are included in, or subordinate to, +sub-families, families, and orders, all united into one class. Thus, the grand +fact in natural history of the subordination of group under group, which, from +its familiarity, does not always sufficiently strike us, is in my judgment +fully explained. + +Naturalists try to arrange the species, genera, and families in each class, on +what is called the Natural System. But what is meant by this system? Some +authors look at it merely as a scheme for arranging together those living +objects which are most alike, and for separating those which are most unlike; +or as an artificial means for enunciating, as briefly as possible, general +propositions,—that is, by one sentence to give the characters common, for +instance, to all mammals, by another those common to all carnivora, by another +those common to the dog-genus, and then by adding a single sentence, a full +description is given of each kind of dog. The ingenuity and utility of this +system are indisputable. But many naturalists think that something more is +meant by the Natural System; they believe that it reveals the plan of the +Creator; but unless it be specified whether order in time or space, or what +else is meant by the plan of the Creator, it seems to me that nothing is thus +added to our knowledge. Such expressions as that famous one of Linnæus, and +which we often meet with in a more or less concealed form, that the characters +do not make the genus, but that the genus gives the characters, seem to imply +that something more is included in our classification, than mere resemblance. I +believe that something more is included; and that propinquity of descent,—the +only known cause of the similarity of organic beings,—is the bond, hidden as it +is by various degrees of modification, which is partially revealed to us by our +classifications. + +Let us now consider the rules followed in classification, and the difficulties +which are encountered on the view that classification either gives some unknown +plan of creation, or is simply a scheme for enunciating general propositions +and of placing together the forms most like each other. It might have been +thought (and was in ancient times thought) that those parts of the structure +which determined the habits of life, and the general place of each being in the +economy of nature, would be of very high importance in classification. Nothing +can be more false. No one regards the external similarity of a mouse to a +shrew, of a dugong to a whale, of a whale to a fish, as of any importance. +These resemblances, though so intimately connected with the whole life of the +being, are ranked as merely “adaptive or analogical characters;” but to the +consideration of these resemblances we shall have to recur. It may even be +given as a general rule, that the less any part of the organisation is +concerned with special habits, the more important it becomes for +classification. As an instance: Owen, in speaking of the dugong, says, “The +generative organs being those which are most remotely related to the habits and +food of an animal, I have always regarded as affording very clear indications +of its true affinities. We are least likely in the modifications of these +organs to mistake a merely adaptive for an essential character.” So with +plants, how remarkable it is that the organs of vegetation, on which their +whole life depends, are of little signification, excepting in the first main +divisions; whereas the organs of reproduction, with their product the seed, are +of paramount importance! + +We must not, therefore, in classifying, trust to resemblances in parts of the +organisation, however important they may be for the welfare of the being in +relation to the outer world. Perhaps from this cause it has partly arisen, that +almost all naturalists lay the greatest stress on resemblances in organs of +high vital or physiological importance. No doubt this view of the +classificatory importance of organs which are important is generally, but by no +means always, true. But their importance for classification, I believe, depends +on their greater constancy throughout large groups of species; and this +constancy depends on such organs having generally been subjected to less change +in the adaptation of the species to their conditions of life. That the mere +physiological importance of an organ does not determine its classificatory +value, is almost shown by the one fact, that in allied groups, in which the +same organ, as we have every reason to suppose, has nearly the same +physiological value, its classificatory value is widely different. No +naturalist can have worked at any group without being struck with this fact; +and it has been most fully acknowledged in the writings of almost every author. +It will suffice to quote the highest authority, Robert Brown, who in speaking +of certain organs in the Proteaceæ, says their generic importance, “like that +of all their parts, not only in this but, as I apprehend, in every natural +family, is very unequal, and in some cases seems to be entirely lost.” Again in +another work he says, the genera of the Connaraceæ “differ in having one or +more ovaria, in the existence or absence of albumen, in the imbricate or +valvular æstivation. Any one of these characters singly is frequently of more +than generic importance, though here even when all taken together they appear +insufficient to separate Cnestis from Connarus.” To give an example amongst +insects, in one great division of the Hymenoptera, the antennæ, as Westwood has +remarked, are most constant in structure; in another division they differ much, +and the differences are of quite subordinate value in classification; yet no +one probably will say that the antennæ in these two divisions of the same order +are of unequal physiological importance. Any number of instances could be given +of the varying importance for classification of the same important organ within +the same group of beings. + +Again, no one will say that rudimentary or atrophied organs are of high +physiological or vital importance; yet, undoubtedly, organs in this condition +are often of high value in classification. No one will dispute that the +rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary +bones of the leg, are highly serviceable in exhibiting the close affinity +between Ruminants and Pachyderms. Robert Brown has strongly insisted on the +fact that the rudimentary florets are of the highest importance in the +classification of the Grasses. + +Numerous instances could be given of characters derived from parts which must +be considered of very trifling physiological importance, but which are +universally admitted as highly serviceable in the definition of whole groups. +For instance, whether or not there is an open passage from the nostrils to the +mouth, the only character, according to Owen, which absolutely distinguishes +fishes and reptiles—the inflection of the angle of the jaws in Marsupials—the +manner in which the wings of insects are folded—mere colour in certain +Algæ—mere pubescence on parts of the flower in grasses—the nature of the dermal +covering, as hair or feathers, in the Vertebrata. If the Ornithorhynchus had +been covered with feathers instead of hair, this external and trifling +character would, I think, have been considered by naturalists as important an +aid in determining the degree of affinity of this strange creature to birds and +reptiles, as an approach in structure in any one internal and important organ. + +The importance, for classification, of trifling characters, mainly depends on +their being correlated with several other characters of more or less +importance. The value indeed of an aggregate of characters is very evident in +natural history. Hence, as has often been remarked, a species may depart from +its allies in several characters, both of high physiological importance and of +almost universal prevalence, and yet leave us in no doubt where it should be +ranked. Hence, also, it has been found, that a classification founded on any +single character, however important that may be, has always failed; for no part +of the organisation is universally constant. The importance of an aggregate of +characters, even when none are important, alone explains, I think, that saying +of Linnæus, that the characters do not give the genus, but the genus gives the +characters; for this saying seems founded on an appreciation of many trifling +points of resemblance, too slight to be defined. Certain plants, belonging to +the Malpighiaceæ, bear perfect and degraded flowers; in the latter, as A. de +Jussieu has remarked, “the greater number of the characters proper to the +species, to the genus, to the family, to the class, disappear, and thus laugh +at our classification.” But when Aspicarpa produced in France, during several +years, only degraded flowers, departing so wonderfully in a number of the most +important points of structure from the proper type of the order, yet M. Richard +sagaciously saw, as Jussieu observes, that this genus should still be retained +amongst the Malpighiaceæ. This case seems to me well to illustrate the spirit +with which our classifications are sometimes necessarily founded. + +Practically when naturalists are at work, they do not trouble themselves about +the physiological value of the characters which they use in defining a group, +or in allocating any particular species. If they find a character nearly +uniform, and common to a great number of forms, and not common to others, they +use it as one of high value; if common to some lesser number, they use it as of +subordinate value. This principle has been broadly confessed by some +naturalists to be the true one; and by none more clearly than by that excellent +botanist, Aug. St. Hilaire. If certain characters are always found correlated +with others, though no apparent bond of connexion can be discovered between +them, especial value is set on them. As in most groups of animals, important +organs, such as those for propelling the blood, or for ærating it, or those for +propagating the race, are found nearly uniform, they are considered as highly +serviceable in classification; but in some groups of animals all these, the +most important vital organs, are found to offer characters of quite subordinate +value. + +We can see why characters derived from the embryo should be of equal importance +with those derived from the adult, for our classifications of course include +all ages of each species. But it is by no means obvious, on the ordinary view, +why the structure of the embryo should be more important for this purpose than +that of the adult, which alone plays its full part in the economy of nature. +Yet it has been strongly urged by those great naturalists, Milne Edwards and +Agassiz, that embryonic characters are the most important of any in the +classification of animals; and this doctrine has very generally been admitted +as true. The same fact holds good with flowering plants, of which the two main +divisions have been founded on characters derived from the embryo,—on the +number and position of the embryonic leaves or cotyledons, and on the mode of +development of the plumule and radicle. In our discussion on embryology, we +shall see why such characters are so valuable, on the view of classification +tacitly including the idea of descent. + +Our classifications are often plainly influenced by chains of affinities. +Nothing can be easier than to define a number of characters common to all +birds; but in the case of crustaceans, such definition has hitherto been found +impossible. There are crustaceans at the opposite ends of the series, which +have hardly a character in common; yet the species at both ends, from being +plainly allied to others, and these to others, and so onwards, can be +recognised as unequivocally belonging to this, and to no other class of the +Articulata. + +Geographical distribution has often been used, though perhaps not quite +logically, in classification, more especially in very large groups of closely +allied forms. Temminck insists on the utility or even necessity of this +practice in certain groups of birds; and it has been followed by several +entomologists and botanists. + +Finally, with respect to the comparative value of the various groups of +species, such as orders, sub-orders, families, sub-families, and genera, they +seem to be, at least at present, almost arbitrary. Several of the best +botanists, such as Mr. Bentham and others, have strongly insisted on their +arbitrary value. Instances could be given amongst plants and insects, of a +group of forms, first ranked by practised naturalists as only a genus, and then +raised to the rank of a sub-family or family; and this has been done, not +because further research has detected important structural differences, at +first overlooked, but because numerous allied species, with slightly different +grades of difference, have been subsequently discovered. + +All the foregoing rules and aids and difficulties in classification are +explained, if I do not greatly deceive myself, on the view that the natural +system is founded on descent with modification; that the characters which +naturalists consider as showing true affinity between any two or more species, +are those which have been inherited from a common parent, and, in so far, all +true classification is genealogical; that community of descent is the hidden +bond which naturalists have been unconsciously seeking, and not some unknown +plan of creation, or the enunciation of general propositions, and the mere +putting together and separating objects more or less alike. + +But I must explain my meaning more fully. I believe that the arrangement of the +groups within each class, in due subordination and relation to the other +groups, must be strictly genealogical in order to be natural; but that the +amount of difference in the several branches or groups, though allied in the +same degree in blood to their common progenitor, may differ greatly, being due +to the different degrees of modification which they have undergone; and this is +expressed by the forms being ranked under different genera, families, sections, +or orders. The reader will best understand what is meant, if he will take the +trouble of referring to the diagram in the fourth chapter. We will suppose the +letters A to L to represent allied genera, which lived during the Silurian +epoch, and these have descended from a species which existed at an unknown +anterior period. Species of three of these genera (A, F, and I) have +transmitted modified descendants to the present day, represented by the fifteen +genera (a^14 to z^14) on the uppermost horizontal line. Now all these modified +descendants from a single species, are represented as related in blood or +descent to the same degree; they may metaphorically be called cousins to the +same millionth degree; yet they differ widely and in different degrees from +each other. The forms descended from A, now broken up into two or three +families, constitute a distinct order from those descended from I, also broken +up into two families. Nor can the existing species, descended from A, be ranked +in the same genus with the parent A; or those from I, with the parent I. But +the existing genus F^14 may be supposed to have been but slightly modified; and +it will then rank with the parent-genus F; just as some few still living +organic beings belong to Silurian genera. So that the amount or value of the +differences between organic beings all related to each other in the same degree +in blood, has come to be widely different. Nevertheless their genealogical +arrangement remains strictly true, not only at the present time, but at each +successive period of descent. All the modified descendants from A will have +inherited something in common from their common parent, as will all the +descendants from I; so will it be with each subordinate branch of descendants, +at each successive period. If, however, we choose to suppose that any of the +descendants of A or of I have been so much modified as to have more or less +completely lost traces of their parentage, in this case, their places in a +natural classification will have been more or less completely lost,—as +sometimes seems to have occurred with existing organisms. All the descendants +of the genus F, along its whole line of descent, are supposed to have been but +little modified, and they yet form a single genus. But this genus, though much +isolated, will still occupy its proper intermediate position; for F originally +was intermediate in character between A and I, and the several genera descended +from these two genera will have inherited to a certain extent their characters. +This natural arrangement is shown, as far as is possible on paper, in the +diagram, but in much too simple a manner. If a branching diagram had not been +used, and only the names of the groups had been written in a linear series, it +would have been still less possible to have given a natural arrangement; and it +is notoriously not possible to represent in a series, on a flat surface, the +affinities which we discover in nature amongst the beings of the same group. +Thus, on the view which I hold, the natural system is genealogical in its +arrangement, like a pedigree; but the degrees of modification which the +different groups have undergone, have to be expressed by ranking them under +different so-called genera, sub-families, families, sections, orders, and +classes. + +It may be worth while to illustrate this view of classification, by taking the +case of languages. If we possessed a perfect pedigree of mankind, a +genealogical arrangement of the races of man would afford the best +classification of the various languages now spoken throughout the world; and if +all extinct languages, and all intermediate and slowly changing dialects, had +to be included, such an arrangement would, I think, be the only possible one. +Yet it might be that some very ancient language had altered little, and had +given rise to few new languages, whilst others (owing to the spreading and +subsequent isolation and states of civilisation of the several races, descended +from a common race) had altered much, and had given rise to many new languages +and dialects. The various degrees of difference in the languages from the same +stock, would have to be expressed by groups subordinate to groups; but the +proper or even only possible arrangement would still be genealogical; and this +would be strictly natural, as it would connect together all languages, extinct +and modern, by the closest affinities, and would give the filiation and origin +of each tongue. + +In confirmation of this view, let us glance at the classification of varieties, +which are believed or known to have descended from one species. These are +grouped under species, with sub-varieties under varieties; and with our +domestic productions, several other grades of difference are requisite, as we +have seen with pigeons. The origin of the existence of groups subordinate to +groups, is the same with varieties as with species, namely, closeness of +descent with various degrees of modification. Nearly the same rules are +followed in classifying varieties, as with species. Authors have insisted on +the necessity of classing varieties on a natural instead of an artificial +system; we are cautioned, for instance, not to class two varieties of the +pine-apple together, merely because their fruit, though the most important +part, happens to be nearly identical; no one puts the swedish and common +turnips together, though the esculent and thickened stems are so similar. +Whatever part is found to be most constant, is used in classing varieties: thus +the great agriculturist Marshall says the horns are very useful for this +purpose with cattle, because they are less variable than the shape or colour of +the body, etc.; whereas with sheep the horns are much less serviceable, because +less constant. In classing varieties, I apprehend if we had a real pedigree, a +genealogical classification would be universally preferred; and it has been +attempted by some authors. For we might feel sure, whether there had been more +or less modification, the principle of inheritance would keep the forms +together which were allied in the greatest number of points. In tumbler +pigeons, though some sub-varieties differ from the others in the important +character of having a longer beak, yet all are kept together from having the +common habit of tumbling; but the short-faced breed has nearly or quite lost +this habit; nevertheless, without any reasoning or thinking on the subject, +these tumblers are kept in the same group, because allied in blood and alike in +some other respects. If it could be proved that the Hottentot had descended +from the Negro, I think he would be classed under the Negro group, however much +he might differ in colour and other important characters from negroes. + +With species in a state of nature, every naturalist has in fact brought descent +into his classification; for he includes in his lowest grade, or that of a +species, the two sexes; and how enormously these sometimes differ in the most +important characters, is known to every naturalist: scarcely a single fact can +be predicated in common of the males and hermaphrodites of certain cirripedes, +when adult, and yet no one dreams of separating them. The naturalist includes +as one species the several larval stages of the same individual, however much +they may differ from each other and from the adult; as he likewise includes the +so-called alternate generations of Steenstrup, which can only in a technical +sense be considered as the same individual. He includes monsters; he includes +varieties, not solely because they closely resemble the parent-form, but +because they are descended from it. He who believes that the cowslip is +descended from the primrose, or conversely, ranks them together as a single +species, and gives a single definition. As soon as three Orchidean forms +(Monochanthus, Myanthus, and Catasetum), which had previously been ranked as +three distinct genera, were known to be sometimes produced on the same spike, +they were immediately included as a single species. But it may be asked, what +ought we to do, if it could be proved that one species of kangaroo had been +produced, by a long course of modification, from a bear? Ought we to rank this +one species with bears, and what should we do with the other species? The +supposition is of course preposterous; and I might answer by the argumentum ad +hominem, and ask what should be done if a perfect kangaroo were seen to come +out of the womb of a bear? According to all analogy, it would be ranked with +bears; but then assuredly all the other species of the kangaroo family would +have to be classed under the bear genus. The whole case is preposterous; for +where there has been close descent in common, there will certainly be close +resemblance or affinity. + +As descent has universally been used in classing together the individuals of +the same species, though the males and females and larvæ are sometimes +extremely different; and as it has been used in classing varieties which have +undergone a certain, and sometimes a considerable amount of modification, may +not this same element of descent have been unconsciously used in grouping +species under genera, and genera under higher groups, though in these cases the +modification has been greater in degree, and has taken a longer time to +complete? I believe it has thus been unconsciously used; and only thus can I +understand the several rules and guides which have been followed by our best +systematists. We have no written pedigrees; we have to make out community of +descent by resemblances of any kind. Therefore we choose those characters +which, as far as we can judge, are the least likely to have been modified in +relation to the conditions of life to which each species has been recently +exposed. Rudimentary structures on this view are as good as, or even sometimes +better than, other parts of the organisation. We care not how trifling a +character may be—let it be the mere inflection of the angle of the jaw, the +manner in which an insect’s wing is folded, whether the skin be covered by hair +or feathers—if it prevail throughout many and different species, especially +those having very different habits of life, it assumes high value; for we can +account for its presence in so many forms with such different habits, only by +its inheritance from a common parent. We may err in this respect in regard to +single points of structure, but when several characters, let them be ever so +trifling, occur together throughout a large group of beings having different +habits, we may feel almost sure, on the theory of descent, that these +characters have been inherited from a common ancestor. And we know that such +correlated or aggregated characters have especial value in classification. + +We can understand why a species or a group of species may depart, in several of +its most important characteristics, from its allies, and yet be safely classed +with them. This may be safely done, and is often done, as long as a sufficient +number of characters, let them be ever so unimportant, betrays the hidden bond +of community of descent. Let two forms have not a single character in common, +yet if these extreme forms are connected together by a chain of intermediate +groups, we may at once infer their community of descent, and we put them all +into the same class. As we find organs of high physiological importance—those +which serve to preserve life under the most diverse conditions of existence—are +generally the most constant, we attach especial value to them; but if these +same organs, in another group or section of a group, are found to differ much, +we at once value them less in our classification. We shall hereafter, I think, +clearly see why embryological characters are of such high classificatory +importance. Geographical distribution may sometimes be brought usefully into +play in classing large and widely-distributed genera, because all the species +of the same genus, inhabiting any distinct and isolated region, have in all +probability descended from the same parents. + +We can understand, on these views, the very important distinction between real +affinities and analogical or adaptive resemblances. Lamarck first called +attention to this distinction, and he has been ably followed by Macleay and +others. The resemblance, in the shape of the body and in the fin-like anterior +limbs, between the dugong, which is a pachydermatous animal, and the whale, and +between both these mammals and fishes, is analogical. Amongst insects there are +innumerable instances: thus Linnæus, misled by external appearances, actually +classed an homopterous insect as a moth. We see something of the same kind even +in our domestic varieties, as in the thickened stems of the common and swedish +turnip. The resemblance of the greyhound and racehorse is hardly more fanciful +than the analogies which have been drawn by some authors between very distinct +animals. On my view of characters being of real importance for classification, +only in so far as they reveal descent, we can clearly understand why analogical +or adaptive character, although of the utmost importance to the welfare of the +being, are almost valueless to the systematist. For animals, belonging to two +most distinct lines of descent, may readily become adapted to similar +conditions, and thus assume a close external resemblance; but such resemblances +will not reveal—will rather tend to conceal their blood-relationship to their +proper lines of descent. We can also understand the apparent paradox, that the +very same characters are analogical when one class or order is compared with +another, but give true affinities when the members of the same class or order +are compared one with another: thus the shape of the body and fin-like limbs +are only analogical when whales are compared with fishes, being adaptations in +both classes for swimming through the water; but the shape of the body and +fin-like limbs serve as characters exhibiting true affinity between the several +members of the whale family; for these cetaceans agree in so many characters, +great and small, that we cannot doubt that they have inherited their general +shape of body and structure of limbs from a common ancestor. So it is with +fishes. + +As members of distinct classes have often been adapted by successive slight +modifications to live under nearly similar circumstances,—to inhabit for +instance the three elements of land, air, and water,—we can perhaps understand +how it is that a numerical parallelism has sometimes been observed between the +sub-groups in distinct classes. A naturalist, struck by a parallelism of this +nature in any one class, by arbitrarily raising or sinking the value of the +groups in other classes (and all our experience shows that this valuation has +hitherto been arbitrary), could easily extend the parallelism over a wide +range; and thus the septenary, quinary, quaternary, and ternary classifications +have probably arisen. + +As the modified descendants of dominant species, belonging to the larger +genera, tend to inherit the advantages, which made the groups to which they +belong large and their parents dominant, they are almost sure to spread widely, +and to seize on more and more places in the economy of nature. The larger and +more dominant groups thus tend to go on increasing in size; and they +consequently supplant many smaller and feebler groups. Thus we can account for +the fact that all organisms, recent and extinct, are included under a few great +orders, under still fewer classes, and all in one great natural system. As +showing how few the higher groups are in number, and how widely spread they are +throughout the world, the fact is striking, that the discovery of Australia has +not added a single insect belonging to a new order; and that in the vegetable +kingdom, as I learn from Dr. Hooker, it has added only two or three orders of +small size. + +In the chapter on geological succession I attempted to show, on the principle +of each group having generally diverged much in character during the +long-continued process of modification, how it is that the more ancient forms +of life often present characters in some slight degree intermediate between +existing groups. A few old and intermediate parent-forms having occasionally +transmitted to the present day descendants but little modified, will give to us +our so-called osculant or aberrant groups. The more aberrant any form is, the +greater must be the number of connecting forms which on my theory have been +exterminated and utterly lost. And we have some evidence of aberrant forms +having suffered severely from extinction, for they are generally represented by +extremely few species; and such species as do occur are generally very distinct +from each other, which again implies extinction. The genera Ornithorhynchus and +Lepidosiren, for example, would not have been less aberrant had each been +represented by a dozen species instead of by a single one; but such richness in +species, as I find after some investigation, does not commonly fall to the lot +of aberrant genera. We can, I think, account for this fact only by looking at +aberrant forms as failing groups conquered by more successful competitors, with +a few members preserved by some unusual coincidence of favourable +circumstances. + +Mr. Waterhouse has remarked that, when a member belonging to one group of +animals exhibits an affinity to a quite distinct group, this affinity in most +cases is general and not special: thus, according to Mr. Waterhouse, of all +Rodents, the bizcacha is most nearly related to Marsupials; but in the points +in which it approaches this order, its relations are general, and not to any +one marsupial species more than to another. As the points of affinity of the +bizcacha to Marsupials are believed to be real and not merely adaptive, they +are due on my theory to inheritance in common. Therefore we must suppose either +that all Rodents, including the bizcacha, branched off from some very ancient +Marsupial, which will have had a character in some degree intermediate with +respect to all existing Marsupials; or that both Rodents and Marsupials +branched off from a common progenitor, and that both groups have since +undergone much modification in divergent directions. On either view we may +suppose that the bizcacha has retained, by inheritance, more of the character +of its ancient progenitor than have other Rodents; and therefore it will not be +specially related to any one existing Marsupial, but indirectly to all or +nearly all Marsupials, from having partially retained the character of their +common progenitor, or of an early member of the group. On the other hand, of +all Marsupials, as Mr. Waterhouse has remarked, the phascolomys resembles most +nearly, not any one species, but the general order of Rodents. In this case, +however, it may be strongly suspected that the resemblance is only analogical, +owing to the phascolomys having become adapted to habits like those of a +Rodent. The elder De Candolle has made nearly similar observations on the +general nature of the affinities of distinct orders of plants. + +On the principle of the multiplication and gradual divergence in character of +the species descended from a common parent, together with their retention by +inheritance of some characters in common, we can understand the excessively +complex and radiating affinities by which all the members of the same family or +higher group are connected together. For the common parent of a whole family of +species, now broken up by extinction into distinct groups and sub-groups, will +have transmitted some of its characters, modified in various ways and degrees, +to all; and the several species will consequently be related to each other by +circuitous lines of affinity of various lengths (as may be seen in the diagram +so often referred to), mounting up through many predecessors. As it is +difficult to show the blood-relationship between the numerous kindred of any +ancient and noble family, even by the aid of a genealogical tree, and almost +impossible to do this without this aid, we can understand the extraordinary +difficulty which naturalists have experienced in describing, without the aid of +a diagram, the various affinities which they perceive between the many living +and extinct members of the same great natural class. + +Extinction, as we have seen in the fourth chapter, has played an important part +in defining and widening the intervals between the several groups in each +class. We may thus account even for the distinctness of whole classes from each +other—for instance, of birds from all other vertebrate animals—by the belief +that many ancient forms of life have been utterly lost, through which the early +progenitors of birds were formerly connected with the early progenitors of the +other vertebrate classes. There has been less entire extinction of the forms of +life which once connected fishes with batrachians. There has been still less in +some other classes, as in that of the Crustacea, for here the most wonderfully +diverse forms are still tied together by a long, but broken, chain of +affinities. Extinction has only separated groups: it has by no means made them; +for if every form which has ever lived on this earth were suddenly to reappear, +though it would be quite impossible to give definitions by which each group +could be distinguished from other groups, as all would blend together by steps +as fine as those between the finest existing varieties, nevertheless a natural +classification, or at least a natural arrangement, would be possible. We shall +see this by turning to the diagram: the letters, A to L, may represent eleven +Silurian genera, some of which have produced large groups of modified +descendants. Every intermediate link between these eleven genera and their +primordial parent, and every intermediate link in each branch and sub-branch of +their descendants, may be supposed to be still alive; and the links to be as +fine as those between the finest varieties. In this case it would be quite +impossible to give any definition by which the several members of the several +groups could be distinguished from their more immediate parents; or these +parents from their ancient and unknown progenitor. Yet the natural arrangement +in the diagram would still hold good; and, on the principle of inheritance, all +the forms descended from A, or from I, would have something in common. In a +tree we can specify this or that branch, though at the actual fork the two +unite and blend together. We could not, as I have said, define the several +groups; but we could pick out types, or forms, representing most of the +characters of each group, whether large or small, and thus give a general idea +of the value of the differences between them. This is what we should be driven +to, if we were ever to succeed in collecting all the forms in any class which +have lived throughout all time and space. We shall certainly never succeed in +making so perfect a collection: nevertheless, in certain classes, we are +tending in this direction; and Milne Edwards has lately insisted, in an able +paper, on the high importance of looking to types, whether or not we can +separate and define the groups to which such types belong. + +Finally, we have seen that natural selection, which results from the struggle +for existence, and which almost inevitably induces extinction and divergence of +character in the many descendants from one dominant parent-species, explains +that great and universal feature in the affinities of all organic beings, +namely, their subordination in group under group. We use the element of descent +in classing the individuals of both sexes and of all ages, although having few +characters in common, under one species; we use descent in classing +acknowledged varieties, however different they may be from their parent; and I +believe this element of descent is the hidden bond of connexion which +naturalists have sought under the term of the Natural System. On this idea of +the natural system being, in so far as it has been perfected, genealogical in +its arrangement, with the grades of difference between the descendants from a +common parent, expressed by the terms genera, families, orders, etc., we can +understand the rules which we are compelled to follow in our classification. We +can understand why we value certain resemblances far more than others; why we +are permitted to use rudimentary and useless organs, or others of trifling +physiological importance; why, in comparing one group with a distinct group, we +summarily reject analogical or adaptive characters, and yet use these same +characters within the limits of the same group. We can clearly see how it is +that all living and extinct forms can be grouped together in one great system; +and how the several members of each class are connected together by the most +complex and radiating lines of affinities. We shall never, probably, +disentangle the inextricable web of affinities between the members of any one +class; but when we have a distinct object in view, and do not look to some +unknown plan of creation, we may hope to make sure but slow progress. + +Morphology.—We have seen that the members of the same class, independently of +their habits of life, resemble each other in the general plan of their +organisation. This resemblance is often expressed by the term “unity of type;” +or by saying that the several parts and organs in the different species of the +class are homologous. The whole subject is included under the general name of +Morphology. This is the most interesting department of natural history, and may +be said to be its very soul. What can be more curious than that the hand of a +man, formed for grasping, that of a mole for digging, the leg of the horse, the +paddle of the porpoise, and the wing of the bat, should all be constructed on +the same pattern, and should include the same bones, in the same relative +positions? Geoffroy St. Hilaire has insisted strongly on the high importance of +relative connexion in homologous organs: the parts may change to almost any +extent in form and size, and yet they always remain connected together in the +same order. We never find, for instance, the bones of the arm and forearm, or +of the thigh and leg, transposed. Hence the same names can be given to the +homologous bones in widely different animals. We see the same great law in the +construction of the mouths of insects: what can be more different than the +immensely long spiral proboscis of a sphinx-moth, the curious folded one of a +bee or bug, and the great jaws of a beetle?—yet all these organs, serving for +such different purposes, are formed by infinitely numerous modifications of an +upper lip, mandibles, and two pairs of maxillæ. Analogous laws govern the +construction of the mouths and limbs of crustaceans. So it is with the flowers +of plants. + +Nothing can be more hopeless than to attempt to explain this similarity of +pattern in members of the same class, by utility or by the doctrine of final +causes. The hopelessness of the attempt has been expressly admitted by Owen in +his most interesting work on the ‘Nature of Limbs.’ On the ordinary view of the +independent creation of each being, we can only say that so it is;—that it has +so pleased the Creator to construct each animal and plant. + +The explanation is manifest on the theory of the natural selection of +successive slight modifications,—each modification being profitable in some way +to the modified form, but often affecting by correlation of growth other parts +of the organisation. In changes of this nature, there will be little or no +tendency to modify the original pattern, or to transpose parts. The bones of a +limb might be shortened and widened to any extent, and become gradually +enveloped in thick membrane, so as to serve as a fin; or a webbed foot might +have all its bones, or certain bones, lengthened to any extent, and the +membrane connecting them increased to any extent, so as to serve as a wing: yet +in all this great amount of modification there will be no tendency to alter the +framework of bones or the relative connexion of the several parts. If we +suppose that the ancient progenitor, the archetype as it may be called, of all +mammals, had its limbs constructed on the existing general pattern, for +whatever purpose they served, we can at once perceive the plain signification +of the homologous construction of the limbs throughout the whole class. So with +the mouths of insects, we have only to suppose that their common progenitor had +an upper lip, mandibles, and two pair of maxillæ, these parts being perhaps +very simple in form; and then natural selection will account for the infinite +diversity in structure and function of the mouths of insects. Nevertheless, it +is conceivable that the general pattern of an organ might become so much +obscured as to be finally lost, by the atrophy and ultimately by the complete +abortion of certain parts, by the soldering together of other parts, and by the +doubling or multiplication of others,—variations which we know to be within the +limits of possibility. In the paddles of the extinct gigantic sea-lizards, and +in the mouths of certain suctorial crustaceans, the general pattern seems to +have been thus to a certain extent obscured. + +There is another and equally curious branch of the present subject; namely, the +comparison not of the same part in different members of a class, but of the +different parts or organs in the same individual. Most physiologists believe +that the bones of the skull are homologous with—that is correspond in number +and in relative connexion with—the elemental parts of a certain number of +vertebræ. The anterior and posterior limbs in each member of the vertebrate and +articulate classes are plainly homologous. We see the same law in comparing the +wonderfully complex jaws and legs in crustaceans. It is familiar to almost +every one, that in a flower the relative position of the sepals, petals, +stamens, and pistils, as well as their intimate structure, are intelligible on +the view that they consist of metamorphosed leaves, arranged in a spire. In +monstrous plants, we often get direct evidence of the possibility of one organ +being transformed into another; and we can actually see in embryonic +crustaceans and in many other animals, and in flowers, that organs, which when +mature become extremely different, are at an early stage of growth exactly +alike. + +How inexplicable are these facts on the ordinary view of creation! Why should +the brain be enclosed in a box composed of such numerous and such +extraordinarily shaped pieces of bone? As Owen has remarked, the benefit +derived from the yielding of the separate pieces in the act of parturition of +mammals, will by no means explain the same construction in the skulls of birds. +Why should similar bones have been created in the formation of the wing and leg +of a bat, used as they are for such totally different purposes? Why should one +crustacean, which has an extremely complex mouth formed of many parts, +consequently always have fewer legs; or conversely, those with many legs have +simpler mouths? Why should the sepals, petals, stamens, and pistils in any +individual flower, though fitted for such widely different purposes, be all +constructed on the same pattern? + +On the theory of natural selection, we can satisfactorily answer these +questions. In the vertebrata, we see a series of internal vertebræ bearing +certain processes and appendages; in the articulata, we see the body divided +into a series of segments, bearing external appendages; and in flowering +plants, we see a series of successive spiral whorls of leaves. An indefinite +repetition of the same part or organ is the common characteristic (as Owen has +observed) of all low or little-modified forms; therefore we may readily believe +that the unknown progenitor of the vertebrata possessed many vertebræ; the +unknown progenitor of the articulata, many segments; and the unknown progenitor +of flowering plants, many spiral whorls of leaves. We have formerly seen that +parts many times repeated are eminently liable to vary in number and structure; +consequently it is quite probable that natural selection, during a +long-continued course of modification, should have seized on a certain number +of the primordially similar elements, many times repeated, and have adapted +them to the most diverse purposes. And as the whole amount of modification will +have been effected by slight successive steps, we need not wonder at +discovering in such parts or organs, a certain degree of fundamental +resemblance, retained by the strong principle of inheritance. + +In the great class of molluscs, though we can homologise the parts of one +species with those of another and distinct species, we can indicate but few +serial homologies; that is, we are seldom enabled to say that one part or organ +is homologous with another in the same individual. And we can understand this +fact; for in molluscs, even in the lowest members of the class, we do not find +nearly so much indefinite repetition of any one part, as we find in the other +great classes of the animal and vegetable kingdoms. + +Naturalists frequently speak of the skull as formed of metamorphosed vertebræ: +the jaws of crabs as metamorphosed legs; the stamens and pistils of flowers as +metamorphosed leaves; but it would in these cases probably be more correct, as +Professor Huxley has remarked, to speak of both skull and vertebræ, both jaws +and legs, etc.,—as having been metamorphosed, not one from the other, but from +some common element. Naturalists, however, use such language only in a +metaphorical sense: they are far from meaning that during a long course of +descent, primordial organs of any kind—vertebræ in the one case and legs in the +other—have actually been modified into skulls or jaws. Yet so strong is the +appearance of a modification of this nature having occurred, that naturalists +can hardly avoid employing language having this plain signification. On my view +these terms may be used literally; and the wonderful fact of the jaws, for +instance, of a crab retaining numerous characters, which they would probably +have retained through inheritance, if they had really been metamorphosed during +a long course of descent from true legs, or from some simple appendage, is +explained. + +Embryology.—It has already been casually remarked that certain organs in the +individual, which when mature become widely different and serve for different +purposes, are in the embryo exactly alike. The embryos, also, of distinct +animals within the same class are often strikingly similar: a better proof of +this cannot be given, than a circumstance mentioned by Agassiz, namely, that +having forgotten to ticket the embryo of some vertebrate animal, he cannot now +tell whether it be that of a mammal, bird, or reptile. The vermiform larvæ of +moths, flies, beetles, etc., resemble each other much more closely than do the +mature insects; but in the case of larvæ, the embryos are active, and have been +adapted for special lines of life. A trace of the law of embryonic resemblance, +sometimes lasts till a rather late age: thus birds of the same genus, and of +closely allied genera, often resemble each other in their first and second +plumage; as we see in the spotted feathers in the thrush group. In the cat +tribe, most of the species are striped or spotted in lines; and stripes can be +plainly distinguished in the whelp of the lion. We occasionally though rarely +see something of this kind in plants: thus the embryonic leaves of the ulex or +furze, and the first leaves of the phyllodineous acaceas, are pinnate or +divided like the ordinary leaves of the leguminosæ. + +The points of structure, in which the embryos of widely different animals of +the same class resemble each other, often have no direct relation to their +conditions of existence. We cannot, for instance, suppose that in the embryos +of the vertebrata the peculiar loop-like course of the arteries near the +branchial slits are related to similar conditions,—in the young mammal which is +nourished in the womb of its mother, in the egg of the bird which is hatched in +a nest, and in the spawn of a frog under water. We have no more reason to +believe in such a relation, than we have to believe that the same bones in the +hand of a man, wing of a bat, and fin of a porpoise, are related to similar +conditions of life. No one will suppose that the stripes on the whelp of a +lion, or the spots on the young blackbird, are of any use to these animals, or +are related to the conditions to which they are exposed. + +The case, however, is different when an animal during any part of its embryonic +career is active, and has to provide for itself. The period of activity may +come on earlier or later in life; but whenever it comes on, the adaptation of +the larva to its conditions of life is just as perfect and as beautiful as in +the adult animal. From such special adaptations, the similarity of the larvæ or +active embryos of allied animals is sometimes much obscured; and cases could be +given of the larvæ of two species, or of two groups of species, differing quite +as much, or even more, from each other than do their adult parents. In most +cases, however, the larvæ, though active, still obey more or less closely the +law of common embryonic resemblance. Cirripedes afford a good instance of this: +even the illustrious Cuvier did not perceive that a barnacle was, as it +certainly is, a crustacean; but a glance at the larva shows this to be the case +in an unmistakeable manner. So again the two main divisions of cirripedes, the +pedunculated and sessile, which differ widely in external appearance, have +larvæ in all their several stages barely distinguishable. + +The embryo in the course of development generally rises in organisation: I use +this expression, though I am aware that it is hardly possible to define clearly +what is meant by the organisation being higher or lower. But no one probably +will dispute that the butterfly is higher than the caterpillar. In some cases, +however, the mature animal is generally considered as lower in the scale than +the larva, as with certain parasitic crustaceans. To refer once again to +cirripedes: the larvæ in the first stage have three pairs of legs, a very +simple single eye, and a probosciformed mouth, with which they feed largely, +for they increase much in size. In the second stage, answering to the chrysalis +stage of butterflies, they have six pairs of beautifully constructed natatory +legs, a pair of magnificent compound eyes, and extremely complex antennæ; but +they have a closed and imperfect mouth, and cannot feed: their function at this +stage is, to search by their well-developed organs of sense, and to reach by +their active powers of swimming, a proper place on which to become attached and +to undergo their final metamorphosis. When this is completed they are fixed for +life: their legs are now converted into prehensile organs; they again obtain a +well-constructed mouth; but they have no antennæ, and their two eyes are now +reconverted into a minute, single, and very simple eye-spot. In this last and +complete state, cirripedes may be considered as either more highly or more +lowly organised than they were in the larval condition. But in some genera the +larvæ become developed either into hermaphrodites having the ordinary +structure, or into what I have called complemental males: and in the latter, +the development has assuredly been retrograde; for the male is a mere sack, +which lives for a short time, and is destitute of mouth, stomach, or other +organ of importance, excepting for reproduction. + +We are so much accustomed to see differences in structure between the embryo +and the adult, and likewise a close similarity in the embryos of widely +different animals within the same class, that we might be led to look at these +facts as necessarily contingent in some manner on growth. But there is no +obvious reason why, for instance, the wing of a bat, or the fin of a porpoise, +should not have been sketched out with all the parts in proper proportion, as +soon as any structure became visible in the embryo. And in some whole groups of +animals and in certain members of other groups, the embryo does not at any +period differ widely from the adult: thus Owen has remarked in regard to +cuttle-fish, “there is no metamorphosis; the cephalopodic character is +manifested long before the parts of the embryo are completed;” and again in +spiders, “there is nothing worthy to be called a metamorphosis.” The larvæ of +insects, whether adapted to the most diverse and active habits, or quite +inactive, being fed by their parents or placed in the midst of proper +nutriment, yet nearly all pass through a similar worm-like stage of +development; but in some few cases, as in that of Aphis, if we look to the +admirable drawings by Professor Huxley of the development of this insect, we +see no trace of the vermiform stage. + +How, then, can we explain these several facts in embryology,—namely the very +general, but not universal difference in structure between the embryo and the +adult;—of parts in the same individual embryo, which ultimately become very +unlike and serve for diverse purposes, being at this early period of growth +alike;—of embryos of different species within the same class, generally, but +not universally, resembling each other;—of the structure of the embryo not +being closely related to its conditions of existence, except when the embryo +becomes at any period of life active and has to provide for itself;—of the +embryo apparently having sometimes a higher organisation than the mature +animal, into which it is developed. I believe that all these facts can be +explained, as follows, on the view of descent with modification. + +It is commonly assumed, perhaps from monstrosities often affecting the embryo +at a very early period, that slight variations necessarily appear at an equally +early period. But we have little evidence on this head—indeed the evidence +rather points the other way; for it is notorious that breeders of cattle, +horses, and various fancy animals, cannot positively tell, until some time +after the animal has been born, what its merits or form will ultimately turn +out. We see this plainly in our own children; we cannot always tell whether the +child will be tall or short, or what its precise features will be. The question +is not, at what period of life any variation has been caused, but at what +period it is fully displayed. The cause may have acted, and I believe generally +has acted, even before the embryo is formed; and the variation may be due to +the male and female sexual elements having been affected by the conditions to +which either parent, or their ancestors, have been exposed. Nevertheless an +effect thus caused at a very early period, even before the formation of the +embryo, may appear late in life; as when an hereditary disease, which appears +in old age alone, has been communicated to the offspring from the reproductive +element of one parent. Or again, as when the horns of cross-bred cattle have +been affected by the shape of the horns of either parent. For the welfare of a +very young animal, as long as it remains in its mother’s womb, or in the egg, +or as long as it is nourished and protected by its parent, it must be quite +unimportant whether most of its characters are fully acquired a little earlier +or later in life. It would not signify, for instance, to a bird which obtained +its food best by having a long beak, whether or not it assumed a beak of this +particular length, as long as it was fed by its parents. Hence, I conclude, +that it is quite possible, that each of the many successive modifications, by +which each species has acquired its present structure, may have supervened at a +not very early period of life; and some direct evidence from our domestic +animals supports this view. But in other cases it is quite possible that each +successive modification, or most of them, may have appeared at an extremely +early period. + +I have stated in the first chapter, that there is some evidence to render it +probable, that at whatever age any variation first appears in the parent, it +tends to reappear at a corresponding age in the offspring. Certain variations +can only appear at corresponding ages, for instance, peculiarities in the +caterpillar, cocoon, or imago states of the silk-moth; or, again, in the horns +of almost full-grown cattle. But further than this, variations which, for all +that we can see, might have appeared earlier or later in life, tend to appear +at a corresponding age in the offspring and parent. I am far from meaning that +this is invariably the case; and I could give a good many cases of variations +(taking the word in the largest sense) which have supervened at an earlier age +in the child than in the parent. + +These two principles, if their truth be admitted, will, I believe, explain all +the above specified leading facts in embryology. But first let us look at a few +analogous cases in domestic varieties. Some authors who have written on Dogs, +maintain that the greyhound and bulldog, though appearing so different, are +really varieties most closely allied, and have probably descended from the same +wild stock; hence I was curious to see how far their puppies differed from each +other: I was told by breeders that they differed just as much as their parents, +and this, judging by the eye, seemed almost to be the case; but on actually +measuring the old dogs and their six-days old puppies, I found that the puppies +had not nearly acquired their full amount of proportional difference. So, +again, I was told that the foals of cart and race-horses differed as much as +the full-grown animals; and this surprised me greatly, as I think it probable +that the difference between these two breeds has been wholly caused by +selection under domestication; but having had careful measurements made of the +dam and of a three-days old colt of a race and heavy cart-horse, I find that +the colts have by no means acquired their full amount of proportional +difference. + +As the evidence appears to me conclusive, that the several domestic breeds of +Pigeon have descended from one wild species, I compared young pigeons of +various breeds, within twelve hours after being hatched; I carefully measured +the proportions (but will not here give details) of the beak, width of mouth, +length of nostril and of eyelid, size of feet and length of leg, in the wild +stock, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now +some of these birds, when mature, differ so extraordinarily in length and form +of beak, that they would, I cannot doubt, be ranked in distinct genera, had +they been natural productions. But when the nestling birds of these several +breeds were placed in a row, though most of them could be distinguished from +each other, yet their proportional differences in the above specified several +points were incomparably less than in the full-grown birds. Some characteristic +points of difference—for instance, that of the width of mouth—could hardly be +detected in the young. But there was one remarkable exception to this rule, for +the young of the short-faced tumbler differed from the young of the wild +rock-pigeon and of the other breeds, in all its proportions, almost exactly as +much as in the adult state. + +The two principles above given seem to me to explain these facts in regard to +the later embryonic stages of our domestic varieties. Fanciers select their +horses, dogs, and pigeons, for breeding, when they are nearly grown up: they +are indifferent whether the desired qualities and structures have been acquired +earlier or later in life, if the full-grown animal possesses them. And the +cases just given, more especially that of pigeons, seem to show that the +characteristic differences which give value to each breed, and which have been +accumulated by man’s selection, have not generally first appeared at an early +period of life, and have been inherited by the offspring at a corresponding not +early period. But the case of the short-faced tumbler, which when twelve hours +old had acquired its proper proportions, proves that this is not the universal +rule; for here the characteristic differences must either have appeared at an +earlier period than usual, or, if not so, the differences must have been +inherited, not at the corresponding, but at an earlier age. + +Now let us apply these facts and the above two principles—which latter, though +not proved true, can be shown to be in some degree probable—to species in a +state of nature. Let us take a genus of birds, descended on my theory from some +one parent-species, and of which the several new species have become modified +through natural selection in accordance with their diverse habits. Then, from +the many slight successive steps of variation having supervened at a rather +late age, and having been inherited at a corresponding age, the young of the +new species of our supposed genus will manifestly tend to resemble each other +much more closely than do the adults, just as we have seen in the case of +pigeons. We may extend this view to whole families or even classes. The +fore-limbs, for instance, which served as legs in the parent-species, may +become, by a long course of modification, adapted in one descendant to act as +hands, in another as paddles, in another as wings; and on the above two +principles—namely of each successive modification supervening at a rather late +age, and being inherited at a corresponding late age—the fore-limbs in the +embryos of the several descendants of the parent-species will still resemble +each other closely, for they will not have been modified. But in each +individual new species, the embryonic fore-limbs will differ greatly from the +fore-limbs in the mature animal; the limbs in the latter having undergone much +modification at a rather late period of life, and having thus been converted +into hands, or paddles, or wings. Whatever influence long-continued exercise or +use on the one hand, and disuse on the other, may have in modifying an organ, +such influence will mainly affect the mature animal, which has come to its full +powers of activity and has to gain its own living; and the effects thus +produced will be inherited at a corresponding mature age. Whereas the young +will remain unmodified, or be modified in a lesser degree, by the effects of +use and disuse. + +In certain cases the successive steps of variation might supervene, from causes +of which we are wholly ignorant, at a very early period of life, or each step +might be inherited at an earlier period than that at which it first appeared. +In either case (as with the short-faced tumbler) the young or embryo would +closely resemble the mature parent-form. We have seen that this is the rule of +development in certain whole groups of animals, as with cuttle-fish and +spiders, and with a few members of the great class of insects, as with Aphis. +With respect to the final cause of the young in these cases not undergoing any +metamorphosis, or closely resembling their parents from their earliest age, we +can see that this would result from the two following contingencies; firstly, +from the young, during a course of modification carried on for many +generations, having to provide for their own wants at a very early stage of +development, and secondly, from their following exactly the same habits of life +with their parents; for in this case, it would be indispensable for the +existence of the species, that the child should be modified at a very early age +in the same manner with its parents, in accordance with their similar habits. +Some further explanation, however, of the embryo not undergoing any +metamorphosis is perhaps requisite. If, on the other hand, it profited the +young to follow habits of life in any degree different from those of their +parent, and consequently to be constructed in a slightly different manner, +then, on the principle of inheritance at corresponding ages, the active young +or larvæ might easily be rendered by natural selection different to any +conceivable extent from their parents. Such differences might, also, become +correlated with successive stages of development; so that the larvæ, in the +first stage, might differ greatly from the larvæ in the second stage, as we +have seen to be the case with cirripedes. The adult might become fitted for +sites or habits, in which organs of locomotion or of the senses, etc., would be +useless; and in this case the final metamorphosis would be said to be +retrograde. + +As all the organic beings, extinct and recent, which have ever lived on this +earth have to be classed together, and as all have been connected by the finest +gradations, the best, or indeed, if our collections were nearly perfect, the +only possible arrangement, would be genealogical. Descent being on my view the +hidden bond of connexion which naturalists have been seeking under the term of +the natural system. On this view we can understand how it is that, in the eyes +of most naturalists, the structure of the embryo is even more important for +classification than that of the adult. For the embryo is the animal in its less +modified state; and in so far it reveals the structure of its progenitor. In +two groups of animal, however much they may at present differ from each other +in structure and habits, if they pass through the same or similar embryonic +stages, we may feel assured that they have both descended from the same or +nearly similar parents, and are therefore in that degree closely related. Thus, +community in embryonic structure reveals community of descent. It will reveal +this community of descent, however much the structure of the adult may have +been modified and obscured; we have seen, for instance, that cirripedes can at +once be recognised by their larvæ as belonging to the great class of +crustaceans. As the embryonic state of each species and group of species +partially shows us the structure of their less modified ancient progenitors, we +can clearly see why ancient and extinct forms of life should resemble the +embryos of their descendants,—our existing species. Agassiz believes this to be +a law of nature; but I am bound to confess that I only hope to see the law +hereafter proved true. It can be proved true in those cases alone in which the +ancient state, now supposed to be represented in many embryos, has not been +obliterated, either by the successive variations in a long course of +modification having supervened at a very early age, or by the variations having +been inherited at an earlier period than that at which they first appeared. It +should also be borne in mind, that the supposed law of resemblance of ancient +forms of life to the embryonic stages of recent forms, may be true, but yet, +owing to the geological record not extending far enough back in time, may +remain for a long period, or for ever, incapable of demonstration. + +Thus, as it seems to me, the leading facts in embryology, which are second in +importance to none in natural history, are explained on the principle of slight +modifications not appearing, in the many descendants from some one ancient +progenitor, at a very early period in the life of each, though perhaps caused +at the earliest, and being inherited at a corresponding not early period. +Embryology rises greatly in interest, when we thus look at the embryo as a +picture, more or less obscured, of the common parent-form of each great class +of animals. + +Rudimentary, atrophied, or aborted organs.—Organs or parts in this strange +condition, bearing the stamp of inutility, are extremely common throughout +nature. For instance, rudimentary mammæ are very general in the males of +mammals: I presume that the “bastard-wing” in birds may be safely considered as +a digit in a rudimentary state: in very many snakes one lobe of the lungs is +rudimentary; in other snakes there are rudiments of the pelvis and hind limbs. +Some of the cases of rudimentary organs are extremely curious; for instance, +the presence of teeth in foetal whales, which when grown up have not a tooth in +their heads; and the presence of teeth, which never cut through the gums, in +the upper jaws of our unborn calves. It has even been stated on good authority +that rudiments of teeth can be detected in the beaks of certain embryonic +birds. Nothing can be plainer than that wings are formed for flight, yet in how +many insects do we see wings so reduced in size as to be utterly incapable of +flight, and not rarely lying under wing-cases, firmly soldered together! + +The meaning of rudimentary organs is often quite unmistakeable: for instance +there are beetles of the same genus (and even of the same species) resembling +each other most closely in all respects, one of which will have full-sized +wings, and another mere rudiments of membrane; and here it is impossible to +doubt, that the rudiments represent wings. Rudimentary organs sometimes retain +their potentiality, and are merely not developed: this seems to be the case +with the mammæ of male mammals, for many instances are on record of these +organs having become well developed in full-grown males, and having secreted +milk. So again there are normally four developed and two rudimentary teats in +the udders of the genus Bos, but in our domestic cows the two sometimes become +developed and give milk. In individual plants of the same species the petals +sometimes occur as mere rudiments, and sometimes in a well-developed state. In +plants with separated sexes, the male flowers often have a rudiment of a +pistil; and Kölreuter found that by crossing such male plants with an +hermaphrodite species, the rudiment of the pistil in the hybrid offspring was +much increased in size; and this shows that the rudiment and the perfect pistil +are essentially alike in nature. + +An organ serving for two purposes, may become rudimentary or utterly aborted +for one, even the more important purpose; and remain perfectly efficient for +the other. Thus in plants, the office of the pistil is to allow the +pollen-tubes to reach the ovules protected in the ovarium at its base. The +pistil consists of a stigma supported on the style; but in some Compositæ, the +male florets, which of course cannot be fecundated, have a pistil, which is in +a rudimentary state, for it is not crowned with a stigma; but the style remains +well developed, and is clothed with hairs as in other compositæ, for the +purpose of brushing the pollen out of the surrounding anthers. Again, an organ +may become rudimentary for its proper purpose, and be used for a distinct +object: in certain fish the swim-bladder seems to be rudimentary for its proper +function of giving buoyancy, but has become converted into a nascent breathing +organ or lung. Other similar instances could be given. + +Rudimentary organs in the individuals of the same species are very liable to +vary in degree of development and in other respects. Moreover, in closely +allied species, the degree to which the same organ has been rendered +rudimentary occasionally differs much. This latter fact is well exemplified in +the state of the wings of the female moths in certain groups. Rudimentary +organs may be utterly aborted; and this implies, that we find in an animal or +plant no trace of an organ, which analogy would lead us to expect to find, and +which is occasionally found in monstrous individuals of the species. Thus in +the snapdragon (antirrhinum) we generally do not find a rudiment of a fifth +stamen; but this may sometimes be seen. In tracing the homologies of the same +part in different members of a class, nothing is more common, or more +necessary, than the use and discovery of rudiments. This is well shown in the +drawings given by Owen of the bones of the leg of the horse, ox, and +rhinoceros. + +It is an important fact that rudimentary organs, such as teeth in the upper +jaws of whales and ruminants, can often be detected in the embryo, but +afterwards wholly disappear. It is also, I believe, a universal rule, that a +rudimentary part or organ is of greater size relatively to the adjoining parts +in the embryo, than in the adult; so that the organ at this early age is less +rudimentary, or even cannot be said to be in any degree rudimentary. Hence, +also, a rudimentary organ in the adult, is often said to have retained its +embryonic condition. + +I have now given the leading facts with respect to rudimentary organs. In +reflecting on them, every one must be struck with astonishment: for the same +reasoning power which tells us plainly that most parts and organs are +exquisitely adapted for certain purposes, tells us with equal plainness that +these rudimentary or atrophied organs, are imperfect and useless. In works on +natural history rudimentary organs are generally said to have been created “for +the sake of symmetry,” or in order “to complete the scheme of nature;” but this +seems to me no explanation, merely a restatement of the fact. Would it be +thought sufficient to say that because planets revolve in elliptic courses +round the sun, satellites follow the same course round the planets, for the +sake of symmetry, and to complete the scheme of nature? An eminent physiologist +accounts for the presence of rudimentary organs, by supposing that they serve +to excrete matter in excess, or injurious to the system; but can we suppose +that the minute papilla, which often represents the pistil in male flowers, and +which is formed merely of cellular tissue, can thus act? Can we suppose that +the formation of rudimentary teeth which are subsequently absorbed, can be of +any service to the rapidly growing embryonic calf by the excretion of precious +phosphate of lime? When a man’s fingers have been amputated, imperfect nails +sometimes appear on the stumps: I could as soon believe that these vestiges of +nails have appeared, not from unknown laws of growth, but in order to excrete +horny matter, as that the rudimentary nails on the fin of the manatee were +formed for this purpose. + +On my view of descent with modification, the origin of rudimentary organs is +simple. We have plenty of cases of rudimentary organs in our domestic +productions,—as the stump of a tail in tailless breeds,—the vestige of an ear +in earless breeds,—the reappearance of minute dangling horns in hornless breeds +of cattle, more especially, according to Youatt, in young animals,—and the +state of the whole flower in the cauliflower. We often see rudiments of various +parts in monsters. But I doubt whether any of these cases throw light on the +origin of rudimentary organs in a state of nature, further than by showing that +rudiments can be produced; for I doubt whether species under nature ever +undergo abrupt changes. I believe that disuse has been the main agency; that it +has led in successive generations to the gradual reduction of various organs, +until they have become rudimentary,—as in the case of the eyes of animals +inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, +which have seldom been forced to take flight, and have ultimately lost the +power of flying. Again, an organ useful under certain conditions, might become +injurious under others, as with the wings of beetles living on small and +exposed islands; and in this case natural selection would continue slowly to +reduce the organ, until it was rendered harmless and rudimentary. + +Any change in function, which can be effected by insensibly small steps, is +within the power of natural selection; so that an organ rendered, during +changed habits of life, useless or injurious for one purpose, might easily be +modified and used for another purpose. Or an organ might be retained for one +alone of its former functions. An organ, when rendered useless, may well be +variable, for its variations cannot be checked by natural selection. At +whatever period of life disuse or selection reduces an organ, and this will +generally be when the being has come to maturity and to its full powers of +action, the principle of inheritance at corresponding ages will reproduce the +organ in its reduced state at the same age, and consequently will seldom affect +or reduce it in the embryo. Thus we can understand the greater relative size of +rudimentary organs in the embryo, and their lesser relative size in the adult. +But if each step of the process of reduction were to be inherited, not at the +corresponding age, but at an extremely early period of life (as we have good +reason to believe to be possible) the rudimentary part would tend to be wholly +lost, and we should have a case of complete abortion. The principle, also, of +economy, explained in a former chapter, by which the materials forming any part +or structure, if not useful to the possessor, will be saved as far as is +possible, will probably often come into play; and this will tend to cause the +entire obliteration of a rudimentary organ. + +As the presence of rudimentary organs is thus due to the tendency in every part +of the organisation, which has long existed, to be inherited—we can understand, +on the genealogical view of classification, how it is that systematists have +found rudimentary parts as useful as, or even sometimes more useful than, parts +of high physiological importance. Rudimentary organs may be compared with the +letters in a word, still retained in the spelling, but become useless in the +pronunciation, but which serve as a clue in seeking for its derivation. On the +view of descent with modification, we may conclude that the existence of organs +in a rudimentary, imperfect, and useless condition, or quite aborted, far from +presenting a strange difficulty, as they assuredly do on the ordinary doctrine +of creation, might even have been anticipated, and can be accounted for by the +laws of inheritance. + +Summary.—In this chapter I have attempted to show, that the subordination of +group to group in all organisms throughout all time; that the nature of the +relationship, by which all living and extinct beings are united by complex, +radiating, and circuitous lines of affinities into one grand system; the rules +followed and the difficulties encountered by naturalists in their +classifications; the value set upon characters, if constant and prevalent, +whether of high vital importance, or of the most trifling importance, or, as in +rudimentary organs, of no importance; the wide opposition in value between +analogical or adaptive characters, and characters of true affinity; and other +such rules;—all naturally follow on the view of the common parentage of those +forms which are considered by naturalists as allied, together with their +modification through natural selection, with its contingencies of extinction +and divergence of character. In considering this view of classification, it +should be borne in mind that the element of descent has been universally used +in ranking together the sexes, ages, and acknowledged varieties of the same +species, however different they may be in structure. If we extend the use of +this element of descent,—the only certainly known cause of similarity in +organic beings,—we shall understand what is meant by the natural system: it is +genealogical in its attempted arrangement, with the grades of acquired +difference marked by the terms varieties, species, genera, families, orders, +and classes. + +On this same view of descent with modification, all the great facts in +Morphology become intelligible,—whether we look to the same pattern displayed +in the homologous organs, to whatever purpose applied, of the different species +of a class; or to the homologous parts constructed on the same pattern in each +individual animal and plant. + +On the principle of successive slight variations, not necessarily or generally +supervening at a very early period of life, and being inherited at a +corresponding period, we can understand the great leading facts in Embryology; +namely, the resemblance in an individual embryo of the homologous parts, which +when matured will become widely different from each other in structure and +function; and the resemblance in different species of a class of the homologous +parts or organs, though fitted in the adult members for purposes as different +as possible. Larvæ are active embryos, which have become specially modified in +relation to their habits of life, through the principle of modifications being +inherited at corresponding ages. On this same principle—and bearing in mind, +that when organs are reduced in size, either from disuse or selection, it will +generally be at that period of life when the being has to provide for its own +wants, and bearing in mind how strong is the principle of inheritance—the +occurrence of rudimentary organs and their final abortion, present to us no +inexplicable difficulties; on the contrary, their presence might have been even +anticipated. The importance of embryological characters and of rudimentary +organs in classification is intelligible, on the view that an arrangement is +only so far natural as it is genealogical. + +Finally, the several classes of facts which have been considered in this +chapter, seem to me to proclaim so plainly, that the innumerable species, +genera, and families of organic beings, with which this world is peopled, have +all descended, each within its own class or group, from common parents, and +have all been modified in the course of descent, that I should without +hesitation adopt this view, even if it were unsupported by other facts or +arguments. + +CHAPTER XIV. +RECAPITULATION AND CONCLUSION. + +Recapitulation of the difficulties on the theory of Natural Selection. +Recapitulation of the general and special circumstances in its favour. Causes +of the general belief in the immutability of species. How far the theory of +natural selection may be extended. Effects of its adoption on the study of +Natural history. Concluding remarks. + +As this whole volume is one long argument, it may be convenient to the reader +to have the leading facts and inferences briefly recapitulated. + +That many and grave objections may be advanced against the theory of descent +with modification through natural selection, I do not deny. I have endeavoured +to give to them their full force. Nothing at first can appear more difficult to +believe than that the more complex organs and instincts should have been +perfected, not by means superior to, though analogous with, human reason, but +by the accumulation of innumerable slight variations, each good for the +individual possessor. Nevertheless, this difficulty, though appearing to our +imagination insuperably great, cannot be considered real if we admit the +following propositions, namely,—that gradations in the perfection of any organ +or instinct, which we may consider, either do now exist or could have existed, +each good of its kind,—that all organs and instincts are, in ever so slight a +degree, variable,—and, lastly, that there is a struggle for existence leading +to the preservation of each profitable deviation of structure or instinct. The +truth of these propositions cannot, I think, be disputed. + +It is, no doubt, extremely difficult even to conjecture by what gradations many +structures have been perfected, more especially amongst broken and failing +groups of organic beings; but we see so many strange gradations in nature, as +is proclaimed by the canon, “Natura non facit saltum,” that we ought to be +extremely cautious in saying that any organ or instinct, or any whole being, +could not have arrived at its present state by many graduated steps. There are, +it must be admitted, cases of special difficulty on the theory of natural +selection; and one of the most curious of these is the existence of two or +three defined castes of workers or sterile females in the same community of +ants; but I have attempted to show how this difficulty can be mastered. + +With respect to the almost universal sterility of species when first crossed, +which forms so remarkable a contrast with the almost universal fertility of +varieties when crossed, I must refer the reader to the recapitulation of the +facts given at the end of the eighth chapter, which seem to me conclusively to +show that this sterility is no more a special endowment than is the incapacity +of two trees to be grafted together, but that it is incidental on +constitutional differences in the reproductive systems of the intercrossed +species. We see the truth of this conclusion in the vast difference in the +result, when the same two species are crossed reciprocally; that is, when one +species is first used as the father and then as the mother. + +The fertility of varieties when intercrossed and of their mongrel offspring +cannot be considered as universal; nor is their very general fertility +surprising when we remember that it is not likely that either their +constitutions or their reproductive systems should have been profoundly +modified. Moreover, most of the varieties which have been experimentised on +have been produced under domestication; and as domestication apparently tends +to eliminate sterility, we ought not to expect it also to produce sterility. + +The sterility of hybrids is a very different case from that of first crosses, +for their reproductive organs are more or less functionally impotent; whereas +in first crosses the organs on both sides are in a perfect condition. As we +continually see that organisms of all kinds are rendered in some degree sterile +from their constitutions having been disturbed by slightly different and new +conditions of life, we need not feel surprise at hybrids being in some degree +sterile, for their constitutions can hardly fail to have been disturbed from +being compounded of two distinct organisations. This parallelism is supported +by another parallel, but directly opposite, class of facts; namely, that the +vigour and fertility of all organic beings are increased by slight changes in +their conditions of life, and that the offspring of slightly modified forms or +varieties acquire from being crossed increased vigour and fertility. So that, +on the one hand, considerable changes in the conditions of life and crosses +between greatly modified forms, lessen fertility; and on the other hand, lesser +changes in the conditions of life and crosses between less modified forms, +increase fertility. + +Turning to geographical distribution, the difficulties encountered on the +theory of descent with modification are grave enough. All the individuals of +the same species, and all the species of the same genus, or even higher group, +must have descended from common parents; and therefore, in however distant and +isolated parts of the world they are now found, they must in the course of +successive generations have passed from some one part to the others. We are +often wholly unable even to conjecture how this could have been effected. Yet, +as we have reason to believe that some species have retained the same specific +form for very long periods, enormously long as measured by years, too much +stress ought not to be laid on the occasional wide diffusion of the same +species; for during very long periods of time there will always be a good +chance for wide migration by many means. A broken or interrupted range may +often be accounted for by the extinction of the species in the intermediate +regions. It cannot be denied that we are as yet very ignorant of the full +extent of the various climatal and geographical changes which have affected the +earth during modern periods; and such changes will obviously have greatly +facilitated migration. As an example, I have attempted to show how potent has +been the influence of the Glacial period on the distribution both of the same +and of representative species throughout the world. We are as yet profoundly +ignorant of the many occasional means of transport. With respect to distinct +species of the same genus inhabiting very distant and isolated regions, as the +process of modification has necessarily been slow, all the means of migration +will have been possible during a very long period; and consequently the +difficulty of the wide diffusion of species of the same genus is in some degree +lessened. + +As on the theory of natural selection an interminable number of intermediate +forms must have existed, linking together all the species in each group by +gradations as fine as our present varieties, it may be asked, Why do we not see +these linking forms all around us? Why are not all organic beings blended +together in an inextricable chaos? With respect to existing forms, we should +remember that we have no right to expect (excepting in rare cases) to discover +directly connecting links between them, but only between each and some extinct +and supplanted form. Even on a wide area, which has during a long period +remained continuous, and of which the climate and other conditions of life +change insensibly in going from a district occupied by one species into another +district occupied by a closely allied species, we have no just right to expect +often to find intermediate varieties in the intermediate zone. For we have +reason to believe that only a few species are undergoing change at any one +period; and all changes are slowly effected. I have also shown that the +intermediate varieties which will at first probably exist in the intermediate +zones, will be liable to be supplanted by the allied forms on either hand; and +the latter, from existing in greater numbers, will generally be modified and +improved at a quicker rate than the intermediate varieties, which exist in +lesser numbers; so that the intermediate varieties will, in the long run, be +supplanted and exterminated. + +On this doctrine of the extermination of an infinitude of connecting links, +between the living and extinct inhabitants of the world, and at each successive +period between the extinct and still older species, why is not every geological +formation charged with such links? Why does not every collection of fossil +remains afford plain evidence of the gradation and mutation of the forms of +life? We meet with no such evidence, and this is the most obvious and forcible +of the many objections which may be urged against my theory. Why, again, do +whole groups of allied species appear, though certainly they often falsely +appear, to have come in suddenly on the several geological stages? Why do we +not find great piles of strata beneath the Silurian system, stored with the +remains of the progenitors of the Silurian groups of fossils? For certainly on +my theory such strata must somewhere have been deposited at these ancient and +utterly unknown epochs in the world’s history. + +I can answer these questions and grave objections only on the supposition that +the geological record is far more imperfect than most geologists believe. It +cannot be objected that there has not been time sufficient for any amount of +organic change; for the lapse of time has been so great as to be utterly +inappreciable by the human intellect. The number of specimens in all our +museums is absolutely as nothing compared with the countless generations of +countless species which certainly have existed. We should not be able to +recognise a species as the parent of any one or more species if we were to +examine them ever so closely, unless we likewise possessed many of the +intermediate links between their past or parent and present states; and these +many links we could hardly ever expect to discover, owing to the imperfection +of the geological record. Numerous existing doubtful forms could be named which +are probably varieties; but who will pretend that in future ages so many fossil +links will be discovered, that naturalists will be able to decide, on the +common view, whether or not these doubtful forms are varieties? As long as most +of the links between any two species are unknown, if any one link or +intermediate variety be discovered, it will simply be classed as another and +distinct species. Only a small portion of the world has been geologically +explored. Only organic beings of certain classes can be preserved in a fossil +condition, at least in any great number. Widely ranging species vary most, and +varieties are often at first local,—both causes rendering the discovery of +intermediate links less likely. Local varieties will not spread into other and +distant regions until they are considerably modified and improved; and when +they do spread, if discovered in a geological formation, they will appear as if +suddenly created there, and will be simply classed as new species. Most +formations have been intermittent in their accumulation; and their duration, I +am inclined to believe, has been shorter than the average duration of specific +forms. Successive formations are separated from each other by enormous blank +intervals of time; for fossiliferous formations, thick enough to resist future +degradation, can be accumulated only where much sediment is deposited on the +subsiding bed of the sea. During the alternate periods of elevation and of +stationary level the record will be blank. During these latter periods there +will probably be more variability in the forms of life; during periods of +subsidence, more extinction. + +With respect to the absence of fossiliferous formations beneath the lowest +Silurian strata, I can only recur to the hypothesis given in the ninth chapter. +That the geological record is imperfect all will admit; but that it is +imperfect to the degree which I require, few will be inclined to admit. If we +look to long enough intervals of time, geology plainly declares that all +species have changed; and they have changed in the manner which my theory +requires, for they have changed slowly and in a graduated manner. We clearly +see this in the fossil remains from consecutive formations invariably being +much more closely related to each other, than are the fossils from formations +distant from each other in time. + +Such is the sum of the several chief objections and difficulties which may +justly be urged against my theory; and I have now briefly recapitulated the +answers and explanations which can be given to them. I have felt these +difficulties far too heavily during many years to doubt their weight. But it +deserves especial notice that the more important objections relate to questions +on which we are confessedly ignorant; nor do we know how ignorant we are. We do +not know all the possible transitional gradations between the simplest and the +most perfect organs; it cannot be pretended that we know all the varied means +of Distribution during the long lapse of years, or that we know how imperfect +the Geological Record is. Grave as these several difficulties are, in my +judgment they do not overthrow the theory of descent with modification. + +Now let us turn to the other side of the argument. Under domestication we see +much variability. This seems to be mainly due to the reproductive system being +eminently susceptible to changes in the conditions of life; so that this +system, when not rendered impotent, fails to reproduce offspring exactly like +the parent-form. Variability is governed by many complex laws,—by correlation +of growth, by use and disuse, and by the direct action of the physical +conditions of life. There is much difficulty in ascertaining how much +modification our domestic productions have undergone; but we may safely infer +that the amount has been large, and that modifications can be inherited for +long periods. As long as the conditions of life remain the same, we have reason +to believe that a modification, which has already been inherited for many +generations, may continue to be inherited for an almost infinite number of +generations. On the other hand we have evidence that variability, when it has +once come into play, does not wholly cease; for new varieties are still +occasionally produced by our most anciently domesticated productions. + +Man does not actually produce variability; he only unintentionally exposes +organic beings to new conditions of life, and then nature acts on the +organisation, and causes variability. But man can and does select the +variations given to him by nature, and thus accumulate them in any desired +manner. He thus adapts animals and plants for his own benefit or pleasure. He +may do this methodically, or he may do it unconsciously by preserving the +individuals most useful to him at the time, without any thought of altering the +breed. It is certain that he can largely influence the character of a breed by +selecting, in each successive generation, individual differences so slight as +to be quite inappreciable by an uneducated eye. This process of selection has +been the great agency in the production of the most distinct and useful +domestic breeds. That many of the breeds produced by man have to a large extent +the character of natural species, is shown by the inextricable doubts whether +very many of them are varieties or aboriginal species. + +There is no obvious reason why the principles which have acted so efficiently +under domestication should not have acted under nature. In the preservation of +favoured individuals and races, during the constantly-recurrent Struggle for +Existence, we see the most powerful and ever-acting means of selection. The +struggle for existence inevitably follows from the high geometrical ratio of +increase which is common to all organic beings. This high rate of increase is +proved by calculation, by the effects of a succession of peculiar seasons, and +by the results of naturalisation, as explained in the third chapter. More +individuals are born than can possibly survive. A grain in the balance will +determine which individual shall live and which shall die,—which variety or +species shall increase in number, and which shall decrease, or finally become +extinct. As the individuals of the same species come in all respects into the +closest competition with each other, the struggle will generally be most severe +between them; it will be almost equally severe between the varieties of the +same species, and next in severity between the species of the same genus. But +the struggle will often be very severe between beings most remote in the scale +of nature. The slightest advantage in one being, at any age or during any +season, over those with which it comes into competition, or better adaptation +in however slight a degree to the surrounding physical conditions, will turn +the balance. + +With animals having separated sexes there will in most cases be a struggle +between the males for possession of the females. The most vigorous individuals, +or those which have most successfully struggled with their conditions of life, +will generally leave most progeny. But success will often depend on having +special weapons or means of defence, or on the charms of the males; and the +slightest advantage will lead to victory. + +As geology plainly proclaims that each land has undergone great physical +changes, we might have expected that organic beings would have varied under +nature, in the same way as they generally have varied under the changed +conditions of domestication. And if there be any variability under nature, it +would be an unaccountable fact if natural selection had not come into play. It +has often been asserted, but the assertion is quite incapable of proof, that +the amount of variation under nature is a strictly limited quantity. Man, +though acting on external characters alone and often capriciously, can produce +within a short period a great result by adding up mere individual differences +in his domestic productions; and every one admits that there are at least +individual differences in species under nature. But, besides such differences, +all naturalists have admitted the existence of varieties, which they think +sufficiently distinct to be worthy of record in systematic works. No one can +draw any clear distinction between individual differences and slight varieties; +or between more plainly marked varieties and sub-species, and species. Let it +be observed how naturalists differ in the rank which they assign to the many +representative forms in Europe and North America. + +If then we have under nature variability and a powerful agent always ready to +act and select, why should we doubt that variations in any way useful to +beings, under their excessively complex relations of life, would be preserved, +accumulated, and inherited? Why, if man can by patience select variations most +useful to himself, should nature fail in selecting variations useful, under +changing conditions of life, to her living products? What limit can be put to +this power, acting during long ages and rigidly scrutinising the whole +constitution, structure, and habits of each creature,—favouring the good and +rejecting the bad? I can see no limit to this power, in slowly and beautifully +adapting each form to the most complex relations of life. The theory of natural +selection, even if we looked no further than this, seems to me to be in itself +probable. I have already recapitulated, as fairly as I could, the opposed +difficulties and objections: now let us turn to the special facts and arguments +in favour of the theory. + +On the view that species are only strongly marked and permanent varieties, and +that each species first existed as a variety, we can see why it is that no line +of demarcation can be drawn between species, commonly supposed to have been +produced by special acts of creation, and varieties which are acknowledged to +have been produced by secondary laws. On this same view we can understand how +it is that in each region where many species of a genus have been produced, and +where they now flourish, these same species should present many varieties; for +where the manufactory of species has been active, we might expect, as a general +rule, to find it still in action; and this is the case if varieties be +incipient species. Moreover, the species of the larger genera, which afford the +greater number of varieties or incipient species, retain to a certain degree +the character of varieties; for they differ from each other by a less amount of +difference than do the species of smaller genera. The closely allied species +also of the larger genera apparently have restricted ranges, and they are +clustered in little groups round other species—in which respects they resemble +varieties. These are strange relations on the view of each species having been +independently created, but are intelligible if all species first existed as +varieties. + +As each species tends by its geometrical ratio of reproduction to increase +inordinately in number; and as the modified descendants of each species will be +enabled to increase by so much the more as they become more diversified in +habits and structure, so as to be enabled to seize on many and widely different +places in the economy of nature, there will be a constant tendency in natural +selection to preserve the most divergent offspring of any one species. Hence +during a long-continued course of modification, the slight differences, +characteristic of varieties of the same species, tend to be augmented into the +greater differences characteristic of species of the same genus. New and +improved varieties will inevitably supplant and exterminate the older, less +improved and intermediate varieties; and thus species are rendered to a large +extent defined and distinct objects. Dominant species belonging to the larger +groups tend to give birth to new and dominant forms; so that each large group +tends to become still larger, and at the same time more divergent in character. +But as all groups cannot thus succeed in increasing in size, for the world +would not hold them, the more dominant groups beat the less dominant. This +tendency in the large groups to go on increasing in size and diverging in +character, together with the almost inevitable contingency of much extinction, +explains the arrangement of all the forms of life, in groups subordinate to +groups, all within a few great classes, which we now see everywhere around us, +and which has prevailed throughout all time. This grand fact of the grouping of +all organic beings seems to me utterly inexplicable on the theory of creation. + +As natural selection acts solely by accumulating slight, successive, favourable +variations, it can produce no great or sudden modification; it can act only by +very short and slow steps. Hence the canon of “Natura non facit saltum,” which +every fresh addition to our knowledge tends to make more strictly correct, is +on this theory simply intelligible. We can plainly see why nature is prodigal +in variety, though niggard in innovation. But why this should be a law of +nature if each species has been independently created, no man can explain. + +Many other facts are, as it seems to me, explicable on this theory. How strange +it is that a bird, under the form of woodpecker, should have been created to +prey on insects on the ground; that upland geese, which never or rarely swim, +should have been created with webbed feet; that a thrush should have been +created to dive and feed on sub-aquatic insects; and that a petrel should have +been created with habits and structure fitting it for the life of an auk or +grebe! and so on in endless other cases. But on the view of each species +constantly trying to increase in number, with natural selection always ready to +adapt the slowly varying descendants of each to any unoccupied or ill-occupied +place in nature, these facts cease to be strange, or perhaps might even have +been anticipated. + +As natural selection acts by competition, it adapts the inhabitants of each +country only in relation to the degree of perfection of their associates; so +that we need feel no surprise at the inhabitants of any one country, although +on the ordinary view supposed to have been specially created and adapted for +that country, being beaten and supplanted by the naturalised productions from +another land. Nor ought we to marvel if all the contrivances in nature be not, +as far as we can judge, absolutely perfect; and if some of them be abhorrent to +our ideas of fitness. We need not marvel at the sting of the bee causing the +bee’s own death; at drones being produced in such vast numbers for one single +act, and being then slaughtered by their sterile sisters; at the astonishing +waste of pollen by our fir-trees; at the instinctive hatred of the queen bee +for her own fertile daughters; at ichneumonidæ feeding within the live bodies +of caterpillars; and at other such cases. The wonder indeed is, on the theory +of natural selection, that more cases of the want of absolute perfection have +not been observed. + +The complex and little known laws governing variation are the same, as far as +we can see, with the laws which have governed the production of so-called +specific forms. In both cases physical conditions seem to have produced but +little direct effect; yet when varieties enter any zone, they occasionally +assume some of the characters of the species proper to that zone. In both +varieties and species, use and disuse seem to have produced some effect; for it +is difficult to resist this conclusion when we look, for instance, at the +logger-headed duck, which has wings incapable of flight, in nearly the same +condition as in the domestic duck; or when we look at the burrowing tucutucu, +which is occasionally blind, and then at certain moles, which are habitually +blind and have their eyes covered with skin; or when we look at the blind +animals inhabiting the dark caves of America and Europe. In both varieties and +species correlation of growth seems to have played a most important part, so +that when one part has been modified other parts are necessarily modified. In +both varieties and species reversions to long-lost characters occur. How +inexplicable on the theory of creation is the occasional appearance of stripes +on the shoulder and legs of the several species of the horse-genus and in their +hybrids! How simply is this fact explained if we believe that these species +have descended from a striped progenitor, in the same manner as the several +domestic breeds of pigeon have descended from the blue and barred rock-pigeon! + +On the ordinary view of each species having been independently created, why +should the specific characters, or those by which the species of the same genus +differ from each other, be more variable than the generic characters in which +they all agree? Why, for instance, should the colour of a flower be more likely +to vary in any one species of a genus, if the other species, supposed to have +been created independently, have differently coloured flowers, than if all the +species of the genus have the same coloured flowers? If species are only +well-marked varieties, of which the characters have become in a high degree +permanent, we can understand this fact; for they have already varied since they +branched off from a common progenitor in certain characters, by which they have +come to be specifically distinct from each other; and therefore these same +characters would be more likely still to be variable than the generic +characters which have been inherited without change for an enormous period. It +is inexplicable on the theory of creation why a part developed in a very +unusual manner in any one species of a genus, and therefore, as we may +naturally infer, of great importance to the species, should be eminently liable +to variation; but, on my view, this part has undergone, since the several +species branched off from a common progenitor, an unusual amount of variability +and modification, and therefore we might expect this part generally to be still +variable. But a part may be developed in the most unusual manner, like the wing +of a bat, and yet not be more variable than any other structure, if the part be +common to many subordinate forms, that is, if it has been inherited for a very +long period; for in this case it will have been rendered constant by +long-continued natural selection. + +Glancing at instincts, marvellous as some are, they offer no greater difficulty +than does corporeal structure on the theory of the natural selection of +successive, slight, but profitable modifications. We can thus understand why +nature moves by graduated steps in endowing different animals of the same class +with their several instincts. I have attempted to show how much light the +principle of gradation throws on the admirable architectural powers of the +hive-bee. Habit no doubt sometimes comes into play in modifying instincts; but +it certainly is not indispensable, as we see, in the case of neuter insects, +which leave no progeny to inherit the effects of long-continued habit. On the +view of all the species of the same genus having descended from a common +parent, and having inherited much in common, we can understand how it is that +allied species, when placed under considerably different conditions of life, +yet should follow nearly the same instincts; why the thrush of South America, +for instance, lines her nest with mud like our British species. On the view of +instincts having been slowly acquired through natural selection we need not +marvel at some instincts being apparently not perfect and liable to mistakes, +and at many instincts causing other animals to suffer. + +If species be only well-marked and permanent varieties, we can at once see why +their crossed offspring should follow the same complex laws in their degrees +and kinds of resemblance to their parents,—in being absorbed into each other by +successive crosses, and in other such points,—as do the crossed offspring of +acknowledged varieties. On the other hand, these would be strange facts if +species have been independently created, and varieties have been produced by +secondary laws. + +If we admit that the geological record is imperfect in an extreme degree, then +such facts as the record gives, support the theory of descent with +modification. New species have come on the stage slowly and at successive +intervals; and the amount of change, after equal intervals of time, is widely +different in different groups. The extinction of species and of whole groups of +species, which has played so conspicuous a part in the history of the organic +world, almost inevitably follows on the principle of natural selection; for old +forms will be supplanted by new and improved forms. Neither single species nor +groups of species reappear when the chain of ordinary generation has once been +broken. The gradual diffusion of dominant forms, with the slow modification of +their descendants, causes the forms of life, after long intervals of time, to +appear as if they had changed simultaneously throughout the world. The fact of +the fossil remains of each formation being in some degree intermediate in +character between the fossils in the formations above and below, is simply +explained by their intermediate position in the chain of descent. The grand +fact that all extinct organic beings belong to the same system with recent +beings, falling either into the same or into intermediate groups, follows from +the living and the extinct being the offspring of common parents. As the groups +which have descended from an ancient progenitor have generally diverged in +character, the progenitor with its early descendants will often be intermediate +in character in comparison with its later descendants; and thus we can see why +the more ancient a fossil is, the oftener it stands in some degree intermediate +between existing and allied groups. Recent forms are generally looked at as +being, in some vague sense, higher than ancient and extinct forms; and they are +in so far higher as the later and more improved forms have conquered the older +and less improved organic beings in the struggle for life. Lastly, the law of +the long endurance of allied forms on the same continent,—of marsupials in +Australia, of edentata in America, and other such cases,—is intelligible, for +within a confined country, the recent and the extinct will naturally be allied +by descent. + +Looking to geographical distribution, if we admit that there has been during +the long course of ages much migration from one part of the world to another, +owing to former climatal and geographical changes and to the many occasional +and unknown means of dispersal, then we can understand, on the theory of +descent with modification, most of the great leading facts in Distribution. We +can see why there should be so striking a parallelism in the distribution of +organic beings throughout space, and in their geological succession throughout +time; for in both cases the beings have been connected by the bond of ordinary +generation, and the means of modification have been the same. We see the full +meaning of the wonderful fact, which must have struck every traveller, namely, +that on the same continent, under the most diverse conditions, under heat and +cold, on mountain and lowland, on deserts and marshes, most of the inhabitants +within each great class are plainly related; for they will generally be +descendants of the same progenitors and early colonists. On this same principle +of former migration, combined in most cases with modification, we can +understand, by the aid of the Glacial period, the identity of some few plants, +and the close alliance of many others, on the most distant mountains, under the +most different climates; and likewise the close alliance of some of the +inhabitants of the sea in the northern and southern temperate zones, though +separated by the whole intertropical ocean. Although two areas may present the +same physical conditions of life, we need feel no surprise at their inhabitants +being widely different, if they have been for a long period completely +separated from each other; for as the relation of organism to organism is the +most important of all relations, and as the two areas will have received +colonists from some third source or from each other, at various periods and in +different proportions, the course of modification in the two areas will +inevitably be different. + +On this view of migration, with subsequent modification, we can see why oceanic +islands should be inhabited by few species, but of these, that many should be +peculiar. We can clearly see why those animals which cannot cross wide spaces +of ocean, as frogs and terrestrial mammals, should not inhabit oceanic islands; +and why, on the other hand, new and peculiar species of bats, which can +traverse the ocean, should so often be found on islands far distant from any +continent. Such facts as the presence of peculiar species of bats, and the +absence of all other mammals, on oceanic islands, are utterly inexplicable on +the theory of independent acts of creation. + +The existence of closely allied or representative species in any two areas, +implies, on the theory of descent with modification, that the same parents +formerly inhabited both areas; and we almost invariably find that wherever many +closely allied species inhabit two areas, some identical species common to both +still exist. Wherever many closely allied yet distinct species occur, many +doubtful forms and varieties of the same species likewise occur. It is a rule +of high generality that the inhabitants of each area are related to the +inhabitants of the nearest source whence immigrants might have been derived. We +see this in nearly all the plants and animals of the Galapagos archipelago, of +Juan Fernandez, and of the other American islands being related in the most +striking manner to the plants and animals of the neighbouring American +mainland; and those of the Cape de Verde archipelago and other African islands +to the African mainland. It must be admitted that these facts receive no +explanation on the theory of creation. + +The fact, as we have seen, that all past and present organic beings constitute +one grand natural system, with group subordinate to group, and with extinct +groups often falling in between recent groups, is intelligible on the theory of +natural selection with its contingencies of extinction and divergence of +character. On these same principles we see how it is, that the mutual +affinities of the species and genera within each class are so complex and +circuitous. We see why certain characters are far more serviceable than others +for classification;—why adaptive characters, though of paramount importance to +the being, are of hardly any importance in classification; why characters +derived from rudimentary parts, though of no service to the being, are often of +high classificatory value; and why embryological characters are the most +valuable of all. The real affinities of all organic beings are due to +inheritance or community of descent. The natural system is a genealogical +arrangement, in which we have to discover the lines of descent by the most +permanent characters, however slight their vital importance may be. + +The framework of bones being the same in the hand of a man, wing of a bat, fin +of the porpoise, and leg of the horse,—the same number of vertebræ forming the +neck of the giraffe and of the elephant,—and innumerable other such facts, at +once explain themselves on the theory of descent with slow and slight +successive modifications. The similarity of pattern in the wing and leg of a +bat, though used for such different purpose,—in the jaws and legs of a crab,—in +the petals, stamens, and pistils of a flower, is likewise intelligible on the +view of the gradual modification of parts or organs, which were alike in the +early progenitor of each class. On the principle of successive variations not +always supervening at an early age, and being inherited at a corresponding not +early period of life, we can clearly see why the embryos of mammals, birds, +reptiles, and fishes should be so closely alike, and should be so unlike the +adult forms. We may cease marvelling at the embryo of an air-breathing mammal +or bird having branchial slits and arteries running in loops, like those in a +fish which has to breathe the air dissolved in water, by the aid of +well-developed branchiæ. + +Disuse, aided sometimes by natural selection, will often tend to reduce an +organ, when it has become useless by changed habits or under changed conditions +of life; and we can clearly understand on this view the meaning of rudimentary +organs. But disuse and selection will generally act on each creature, when it +has come to maturity and has to play its full part in the struggle for +existence, and will thus have little power of acting on an organ during early +life; hence the organ will not be much reduced or rendered rudimentary at this +early age. The calf, for instance, has inherited teeth, which never cut through +the gums of the upper jaw, from an early progenitor having well-developed +teeth; and we may believe, that the teeth in the mature animal were reduced, +during successive generations, by disuse or by the tongue and palate having +been fitted by natural selection to browse without their aid; whereas in the +calf, the teeth have been left untouched by selection or disuse, and on the +principle of inheritance at corresponding ages have been inherited from a +remote period to the present day. On the view of each organic being and each +separate organ having been specially created, how utterly inexplicable it is +that parts, like the teeth in the embryonic calf or like the shrivelled wings +under the soldered wing-covers of some beetles, should thus so frequently bear +the plain stamp of inutility! Nature may be said to have taken pains to reveal, +by rudimentary organs and by homologous structures, her scheme of modification, +which it seems that we wilfully will not understand. + +I have now recapitulated the chief facts and considerations which have +thoroughly convinced me that species have changed, and are still slowly +changing by the preservation and accumulation of successive slight favourable +variations. Why, it may be asked, have all the most eminent living naturalists +and geologists rejected this view of the mutability of species? It cannot be +asserted that organic beings in a state of nature are subject to no variation; +it cannot be proved that the amount of variation in the course of long ages is +a limited quantity; no clear distinction has been, or can be, drawn between +species and well-marked varieties. It cannot be maintained that species when +intercrossed are invariably sterile, and varieties invariably fertile; or that +sterility is a special endowment and sign of creation. The belief that species +were immutable productions was almost unavoidable as long as the history of the +world was thought to be of short duration; and now that we have acquired some +idea of the lapse of time, we are too apt to assume, without proof, that the +geological record is so perfect that it would have afforded us plain evidence +of the mutation of species, if they had undergone mutation. + +But the chief cause of our natural unwillingness to admit that one species has +given birth to other and distinct species, is that we are always slow in +admitting any great change of which we do not see the intermediate steps. The +difficulty is the same as that felt by so many geologists, when Lyell first +insisted that long lines of inland cliffs had been formed, and great valleys +excavated, by the slow action of the coast-waves. The mind cannot possibly +grasp the full meaning of the term of a hundred million years; it cannot add up +and perceive the full effects of many slight variations, accumulated during an +almost infinite number of generations. + +Although I am fully convinced of the truth of the views given in this volume +under the form of an abstract, I by no means expect to convince experienced +naturalists whose minds are stocked with a multitude of facts all viewed, +during a long course of years, from a point of view directly opposite to mine. +It is so easy to hide our ignorance under such expressions as the “plan of +creation,” “unity of design,” etc., and to think that we give an explanation +when we only restate a fact. Any one whose disposition leads him to attach more +weight to unexplained difficulties than to the explanation of a certain number +of facts will certainly reject my theory. A few naturalists, endowed with much +flexibility of mind, and who have already begun to doubt on the immutability of +species, may be influenced by this volume; but I look with confidence to the +future, to young and rising naturalists, who will be able to view both sides of +the question with impartiality. Whoever is led to believe that species are +mutable will do good service by conscientiously expressing his conviction; for +only thus can the load of prejudice by which this subject is overwhelmed be +removed. + +Several eminent naturalists have of late published their belief that a +multitude of reputed species in each genus are not real species; but that other +species are real, that is, have been independently created. This seems to me a +strange conclusion to arrive at. They admit that a multitude of forms, which +till lately they themselves thought were special creations, and which are still +thus looked at by the majority of naturalists, and which consequently have +every external characteristic feature of true species,—they admit that these +have been produced by variation, but they refuse to extend the same view to +other and very slightly different forms. Nevertheless they do not pretend that +they can define, or even conjecture, which are the created forms of life, and +which are those produced by secondary laws. They admit variation as a vera +causa in one case, they arbitrarily reject it in another, without assigning any +distinction in the two cases. The day will come when this will be given as a +curious illustration of the blindness of preconceived opinion. These authors +seem no more startled at a miraculous act of creation than at an ordinary +birth. But do they really believe that at innumerable periods in the earth’s +history certain elemental atoms have been commanded suddenly to flash into +living tissues? Do they believe that at each supposed act of creation one +individual or many were produced? Were all the infinitely numerous kinds of +animals and plants created as eggs or seed, or as full grown? and in the case +of mammals, were they created bearing the false marks of nourishment from the +mother’s womb? Although naturalists very properly demand a full explanation of +every difficulty from those who believe in the mutability of species, on their +own side they ignore the whole subject of the first appearance of species in +what they consider reverent silence. + +It may be asked how far I extend the doctrine of the modification of species. +The question is difficult to answer, because the more distinct the forms are +which we may consider, by so much the arguments fall away in force. But some +arguments of the greatest weight extend very far. All the members of whole +classes can be connected together by chains of affinities, and all can be +classified on the same principle, in groups subordinate to groups. Fossil +remains sometimes tend to fill up very wide intervals between existing orders. +Organs in a rudimentary condition plainly show that an early progenitor had the +organ in a fully developed state; and this in some instances necessarily +implies an enormous amount of modification in the descendants. Throughout whole +classes various structures are formed on the same pattern, and at an embryonic +age the species closely resemble each other. Therefore I cannot doubt that the +theory of descent with modification embraces all the members of the same class. +I believe that animals have descended from at most only four or five +progenitors, and plants from an equal or lesser number. + +Analogy would lead me one step further, namely, to the belief that all animals +and plants have descended from some one prototype. But analogy may be a +deceitful guide. Nevertheless all living things have much in common, in their +chemical composition, their germinal vesicles, their cellular structure, and +their laws of growth and reproduction. We see this even in so trifling a +circumstance as that the same poison often similarly affects plants and +animals; or that the poison secreted by the gall-fly produces monstrous growths +on the wild rose or oak-tree. Therefore I should infer from analogy that +probably all the organic beings which have ever lived on this earth have +descended from some one primordial form, into which life was first breathed. + +When the views entertained in this volume on the origin of species, or when +analogous views are generally admitted, we can dimly foresee that there will be +a considerable revolution in natural history. Systematists will be able to +pursue their labours as at present; but they will not be incessantly haunted by +the shadowy doubt whether this or that form be in essence a species. This I +feel sure, and I speak after experience, will be no slight relief. The endless +disputes whether or not some fifty species of British brambles are true species +will cease. Systematists will have only to decide (not that this will be easy) +whether any form be sufficiently constant and distinct from other forms, to be +capable of definition; and if definable, whether the differences be +sufficiently important to deserve a specific name. This latter point will +become a far more essential consideration than it is at present; for +differences, however slight, between any two forms, if not blended by +intermediate gradations, are looked at by most naturalists as sufficient to +raise both forms to the rank of species. Hereafter we shall be compelled to +acknowledge that the only distinction between species and well-marked varieties +is, that the latter are known, or believed, to be connected at the present day +by intermediate gradations, whereas species were formerly thus connected. +Hence, without quite rejecting the consideration of the present existence of +intermediate gradations between any two forms, we shall be led to weigh more +carefully and to value higher the actual amount of difference between them. It +is quite possible that forms now generally acknowledged to be merely varieties +may hereafter be thought worthy of specific names, as with the primrose and +cowslip; and in this case scientific and common language will come into +accordance. In short, we shall have to treat species in the same manner as +those naturalists treat genera, who admit that genera are merely artificial +combinations made for convenience. This may not be a cheering prospect; but we +shall at least be freed from the vain search for the undiscovered and +undiscoverable essence of the term species. + +The other and more general departments of natural history will rise greatly in +interest. The terms used by naturalists of affinity, relationship, community of +type, paternity, morphology, adaptive characters, rudimentary and aborted +organs, etc., will cease to be metaphorical, and will have a plain +signification. When we no longer look at an organic being as a savage looks at +a ship, as at something wholly beyond his comprehension; when we regard every +production of nature as one which has had a history; when we contemplate every +complex structure and instinct as the summing up of many contrivances, each +useful to the possessor, nearly in the same way as when we look at any great +mechanical invention as the summing up of the labour, the experience, the +reason, and even the blunders of numerous workmen; when we thus view each +organic being, how far more interesting, I speak from experience, will the +study of natural history become! + +A grand and almost untrodden field of inquiry will be opened, on the causes and +laws of variation, on correlation of growth, on the effects of use and disuse, +on the direct action of external conditions, and so forth. The study of +domestic productions will rise immensely in value. A new variety raised by man +will be a far more important and interesting subject for study than one more +species added to the infinitude of already recorded species. Our +classifications will come to be, as far as they can be so made, genealogies; +and will then truly give what may be called the plan of creation. The rules for +classifying will no doubt become simpler when we have a definite object in +view. We possess no pedigrees or armorial bearings; and we have to discover and +trace the many diverging lines of descent in our natural genealogies, by +characters of any kind which have long been inherited. Rudimentary organs will +speak infallibly with respect to the nature of long-lost structures. Species +and groups of species, which are called aberrant, and which may fancifully be +called living fossils, will aid us in forming a picture of the ancient forms of +life. Embryology will reveal to us the structure, in some degree obscured, of +the prototypes of each great class. + +When we can feel assured that all the individuals of the same species, and all +the closely allied species of most genera, have within a not very remote period +descended from one parent, and have migrated from some one birthplace; and when +we better know the many means of migration, then, by the light which geology +now throws, and will continue to throw, on former changes of climate and of the +level of the land, we shall surely be enabled to trace in an admirable manner +the former migrations of the inhabitants of the whole world. Even at present, +by comparing the differences of the inhabitants of the sea on the opposite +sides of a continent, and the nature of the various inhabitants of that +continent in relation to their apparent means of immigration, some light can be +thrown on ancient geography. + +The noble science of Geology loses glory from the extreme imperfection of the +record. The crust of the earth with its embedded remains must not be looked at +as a well-filled museum, but as a poor collection made at hazard and at rare +intervals. The accumulation of each great fossiliferous formation will be +recognised as having depended on an unusual concurrence of circumstances, and +the blank intervals between the successive stages as having been of vast +duration. But we shall be able to gauge with some security the duration of +these intervals by a comparison of the preceding and succeeding organic forms. +We must be cautious in attempting to correlate as strictly contemporaneous two +formations, which include few identical species, by the general succession of +their forms of life. As species are produced and exterminated by slowly acting +and still existing causes, and not by miraculous acts of creation and by +catastrophes; and as the most important of all causes of organic change is one +which is almost independent of altered and perhaps suddenly altered physical +conditions, namely, the mutual relation of organism to organism,—the +improvement of one being entailing the improvement or the extermination of +others; it follows, that the amount of organic change in the fossils of +consecutive formations probably serves as a fair measure of the lapse of actual +time. A number of species, however, keeping in a body might remain for a long +period unchanged, whilst within this same period, several of these species, by +migrating into new countries and coming into competition with foreign +associates, might become modified; so that we must not overrate the accuracy of +organic change as a measure of time. During early periods of the earth’s +history, when the forms of life were probably fewer and simpler, the rate of +change was probably slower; and at the first dawn of life, when very few forms +of the simplest structure existed, the rate of change may have been slow in an +extreme degree. The whole history of the world, as at present known, although +of a length quite incomprehensible by us, will hereafter be recognised as a +mere fragment of time, compared with the ages which have elapsed since the +first creature, the progenitor of innumerable extinct and living descendants, +was created. + +In the distant future I see open fields for far more important researches. +Psychology will be based on a new foundation, that of the necessary acquirement +of each mental power and capacity by gradation. Light will be thrown on the +origin of man and his history. + +Authors of the highest eminence seem to be fully satisfied with the view that +each species has been independently created. To my mind it accords better with +what we know of the laws impressed on matter by the Creator, that the +production and extinction of the past and present inhabitants of the world +should have been due to secondary causes, like those determining the birth and +death of the individual. When I view all beings not as special creations, but +as the lineal descendants of some few beings which lived long before the first +bed of the Silurian system was deposited, they seem to me to become ennobled. +Judging from the past, we may safely infer that not one living species will +transmit its unaltered likeness to a distant futurity. And of the species now +living very few will transmit progeny of any kind to a far distant futurity; +for the manner in which all organic beings are grouped, shows that the greater +number of species of each genus, and all the species of many genera, have left +no descendants, but have become utterly extinct. We can so far take a prophetic +glance into futurity as to foretel that it will be the common and widely-spread +species, belonging to the larger and dominant groups, which will ultimately +prevail and procreate new and dominant species. As all the living forms of life +are the lineal descendants of those which lived long before the Silurian epoch, +we may feel certain that the ordinary succession by generation has never once +been broken, and that no cataclysm has desolated the whole world. Hence we may +look with some confidence to a secure future of equally inappreciable length. +And as natural selection works solely by and for the good of each being, all +corporeal and mental endowments will tend to progress towards perfection. + +It is interesting to contemplate an entangled bank, clothed with many plants of +many kinds, with birds singing on the bushes, with various insects flitting +about, and with worms crawling through the damp earth, and to reflect that +these elaborately constructed forms, so different from each other, and +dependent on each other in so complex a manner, have all been produced by laws +acting around us. These laws, taken in the largest sense, being Growth with +Reproduction; Inheritance which is almost implied by reproduction; Variability +from the indirect and direct action of the external conditions of life, and +from use and disuse; a Ratio of Increase so high as to lead to a Struggle for +Life, and as a consequence to Natural Selection, entailing Divergence of +Character and the Extinction of less-improved forms. Thus, from the war of +nature, from famine and death, the most exalted object which we are capable of +conceiving, namely, the production of the higher animals, directly follows. +There is grandeur in this view of life, with its several powers, having been +originally breathed into a few forms or into one; and that, whilst this planet +has gone cycling on according to the fixed law of gravity, from so simple a +beginning endless forms most beautiful and most wonderful have been, and are +being, evolved. + +INDEX. + +Aberrant groups, 429. + +Abyssinia, plants of, 375. + +Acclimatisation, 139. + +Affinities: +of extinct species, 329. +of organic beings, 411. + +Agassiz: +on Amblyopsis, 139. +on groups of species suddenly appearing, 302, 305. +on embryological succession, 338. +on the glacial period, 366. +on embryological characters, 418. +on the embryos of vertebrata, 439. +on parallelism of embryological development and geological succession, 449. + +Algæ of New Zealand, 376. + +Alligators, males, fighting, 88. + +Amblyopsis, blind fish, 139. + +America, North: +productions allied to those of Europe, 371. +boulders and glaciers of, 373. +South, no modern formations on west coast, 290. + +Ammonites, sudden extinction of, 321. + +Anagallis, sterility of, 247. + +Analogy of variations, 159. + +Ancylus, 386. + +Animals: +not domesticated from being variable, 17. +domestic, descended from several stocks, 19. +acclimatisation of, 141. +of Australia, 116. +with thicker fur in cold climates, 133. +blind, in caves, 137. +extinct, of Australia, 339. + +Anomma, 240. + +Antarctic islands, ancient flora of, 399. + +Antirrhinum, 161. + +Ants: +attending aphides, 211. +slave-making instinct, 219. + +Ants, neuter, structure of, 236. + +Aphides attended by ants, 211. + +Aphis, development of, 442. + +Apteryx, 182. + +Arab horses, 35. + +Aralo-Caspian Sea, 339. + +Archiac, M. de, on the succession of species, 325. + +Artichoke, Jerusalem, 142. + +Ascension, plants of, 389. + +Asclepias, pollen of, 193. + +Asparagus, 359. + +Aspicarpa, 417. + +Asses, striped, 163. + +Ateuchus, 135. + +Audubon: +on habits of frigate-bird, 185. +on variation in birds’-nests, 212. +on heron eating seeds, 387. + +Australia: +animals of, 116. +dogs of, 215. +extinct animals of, 339. +European plants in, 375. + +Azara on flies destroying cattle, 72. + +Azores, flora of, 363. + +Babington, Mr., on British plants, 48. + +Balancement of growth, 147. + +Bamboo with hooks, 197. + +Barberry, flowers of, 98. + +Barrande, M.: +on Silurian colonies, 313. +on the succession of species, 325. +on parallelism of palæozoic formations, 328. +on affinities of ancient species, 330. + +Barriers, importance of, 347. + +Batrachians on islands, 393. + +Bats: +how structure acquired, 180. +distribution of, 394. + +Bear, catching water-insects, 184. + +Bee: +sting of, 202. +queen, killing rivals, 202. + +Bees fertilising flowers, 73. + +Bees: +hive, not sucking the red clover, 95. +cell-making instinct, 224. +humble, cells of, 225. +parasitic, 218. + +Beetles: +wingless, in Madeira, 135. +with deficient tarsi, 135. + +Bentham, Mr.: +on British plants, 48. +on classification, 419. + +Berkeley, Mr., on seeds in salt-water, 358. + +Bermuda, birds of, 391. + +Birds: +acquiring fear, 212. +annually cross the Atlantic, 364. +colour of, on continents, 132. +fossil, in caves of Brazil, 339. +of Madeira, Bermuda, and Galapagos, 390. +song of males, 89. +transporting seeds, 361. +waders, 386. +wingless, 134, 182. +with traces of embryonic teeth, 451. + +Bizcacha, 349. +affinities of, 429. + +Bladder for swimming in fish, 190. + +Blindness of cave animals, 137, + +Blyth, Mr.: +on distinctness of Indian cattle, 18. +on striped Hemionus, 163. +on crossed geese, 253. + +Boar, shoulder-pad of, 88. + +Borrow, Mr., on the Spanish pointer, 35. + +Bory St. Vincent on Batrachians, 393. + +Bosquet, M., on fossil Chthamalus, 304. + +Boulders, erratic, on the Azores, 363. + +Branchiæ, 190. + +Brent, Mr.: +on house-tumblers, 214. +on hawks killing pigeons, 362. + +Brewer, Dr., on American cuckoo, 217. + +Britain, mammals of, 395. + +Bronn on duration of specific forms, 293. + +Brown, Robert, on classification, 414. + +Buckman on variation in plants, 10. + +Buzareingues on sterility of varieties, 270. + +Cabbage, varieties of, crossed, 99. + +Calceolaria, 251. + +Canary-birds, sterility of hybrids, 252. + +Cape de Verde islands, 398. + +Cape of Good Hope, plants of, 110, 375. + +Carrier-pigeons killed by hawks, 362. + +Cassini on flowers of compositæ, 145. + +Catasetum, 424. + +Cats: +with blue eyes, deaf, 12. +variation in habits of, 91. +curling tail when going to spring, 201. + +Cattle: +destroying fir-trees, 71. +destroyed by flies in La Plata, 72. +breeds of, locally extinct, 111. +fertility of Indian and European breeds, 254. + +Cave, inhabitants of, blind, 137. + +Centres of creation, 352. + +Cephalopodæ, development of, 442. + +Cervulus, 253. + +Cetacea, teeth and hair, 144. + +Ceylon, plants of, 375. + +Chalk formation, 322. + +Characters: +divergence of, 111. +sexual, variable, 156. +adaptive or analogical, 427. + +Charlock, 76. + +Checks: +to increase, 67. +mutual, 71. + +Chickens, instinctive tameness of, 216. + +Chthamalinæ, 288. + +Chthamalus, cretacean species of, 304. + +Circumstances favourable: +to selection of domestic products, 40. +to natural selection, 101. + +Cirripedes: +capable of crossing, 101. +carapace aborted, 148. +their ovigerous frena, 192. +fossil, 304. +larvæ of, 440. + +Classification, 413. + +Clift, Mr., on the succession of types, 339. + +Climate: +effects of, in checking increase of beings, 68. +adaptation of, to organisms, 139. + +Cobites, intestine of, 190. + +Cockroach, 76. + +Collections, palæontological, poor, 287. + +Colour: +influenced by climate, 132. +in relation to attacks by flies, 198. + +Columba livia, parent of domestic pigeons, 23. + +Colymbetes, 386. + +Compensation of growth, 147. + +Compositæ: +outer and inner florets of, 144. +male flowers of, 451. + +Conclusion, general, 480. + +Conditions, slight changes in, favourable to fertility, 267. + +Coot, 185. + +Coral: +islands, seeds drifted to, 360. +reefs, indicating movements of earth, 309. + +Corn-crake, 185. + +Correlation: +of growth in domestic productions, 11. +of growth, 143, 198. + +Cowslip, 49. + +Creation, single centres of, 352. + +Crinum, 250. + +Crosses, reciprocal, 258. + +Crossing: +of domestic animals, importance in altering breeds, 20. +advantages of, 96. +unfavourable to selection, 102. + +Crustacea of New Zealand, 376. + +Crustacean, blind, 137. + +Cryptocerus, 238. + +Ctenomys, blind, 137. + +Cuckoo, instinct of, 216. + +Currants, grafts of, 262. + +Currents of sea, rate of, 359. + +Cuvier: +on conditions of existence, 206. +on fossil monkeys, 303. + +Cuvier, Fred., on instinct, 208. + +Dana, Professor: +on blind cave-animals, 139. +on relations of crustaceans of Japan, 372. +on crustaceans of New Zealand, 376. + +De Candolle: +on struggle for existence, 62. +on umbelliferæ, 146. +on general affinities, 430. + +De Candolle, Alph.: +on low plants, widely dispersed, 406. +on widely-ranging plants being variable, 53. +on naturalisation, 115. +on winged seeds, 146. +on Alpine species suddenly becoming rare, 175. +on distribution of plants with large seeds, 360. +on vegetation of Australia, 379. +on fresh-water plants, 386. +on insular plants, 389. + +Degradation of coast-rocks, 282. + +Denudation: +rate of, 285. +of oldest rocks, 308. + +Development of ancient forms, 336. + +Devonian system, 334. + +Dianthus, fertility of crosses, 256. + +Dirt on feet of birds, 362. + +Dispersal: +means of, 356. +during glacial period, 365. + +Distribution: +geographical, 346. +means of, 356. + +Disuse, effects of, under nature, 134. + +Divergence of character, 111. + +Division, physiological, of labour, 115. + +Dogs: +hairless, with imperfect teeth, 12. +descended from several wild stocks, 18. +domestic instincts of, 213. +inherited civilisation of, 215. +fertility of breeds together, 254. +of crosses, 268. +proportions of, when young, 444. + +Domestication, variation under, 7. + +Downing, Mr., on fruit-trees in America, 85. + +Downs, North and South, 285. + +Dragon-flies, intestines of, 190. + +Drift-timber, 360. + +Driver-ant, 240. + +Drones killed by other bees, 202. + +Duck: +domestic, wings of, reduced, 11. +logger-headed, 182. + +Duckweed, 385. + +Dugong, affinities of, 414. + +Dung-beetles with deficient tarsi, 135. + +Dyticus, 386. + +Earl, Mr. W., on the Malay Archipelago, 395. + +Ears: +drooping, in domestic animals, 11. +rudimentary, 454. + +Earth, seeds in roots of trees, 361. + +Eciton, 238. + +Economy of organisation, 147. + +Edentata: +teeth and hair, 144. +fossil species of, 339. + +Edwards, Milne: +on physiological divisions of labour, 115. +on gradations of structure, 194. +on embryological characters, 418. + +Eggs, young birds escaping from, 87. + +Electric organs, 192. + +Elephant: +rate of increase, 64. +of glacial period, 141. + +Embryology, 439. + +Existence: +struggle for, 60. +conditions of, 206. + +Extinction: +as bearing on natural selection, 109. +of domestic varieties, 111. +317. + +Eye: +structure of, 187. +correction for aberration, 202. + +Eyes reduced in moles, 137. + +Fabre, M., on parasitic sphex, 218. + +Falconer, Dr.: +on naturalization of plants in India, 65. +on fossil crocodile, 313. +on elephants and mastodons, 334. +and Cautley on mammals of sub-Himalayan beds, 340. + +Falkland Island, wolf of, 393. + +Faults, 285. + +Faunas, marine, 348. + +Fear, instinctive, in birds, 212. + +Feet of birds, young molluscs adhering to, 385. + +Fertility: +of hybrids, 249. +from slight changes in conditions, 267. +of crossed varieties, 267. + +Fir-trees: +destroyed by cattle, 71. +pollen of, 203. + +Fish: +flying, 182. +teleostean, sudden appearance of, 305. +eating seeds, 362, 387. +fresh-water, distribution of, 384. + +Fishes: +ganoid, now confined to fresh water, 107. +electric organs of, 192. +ganoid, living in fresh water, 321. +of southern hemisphere, 376. + +Flight, powers of, how acquired, 182. + +Flowers: +structure of, in relation to crossing, 97. +of compositæ and umbelliferæ, 144. + +Forbes, E.: +on colours of shells, 132. +on abrupt range of shells in depth, 175. +on poorness of palæontological collections, 287. +on continuous succession of genera, 316. +on continental extensions, 357. +on distribution during glacial period, 366 +on parallelism in time and space, 409. + +Forests, changes in, in America, 74. + +Formation, Devonian, 334. + +Formations: +thickness of, in Britain, 284. +intermittent, 290. + +Formica rufescens, 219. + +Formica sanguinea, 219. + +Formica flava, neuter of, 239. + +Frena, ovigerous, of cirripedes, 192. + +Fresh-water productions, dispersal of, 383. + +Fries on species in large genera being closely allied to other species, 57. + +Frigate-bird, 185. + +Frogs on islands, 393. + +Fruit-trees: +gradual improvement of, 37. +in United States, 85. +varieties of, acclimatised in United States, 142. + +Fuci, crossed, 258. + +Fur, thicker in cold climates, 133. + +Furze, 439. + +Galapagos Archipelago: +birds of, 390. +productions of, 398, 400. + +Galeopithecus, 181. + +Game, increase of, checked by vermin, 68. + +Gärtner: +on sterility of hybrids, 247, 255. +on reciprocal crosses, 258. +on crossed maize and verbascum, 270. +on comparison of hybrids and mongrels, 272. + +Geese: +fertility when crossed, 253. +upland, 185. + +Genealogy important in classification, 425. + +Geoffrey St. Hilaire: +on balancement, 147. +on homologous organs, 434. + +Geoffrey St. Hilaire, Isidore: +on variability of repeated parts, 149. +on correlation in monstrosities, 11. +on correlation, 144. +on variable parts being often monstrous, 155. + +Geographical distribution, 346. + +Geography, ancient, 487. + +Geology: +future progress of, 487. +imperfection of the record, 279. + +Giraffe, tail of, 195. + +Glacial period, 365. + +Gmelin on distribution, 365. + +Gnathodon, fossil, 368. + +Godwin-Austen, Mr., on the Malay Archipelago, 299. + +Goethe on compensation of growth, 147. + +Gooseberry, grafts of, 262. + +Gould, Dr. A., on land-shells, 397. + +Gould, Mr.: +on colours of birds, 132. +on birds of the Galapagos, 398. +on distribution of genera of birds, 404. + +Gourds, crossed, 270. + +Grafts, capacity of, 261. + +Grasses, varieties of, 113. + +Gray, Dr. Asa: +on trees of United States, 100. +on naturalised plants in the United States, 115. +on rarity of intermediate varieties, 176. +on Alpine plants, 365. + +Gray, Dr. J. E., on striped mule, 165. + +Grebe, 185. + +Groups, aberrant, 429. + +Grouse: +colours of, 84. +red, a doubtful species, 49. + +Growth: +compensation of, 147. +correlation of, in domestic products, 11. +correlation of, 143. + +Habit: +effect of, under domestication, 11. +effect of, under nature, 134. +diversified, of same species, 183. + +Hair and teeth, correlated, 144. + +Harcourt, Mr. E. V., on the birds of Madeira, 391. + +Hartung, M., on boulders in the Azores, 363. + +Hazel-nuts, 359. + +Hearne on habits of bears, 184. + +Heath, changes in vegetation, 72, + +Heer, O., on plants of Madeira, 107. + +Helix pomatia, 397. + +Helosciadium, 359. + +Hemionus, striped, 163. + +Herbert, W.: +on struggle for existence, 62. +on sterility of hybrids, 249. + +Hermaphrodites crossing, 96. + +Heron eating seed, 387. + +Heron, Sir R., on peacocks, 89. + +Heusinger on white animals not poisoned by certain plants, 12. + +Hewitt, Mr., on sterility of first crosses, 264. + +Himalaya: +glaciers of, 373. +plants of, 375. + +Hippeastrum, 250. + +Holly-trees, sexes of, 93. + +Hollyhock, varieties of, crossed, 271. + +Hooker, Dr., on trees of New Zealand, 100. + +Hooker, Dr.: +on acclimatisation of Himalayan trees, 140. +on flowers of umbelliferæ, 145. +on glaciers of Himalaya, 373. +on algæ of New Zealand, 376. +on vegetation at the base of the Himalaya, 378. +on plants of Tierra del Fuego, 374, 378. +on Australian plants, 375, 399. +on relations of flora of South America, 379. +on flora of the Antarctic lands, 381, 399. +on the plants of the Galapagos, 391, 398. + +Hooks: +on bamboos, 197. +to seeds on islands, 392. + +Horner, Mr., on the antiquity of Egyptians, 18. + +Horns, rudimentary, 454. + +Horse, fossil, in La Plata, 318. + +Horses: +destroyed by flies in La Plata, 72. +striped, 163. +proportions of, when young, 445. + +Horticulturists, selection applied by, 32. + +Huber on cells of bees, 230. + +Huber, P.: +on reason blended with instinct, 208. +on habitual nature of instincts, 208. +on slave making ants, 219. +on Melipona domestica, 225. + +Humble-bees, cells of, 225. + +Hunter, J., on secondary sexual characters, 150. + +Hutton, Captain, on crossed geese, 253. + +Huxley, Professor: +on structure of hermaphrodites, 101. +on embryological succession, 338. +on homologous organs, 438. +on the development of aphis, 442. + +Hybrids and mongrels compared, 272. + +Hybridism, 245. + +Hydra, structure of, 190. + +Ibla, 148. + +Icebergs transporting seeds, 363. + +Increase, rate of, 63. + +Individuals: +numbers favourable to selection, 102. +many, whether simultaneously created, 356. + +Inheritance: +laws of, 12. +at corresponding ages, 14, 86. + +Insects: +colour of, fitted for habitations, 84. +sea-side, colours of, 132. +blind, in caves, 138. +luminous, 193. +neuter, 236. + +Instinct, 207. + +Instincts, domestic, 213. + +Intercrossing, advantages of, 96. + +Islands, oceanic, 388. + +Isolation favourable to selection, 104. + +Japan, productions of, 372. + +Java, plants of, 375. + +Jones, Mr. J. M., on the birds of Bermuda, 391. + +Jussieu on classification, 417. + +Kentucky, caves of, 137. + +Kerguelen-land, flora of, 381, 399. + +Kidney-bean, acclimatisation of, 142. + +Kidneys of birds, 144. + +Kirby on tarsi deficient in beetles, 135. + +Knight, Andrew, on cause of variation, 7. + +Kölreuter: +on the barberry, 98. +on sterility of hybrids, 247. +on reciprocal crosses, 258. +on crossed varieties of nicotiana, 271. +on crossing male and hermaphrodite flowers, 451. + +Lamarck on adaptive characters, 427. + +Land-shells: +distribution of, 397. +of Madeira, naturalised, 402. + +Languages, classification of, 422. + +Lapse, great, of time, 282. + +Larvæ, 440. + +Laurel, nectar secreted by the leaves, 92. + +Laws of variation, 131. + +Leech, varieties of, 76. + +Leguminosæ, nectar secreted by glands, 92. + +Lepidosiren, 107, 330. + +Life, struggle for, 60. + +Lingula, Silurian, 306. + +Linnæus, aphorism of, 413. + +Lion: +mane of, 88. +young of, striped, 439. + +Lobelia fulgens, 73, 98. + +Lobelia, sterility of crosses, 250. + +Loess of the Rhine, 384. + +Lowness of structure connected with variability, 149. + +Lowness, related to wide distribution, 406. + +Lubbock, Mr., on the nerves of coccus, 46. + +Lucas, Dr. P.: +on inheritance, 12. +on resemblance of child to parent, 275. + +Lund and Clausen on fossils of Brazil, 339. + +Lyell, Sir C.: +on the struggle for existence, 62. +on modern changes of the earth, 95. +on measure of denudation, 283. +on a carboniferous land-shell, 289. +on fossil whales, 303. +on strata beneath Silurian system, 307. +on the imperfection of the geological record, 310. +on the appearance of species, 312. +on Barrande’s colonies, 313. +on tertiary formations of Europe and North America, 323. +on parallelism of tertiary formations, 328. +on transport of seeds by icebergs, 363. +on great alternations of climate, 382. +on the distribution of fresh-water shells, 385. +on land-shells of Madeira, 402. + +Lyell and Dawson on fossilized trees in Nova Scotia, 296. + +Macleay on analogical characters, 427. + +Madeira: +plants of, 107. +beetles of, wingless, 135. +fossil land-shells of, 339. +birds of, 390. + +Magpie tame in Norway, 212. + +Maize, crossed, 270. + +Malay Archipelago: +compared with Europe, 299. +mammals of, 395. + +Malpighiaceæ, 417. + +Mammæ, rudimentary, 451. + +Mammals: +fossil, in secondary formation, 303. +insular, 393. + +Man, origin of races of, 199. + +Manatee, rudimentary nails of, 454. + +Marsupials: +of Australia, 116. +fossil species of, 339. + +Martens, M., experiment on seeds, 360. + +Martin, Mr. W. C., on striped mules, 165. + +Matteuchi on the electric organs of rays, 193. + +Matthiola, reciprocal crosses of, 258. + +Means of dispersal, 356. + +Melipona domestica, 225. + +Metamorphism of oldest rocks, 308. + +Mice: +destroying bees, 74. +acclimatisation of, 141. + +Migration, bears on first appearance of fossils, 296. + +Miller, Professor, on the cells of bees, 226. + +Mirabilis, crosses of, 258. + +Missel-thrush, 76. + +Misseltoe, complex relations of, 3. + +Mississippi, rate of deposition at mouth, 284. + +Mocking-thrush of the Galapagos, 402. + +Modification of species, how far applicable, 483. + +Moles, blind, 137. + +Mongrels: +fertility and sterility of, 267. +and hybrids compared, 272. + +Monkeys, fossil, 303, + +Monocanthus, 424. + +Mons, Van, on the origin of fruit-trees, 29, 39. + +Moquin-Tandon on sea-side plants, 132. + +Morphology, 434. + +Mozart, musical powers of, 209. + +Mud, seeds in, 386. + +Mules, striped, 165. + +Müller, Dr. F., on Alpine Australian plants, 375. + +Murchison, Sir R.: +on the formations of Russia, 289. +on azoic formations, 307. +on extinction, 317. + +Mustela vison, 179. + +Myanthus, 424. + +Myrmecocystus, 238. + +Myrmica, eyes of, 240. + +Nails, rudimentary, 453. + +Natural history: +future progress of, 484. +selection, 80. +system, 413. + +Naturalisation: +of forms distinct from the indigenous species, 115. +in New Zealand, 201. + +Nautilus, Silurian, 306. + +Nectar of plants, 92. + +Nectaries, how formed, 92. + +Nelumbium luteum, 387. + +Nests, variation in, 212. + +Neuter insects, 236. + +Newman, Mr., on humble-bees, 74. + +New Zealand: +productions of, not perfect, 201. +naturalised products of, 337. +fossil birds of, 339. +glacial action in, 373. +crustaceans of, 376. +algæ of, 376. +number of plants of, 389. +flora of, 399. + +Nicotiana: +crossed varieties of, 271. +certain species very sterile, 257. + +Noble, Mr., on fertility of Rhododendron, 251. + +Nodules, phosphatic, in azoic rocks, 307. + +Oak, varieties of, 50. + +Onites apelles, 135. + +Orchis, pollen of, 193. + +Organs: +of extreme perfection, 186. +electric, of fishes, 192. +of little importance, 194. +homologous, 434. +rudiments of, 450. + +Ornithorhynchus, 107, 416. + +Ostrich: +not capable of flight, 134. +habit of laying eggs together, 218. +American, two species of, 349. + +Otter, habits of, how acquired, 179. + +Ouzel, water, 185. + +Owen, Professor: +on birds not flying, 134. +on vegetative repetition, 149. +on variable length of arms in ourang-outang, 150. +on the swim-bladder of fishes, 191. +on electric organs, 192. +on fossil horse of La Plata, 319. +on relations of ruminants and pachyderms, 329. +on fossil birds of New Zealand, 339. +on succession of types, 339. +on affinities of the dugong, 414. +on homologous organs, 435. +on the metamorphosis of cephalopods and spiders, 442. + +Pacific Ocean, faunas of, 348. + +Paley on no organ formed to give pain, 201. + +Pallas on the fertility of the wild stocks of domestic animals, 253. + +Paraguay, cattle destroyed by flies, 72. + +Parasites, 217. + +Partridge, dirt on feet, 362. + +Parts: +greatly developed, variable, 150. +degrees of utility of, 201. + +Parus major, 183. + +Passiflora, 251. + +Peaches in United States, 85. + +Pear, grafts of, 261. + +Pelargonium: +flowers of, 145. +sterility of, 251. + +Pelvis of women, 144. + +Peloria, 145. + +Period, glacial, 365. + +Petrels, habits of, 184. + +Phasianus, fertility of hybrids, 253. + +Pheasant, young, wild, 216. + +Philippi on tertiary species in Sicily, 312. + +Pictet, Professor: +on groups of species suddenly appearing, 302, 305. +on rate of organic change, 313. +on continuous succession of genera, 316. +on close alliance of fossils in consecutive formations, 335. +on embryological succession, 338. + +Pierce, Mr., on varieties of wolves, 91. + +Pigeons: +with feathered feet and skin between toes, 12. +breeds described, and origin of, 20. +breeds of, how produced, 39, 42. +tumbler, not being able to get out of egg, 87. +reverting to blue colour, 160. +instinct of tumbling, 214. +carriers, killed by hawks, 362. +young of, 445. + +Pistil, rudimentary, 451. + +Plants: +poisonous, not affecting certain coloured animals, 12. +selection applied to, 32. +gradual improvement of, 37. +not improved in barbarous countries, 38. +destroyed by insects, 67. +in midst of range, have to struggle with other plants, 77. +nectar of, 92. +fleshy, on sea-shores, 132. +fresh-water, distribution of, 386. +low in scale, widely distributed, 406. + +Plumage, laws of change in sexes of birds, 89. + +Plums in the United States, 85. + +Pointer dog: +origin of, 35. +habits of, 213. + +Poison not affecting certain coloured animals, 12. + +Poison, similar effect of, on animals and plants, 484. + +Pollen of fir-trees, 203, + +Poole, Col., on striped hemionus, 163. + +Potamogeton, 387. + +Prestwich, Mr., on English and French eocene formations, 328. + +Primrose, 49. +sterility of, 247. + +Primula, varieties of, 49. + +Proteolepas, 148. + +Proteus, 139. + +Psychology, future progress of, 488. + +Quagga, striped, 165. + +Quince, grafts of, 261. + +Rabbit, disposition of young, 215. + +Races, domestic, characters of, 16. + +Race-horses: +Arab, 35. +English, 356. + +Ramond on plants of Pyrenees, 368. + +Ramsay, Professor: +on thickness of the British formations, 284. +on faults, 285. + +Ratio of increase, 63. + +Rats: +supplanting each other, 76. +acclimatisation of, 141. +blind in cave, 137. + +Rattle-snake, 201. + +Reason and instinct, 208. + +Recapitulation, general, 459. + +Reciprocity of crosses, 258. + +Record, geological, imperfect, 279. + +Rengger on flies destroying cattle, 72. + +Reproduction, rate of, 63. + +Resemblance to parents in mongrels and hybrids, 273. + +Reversion: +law of inheritance, 14. +in pigeons to blue colour, 160. + +Rhododendron, sterility of, 251. + +Richard, Professor, on Aspicarpa, 417. + +Richardson, Sir J.: +on structure of squirrels, 180. +on fishes of the southern hemisphere, 376. + +Robinia, grafts of, 262. + +Rodents, blind, 137. + +Rudimentary organs, 450. + +Rudiments important for classification, 416. + +Sageret on grafts, 262. + +Salmons, males fighting, and hooked jaws of, 88. + +Salt-water, how far injurious to seeds, 358. + +Saurophagus sulphuratus, 183. + +Schiödte on blind insects, 138. + +Schlegel on snakes, 144. + +Sea-water, how far injurious to seeds, 358. + +Sebright, Sir J.: +on crossed animals, 20. +on selection of pigeons, 31. + +Sedgwick, Professor, on groups of species suddenly appearing, 302. + +Seedlings destroyed by insects, 67. + +Seeds: +nutriment in, 77. +winged, 146. +power of resisting salt-water, 358. +in crops and intestines of birds, 361. +eaten by fish, 362, 387. +in mud, 386. +hooked, on islands, 392. + +Selection: +of domestic products, 29. +principle not of recent origin, 33. +unconscious, 34. +natural, 80. +sexual, 87. +natural, circumstances favourable to, 101. + +Sexes, relations of, 87. + +Sexual: +characters variable, 156. +selection, 87. + +Sheep: +Merino, their selection, 31. +two sub-breeds unintentionally produced, 36. +mountain, varieties of, 76. + +Shells: +colours of, 132. +littoral, seldom embedded, 288. +fresh-water, dispersal of, 385. +of Madeira, 391. +land, distribution of, 397. + +Silene, fertility of crosses, 257. + +Silliman, Professor, on blind rat, 137. + +Skulls of young mammals, 197, 437. + +Slave-making instinct, 219. + +Smith, Col. Hamilton, on striped horses, 164. + +Smith, Mr. Fred.: +on slave-making ants, 219. +on neuter ants, 239. + +Smith, Mr., of Jordan Hill, on the degradation of coast-rocks, 283. + +Snap-dragon, 161. + +Somerville, Lord, on selection of sheep, 31. + +Sorbus, grafts of, 262. + +Spaniel, King Charles’s breed, 35. + +Species: +polymorphic, 46. +common, variable, 53. +in large genera variable, 54. +groups of, suddenly appearing, 302, 306. +beneath Silurian formations, 306. +successively appearing, 312. +changing simultaneously throughout the world, 322. + +Spencer, Lord, on increase in size of cattle, 35. + +Sphex, parasitic, 218. + +Spiders, development of, 442. + +Spitz-dog crossed with fox, 268. + +Sports in plants, 9. + +Sprengel, C. C.: +on crossing, 98. +on ray-florets, 145. + +Squirrels, gradations in structure, 180. + +Staffordshire, heath, changes in, 72. + +Stag-beetles, fighting, 88. + +Sterility: +from changed conditions of life, 9. +of hybrids, 246. +laws of, 254. +causes of, 263. +from unfavourable conditions, 265. +of certain varieties, 269. + +St. Helena, productions of, 389. + +St. Hilaire, Aug., on classification, 418. + +St. John, Mr., on habits of cats, 91. + +Sting of bee, 202. + +Stocks, aboriginal, of domestic animals, 18, + +Strata, thickness of, in Britain, 284. + +Stripes on horses, 163. + +Structure, degrees of utility of, 201. + +Struggle for existence, 60. + +Succession, geological, 312. + +Succession of types in same areas, 338. + +Swallow, one species supplanting another, 76. + +Swim-bladder, 190. + +System, natural, 413. + +Tail: +of giraffe, 195. +of aquatic animals, 196. +rudimentary, 454. + +Tarsi deficient, 135. + +Tausch on umbelliferous flowers, 146. + +Teeth and hair: +correlated, 144. +embryonic, traces of, in birds, 451. +rudimentary, in embryonic calf, 450, 480. + +Tegetmeier, Mr., on cells of bees, 228, 233. + +Temminck on distribution aiding classification, 419. + +Thouin on grafts, 262. + +Thrush: +aquatic species of, 185. +mocking, of the Galapagos, 402. +young of, spotted, 439. +nest of, 243. + +Thuret, >M., on crossed fuci, 258. + +Thwaites, Mr., on acclimatisation, 140. + +Tierra del Fuego: +dogs of, 215. +plants of, 374, 378. + +Timber-drift, 360. + +Time, lapse of, 282. + +Titmouse, 183. + +Toads on islands, 393. + +Tobacco, crossed varieties of, 271. + +Tomes, Mr., on the distribution of bats, 394. + +Transitions in varieties rare, 172. + +Trees: +on islands belong to peculiar orders, 392. +with separated sexes, 99. + +Trifolium pratense, 73, 94. + +Trifolium incarnatum, 94. + +Trigonia, 321. + +Trilobites, 306. +sudden extinction of, 321. + +Troglodytes, 243. + +Tucutucu, blind, 137. + +Tumbler pigeons: +habits of, hereditary, 214. +young of, 446. + +Turkey-cock, brush of hair on breast, 90. + +Turkey: +naked skin on head, 197. +young, wild, 216. + +Turnip and cabbage, analogous variations of, 159. + +Type, unity of, 206. + +Types, succession of, in same areas, 338. + +Udders: +enlarged by use, 11. +rudimentary, 451. + +Ulex, young leaves of, 439. + +Umbelliferæ, outer and inner florets of, 144. + +Unity of type, 206. + +Use: +effects of, under domestication, 11. +effects of, in a state of nature, 134. + +Utility, how far important in the construction of each part, 199. + +Valenciennes on fresh-water fish, 384. + +Variability of mongrels and hybrids, 274. + +Variation: +under domestication, 7. +caused by reproductive system being affected by conditions of life, 8. +under nature, 44. +laws of, 131. + +Variations: +appear at corresponding ages, 14, 86. +analogous in distinct species, 159. + +Varieties: +natural, 44. +struggle between, 75. +domestic, extinction of, 111. +transitional, rarity of, 172. +when crossed, fertile, 267. +when crossed, sterile, 269. +classification of, 423. + +Verbascum: +sterility of, 251. +varieties of, crossed, 270. + +Verneuil, M. de, on the succession of species, 325. + +Viola tricolor, 73. + +Volcanic islands, denudation of, 284. + +Vulture, naked skin on head, 197. + +Wading-birds, 386. + +Wallace, Mr.: +on origin of species, 2. +on law of geographical distribution, 355. +on the Malay Archipelago, 395. + +Wasp, sting of, 202. + +Water, fresh, productions of, 383. + +Water-hen, 185. + +Waterhouse, Mr.: +on Australian marsupials, 116. +on greatly developed parts being variable, 150. +on the cells of bees, 225. +on general affinities, 429. + +Water-ouzel, 185. + +Watson, Mr. H. C.: +on range of varieties of British plants, 58. +on acclimatisation, 140. +on flora of Azores, 363. +on Alpine plants, 367, 376. +on rarity of intermediate varieties, 176. + +Weald, denudation of, 285. + +Web of feet in water-birds, 185. + +West Indian islands, mammals of, 395. + +Westwood: +on species in large genera being closely allied to others, 57. +on the tarsi of Engidæ, 157. +on the antennæ of hymenopterous insects, 416. + +Whales, fossil, 303. + +Wheat, varieties of, 113. + +White Mountains, flora of, 365. + +Wings, reduction of size, 134. + +Wings: +of insects homologous with branchiæ, 191. +rudimentary, in insects, 451. + +Wolf: +crossed with dog, 214. +of Falkland Isles, 393. + +Wollaston, Mr.: +on varieties of insects, 48. +on fossil varieties of land-shells in Madeira, 52. +on colours of insects on sea-shore, 132. +on wingless beetles, 135. +on rarity of intermediate varieties, 176. +on insular insects, 389. +on land-shells of Madeira, naturalised, 402. + +Wolves, varieties of, 90. + +Woodpecker: +habits of, 184. +green colour of, 197. + +Woodward, Mr.: +on the duration of specific forms, 293. +on the continuous succession of genera, 316. +on the succession of types, 339. + +World, species changing simultaneously throughout, 322. + +Wrens, nest of, 243. + +Youatt, Mr.: +on selection, 31. +on sub-breeds of sheep, 36. +on rudimentary horns in young cattle, 454. + +Zebra, stripes on, 163. + +*** END OF THE PROJECT GUTENBERG EBOOK 1228 *** diff --git a/darwin/darwin_quotes b/darwin/darwin_quotes @@ -0,0 +1,151 @@ +name: Darwin +full_name: Charles Darwin +titre: +tags: + +## informations générales +Pour l'instant je distingue trois typologies de documents: +- théorie publiée (OS, Human Descent, … ) +- lettres adressée +- notes personnelle (notebook B) + + + "I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable” (Darwin to Hooker, 11 January 1844)" + → https://www.darwinproject.ac.uk/letter/?docId=letters/DCP-LETT-729.xml + +NOT DARWIN, Brown Janet on Darwin: +For Sedgwick, as for the others, there was no apparent +disharmony between science and religion. Science, in a sense,was religion. » +Janet BROWNE, Charles Darwin: A Biography, vol. 1 : Voyaging, +Princeton, Princeton University Press, 1996 +# science was religion, doute dans la foi dans la religion (far fetched mais à laissé de côté) + + +Thierry HOQUET, Darwin contre Darwin : +comment lire L’Origine des espèces ?, Paris, +Seuil, 2009, p. 92 +« Nul ne semble percevoir que Darwin +transforme la question de l’origine et la +déplace : de la dérivation à partir d’un +point source aux mécanismes qui +permettent la production efficace des + + +notebook B; p.101 +« Astronomers might +formerly have said that God +ordered each planet to move +in its particular destiny. — In +same manner God orders +each animal created with +certain form in certain +country, but how much more +simple & sublime powers let +attraction act according to +certain law such are +inevitable consequen[ces]. +Let animals be created, then +by the fixed laws of +generation, such will be their +successors. — » +espèces […] » + +Darwin, OS 1859, p. 6, trad. ES +« Je suis pleinement convaincu que les espèces ne +sont pas immuables […]. En outre, je suis +convaincu que la sélection naturelle a été le +moyen principal mais non exclusif de la +modification » + + +(Darwin, OS 1859, p. 242); aboutissement? accumulation doute du D +“I am surprised that no one has advanced this +demonstrative case of neuter insects, against the +well-known doctrine of Lamarck.” +(Darwin, OS 1859, p. 242) + +Définition de la sélection naturelle par Darwin, p. 81 de l’OS +« can we doubt […] that +individuals having any +advantage, however slight, over +others, would have the best +chance of surviving and of +procreating their kind? On the +other hand, we may feel sure +that any variation in the least +degree injurious would be rigidly +destroyed. This preservation +of favourable variations and +the rejection of injurious +variations, I call Natural +Selection. » + +OS, p.4 ; speculation en lien avec philo naturelle au XIXe J-Y.Cariou +« At the commencement of my observations it +seemed to me probable that a careful study of +domesticated animals and of cultivated plants +would offer the best chance of making out this +obscure problem. Nor have I been disappointed; +in this and in all other perplexing cases I have +invariably found that our knowledge, imperfect +though it be, of variation under domestication, +afforded the best and safest clue. I may venture +to express my conviction of the high value of +such studies, although they have been very +commonly neglected by naturalists. » +« Au commencement de mes observations, il me +paraissait probable qu’une étude attentive des +animaux domestiques et des plantes cultivées +m’offrirait une excellente occasion d’y voir clair sur +ce problème obscur. Je n’ai pas été déçu. Dans ce +cas troublant comme en tous les autres, j’ai +invariablement constaté que notre connaissance, +aussi imparfaite qu’elle fût, de la variation sous +domestication me procurait les meilleurs et les plus +sûrs indices. Qu’on me permette d’exprimer ma +conviction de la haute valeur de telles études, bien +qu’elles aient été très communément négligées par +les naturalistes. » (trad. Thierry Hoquet) + +Cf. lettre de Darwin à Hooker du 24 décembre 1856 : +« How I do wish I lived near you to discuss matters with. – I have just been +comparing definitions of species, & stating briefly how systematic +naturalists work out their subject: – Aquilegia in F. Indica was a capital +example for me. – It is really laughable to see what different ideas are +prominent in various naturalists minds, when they speak of “species” in +some resemblance is everything & descent of little weight – in some +resemblance seems to go for nothing & Creation the reigning idea – in +some descent the key – in some sterility an unfailing test, with others not +worth a farthing. It all comes, I believe, from trying to define the +undefinable. » + +OS.p.467 +« It is certain that [man] can +largely influence the character of +a breed by selecting, in each +successive generation, individual +differences so slight as to be quite +inappreciable by an uneducated +eye. This process of selection has +been the great agency in the +production of the most distinct +and useful domestic breeds. » + +Ernst Haeckel, Generelle Morphologie der Organismen. Allgemeine Grundzüge der organischen Formen- +Wissenschaft, mechanisch begründet durch die von Charles Darwin reformirte Descendenz-Theorie, Berlin, +Verlag von Georg Reimer, 1866, Bd. 1, Vorwort, p. XV, trad. ES +« Toutes les conceptions morphologiques générales en zoologie et en botanique sont +encore sous la coupe d’une corporation savante qu’on ne peut comparer qu’à la +scolastique du Moyen-Âge. Le dogme et l’autorité, en se conjurant pour réprimer +toute pensée libre et toute connaissance immédiate de la nature, ont érigé une +double et même une triple muraille de Chine de préjugés de toutes espèces autour +de la forteresse de la morphologie organique, dans laquelle la croyance aux miracles, +réprimée partout ailleurs, se réfugie à présent comme dans son dernier bastion. +Nous entrons pourtant dans ce combat sans crainte et sûr de la victoire. L’issue ne +peut plus faire de doute depuis que Charles Darwin a trouvé la clef de cette +forteresse il y a sept ans, et depuis que, grâce à son admirable théorie de la sélection, +la théorie de la descendance établie par Wolfgang Goethe et Jean Lamarck s’est +transformée en arme de conquête victorieuse. » +Ernst Haeckel, Generelle Morphologie der Organismen. Allgemeine Grundzüge der organischen Formen- +Wissenschaft, mechanisch begründet durch die von Charles Darwin reformirte Descendenz-Theorie, Berlin, +Verlag von Georg Reimer, 1866, Bd. 1, Vorwort, p. XV, trad. ES diff --git a/darwin/darwindoubt b/darwin/darwindoubt @@ -0,0 +1,998 @@ +grep -ni -A 2 -B 2 "doubt" git_mxsbag1/darwin/darwin/corpus/OS.txt + +99-Variability. +100-Individual Differences. +101:Doubtful species. +102-Wide ranging, much diffused, and common species vary most. +103-Species of the larger genera in any country vary more than the species of the +-- +317-This Abstract, which I now publish, must necessarily be imperfect. I cannot +318-here give references and authorities for my several statements; and I must +319:trust to the reader reposing some confidence in my accuracy. No doubt errors +320-will have crept in, though I hope I have always been cautious in trusting to +321-good authorities alone. I can here give only the general conclusions at which I +-- +424-Still less do we know of the mutual relations of the innumerable inhabitants of +425-the world during the many past geological epochs in its history. Although much +426:remains obscure, and will long remain obscure, I can entertain no doubt, after +427-the most deliberate study and dispassionate judgment of which I am capable, +428-that the view which most naturalists entertain, and which I formerly +-- +591-those of considerable physiological importance, is endless. Dr. Prosper Lucas’s +592-treatise, in two large volumes, is the fullest and the best on this subject. No +593:breeder doubts how strong is the tendency to inheritance: like produces like is +594:his fundamental belief: doubts have been thrown on this principle by +595-theoretical writers alone. When a deviation appears not unfrequently, and we +596-see it in the father and child, we cannot tell whether it may not be due to the +-- +684-as mere varieties, and by other competent judges as the descendants of +685-aboriginally distinct species. If any marked distinction existed between +686:domestic races and species, this source of doubt could not so perpetually +687-recur. It has often been stated that domestic races do not differ from each +688-other in characters of generic value. I think it could be shown that this +-- +694- +695-When we attempt to estimate the amount of structural difference between the +696:domestic races of the same species, we are soon involved in doubt, from not +697-knowing whether they have descended from one or several parent-species. This +698-point, if it could be cleared up, would be interesting; if, for instance, it +699-could be shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, +700-which we all know propagate their kind so truly, were the offspring of any +701:single species, then such facts would have great weight in making us doubt +702-about the immutability of the many very closely allied and natural species—for +703-instance, of the many foxes—inhabiting different quarters of the world. I do +-- +714-little variability of the ass or guinea-fowl, or the small power of endurance +715-of warmth by the rein-deer, or of cold by the common camel, prevented their +716:domestication? I cannot doubt that if other animals and plants, equal in number +717-to our domesticated productions, and belonging to equally diverse classes and +718-countries, were taken from a state of nature, and could be made to breed for an +-- +746-several competent judges believe that these latter have had more than one wild +747-parent. With respect to horses, from reasons which I cannot give here, I am +748:doubtfully inclined to believe, in opposition to several authors, that all the +749-races have descended from one wild stock. Mr. Blyth, whose opinion, from his +750-large and varied stores of knowledge, I should value more than that of almost +751-any one, thinks that all the breeds of poultry have proceeded from the common +752-wild Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the breeds +753:of which differ considerably from each other in structure, I do not doubt that +754-they all have descended from the common wild duck and rabbit. +755- +-- +769-as distinct parent-stocks? So it is in India. Even in the case of the domestic +770-dogs of the whole world, which I fully admit have probably descended from +771:several wild species, I cannot doubt that there has been an immense amount of +772-inherited variation. Who can believe that animals closely resembling the +773-Italian greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, etc.—so +-- +779-extreme forms, as the Italian greyhound, bloodhound, bull-dog, etc., in the +780-wild state. Moreover, the possibility of making distinct races by crossing has +781:been greatly exaggerated. There can be no doubt that a race may be modified by +782-occasional crosses, if aided by the careful selection of those individual +783-mongrels, which present any desired character; but that a race could be +-- +965-occasionally appearing in all the breeds, both when kept pure and when crossed; +966-the mongrel offspring being perfectly fertile;—from these several reasons, +967:taken together, I can feel no doubt that all our domestic breeds have descended +968-from the Columba livia with its geographical sub-species. +969- +-- +1131-ever, perhaps never, the case. The laws of correlation of growth, the +1132-importance of which should never be overlooked, will ensure some differences; +1133:but, as a general rule, I cannot doubt that the continued selection of slight +1134-variations, either in the leaves, the flowers, or the fruit, will produce races +1135-differing from each other chiefly in these characters. +-- +1167-man who intends keeping pointers naturally tries to get as good dogs as he can, +1168-and afterwards breeds from his own best dogs, but he has no wish or expectation +1169:of permanently altering the breed. Nevertheless I cannot doubt that this +1170-process, continued during centuries, would improve and modify any breed, in the +1171-same way as Bakewell, Collins, etc., by this very same process, only carried on +-- +1234-quality. I have seen great surprise expressed in horticultural works at the +1235-wonderful skill of gardeners, in having produced such splendid results from +1236:such poor materials; but the art, I cannot doubt, has been simple, and, as far +1237-as the final result is concerned, has been followed almost unconsciously. It +1238-has consisted in always cultivating the best known variety, sowing its seeds, +-- +1282-and the more abnormal or unusual any character was when it first appeared, the +1283-more likely it would be to catch his attention. But to use such an expression +1284:as trying to make a fantail, is, I have no doubt, in most cases, utterly +1285-incorrect. The man who first selected a pigeon with a slightly larger tail, +1286-never dreamed what the descendants of that pigeon would become through +-- +1349-Unless such attention be paid nothing can be effected. I have seen it gravely +1350-remarked, that it was most fortunate that the strawberry began to vary just +1351:when gardeners began to attend closely to this plant. No doubt the strawberry +1352-had always varied since it was cultivated, but the slight varieties had been +1353-neglected. As soon, however, as gardeners picked out individual plants with +-- +1370-and, although so much valued by women and children, we hardly ever see a +1371-distinct breed kept up; such breeds as we do sometimes see are almost always +1372:imported from some other country, often from islands. Although I do not doubt +1373-that some domestic animals vary less than others, yet the rarity or absence of +1374-distinct breeds of the cat, the donkey, peacock, goose, etc., may be attributed +-- +1389-correlation of growth. Something may be attributed to the direct action of the +1390-conditions of life. Something must be attributed to use and disuse. The final +1391:result is thus rendered infinitely complex. In some cases, I do not doubt that +1392-the intercrossing of species, aboriginally distinct, has played an important +1393-part in the origin of our domestic productions. When in any country several +1394-domestic breeds have once been established, their occasional intercrossing, +1395:with the aid of selection, has, no doubt, largely aided in the formation of new +1396-sub-breeds; but the importance of the crossing of varieties has, I believe, +1397-been greatly exaggerated, both in regard to animals and to those plants which +-- +1409-VARIATION UNDER NATURE. +1410- +1411:Variability. Individual differences. Doubtful species. Wide ranging, much +1412-diffused, and common species vary most. Species of the larger genera in any +1413-country vary more than the species of the smaller genera. Many of the species +-- +1490-to them by intermediate gradations, that naturalists do not like to rank them +1491-as distinct species, are in several respects the most important for us. We have +1492:every reason to believe that many of these doubtful and closely-allied forms +1493-have permanently retained their characters in their own country for a long +1494-time; for as long, as far as we know, as have good and true species. +-- +1504-found, but because analogy leads the observer to suppose either that they do +1505-now somewhere exist, or may formerly have existed; and here a wide door for the +1506:entry of doubt and conjecture is opened. +1507- +1508-Hence, in determining whether a form should be ranked as a species or a +-- +1512-named which have not been ranked as species by at least some competent judges. +1513- +1514:That varieties of this doubtful nature are far from uncommon cannot be +1515-disputed. Compare the several floras of Great Britain, of France or of the +1516-United States, drawn up by different botanists, and see what a surprising +-- +1523-species, and he has entirely omitted several highly polymorphic genera. Under +1524-genera, including the most polymorphic forms, Mr. Babington gives 251 species, +1525:whereas Mr. Bentham gives only 112,—a difference of 139 doubtful forms! Amongst +1526:animals which unite for each birth, and which are highly locomotive, doubtful +1527-forms, ranked by one zoologist as a species and by another as a variety, can +1528-rarely be found within the same country, but are common in separated areas. How +1529-many of those birds and insects in North America and Europe, which differ very +1530-slightly from each other, have been ranked by one eminent naturalist as +1531:undoubted species, and by another as varieties, or, as they are often called, +1532-as geographical races! Many years ago, when comparing, and seeing others +1533-compare, the birds from the separate islands of the Galapagos Archipelago, both +-- +1536-varieties. On the islets of the little Madeira group there are many insects +1537-which are characterized as varieties in Mr. Wollaston’s admirable work, but +1538:which it cannot be doubted would be ranked as distinct species by many +1539-entomologists. Even Ireland has a few animals, now generally regarded as +1540-varieties, but which have been ranked as species by some zoologists. Several +1541-most experienced ornithologists consider our British red grouse as only a +1542-strongly-marked race of a Norwegian species, whereas the greater number rank it +1543:as an undoubted species peculiar to Great Britain. A wide distance between the +1544:homes of two doubtful forms leads many naturalists to rank both as distinct +1545-species; but what distance, it has been well asked, will suffice? if that +1546-between America and Europe is ample, will that between the Continent and the +-- +1552-has been generally accepted, is vainly to beat the air. +1553- +1554:Many of the cases of strongly-marked varieties or doubtful species well deserve +1555-consideration; for several interesting lines of argument, from geographical +1556-distribution, analogical variation, hybridism, etc., have been brought to bear +-- +1564-Gärtner, they can be crossed only with much difficulty. We could hardly wish +1565-for better evidence of the two forms being specifically distinct. On the other +1566:hand, they are united by many intermediate links, and it is very doubtful +1567-whether these links are hybrids; and there is, as it seems to me, an +1568-overwhelming amount of experimental evidence, showing that they descend from +-- +1570- +1571-Close investigation, in most cases, will bring naturalists to an agreement how +1572:to rank doubtful forms. Yet it must be confessed, that it is in the best-known +1573:countries that we find the greatest number of forms of doubtful value. I have +1574-been struck with the fact, that if any animal or plant in a state of nature be +1575-highly useful to man, or from any cause closely attract his attention, +-- +1588-how very generally there is some variation. But if he confine his attention to +1589-one class within one country, he will soon make up his mind how to rank most of +1590:the doubtful forms. His general tendency will be to make many species, for he +1591-will become impressed, just like the pigeon or poultry-fancier before alluded +1592-to, with the amount of difference in the forms which he is continually +-- +1600-this admission will often be disputed by other naturalists. When, moreover, he +1601-comes to study allied forms brought from countries not now continuous, in which +1602:case he can hardly hope to find the intermediate links between his doubtful +1603-forms, he will have to trust almost entirely to analogy, and his difficulties +1604-will rise to a climax. +-- +1747-infallible criterion by which to distinguish species and well-marked varieties; +1748-and in those cases in which intermediate links have not been found between +1749:doubtful forms, naturalists are compelled to come to a determination by the +1750-amount of difference between them, judging by analogy whether or not the amount +1751-suffices to raise one or both to the rank of species. Hence the amount of +-- +1772-generally clustered like satellites around certain other species. And what are +1773-varieties but groups of forms, unequally related to each other, and clustered +1774:round certain forms—that is, round their parent-species? Undoubtedly there is +1775-one most important point of difference between varieties and species; namely, +1776-that the amount of difference between varieties, when compared with each other +-- +1790-London Catalogue of plants (4th edition) 63 plants which are therein ranked as +1791-species, but which he considers as so closely allied to other species as to be +1792:of doubtful value: these 63 reputed species range on an average over 6.9 of the +1793-provinces into which Mr. Watson has divided Great Britain. Now, in this same +1794-catalogue, 53 acknowledged varieties are recorded, and these range over 7.7 +-- +1796-provinces. So that the acknowledged varieties have very nearly the same +1797-restricted average range, as have those very closely allied forms, marked for +1798:me by Mr. Watson as doubtful species, but which are almost universally ranked +1799-by British botanists as good and true species. +1800- +-- +1841-has been seen in the last chapter that amongst organic beings in a state of +1842-nature there is some individual variability; indeed I am not aware that this +1843:has ever been disputed. It is immaterial for us whether a multitude of doubtful +1844-forms be called species or sub-species or varieties; what rank, for instance, +1845:the two or three hundred doubtful forms of British plants are entitled to hold, +1846-if the existence of any well-marked varieties be admitted. But the mere +1847-existence of individual variability and of some few well-marked varieties, +-- +2045-each can increase; but very frequently it is not the obtaining food, but the +2046-serving as prey to other animals, which determines the average numbers of a +2047:species. Thus, there seems to be little doubt that the stock of partridges, +2048-grouse, and hares on any large estate depends chiefly on the destruction of +2049-vermin. If not one head of game were shot during the next twenty years in +-- +2195-least highly beneficial to the fertilisation of our clovers; but humble-bees +2196-alone visit the common red clover (Trifolium pratense), as other bees cannot +2197:reach the nectar. Hence I have very little doubt, that if the whole genus of +2198-humble-bees became extinct or very rare in England, the heartsease and red +2199-clover would become very rare, or wholly disappear. The number of humble-bees +-- +2252-starve out other mountain-varieties, so that they cannot be kept together. The +2253-same result has followed from keeping together different varieties of the +2254:medicinal leech. It may even be doubted whether the varieties of any one of our +2255-domestic plants or animals have so exactly the same strength, habits, and +2256-constitution, that the original proportions of a mixed stock could be kept up +-- +2283-plumed seed of the dandelion, and in the flattened and fringed legs of the +2284-water-beetle, the relation seems at first confined to the elements of air and +2285:water. Yet the advantage of plumed seeds no doubt stands in the closest +2286-relation to the land being already thickly clothed by other plants; so that the +2287-seeds may be widely distributed and fall on unoccupied ground. In the +-- +2356-close-fitting are the mutual relations of all organic beings to each other and +2357-to their physical conditions of life. Can it, then, be thought improbable, +2358:seeing that variations useful to man have undoubtedly occurred, that other +2359-variations useful in some way to each being in the great and complex battle of +2360-life, should sometimes occur in the course of thousands of generations? If such +2361:do occur, can we doubt (remembering that many more individuals are born than +2362-can possibly survive) that individuals having any advantage, however slight, +2363-over others, would have the best chance of surviving and of procreating their +-- +2464-from birds of prey; and hawks are guided by eyesight to their prey,—so much so, +2465-that on parts of the Continent persons are warned not to keep white pigeons, as +2466:being the most liable to destruction. Hence I can see no reason to doubt that +2467-natural selection might be most effective in giving the proper colour to each +2468-kind of grouse, and in keeping that colour, when once acquired, true and +-- +2506-his cotton-trees. Natural selection may modify and adapt the larva of an insect +2507-to a score of contingencies, wholly different from those which concern the +2508:mature insect. These modifications will no doubt affect, through the laws of +2509-correlation, the structure of the adult; and probably in the case of those +2510-insects which live only for a few hours, and which never feed, a large part of +-- +2578-weak means: I cannot here enter on the details necessary to support this view; +2579-but if man can in a short time give elegant carriage and beauty to his bantams, +2580:according to his standard of beauty, I can see no good reason to doubt that +2581-female birds, by selecting, during thousands of generations, the most melodious +2582-or beautiful males, according to their standard of beauty, might produce a +-- +2611-from any change in the country increased in numbers, or that other prey had +2612-decreased in numbers, during that season of the year when the wolf is hardest +2613:pressed for food. I can under such circumstances see no reason to doubt that +2614-the swiftest and slimmest wolves would have the best chance of surviving, and +2615-so be preserved or selected,—provided always that they retained strength to +2616-master their prey at this or at some other period of the year, when they might +2617:be compelled to prey on other animals. I can see no more reason to doubt this, +2618-than that man can improve the fleetness of his greyhounds by careful and +2619-methodical selection, or by that unconscious selection which results from each +-- +2693-case: as soon as the plant had been rendered so highly attractive to insects +2694-that pollen was regularly carried from flower to flower, another process might +2695:commence. No naturalist doubts the advantage of what has been called the +2696-“physiological division of labour;” hence we may believe that it would be +2697-advantageous to a plant to produce stamens alone in one flower or on one whole +-- +2713-the nectar at the bases of certain flowers, which they can, with a very little +2714-more trouble, enter by the mouth. Bearing such facts in mind, I can see no +2715:reason to doubt that an accidental deviation in the size and form of the body, +2716-or in the curvature and length of the proboscis, etc., far too slight to be +2717-appreciated by us, might profit a bee or other insect, so that an individual so +-- +2809-When the stamens of a flower suddenly spring towards the pistil, or slowly move +2810-one after the other towards it, the contrivance seems adapted solely to ensure +2811:self-fertilisation; and no doubt it is useful for this end: but, the agency of +2812-insects is often required to cause the stamens to spring forward, as Kölreuter +2813-has shown to be the case with the barberry; and curiously in this very genus, +-- +3011-doing. +3012- +3013:Although I do not doubt that isolation is of considerable importance in the +3014-production of new species, on the whole I am inclined to believe that largeness +3015-of area is of more importance, more especially in the production of species, +-- +3159-important facts. In the first place, varieties, even strongly-marked ones, +3160-though having somewhat of the character of species—as is shown by the hopeless +3161:doubts in many cases how to rank them—yet certainly differ from each other far +3162-less than do good and distinct species. Nevertheless, according to my view, +3163-varieties are species in the process of formation, or are, as I have called +-- +3227-variety of grass is annually sowing almost countless seeds; and thus, as it may +3228-be said, is striving its utmost to increase its numbers. Consequently, I cannot +3229:doubt that in the course of many thousands of generations, the most distinct +3230-varieties of any one species of grass would always have the best chance of +3231-succeeding and of increasing in numbers, and thus of supplanting the less +-- +3283-fact, the same as that of the physiological division of labour in the organs of +3284-the same individual body—a subject so well elucidated by Milne Edwards. No +3285:physiologist doubts that a stomach by being adapted to digest vegetable matter +3286-alone, or flesh alone, draws most nutriment from these substances. So in the +3287-general economy of any land, the more widely and perfectly the animals and +-- +3289-of individuals be capable of there supporting themselves. A set of animals, +3290-with their organisation but little diversified, could hardly compete with a set +3291:more perfectly diversified in structure. It may be doubted, for instance, +3292-whether the Australian marsupials, which are divided into groups differing but +3293-little from each other, and feebly representing, as Mr. Waterhouse and others +-- +3385-improved branches: this is represented in the diagram by some of the lower +3386-branches not reaching to the upper horizontal lines. In some cases I do not +3387:doubt that the process of modification will be confined to a single line of +3388-descent, and the number of the descendants will not be increased; although the +3389-amount of divergent modification may have been increased in the successive +-- +3400-amount of change between each horizontal line in our diagram to be excessively +3401-small, these three forms may still be only well-marked varieties; or they may +3402:have arrived at the doubtful category of sub-species; but we have only to +3403-suppose the steps in the process of modification to be more numerous or greater +3404-in amount, to convert these three forms into well-defined species: thus the +-- +3703- +3704-How much direct effect difference of climate, food, etc., produces on any being +3705:is extremely doubtful. My impression is, that the effect is extremely small in +3706-the case of animals, but perhaps rather more in that of plants. We may, at +3707-least, safely conclude that such influences cannot have produced the many +-- +3756- +3757-Effects of Use and Disuse.—From the facts alluded to in the first chapter, I +3758:think there can be little doubt that use in our domestic animals strengthens +3759-and enlarges certain parts, and disuse diminishes them; and that such +3760-modifications are inherited. Under free nature, we can have no standard of +-- +3955-selecting so many breeds and sub-breeds with constitutions specially fitted for +3956-their own districts: the result must, I think, be due to habit. On the other +3957:hand, I can see no reason to doubt that natural selection will continually tend +3958-to preserve those individuals which are born with constitutions best adapted to +3959-their native countries. In treatises on many kinds of cultivated plants, +-- +3995-front and hind legs, and even in the jaws and limbs, varying together, for the +3996-lower jaw is believed to be homologous with the limbs. These tendencies, I do +3997:not doubt, may be mastered more or less completely by natural selection: thus a +3998-family of stags once existed with an antler only on one side; and if this had +3999-been of any great use to the breed it might probably have been rendered +-- +4076-modifications, having been transmitted to a whole group of descendants with +4077-diverse habits, would naturally be thought to be correlated in some necessary +4078:manner. So, again, I do not doubt that some apparent correlations, occurring +4079-throughout whole orders, are entirely due to the manner alone in which natural +4080-selection can act. For instance, Alph. De Candolle has remarked that winged +-- +4191-characters, may be due to the great variability of these characters, whether or +4192-not displayed in any unusual manner—of which fact I think there can be little +4193:doubt. But that our rule is not confined to secondary sexual characters is +4194-clearly shown in the case of hermaphrodite cirripedes; and I may here add, that +4195-I particularly attended to Mr. Waterhouse’s remark, whilst investigating this +-- +4266-on the one hand, and the tendency to reversion and variability on the other +4267-hand, will in the course of time cease; and that the most abnormally developed +4268:organs may be made constant, I can see no reason to doubt. Hence when an organ, +4269-however abnormal it may be, has been transmitted in approximately the same +4270-condition to many modified descendants, as in the case of the wing of the bat, +-- +4302-physiological importance may remain the same. Something of the same kind +4303-applies to monstrosities: at least Is. Geoffroy St. Hilaire seems to entertain +4304:no doubt, that the more an organ normally differs in the different species of +4305-the same group, the more subject it is to individual anomalies. +4306- +-- +4403-distinct races. The frequent presence of fourteen or even sixteen tail-feathers +4404-in the pouter, may be considered as a variation representing the normal +4405:structure of another race, the fantail. I presume that no one will doubt that +4406-all such analogous variations are due to the several races of the pigeon having +4407-inherited from a common parent the same constitution and tendency to variation, +-- +4422-white rump, a bar at the end of the tail, with the outer feathers externally +4423-edged near their bases with white. As all these marks are characteristic of the +4424:parent rock-pigeon, I presume that no one will doubt that this is a case of +4425-reversion, and not of a new yet analogous variation appearing in the several +4426-breeds. We may I think confidently come to this conclusion, because, as we have +-- +4431-of crossing on the laws of inheritance. +4432- +4433:No doubt it is a very surprising fact that characters should reappear after +4434-having been lost for many, perhaps for hundreds of generations. But when a +4435-breed has been crossed only once by some other breed, the offspring +-- +4480-especially we might have inferred this, from the blue colour and marks so often +4481-appearing when distinct breeds of diverse colours are crossed. Hence, though +4482:under nature it must generally be left doubtful, what cases are reversions to +4483-an anciently existing character, and what are new but analogous variations, yet +4484-we ought, on my theory, sometimes to find the varying offspring of a species +4485-assuming characters (either from reversion or from analogous variation) which +4486:already occur in some other members of the same group. And this undoubtedly is +4487-the case in nature. +4488- +-- +4491-other species of the same genus. A considerable catalogue, also, could be given +4492-of forms intermediate between two other forms, which themselves must be +4493:doubtfully ranked as either varieties or species; and this shows, unless all +4494-these forms be considered as independently created species, that the one in +4495-varying has assumed some of the characters of the other, so as to produce the +-- +4787-at present. But I will pass over this way of escaping from the difficulty; for +4788-I believe that many perfectly defined species have been formed on strictly +4789:continuous areas; though I do not doubt that the formerly broken condition of +4790-areas now continuous has played an important part in the formation of new +4791-species, more especially with freely-crossing and wandering animals. +-- +4946-squirrels have their limbs and even the base of the tail united by a broad +4947-expanse of skin, which serves as a parachute and allows them to glide through +4948:the air to an astonishing distance from tree to tree. We cannot doubt that each +4949-structure is of use to each kind of squirrel in its own country, by enabling it +4950-to escape birds or beasts of prey, or to collect food more quickly, or, as +-- +5187-If it could be demonstrated that any complex organ existed, which could not +5188-possibly have been formed by numerous, successive, slight modifications, my +5189:theory would absolutely break down. But I can find out no such case. No doubt +5190-many organs exist of which we do not know the transitional grades, more +5191-especially if we look to much-isolated species, round which, according to my +-- +5228-swimbladder into a lung, or organ used exclusively for respiration. +5229- +5230:I can, indeed, hardly doubt that all vertebrate animals having true lungs have +5231-descended by ordinary generation from an ancient prototype, of which we know +5232-nothing, furnished with a floating apparatus or swimbladder. We can thus, as I +-- +5256-think no one will dispute that the ovigerous frena in the one family are +5257-strictly homologous with the branchiæ of the other family; indeed, they +5258:graduate into each other. Therefore I do not doubt that little folds of skin, +5259-which originally served as ovigerous frena, but which, likewise, very slightly +5260-aided the act of respiration, have been gradually converted by natural +-- +5268-Although we must be extremely cautious in concluding that any organ could not +5269-possibly have been produced by successive transitional gradations, yet, +5270:undoubtedly, grave cases of difficulty occur, some of which will be discussed +5271-in my future work. +5272- +-- +5387-was a beautiful adaptation to hide this tree-frequenting bird from its enemies; +5388-and consequently that it was a character of importance and might have been +5389:acquired through natural selection; as it is, I have no doubt that the colour +5390-is due to some quite distinct cause, probably to sexual selection. A trailing +5391-bamboo in the Malay Archipelago climbs the loftiest trees by the aid of +5392-exquisitely constructed hooks clustered around the ends of the branches, and +5393:this contrivance, no doubt, is of the highest service to the plant; but as we +5394-see nearly similar hooks on many trees which are not climbers, the hooks on the +5395-bamboo may have arisen from unknown laws of growth, and have been subsequently +-- +5401-of the clean-feeding male turkey is likewise naked. The sutures in the skulls +5402-of young mammals have been advanced as a beautiful adaptation for aiding +5403:parturition, and no doubt they facilitate, or may be indispensable for this +5404-act; but as sutures occur in the skulls of young birds and reptiles, which have +5405-only to escape from a broken egg, we may infer that this structure has arisen +-- +5448-possessors. Physical conditions probably have had some little effect on +5449-structure, quite independently of any good thus gained. Correlation of growth +5450:has no doubt played a most important part, and a useful modification of one +5451-part will often have entailed on other parts diversified changes of no direct +5452-use. So again characters which formerly were useful, or which formerly had +-- +5464-special use to these animals. We may safely attribute these structures to +5465-inheritance. But to the progenitor of the upland goose and of the frigate-bird, +5466:webbed feet no doubt were as useful as they now are to the most aquatic of +5467-existing birds. So we may believe that the progenitor of the seal had not a +5468-flipper, but a foot with five toes fitted for walking or grasping; and we may +-- +5538-hatred of the queen-bee, which urges her instantly to destroy the young queens +5539-her daughters as soon as born, or to perish herself in the combat; for +5540:undoubtedly this is for the good of the community; and maternal love or +5541-maternal hatred, though the latter fortunately is most rare, is all the same to +5542-the inexorable principle of natural selection. If we admire the several +-- +5720-all the most complex and wonderful instincts have originated. As modifications +5721-of corporeal structure arise from, and are increased by, use or habit, and are +5722:diminished or lost by disuse, so I do not doubt it has been with instincts. But +5723-I believe that the effects of habit are of quite subordinate importance to the +5724-effects of the natural selection of what may be called accidental variations of +-- +5810-disposition and tastes, and likewise of the oddest tricks, associated with +5811-certain frames of mind or periods of time. But let us look to the familiar case +5812:of the several breeds of dogs: it cannot be doubted that young pointers (I have +5813-myself seen a striking instance) will sometimes point and even back other dogs +5814-the very first time that they are taken out; retrieving is certainly in some +-- +5851-made tumblers what they now are; and near Glasgow there are house-tumblers, as +5852-I hear from Mr. Brent, which cannot fly eighteen inches high without going head +5853:over heels. It may be doubted whether any one would have thought of training a +5854-dog to point, had not some one dog naturally shown a tendency in this line; and +5855-this is known occasionally to happen, as I once saw in a pure terrier. When the +-- +5869-that is, never wish to sit on their eggs. Familiarity alone prevents our seeing +5870-how universally and largely the minds of our domestic animals have been +5871:modified by domestication. It is scarcely possible to doubt that the love of +5872-man has become instinctive in the dog. All wolves, foxes, jackals, and species +5873-of the cat genus, when kept tame, are most eager to attack poultry, sheep, and +-- +5876-where the savages do not keep these domestic animals. How rarely, on the other +5877-hand, do our civilised dogs, even when quite young, require to be taught not to +5878:attack poultry, sheep, and pigs! No doubt they occasionally do make an attack, +5879-and are then beaten; and if not cured, they are destroyed; so that habit, with +5880-some degree of selection, has probably concurred in civilising by inheritance +5881-our dogs. On the other hand, young chickens have lost, wholly by habit, that +5882:fear of the dog and cat which no doubt was originally instinctive in them, in +5883-the same way as it is so plainly instinctive in young pheasants, though reared +5884-under a hen. It is not that chickens have lost all fear, but fear only of dogs +-- +5991-fully trusting to the statements of Huber and Mr. Smith, I tried to approach +5992-the subject in a sceptical frame of mind, as any one may well be excused for +5993:doubting the truth of so extraordinary and odious an instinct as that of making +5994-slaves. Hence I will give the observations which I have myself made, in some +5995-little detail. I opened fourteen nests of F. sanguinea, and found a few slaves +-- +6367-a country; and let us further suppose that the community lived throughout the +6368-winter, and consequently required a store of honey: there can in this case be +6369:no doubt that it would be an advantage to our humble-bee, if a slight +6370-modification of her instinct led her to make her waxen cells near together, so +6371-as to intersect a little; for a wall in common even to two adjoining cells, +-- +6398-struggle for existence. +6399- +6400:No doubt many instincts of very difficult explanation could be opposed to the +6401-theory of natural selection,—cases, in which we cannot see how an instinct +6402-could possibly have originated; cases, in which no intermediate gradations are +-- +6461-marbled together; the animal has been slaughtered, but the breeder goes with +6462-confidence to the same family. I have such faith in the powers of selection, +6463:that I do not doubt that a breed of cattle, always yielding oxen with +6464-extraordinarily long horns, could be slowly formed by carefully watching which +6465-individual bulls and cows, when matched, produced oxen with the longest horns; +-- +6690-experimentised on by Gärtner were potted, and apparently were kept in a chamber +6691-in his house. That these processes are often injurious to the fertility of a +6692:plant cannot be doubted; for Gärtner gives in his table about a score of cases +6693-of plants which he castrated, and artificially fertilised with their own +6694-pollen, and (excluding all cases such as the Leguminosæ, in which there is an +-- +6700-arvensis and coerulea), which the best botanists rank as varieties, absolutely +6701-sterile together; and as he came to the same conclusion in several other +6702:analogous cases; it seems to me that we may well be permitted to doubt whether +6703-many other species are really so sterile, when intercrossed, as Gärtner +6704-believes. +-- +6714-instructive to compare—but I have not space here to enter on details—the +6715-evidence advanced by our best botanists on the question whether certain +6716:doubtful forms should be ranked as species or varieties, with the evidence from +6717-fertility adduced by different hybridisers, or by the same author, from +6718-experiments made during different years. It can thus be shown that neither +6719-sterility nor fertility affords any clear distinction between species and +6720-varieties; but that the evidence from this source graduates away, and is +6721:doubtful in the same degree as is the evidence derived from other +6722-constitutional and structural differences. +6723- +-- +6726-either pure parent, for six or seven, and in one case for ten generations, yet +6727-he asserts positively that their fertility never increased, but generally +6728:greatly decreased. I do not doubt that this is usually the case, and that the +6729-fertility often suddenly decreases in the first few generations. Nevertheless I +6730-believe that in all these experiments the fertility has been diminished by an +-- +6732-a body of facts, showing that close interbreeding lessens fertility, and, on +6733-the other hand, that an occasional cross with a distinct individual or variety +6734:increases fertility, that I cannot doubt the correctness of this almost +6735-universal belief amongst breeders. Hybrids are seldom raised by +6736-experimentalists in great numbers; and as the parent-species, or other allied +-- +6826-then we may infer that animals more widely separated in the scale of nature can +6827-be more easily crossed than in the case of plants; but the hybrids themselves +6828:are, I think, more sterile. I doubt whether any case of a perfectly fertile +6829-hybrid animal can be considered as thoroughly well authenticated. It should, +6830-however, be borne in mind that, owing to few animals breeding freely under +-- +6871-several wild stocks; yet, with perhaps the exception of certain indigenous +6872-domestic dogs of South America, all are quite fertile together; and analogy +6873:makes me greatly doubt, whether the several aboriginal species would at first +6874-have freely bred together and have produced quite fertile hybrids. So again +6875-there is reason to believe that our European and the humped Indian cattle are +-- +7237-and cowslip, which are considered by many of our best botanists as varieties, +7238-are said by Gärtner not to be quite fertile when crossed, and he consequently +7239:ranks them as undoubted species. If we thus argue in a circle, the fertility of +7240-all varieties produced under nature will assuredly have to be granted. +7241- +7242-If we turn to varieties, produced, or supposed to have been produced, under +7243:domestication, we are still involved in doubt. For when it is stated, for +7244-instance, that the German Spitz dog unites more easily than other dogs with +7245-foxes, or that certain South American indigenous domestic dogs do not readily +-- +7694-flanks of which the overlying sedimentary deposits might have accumulated in +7695-thinner masses than elsewhere, the above estimate would be erroneous; but this +7696:source of doubt probably would not greatly affect the estimate as applied to +7697-the western extremity of the district. If, then, we knew the rate at which the +7698-sea commonly wears away a line of cliff of any given height, we could measure +-- +7703-same as if we were to assume a cliff one yard in height to be eaten back along +7704-a whole line of coast at the rate of one yard in nearly every twenty-two years. +7705:I doubt whether any rock, even as soft as chalk, would yield at this rate +7706:excepting on the most exposed coasts; though no doubt the degradation of a +7707-lofty cliff would be more rapid from the breakage of the fallen fragments. On +7708-the other hand, I do not believe that any line of coast, ten or twenty miles in +-- +7807-though the supply of sediment must for ages have been great, from the enormous +7808-degradation of the coast-rocks and from muddy streams entering the sea. The +7809:explanation, no doubt, is, that the littoral and sub-littoral deposits are +7810-continually worn away, as soon as they are brought up by the slow and gradual +7811-rising of the land within the grinding action of the coast-waves. +-- +7868-linking forms. +7869- +7870:From the foregoing considerations it cannot be doubted that the geological +7871-record, viewed as a whole, is extremely imperfect; but if we confine our +7872-attention to any one formation, it becomes more difficult to understand, why we +-- +7910-whole geological period; and likewise to reflect on the great changes of level, +7911-on the inordinately great change of climate, on the prodigious lapse of time, +7912:all included within this same glacial period. Yet it may be doubted whether in +7913-any quarter of the world, sedimentary deposits, including fossil remains, have +7914-gone on accumulating within the same area during the whole of this period. It +-- +8076-geological record would then be least perfect. +8077- +8078:It may be doubted whether the duration of any one great period of subsidence +8079-over the whole or part of the archipelago, together with a contemporaneous +8080-accumulation of sediment, would exceed the average duration of the same +-- +8227-which have convinced me that all the existing species of the same group have +8228-descended from one progenitor, apply with nearly equal force to the earliest +8229:known species. For instance, I cannot doubt that all the Silurian trilobites +8230-have descended from some one crustacean, which must have lived long before the +8231-Silurian age, and which probably differed greatly from any known animal. Some +-- +8256-lowest azoic rocks, probably indicates the former existence of life at these +8257-periods. But the difficulty of understanding the absence of vast piles of +8258:fossiliferous strata, which on my theory no doubt were somewhere accumulated +8259-before the Silurian epoch, is very great. If these most ancient beds had been +8260-wholly worn away by denudation, or obliterated by metamorphic action, we ought +-- +8292-oceans now extend, oceans have extended from the remotest period of which we +8293-have any record; and on the other hand, that where continents now exist, large +8294:tracts of land have existed, subjected no doubt to great oscillations of level, +8295-since the earliest silurian period. The coloured map appended to my volume on +8296-Coral Reefs, led me to conclude that the great oceans are still mainly areas of +-- +8321-groups of species appear in our European formations; the almost entire absence, +8322-as at present known, of fossiliferous formations beneath the Silurian strata, +8323:are all undoubtedly of the gravest nature. We see this in the plainest manner +8324-by the fact that all the most eminent palæontologists, namely Cuvier, Owen, +8325-Agassiz, Barrande, Falconer, E. Forbes, etc., and all our greatest geologists, +-- +8327-maintained the immutability of species. But I have reason to believe that one +8328-great authority, Sir Charles Lyell, from further reflexion entertains grave +8329:doubts on this subject. I feel how rash it is to differ from these great +8330-authorities, to whom, with others, we owe all our knowledge. Those who think +8331-the natural geological record in any degree perfect, and who do not attach much +8332-weight to the facts and arguments of other kinds given in this volume, will +8333:undoubtedly at once reject my theory. For my part, following out Lyell’s +8334-metaphor, I look at the natural geological record, as a history of the world +8335-imperfectly kept, and written in a changing dialect; of this history we possess +-- +8364-year tends to fill up the blanks between them, and to make the percentage +8365-system of lost and new forms more gradual. In some of the most recent beds, +8366:though undoubtedly of high antiquity if measured by years, only one or two +8367-species are lost forms, and only one or two are new forms, having here appeared +8368-for the first time, either locally, or, as far as we know, on the face of the +-- +8438-We can clearly understand why a species when once lost should never reappear, +8439-even if the very same conditions of life, organic and inorganic, should recur. +8440:For though the offspring of one species might be adapted (and no doubt this has +8441-occurred in innumerable instances) to fill the exact place of another species +8442-in the economy of nature, and thus supplant it; yet the two forms—the old and +-- +8652-of the world: we have not sufficient data to judge whether the productions of +8653-the land and of fresh water change at distant points in the same parallel +8654:manner. We may doubt whether they have thus changed: if the Megatherium, +8655-Mylodon, Macrauchenia, and Toxodon had been brought to Europe from La Plata, +8656-without any information in regard to their geological position, no one would +-- +8676-North America would hereafter be liable to be classed with somewhat older +8677-European beds. Nevertheless, looking to a remotely future epoch, there can, I +8678:think, be little doubt that all the more modern marine formations, namely, the +8679-upper pliocene, the pleistocene and strictly modern beds, of Europe, North and +8680-South America, and Australia, from containing fossil remains in some degree +-- +8837-It is a common belief that the more ancient a form is, by so much the more it +8838-tends to connect by some of its characters groups now widely separated from +8839:each other. This remark no doubt must be restricted to those groups which have +8840-undergone much change in the course of geological ages; and it would be +8841-difficult to prove the truth of the proposition, for every now and then even a +-- +8933-modification, during the long and blank intervals between the successive +8934-formations. Subject to these allowances, the fauna of each geological period +8935:undoubtedly is intermediate in character, between the preceding and succeeding +8936-faunas. I need give only one instance, namely, the manner in which the fossils +8937-of the Devonian system, when this system was first discovered, were at once +-- +9006-the existing inhabitants of the same or some other quarter, the eocene fauna or +9007-flora would certainly be beaten and exterminated; as would a secondary fauna by +9008:an eocene, and a palæozoic fauna by a secondary fauna. I do not doubt that this +9009-process of improvement has affected in a marked and sensible manner the +9010-organisation of the more recent and victorious forms of life, in comparison +-- +9018-there, and would exterminate many of the natives. On the other hand, from what +9019-we see now occurring in New Zealand, and from hardly a single inhabitant of the +9020:southern hemisphere having become wild in any part of Europe, we may doubt, if +9021-all the productions of New Zealand were set free in Great Britain, whether any +9022-considerable number would be enabled to seize on places now occupied by our +-- +9394-We are thus brought to the question which has been largely discussed by +9395-naturalists, namely, whether species have been created at one or more points of +9396:the earth’s surface. Undoubtedly there are very many cases of extreme +9397-difficulty, in understanding how the same species could possibly have migrated +9398-from some one point to the several distant and isolated points, where now +-- +9433-each species having been produced in one area alone, and having subsequently +9434-migrated from that area as far as its powers of migration and subsistence under +9435:past and present conditions permitted, is the most probable. Undoubtedly many +9436-cases occur, in which we cannot explain how the same species could have passed +9437-from one point to the other. But the geographical and climatal changes, which +-- +9606-caused some of them to have floated much longer. The result was that 18/98 of +9607-his seeds floated for 42 days, and were then capable of germination. But I do +9608:not doubt that plants exposed to the waves would float for a less time than +9609-those protected from violent movement as in our experiments. Therefore it would +9610-perhaps be safer to assume that the seeds of about 10/100 plants of a flora, +-- +9672-showing that soil almost everywhere is charged with seeds. Reflect for a moment +9673-on the millions of quails which annually cross the Mediterranean; and can we +9674:doubt that the earth adhering to their feet would sometimes include a few +9675-minute seeds? But I shall presently have to recur to this subject. +9676- +9677-As icebergs are known to be sometimes loaded with earth and stones, and have +9678:even carried brushwood, bones, and the nest of a land-bird, I can hardly doubt +9679-that they must occasionally have transported seeds from one part to another of +9680-the arctic and antarctic regions, as suggested by Lyell; and during the Glacial +-- +9693- +9694-Considering that the several above means of transport, and that several other +9695:means, which without doubt remain to be discovered, have been in action year +9696-after year, for centuries and tens of thousands of years, it would I think be a +9697-marvellous fact if many plants had not thus become widely transported. These +-- +9720-few centuries, through occasional means of transport, immigrants from Europe or +9721-any other continent, that a poorly-stocked island, though standing more remote +9722:from the mainland, would not receive colonists by similar means. I do not doubt +9723-that out of twenty seeds or animals transported to an island, even if far less +9724-well-stocked than Britain, scarcely more than one would be so well fitted to +-- +9833-find has been the case; for if we compare the present Alpine plants and animals +9834-of the several great European mountain-ranges, though very many of the species +9835:are identically the same, some present varieties, some are ranked as doubtful +9836-forms, and some few are distinct yet closely allied or representative species. +9837- +-- +10025-towards equatorial latitudes, the Alpine or mountain floras really become less +10026-and less arctic.” Many of the forms living on the mountains of the warmer +10027:regions of the earth and in the southern hemisphere are of doubtful value, +10028-being ranked by some naturalists as specifically distinct, by others as +10029-varieties; but some are certainly identical, and many, though closely related +-- +10065-flowering plants, about forty-six in number, common to Tierra del Fuego and to +10066-Europe still exist in North America, which must have lain on the line of march. +10067:But I do not doubt that some temperate productions entered and crossed even the +10068-lowlands of the tropics at the period when the cold was most intense,—when +10069-arctic forms had migrated some twenty-five degrees of latitude from their +-- +10114-Europe during the last two or three centuries from La Plata, and during the +10115-last thirty or forty years from Australia. Something of the same kind must have +10116:occurred on the intertropical mountains: no doubt before the Glacial period +10117-they were stocked with endemic Alpine forms; but these have almost everywhere +10118-largely yielded to the more dominant forms, generated in the larger areas and +-- +10223-completely prevented their inosculation, seems to lead to this same conclusion. +10224-With respect to allied fresh-water fish occurring at very distant points of the +10225:world, no doubt there are many cases which cannot at present be explained: but +10226-some fresh-water fish belong to very ancient forms, and in such cases there +10227-will have been ample time for great geographical changes, and consequently time +-- +10239-they are immediately killed by sea water, as are the adults. I could not even +10240-understand how some naturalised species have rapidly spread throughout the same +10241:country. But two facts, which I have observed—and no doubt many others remain +10242-to be observed—throw some light on this subject. When a duck suddenly emerges +10243-from a pond covered with duck-weed, I have twice seen these little plants +-- +10409-are elsewhere. Such cases are generally accounted for by the physical +10410-conditions of the islands; but this explanation seems to me not a little +10411:doubtful. Facility of immigration, I believe, has been at least as important as +10412-the nature of the conditions. +10413- +-- +10452-Mammals offer another and similar case. I have carefully searched the oldest +10453-voyages, but have not finished my search; as yet I have not found a single +10454:instance, free from doubt, of a terrestrial mammal (excluding domesticated +10455-animals kept by the natives) inhabiting an island situated above 300 miles from +10456-a continent or great continental island; and many islands situated at a much +-- +10496-and this space separates two widely distinct mammalian faunas. On either side +10497-the islands are situated on moderately deep submarine banks, and they are +10498:inhabited by closely allied or identical quadrupeds. No doubt some few +10499-anomalies occur in this great archipelago, and there is much difficulty in +10500-forming a judgment in some cases owing to the probable naturalisation of +-- +10609-the south-western corner of Australia and of the Cape of Good Hope, is a far +10610-more remarkable case, and is at present inexplicable: but this affinity is +10611:confined to the plants, and will, I do not doubt, be some day explained. +10612- +10613-The law which causes the inhabitants of an archipelago, though specifically +-- +10641-plant to another island. Hence when in former times an immigrant settled on any +10642-one or more of the islands, or when it subsequently spread from one island to +10643:another, it would undoubtedly be exposed to different conditions of life in the +10644-different islands, for it would have to compete with different sets of +10645-organisms: a plant, for instance, would find the best-fitted ground more +-- +10665-we often take, I think, an erroneous view of the probability of closely allied +10666-species invading each other’s territory, when put into free intercommunication. +10667:Undoubtedly if one species has any advantage whatever over another, it will in +10668-a very brief time wholly or in part supplant it; but if both are equally well +10669-fitted for their own places in nature, both probably will hold their own places +-- +10690-Porto Santo to Madeira, yet this latter island has not become colonised by the +10691-Porto Santo species: nevertheless both islands have been colonised by some +10692:European land-shells, which no doubt had some advantage over the indigenous +10693-species. From these considerations I think we need not greatly marvel at the +10694-endemic and representative species, which inhabit the several islands of the +-- +10723-between the two regions. And wherever many closely-allied species occur, there +10724-will be found many forms which some naturalists rank as distinct species, and +10725:some as varieties; these doubtful forms showing us the steps in the process of +10726-modification. +10727- +-- +10731-is shown in another and more general way. Mr. Gould remarked to me long ago, +10732-that in those genera of birds which range over the world, many of the species +10733:have very wide ranges. I can hardly doubt that this rule is generally true, +10734-though it would be difficult to prove it. Amongst mammals, we see it strikingly +10735-displayed in Bats, and in a lesser degree in the Felidæ and Canidæ. We see it, +-- +10861-derived. We can see why in two areas, however distant from each other, there +10862-should be a correlation, in the presence of identical species, of varieties, of +10863:doubtful species, and of distinct but representative species. +10864- +10865-As the late Edward Forbes often insisted, there is a striking parallelism in +-- +11005-relation to the outer world. Perhaps from this cause it has partly arisen, that +11006-almost all naturalists lay the greatest stress on resemblances in organs of +11007:high vital or physiological importance. No doubt this view of the +11008-classificatory importance of organs which are important is generally, but by no +11009-means always, true. But their importance for classification, I believe, depends +-- +11035- +11036-Again, no one will say that rudimentary or atrophied organs are of high +11037:physiological or vital importance; yet, undoubtedly, organs in this condition +11038-are often of high value in classification. No one will dispute that the +11039-rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary +-- +11062-natural history. Hence, as has often been remarked, a species may depart from +11063-its allies in several characters, both of high physiological importance and of +11064:almost universal prevalence, and yet leave us in no doubt where it should be +11065-ranked. Hence, also, it has been found, that a classification founded on any +11066-single character, however important that may be, has always failed; for no part +-- +11346-fin-like limbs serve as characters exhibiting true affinity between the several +11347-members of the whale family; for these cetaceans agree in so many characters, +11348:great and small, that we cannot doubt that they have inherited their general +11349-shape of body and structure of limbs from a common ancestor. So it is with +11350-fishes. +-- +11807-stock, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now +11808-some of these birds, when mature, differ so extraordinarily in length and form +11809:of beak, that they would, I cannot doubt, be ranked in distinct genera, had +11810-they been natural productions. But when the nestling birds of these several +11811-breeds were placed in a row, though most of them could be distinguished from +-- +11951-each other most closely in all respects, one of which will have full-sized +11952-wings, and another mere rudiments of membrane; and here it is impossible to +11953:doubt, that the rudiments represent wings. Rudimentary organs sometimes retain +11954-their potentiality, and are merely not developed: this seems to be the case +11955-with the mammæ of male mammals, for many instances are on record of these +-- +12032-of cattle, more especially, according to Youatt, in young animals,—and the +12033-state of the whole flower in the cauliflower. We often see rudiments of various +12034:parts in monsters. But I doubt whether any of these cases throw light on the +12035-origin of rudimentary organs in a state of nature, further than by showing that +12036:rudiments can be produced; for I doubt whether species under nature ever +12037-undergo abrupt changes. I believe that disuse has been the main agency; that it +12038-has led in successive generations to the gradual reduction of various organs, +-- +12161-truth of these propositions cannot, I think, be disputed. +12162- +12163:It is, no doubt, extremely difficult even to conjecture by what gradations many +12164-structures have been perfected, more especially amongst broken and failing +12165-groups of organic beings; but we see so many strange gradations in nature, as +-- +12280-intermediate links between their past or parent and present states; and these +12281-many links we could hardly ever expect to discover, owing to the imperfection +12282:of the geological record. Numerous existing doubtful forms could be named which +12283-are probably varieties; but who will pretend that in future ages so many fossil +12284-links will be discovered, that naturalists will be able to decide, on the +12285:common view, whether or not these doubtful forms are varieties? As long as most +12286-of the links between any two species are unknown, if any one link or +12287-intermediate variety be discovered, it will simply be classed as another and +-- +12318-justly be urged against my theory; and I have now briefly recapitulated the +12319-answers and explanations which can be given to them. I have felt these +12320:difficulties far too heavily during many years to doubt their weight. But it +12321-deserves especial notice that the more important objections relate to questions +12322-on which we are confessedly ignorant; nor do we know how ignorant we are. We do +-- +12355-been the great agency in the production of the most distinct and useful +12356-domestic breeds. That many of the breeds produced by man have to a large extent +12357:the character of natural species, is shown by the inextricable doubts whether +12358-very many of them are varieties or aboriginal species. +12359- +-- +12405- +12406-If then we have under nature variability and a powerful agent always ready to +12407:act and select, why should we doubt that variations in any way useful to +12408-beings, under their excessively complex relations of life, would be preserved, +12409-accumulated, and inherited? Why, if man can by patience select variations most +-- +12549-with their several instincts. I have attempted to show how much light the +12550-principle of gradation throws on the admirable architectural powers of the +12551:hive-bee. Habit no doubt sometimes comes into play in modifying instincts; but +12552-it certainly is not indispensable, as we see, in the case of neuter insects, +12553-which leave no progeny to inherit the effects of long-continued habit. On the +-- +12643-closely allied species inhabit two areas, some identical species common to both +12644-still exist. Wherever many closely allied yet distinct species occur, many +12645:doubtful forms and varieties of the same species likewise occur. It is a rule +12646-of high generality that the inhabitants of each area are related to the +12647-inhabitants of the nearest source whence immigrants might have been derived. We +-- +12744-weight to unexplained difficulties than to the explanation of a certain number +12745-of facts will certainly reject my theory. A few naturalists, endowed with much +12746:flexibility of mind, and who have already begun to doubt on the immutability of +12747-species, may be influenced by this volume; but I look with confidence to the +12748-future, to young and rising naturalists, who will be able to view both sides of +-- +12789-implies an enormous amount of modification in the descendants. Throughout whole +12790-classes various structures are formed on the same pattern, and at an embryonic +12791:age the species closely resemble each other. Therefore I cannot doubt that the +12792-theory of descent with modification embraces all the members of the same class. +12793-I believe that animals have descended from at most only four or five +-- +12809-a considerable revolution in natural history. Systematists will be able to +12810-pursue their labours as at present; but they will not be incessantly haunted by +12811:the shadowy doubt whether this or that form be in essence a species. This I +12812-feel sure, and I speak after experience, will be no slight relief. The endless +12813-disputes whether or not some fifty species of British brambles are true species +-- +12857-classifications will come to be, as far as they can be so made, genealogies; +12858-and will then truly give what may be called the plan of creation. The rules for +12859:classifying will no doubt become simpler when we have a definite object in +12860-view. We possess no pedigrees or armorial bearings; and we have to discover and +12861-trace the many diverging lines of descent in our natural genealogies, by +-- +13605-Grouse: +13606-colours of, 84. +13607:red, a doubtful species, 49. +13608- +13609-Growth: + diff --git a/darwin/gayon b/darwin/gayon @@ -0,0 +1 @@ +head jauss diff --git a/darwin/herschel b/darwin/herschel @@ -0,0 +1,11 @@ +name: Herschel +full_name: John Herschel (1792-1871) +titre: +tags: philosophie_sciences + + +Vera causa selon Herschel : +• Cause réelle +• Cause compétente (pour produire le +phénomène) +• Cause responsable (du phénomène) diff --git a/darwin/jauss b/darwin/jauss @@ -0,0 +1,11 @@ +name: Jauss +full_name: Hans-Robert Jauss (1921-1997) +titre: Literaturgeschichte als Provokation der Literaturwissenschaft (= l’histoire littéraire comme provocation pour la théorie littéraire) +tags: herméneutique + +Un texte n’est à l’origine qu’un +ensemble de potentialités, qui +doivent ensuite être actualisées +par des lectures. + +CHEZ JAUSS, UN PRÉSUPPOSÉ PHILOSOPHIQUE : Le texte n’est rien en lui-même diff --git a/darwin/structure_doubt_note.txt b/darwin/structure_doubt_note.txt @@ -0,0 +1 @@ +assertif diff --git a/darwin/w_intro.md b/darwin/w_intro.md @@ -0,0 +1,29 @@ +title: Darwin's Doubt +path: +author: vi +summary: +tags: +date: 2025-06-11 15:49:52 +lang:fr + +# [%title] + +Par quel endroit on veut rentrer ? On est sur du darwin, et par simplicité on veut faire une étude de cas autour du doute sur son travail et éventuellement celui de ses contemporains. + + "The History of Error is quite unimportant" + Darwin Correspondence Project, “Letter no. 2646,” accessed on 13 June 2025, https://www.darwinproject.ac.uk/letter/?docId=letters/DCP-LETT-2646.xml + +bon on y va + +Le 9 janvier 1860, Charles Darwin adresse à Thomas Huxley cette Lettre qu'il commence par cette proposition : «The History of Error is quite unimportant». L'erreur en question est celle des théories Jean-Baptiste Lamarck et dans cette lettre est attaquée la loi d'usage et non-usage (tk); l'auteur de *The Origin of Species* (1860) publié quarante-deux jours plus tôt le 28 novembre 1859, qualifie de sans intérêt le travail de son collègue français. + + +histoire des sciences controverses, erreurs + +## revue littérature + +## énonciation problématisation + +## méthode + +fin