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diff --git a/2009-h/2009-h.htm b/2009-h/2009-h.htm new file mode 100644 index 0000000..3335c55 --- /dev/null +++ b/2009-h/2009-h.htm @@ -0,0 +1,23534 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" +"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> +<meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> +<meta http-equiv="Content-Style-Type" content="text/css" /> +<title>The Project Gutenberg eBook of On the Origin of Species, Sixth Edition by Charles Darwin</title> + +<style type="text/css" xml:space="preserve"> + +body { margin-left: 20%; + margin-right: 20%; + text-align: justify; } + +h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight: +normal; line-height: 1.5; margin-top: .5em; margin-bottom: .5em;} + +h1 {font-size: 300%; + margin-top: 0.6em; + margin-bottom: 0.6em; + letter-spacing: 0.12em; + word-spacing: 0.2em; + text-indent: 0em;} +h2 {font-size: 150%; margin-top: 2em; margin-bottom: 1em;} +h3 {font-size: 130%; margin-top: 1em;} +h4 {font-size: 120%;} +h5 {font-size: 110%;} + +.no-break {page-break-before: avoid;} /* for epubs */ + +div.chapter {page-break-before: always; margin-top: 4em;} + +hr {width: 80%; margin-top: 2em; margin-bottom: 2em;} + +p {text-indent: 1em; + margin-top: 0.25em; + margin-bottom: 0.25em; } + +.p2 {margin-top: 2em;} + +p.letter {text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.noindent {text-indent: 0% } + +p.center {text-align: center; + text-indent: 0em; + margin-top: 1em; + margin-bottom: 1em; } + +p.right {text-align: right; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.footnote {font-size: 90%; + text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +sup { vertical-align: top; font-size: 0.6em; } + +div.fig { display:block; + margin:0 auto; + text-align:center; + margin-top: 1em; + margin-bottom: 1em;} + +a:link {color:blue; text-decoration:none} +a:visited {color:blue; text-decoration:none} +a:hover {color:red} + +</style> + </head> + <body> + +<div style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of On the Origin of Species, by Charles Darwin</div> +<div style='display:block; margin:1em 0'> +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 <a href="https://www.gutenberg.org">www.gutenberg.org</a>. 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. +</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: On the Origin of Species<br /> +6th Edition</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Charles Darwin</div> +<div style='display:block; margin:1em 0'>Release Date: December, 1999 [eBook #2009]<br /> +[Most recently updated: November 9, 2022]</div> +<div style='display:block; margin:1em 0'>Language: English</div> +<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div> +<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Sue Asscher and David Widger</div> +<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK ON THE ORIGIN OF SPECIES ***</div> + +<p class="center"> +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.<br /> +Click on any of the filenumbers below to quickly view each ebook. +</p> + +<table summary="" style="margin-right: auto; margin-left: auto" cellpadding="4" border="3"> + +<tr><td> + <b><a href="http://www.gutenberg.org/files/1228/1228-h/1228-h.htm"> +1228</a> </b> </td><td>1859, First Edition +</td></tr> + +<tr><td> + <b><a href="http://www.gutenberg.org/files/22764/22764-h/22764-h.htm"> +22764</a></b></td><td>1860, Second Edition +</td></tr> + +<tr><td> + <b><a href="http://www.gutenberg.org/files/2009/2009-h/2009-h.htm"> +2009</a></b> </td><td>1872, Sixth Edition, considered the definitive edition. +</td></tr> + +</table> + +<h1>On<br />the Origin of Species</h1> + +<h4>BY MEANS OF NATURAL SELECTION,</h4> + +<h3>OR THE<br />PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE.</h3> + +<h2 class="no-break">By Charles Darwin, M.A., F.R.S.,</h2> + +<h4>Author of “The Descent of Man,” etc., etc.</h4> + +<h3>Sixth London Edition, with all Additions and Corrections.</h3> + +<hr /> + +<p> +“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.” +</p> + +<p class="right"> +W<small>HEWELL</small>: <i>Bridgewater Treatise</i>. +</p> + +<p> +“The only distinct meaning of the word ‘natural’ is +<i>stated</i>, <i>fixed</i> or <i>settled;</i> since what is natural as much +requires and presupposes an intelligent agent to render it so, <i>i.e.</i>, to +effect it continually or at stated times, as what is supernatural or miraculous +does to effect it for once.” +</p> + +<p class="right"> +B<small>UTLER</small>: <i>Analogy of Revealed Religion</i>. +</p> + +<p> +“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.” +</p> + +<p class="right"> +B<small>ACON</small>: <i>Advancement of Learning</i>. +</p> + +<hr /> + +<div class="chapter"> + +<h2><a name="Pagexiii"></a><a name="link2H_4_0001" id="link2H_4_0001"></a> +AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES, +PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK.</h2> + +<p> +I will here give a brief sketch of the progress of opinion on the Origin of +Species. Until recently the great majority of naturalists believed that species +were immutable productions, and had been separately created. This view has been +ably maintained by many authors. Some few naturalists, on the other hand, have +believed that species undergo modification, and that the existing forms of life +are the descendants by true generation of pre existing forms. Passing over +allusions to the subject in the classical writers,<a href="#fn1" name="fnref1" id="fnref1"><sup>[1]</sup></a> +the first author who in modern times has treated it in a scientific spirit was +Buffon. But as his opinions fluctuated greatly at different periods, and as he +does not enter on the causes or means of the transformation of species, I need +not here enter on details. +</p> + +<p class="footnote"> +<a name="fn1" id="fn1"></a> <a href="#fnref1">[1]</a> Aristotle, in his +“Physicæ Auscultationes” (lib.2, cap.8, s.2), after remarking that +rain does not fall in order to make the corn grow, any more than it falls to +spoil the farmer’s corn when threshed out of doors, applies the same +argument to organisation; and adds (as translated by Mr. Clair Grece, who first +pointed out the passage to me), “So what hinders the different parts (of +the body) from having this merely accidental relation in nature? as the teeth, +for example, grow by necessity, the front ones sharp, adapted for dividing, and +the grinders flat, and serviceable for masticating the food; since they were +not made for the sake of this, but it was the result of accident. And in like +manner as to other parts in which there appears to exist an adaptation to an +end. Wheresoever, therefore, all things together (that is all the parts of one +whole) happened like as if they were made for the sake of something, these were +preserved, having been appropriately constituted by an internal spontaneity; +and whatsoever things were not thus constituted, perished and still +perish.” We here see the principle of natural selection shadowed forth, +but how little Aristotle fully comprehended the principle, is shown by his +remarks on the formation of the teeth. +</p> + +<p> +Lamarck was the first man whose conclusions on the subject excited much +attention. This justly celebrated naturalist first published his views in 1801; +he much enlarged them in 1809 in his “Philosophie Zoologique”, and +subsequently, 1815, in the Introduction to his “Hist. Nat. des Animaux +sans Vertébres”. In these +<a name="Pagexiv"></a> +works he up holds the doctrine that all species, including man, are descended +from other species. He first did the eminent service of arousing attention to +the probability of all change in the organic, as well as in the inorganic +world, being the result of law, and not of miraculous interposition. Lamarck +seems to have been chiefly led to his conclusion on the gradual change of +species, by the difficulty of distinguishing species and varieties, by the +almost perfect gradation of forms in certain groups, and by the analogy of +domestic productions. With respect to the means of modification, he attributed +something to the direct action of the physical conditions of life, something to +the crossing of already existing forms, and much to use and disuse, that is, to +the effects of habit. To this latter agency he seems to attribute all the +beautiful adaptations in nature; such as the long neck of the giraffe for +browsing on the branches of trees. But he likewise believed in a law of +progressive development, and as all the forms of life thus tend to progress, in +order to account for the existence at the present day of simple productions, he +maintains that such forms are now spontaneously generated.<a href="#fn2" name="fnref2" id="fnref2"><sup>[2]</sup></a> +</p> + +<p class="footnote"> +<a name="fn2" id="fn2"></a> <a href="#fnref2">[2]</a> I have taken the date of +the first publication of Lamarck from Isidore Geoffroy Saint-Hilaire’s +(“Hist. Nat. Générale”, tom. ii. page 405, 1859) excellent history +of opinion on this subject. In this work a full account is given of +Buffon’s conclusions on the same subject. It is curious how largely my +grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous grounds of +opinion of Lamarck in his “Zoonomia” (vol. i. pages 500-510), +published in 1794. According to Isid. Geoffroy there is no doubt that Goethe +was an extreme partisan of similar views, as shown in the introduction to a +work written in 1794 and 1795, but not published till long afterward; he has +pointedly remarked (“Goethe als Naturforscher”, von Dr. Karl +Meding, s. 34) that the future question for naturalists will be how, for +instance, cattle got their horns and not for what they are used. It is rather a +singular instance of the manner in which similar views arise at about the same +time, that Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint-Hilaire +(as we shall immediately see) in France, came to the same conclusion on the +origin of species, in the years 1794-5. +</p> + +<p> +Geoffroy Saint-Hilaire, as is stated in his “Life”, written by his +son, suspected, as early as 1795, that what we call species are various +degenerations of the same type. It was not until 1828 that he published his +conviction that the same forms have not been perpetuated since the origin of +all things. Geoffroy seems to have relied chiefly on the conditions of life, or +the “<i>monde ambiant</i>” as the cause of change. He was cautious +in drawing conclusions, and did not believe that existing species are now +undergoing modification; and, as his son adds, “C’est donc un +problème à réserver entièrement à l’avenir, supposé même que +l’avenir doive avoir prise sur lui.” +</p> + +<p> +<a name="Pagexv"></a> +In 1813 Dr. W.C. Wells read before the Royal Society “An Account of a +White Female, part of whose skin resembles that of a Negro”; but his +paper was not published until his famous “Two Essays upon Dew and Single +Vision” appeared in 1818. In this paper he distinctly recognises the +principle of natural selection, and this is the first recognition which has +been indicated; but he applies it only to the races of man, and to certain +characters alone. After remarking that negroes and mulattoes enjoy an immunity +from certain tropical diseases, he observes, firstly, that all animals tend to +vary in some degree, and, secondly, that agriculturists improve their +domesticated animals by selection; and then, he adds, but what is done in this +latter case “by art, seems to be done with equal efficacy, though more +slowly, by nature, in the formation of varieties of mankind, fitted for the +country which they inhabit. Of the accidental varieties of man, which would +occur among the first few and scattered inhabitants of the middle regions of +Africa, some one would be better fitted than others to bear the diseases of the +country. This race would consequently multiply, while the others would +decrease; not only from their in ability to sustain the attacks of disease, but +from their incapacity of contending with their more vigorous neighbours. The +colour of this vigorous race I take for granted, from what has been already +said, would be dark. But the same disposition to form varieties still existing, +a darker and a darker race would in the course of time occur: and as the +darkest would be the best fitted for the climate, this would at length become +the most prevalent, if not the only race, in the particular country in which it +had originated.” He then extends these same views to the white +inhabitants of colder climates. I am indebted to Mr. Rowley, of the United +States, for having called my attention, through Mr. Brace, to the above passage +of Dr. Wells’ work. +</p> + +<p> +The Hon. and Rev. W. Herbert, afterward Dean of Manchester, in the fourth +volume of the “Horticultural Transactions”, 1822, and in his work +on the “Amaryllidaceæ” (1837, pages 19, 339), declares that +“horticultural experiments have established, beyond the possibility of +refutation, that botanical species are only a higher and more permanent class +of varieties.” He extends the same view to animals. The dean believes +that single species of each genus were created in an originally highly plastic +condition, and that these have produced, chiefly by inter-crossing, but +likewise by variation, all our existing species. +</p> + +<p> +In 1826 Professor Grant, in the concluding paragraph in his well-known paper +(“Edinburgh Philosophical Journal”, vol. XIV, page 283) on the +Spongilla, clearly declares his belief that species are +<a name="Pagexvi"></a> +descended from other species, and that they become improved in the course of +modification. This same view was given in his Fifty-fifth Lecture, published in +the “Lancet” in 1834. +</p> + +<p> +In 1831 Mr. Patrick Matthew published his work on “Naval Timber and +Arboriculture”, in which he gives precisely the same view on the origin +of species as that (presently to be alluded to) propounded by Mr. Wallace and +myself in the “Linnean Journal”, and as that enlarged in the +present volume. Unfortunately the view was given by Mr. Matthew very briefly in +scattered passages in an appendix to a work on a different subject, so that it +remained unnoticed until Mr. Matthew himself drew attention to it in the +“Gardeners’ Chronicle”, on April 7, 1860. The differences of +Mr. Matthew’s views from mine are not of much importance: he seems to +consider that the world was nearly depopulated at successive periods, and then +restocked; and he gives as an alternative, that new forms may be generated +“without the presence of any mold or germ of former aggregates.” I +am not sure that I understand some passages; but it seems that he attributes +much influence to the direct action of the conditions of life. He clearly saw, +however, the full force of the principle of natural selection. +</p> + +<p> +The celebrated geologist and naturalist, Von Buch, in his excellent +“Description Physique des Isles Canaries” (1836, page 147), clearly +expresses his belief that varieties slowly become changed into permanent +species, which are no longer capable of intercrossing. +</p> + +<p> +Rafinesque, in his “New Flora of North America”, published in 1836, +wrote (page 6) as follows: “All species might have been varieties once, +and many varieties are gradually becoming species by assuming constant and +peculiar characters;” but further on (page 18) he adds, “except the +original types or ancestors of the genus.” +</p> + +<p> +In 1843-44 Professor Haldeman (“Boston Journal of Nat. Hist. U. +States”, vol. iv, page 468) has ably given the arguments for and against +the hypothesis of the development and modification of species: he seems to lean +toward the side of change. +</p> + +<p> +The “Vestiges of Creation” appeared in 1844. In the tenth and much +improved edition (1853) the anonymous author says (page 155): “The +proposition determined on after much consideration is, that the several series +of animated beings, from the simplest and oldest up to the highest and most +recent, are, under the providence of God, the results, <i>first</i>, of an +impulse which has been imparted to the forms of life, advancing them, in +definite times, by generation, through grades of organisation terminating in +the +<a name="Pagexvii"></a> +highest dicotyledons and vertebrata, these grades being few in number, and +generally marked by intervals of organic character, which we find to be a +practical difficulty in ascertaining affinities; <i>second</i>, of another +impulse connected with the vital forces, tending, in the course of generations, +to modify organic structures in accordance with external circumstances, as +food, the nature of the habitat, and the meteoric agencies, these being the +‘adaptations’ of the natural theologian.” The author +apparently believes that organisation progresses by sudden leaps, but that the +effects produced by the conditions of life are gradual. He argues with much +force on general grounds that species are not immutable productions. But I +cannot see how the two supposed “impulses” account in a scientific +sense for the numerous and beautiful coadaptations which we see throughout +nature; I cannot see that we thus gain any insight how, for instance, a +woodpecker has become adapted to its peculiar habits of life. The work, from +its powerful and brilliant style, though displaying in the early editions +little accurate knowledge and a great want of scientific caution, immediately +had a very wide circulation. In my opinion it has done excellent service in +this country in calling attention to the subject, in removing prejudice, and in +thus preparing the ground for the reception of analogous views. +</p> + +<p> +In 1846 the veteran geologist M.J. d’Omalius d’Halloy published in +an excellent though short paper (“Bulletins de l’Acad. Roy. +Bruxelles”, tom. xiii, page 581) his opinion that it is more probable +that new species have been produced by descent with modification than that they +have been separately created: the author first promulgated this opinion in +1831. +</p> + +<p> +Professor Owen, in 1849 (“Nature of Limbs”, page 86), wrote as +follows: “The archetypal idea was manifested in the flesh under diverse +such modifications, upon this planet, long prior to the existence of those +animal species that actually exemplify it. To what natural laws or secondary +causes the orderly succession and progression of such organic phenomena may +have been committed, we, as yet, are ignorant.” In his address to the +British Association, in 1858, he speaks (page li) of “the axiom of the +continuous operation of creative power, or of the ordained becoming of living +things.” Further on (page xc), after referring to geographical +distribution, he adds, “These phenomena shake our confidence in the +conclusion that the Apteryx of New Zealand and the Red Grouse of England were +distinct creations in and for those islands respectively. Always, also, it may +be well to bear in mind that by the word ‘creation’ the zoologist +means ‘a process he knows not +<a name="Pagexviii"></a> +what.’” He amplifies this idea by adding that when such cases as +that of the Red Grouse are “enumerated by the zoologist as evidence of +distinct creation of the bird in and for such islands, he chiefly expresses +that he knows not how the Red Grouse came to be there, and there exclusively; +signifying also, by this mode of expressing such ignorance, his belief that +both the bird and the islands owed their origin to a great first Creative +Cause.” If we interpret these sentences given in the same address, one by +the other, it appears that this eminent philosopher felt in 1858 his confidence +shaken that the Apteryx and the Red Grouse first appeared in their respective +homes “he knew not how,” or by some process “he knew not +what.” +</p> + +<p> +This Address was delivered after the papers by Mr. Wallace and myself on the +Origin of Species, presently to be referred to, had been read before the +Linnean Society. When the first edition of this work was published, I was so +completely deceived, as were many others, by such expressions as “the +continuous operation of creative power,” that I included Professor Owen +with other palæontologists as being firmly convinced of the immutability of +species; but it appears (“Anat. of Vertebrates”, vol. iii, page +796) that this was on my part a preposterous error. In the last edition of this +work I inferred, and the inference still seems to me perfectly just, from a +passage beginning with the words “no doubt the type-form,” +&c.(Ibid., vol. i, page xxxv), that Professor Owen admitted that natural +selection may have done something in the formation of a new species; but this +it appears (Ibid., vol. iii. page 798) is inaccurate and without evidence. I +also gave some extracts from a correspondence between Professor Owen and the +editor of the “London Review”, from which it appeared manifest to +the editor as well as to myself, that Professor Owen claimed to have +promulgated the theory of natural selection before I had done so; and I +expressed my surprise and satisfaction at this announcement; but as far as it +is possible to understand certain recently published passages (Ibid., vol. iii. +page 798) I have either partially or wholly again fallen into error. It is +consolatory to me that others find Professor Owen’s controversial +writings as difficult to understand and to reconcile with each other, as I do. +As far as the mere enunciation of the principle of natural selection is +concerned, it is quite immaterial whether or not Professor Owen preceded me, +for both of us, as shown in this historical sketch, were long ago preceded by +Dr. Wells and Mr. Matthews. +</p> + +<p> +M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850 (of which +a Résumé appeared in the “Revue et Mag. de +<a name="Pagexix"></a> +Zoolog.”, Jan., 1851), briefly gives his reason for believing that +specific characters “sont fixés, pour chaque espèce, tant qu’elle +se perpétue au milieu des mêmes circonstances: ils se modifient, si les +circonstances ambiantes viennent à changer. En résumé, +<i>l’observation</i> des animaux sauvages démontre deja la variabilité +<i>limitée</i> des espèces. Les <i>expériences</i> sur les animaux sauvages +devenus domestiques, et sur les animaux domestiques redevenus sauvages, la +démontrent plus clairment encore. Ces mêmes expériences prouvent, de plus, que +les différences produites peuvent être de <i>valeur générique</i>.” In +his “Hist. Nat. Générale” (tom. ii, page 430, 1859) he amplifies +analogous conclusions. +</p> + +<p> +From a circular lately issued it appears that Dr. Freke, in 1851 (“Dublin +Medical Press”, page 322), propounded the doctrine that all organic +beings have descended from one primordial form. His grounds of belief and +treatment of the subject are wholly different from mine; but as Dr. Freke has +now (1861) published his Essay on the “Origin of Species by means of +Organic Affinity”, the difficult attempt to give any idea of his views +would be superfluous on my part. +</p> + +<p> +Mr. Herbert Spencer, in an Essay (originally published in the +“Leader”, March, 1852, and republished in his “Essays”, +in 1858), has contrasted the theories of the Creation and the Development of +organic beings with remarkable skill and force. He argues from the analogy of +domestic productions, from the changes which the embryos of many species +undergo, from the difficulty of distinguishing species and varieties, and from +the principle of general gradation, that species have been modified; and he +attributes the modification to the change of circumstances. The author (1855) +has also treated Psychology on the principle of the necessary acquirement of +each mental power and capacity by gradation. +</p> + +<p> +In 1852 M. Naudin, a distinguished botanist, expressly stated, in an admirable +paper on the Origin of Species (“Revue Horticole”, page 102; since +partly republished in the “Nouvelles Archives du Muséum”, tom. i, +p. 171), his belief that species are formed in an analogous manner as varieties +are under cultivation; and the latter process he attributes to man’s +power of selection. But he does not show how selection acts under nature. He +believes, like Dean Herbert, that species, when nascent, were more plastic than +at present. He lays weight on what he calls the principle of finality, +“puissance mystérieuse, indéterminée; fatalité pour les uns; pour les +autres volonté providentielle, dont l’action incessante sur les êtres +vivantes détermine, à toutes les époques de l’existence du monde, la +forme, le volume, et la durée de chacun d’eux, en raison +<a name="Pagexx"></a> +de sa destinée dans l’ordre de choses dont il fait partie. C’est +cette puissance qui harmonise chaque membre à l’ensemble, en +l’appropriant à la fonction qu’il doit remplir dans +l’organisme général de la nature, fonction qui est pour lui sa raison +d’être.”<a href="#fn3" name="fnref3" id="fnref3"><sup>[3]</sup></a> +</p> + +<p class="footnote"> +<a name="fn3" id="fn3"></a> <a href="#fnref3">[3]</a> From references in +Bronn’s “Untersuchungen über die Entwickelungs-Gesetze”, it +appears that the celebrated botanist and palæontologist Unger published, in +1852, his belief that species undergo development and modification. Dalton, +likewise, in Pander and Dalton’s work on Fossil Sloths, expressed, in +1821, a similar belief. Similar views have, as is well known, been maintained +by Oken in his mystical “Natur-Philosophie”. From other references +in Godron’s work “Sur l’Espèce”, it seems that Bory St. +Vincent, Burdach, Poiret and Fries, have all admitted that new species are +continually being produced.<br/> + I may add, that of the thirty-four authors named in this Historical Sketch, +who believe in the modification of species, or at least disbelieve in separate +acts of creation, twenty-seven have written on special branches of natural +history or geology. +</p> + +<p> +In 1853 a celebrated geologist, Count Keyserling (“Bulletin de la Soc. +Geolog.”, 2nd Ser., tom. x, page 357), suggested that as new diseases, +supposed to have been caused by some miasma have arisen and spread over the +world, so at certain periods the germs of existing species may have been +chemically affected by circumambient molecules of a particular nature, and thus +have given rise to new forms. +</p> + +<p> +In this same year, 1853, Dr. Schaaffhausen published an excellent pamphlet +(“Verhand. des Naturhist. Vereins der Preuss. Rheinlands”, +&c.), in which he maintains the development of organic forms on the earth. +He infers that many species have kept true for long periods, whereas a few have +become modified. The distinction of species he explains by the destruction of +intermediate graduated forms. “Thus living plants and animals are not +separated from the extinct by new creations, but are to be regarded as their +descendants through continued reproduction.” +</p> + +<p> +A well-known French botanist, M. Lecoq, writes in 1854 (“Etudes sur +Géograph.” Bot. tom. i, page 250), “On voit que nos recherches sur +la fixité ou la variation de l’espéce, nous conduisent directement aux +idées émises par deux hommes justement célèbres, Geoffroy Saint-Hilaire et +Goethe.” Some other passages scattered through M. Lecoq’s large +work make it a little doubtful how far he extends his views on the modification +of species. +</p> + +<p> +The “Philosophy of Creation” has been treated in a masterly manner +by the Rev. Baden Powell, in his “Essays on the Unity of Worlds”, +1855. Nothing can be more striking than the manner in which he shows that the +introduction of new species is “a regular, +<a name="Pagexxi"></a> +not a casual phenomenon,” or, as Sir John Herschel expresses it, “a +natural in contradistinction to a miraculous process.” +</p> + +<p> +The third volume of the “Journal of the Linnean Society” contains +papers, read July 1, 1858, by Mr. Wallace and myself, in which, as stated in +the introductory remarks to this volume, the theory of Natural Selection is +promulgated by Mr. Wallace with admirable force and clearness. +</p> + +<p> +Von Baer, toward whom all zoologists feel so profound a respect, expressed +about the year 1859 (see Prof. Rudolph Wagner, +“Zoologisch-Anthropologische Untersuchungen”, 1861, s. 51) his +conviction, chiefly grounded on the laws of geographical distribution, that +forms now perfectly distinct have descended from a single parent-form. +</p> + +<p> +In June, 1859, Professor Huxley gave a lecture before the Royal Institution on +the ‘Persistent Types of Animal Life’. Referring to such cases, he +remarks, “It is difficult to comprehend the meaning of such facts as +these, if we suppose that each species of animal and plant, or each great type +of organisation, was formed and placed upon the surface of the globe at long +intervals by a distinct act of creative power; and it is well to recollect that +such an assumption is as unsupported by tradition or revelation as it is +opposed to the general analogy of nature. If, on the other hand, we view +‘Persistent Types’ in relation to that hypothesis which supposes +the species living at any time to be the result of the gradual modification of +pre-existing species, a hypothesis which, though unproven, and sadly damaged by +some of its supporters, is yet the only one to which physiology lends any +countenance; their existence would seem to show that the amount of modification +which living beings have undergone during geological time is but very small in +relation to the whole series of changes which they have suffered.” +</p> + +<p> +In December, 1859, Dr. Hooker published his “Introduction to the +Australian Flora”. In the first part of this great work he admits the +truth of the descent and modification of species, and supports this doctrine by +many original observations. +</p> + +<p> +The first edition of this work was published on November 24, 1859, and the +second edition on January 7, 1860. +</p> + +<hr /> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>Contents</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#link2H_4_0001">AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES</a></td> +</tr> + +<tr> +<td> <a href="#link2H_INTR">INTRODUCTION.</a><br /><br /></td> +</tr> + +<tr> +<td> <a href="#link2HCH0001">CHAPTER I.</a> VARIATION UNDER DOMESTICATION</td> +</tr> + +<tr> +<td> <a href="#link2HCH0002">CHAPTER II.</a> VARIATION UNDER NATURE</td> +</tr> + +<tr> +<td> <a href="#link2HCH0003">CHAPTER III.</a> STRUGGLE FOR EXISTENCE</td> +</tr> + +<tr> +<td> <a href="#link2HCH0004">CHAPTER IV.</a> NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST</td> +</tr> + +<tr> +<td> <a href="#link2HCH0005">CHAPTER V.</a> LAWS OF VARIATION</td> +</tr> + +<tr> +<td> <a href="#link2HCH0006">CHAPTER VI.</a> DIFFICULTIES OF THE THEORY</td> +</tr> + +<tr> +<td> <a href="#link2HCH0007">CHAPTER VII.</a> MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION</td> +</tr> + +<tr> +<td> <a href="#link2HCH0008">CHAPTER VIII.</a> INSTINCT</td> +</tr> + +<tr> +<td> <a href="#link2HCH0009">CHAPTER IX.</a> HYBRIDISM</td> +</tr> + +<tr> +<td> <a href="#link2HCH0010">CHAPTER X.</a> ON THE IMPERFECTION OF THE GEOLOGICAL RECORD</td> +</tr> + +<tr> +<td> <a href="#link2HCH0011">CHAPTER XI.</a> ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS</td> +</tr> + +<tr> +<td> <a href="#link2HCH0012">CHAPTER XII.</a> GEOGRAPHICAL DISTRIBUTION</td> +</tr> + +<tr> +<td> <a href="#link2HCH0013">CHAPTER XIII.</a> GEOGRAPHICAL DISTRIBUTION—<i>continued</i></td> +</tr> + +<tr> +<td> <a href="#link2HCH0014">CHAPTER XIV.</a> MUTUAL AFFINITIES OF ORGANIC BEINGS</td> +</tr> + +<tr> +<td> <a href="#link2HCH0015">CHAPTER XV.</a> RECAPITULATION AND CONCLUSION<br /><br /></td> +</tr> + +<tr> +<td> <a href="#link2H_GLOS">GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.</a></td> +</tr> + +<tr> +<td> <a href="#link2H_4_0029">INDEX.</a></td> +</tr> + +</table> + +<hr /> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>DETAILED CONTENTS.</h2> + +<p class="noindent"> +<a href="#link2H_INTR">INTRODUCTION</a><br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0001">CHAPTER I.</a><br/> +VARIATION UNDER DOMESTICATION.<br/> +Causes of Variability—Effects of Habit and the use or disuse of +Parts—Correlated Variation—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—Principles +of Selection, anciently followed, their Effects—Methodical and +Unconscious Selection—Unknown Origin of our Domestic +Productions—Circumstances favourable to Man’s power of Selection.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0002">CHAPTER II.</a><br/> +VARIATION UNDER NATURE.<br/> +Variability—Individual Differences—Doubtful species—Wide +ranging, much diffused, and common species, vary most—Species of the +larger genera in each country vary more frequently 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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0003">CHAPTER III.</a><br/> +STRUGGLE FOR EXISTENCE.<br/> +Its bearing on natural selection—The term used in a wide +sense—Geometrical ratio 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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0004">CHAPTER IV.</a><br/> +NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.<br/> +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 the results of 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—Advance in organisation—Low forms +preserved—Convergence of character—Indefinite multiplication of +species—Summary.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0005">CHAPTER V.</a><br/> +LAWS OF VARIATION.<br/> +Effects of changed conditions—Use and disuse, combined with natural +selection; organs of flight and of +vision—Acclimatisation—Correlated variation—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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0006">CHAPTER VI.</a><br/> +DIFFICULTIES OF THE THEORY.<br/> +Difficulties of the theory of descent with modification—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—Modes 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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0007">CHAPTER VII.</a><br/> +MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.<br/> +Longevity—Modifications not necessarily simultaneous—Modifications +apparently of no direct service—Progressive development—Characters +of small functional importance, the most constant—Supposed incompetence +of natural selection to account for the incipient stages of useful +structures—Causes which interfere with the acquisition through natural +selection of useful structures—Gradations of structure with changed +functions—Widely different organs in members of the same class, developed +from one and the same source—Reasons for disbelieving in great and abrupt +modifications.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0008">CHAPTER VIII.</a><br/> +INSTINCT.<br/> +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, molothrus, +ostrich, and parasitic bees—Slave-making ants—Hive-bee, its +cell-making instinct—Changes of instinct and structure not necessarily +simultaneous—Difficulties on the theory of the Natural Selection of +instincts—Neuter or sterile insects—Summary.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0009">CHAPTER IX.</a><br/> +HYBRIDISM.<br/> +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, not accumulated by natural selection—Causes of the sterility +of first crosses and of hybrids—Parallelism between the effects of +changed conditions of life and of crossing—Dimorphism and +Trimorphism—Fertility of varieties when crossed and of their mongrel +offspring not universal—Hybrids and mongrels compared independently of +their fertility—Summary.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0010">CHAPTER X.</a><br/> +ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.<br/> +On the absence of intermediate varieties at the present day—On the nature +of extinct intermediate varieties; on their number—On the lapse of time, +as inferred from the rate of denudation and of deposition—On the lapse of +time as estimated in years—On the poorness of our palæontological +collections—On the intermittence of geological formations—On the +denudation of granitic areas—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—Antiquity of the habitable earth.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0011">CHAPTER XI.</a><br/> +ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.<br/> +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 chapter.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0012">CHAPTER XII.</a><br/> +GEOGRAPHICAL DISTRIBUTION.<br/> +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—Alternate Glacial periods +in the north and south.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0013">CHAPTER XIII.</a><br/> +GEOGRAPHICAL DISTRIBUTION—<i>continued</i>.<br/> +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 chapter.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0014">CHAPTER XIV.</a><br/> +MUTUAL AFFINITIES OF ORGANIC BEINGS:<br/> +MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS.<br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0015">CHAPTER XV.</a><br/> +RECAPITULATION AND CONCLUSION.<br/> +Recapitulation of the objections to 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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2H_GLOS">GLOSSARY OF SCIENTIFIC TERMS.</a><br/><br/> +</p> + +<p class="noindent"> +<a href="#link2H_4_0029">INDEX.</a><br/> +</p> + +<hr /> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page01"></a>ORIGIN OF SPECIES.</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2H_INTR" id="link2H_INTR"></a> +INTRODUCTION.</h2> + +<p> +When on board H.M.S. Beagle, as naturalist, I was much struck with certain +facts in the distribution of the organic beings inhabiting South America, and +in the geological relations of the present to the past inhabitants of that +continent. These facts, as will be seen in the latter chapters of this volume, +seemed 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. +</p> + +<p> +My work is now (1859) nearly finished; but as it will take me many 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. In 1858 he sent 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. +</p> + +<p> +This abstract, which I now publish, must necessarily be imperfect. I cannot +here give references and authorities for my +<a name="Page02"></a> +several statements; and I must trust to the reader reposing some confidence in +my accuracy. No doubt errors may 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 is here impossible. +</p> + +<p> +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. +</p> + +<p> +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 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 justly excites our admiration. Naturalists continually +refer to external conditions, such as climate, food, &c., as the only +possible cause of variation. In one 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 mistletoe, 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 +<a name="Page03"></a> +of external conditions, or of habit, or of the volition of the plant itself. +</p> + +<p> +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. +</p> + +<p> +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 among all organic beings throughout the world, which +inevitably follows from the high geometrical ratio of their increase, will be +considered. 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 +<i>naturally selected</i>. From the strong principle of inheritance, any +selected variety will tend to propagate its new and modified form. +</p> + +<p> +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 leads +to what I have called divergence of character. In the next chapter I shall +discuss the complex and little known laws of variation. In the five succeeding +chapters, the most apparent and gravest difficulties in accepting the theory +will be given: namely, first, the difficulties of transitions, or how a +<a name="Page04"></a> +simple being or a simple organ can be changed and perfected into a highly +developed being or into an 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 twelfth and thirteenth, their geographical distribution throughout space; +in the fourteenth, 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. +</p> + +<p> +No one ought to feel surprise at much remaining as yet unexplained in regard to +the origin of species and varieties, if he make due allowance for our profound +ignorance in regard to the mutual relations of the many 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 until recently entertained, 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 +most important, but not the exclusive, means of modification. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0001" id="link2HCH0001"></a> +<a name="Page05"></a>CHAPTER I.<br/> +VARIATION UNDER DOMESTICATION.</h2> + +<p class="letter"> +Causes of Variability—Effects of Habit and the use and disuse of +Parts—Correlated Variation—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—Principles +of Selection, anciently followed, their Effects—Methodical and +Unconscious Selection—Unknown Origin of our Domestic +Productions—Circumstances favourable to Man’s power of Selection. +</p> + +<p class="center"> +<i>Causes of Variability.</i> +</p> + +<p> +When we compare 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 more from each other than do the individuals of any +one species or variety in a state of nature. And if 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, we are +driven to conclude that this great variability is 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 had been exposed +under nature. There is, also, some probability in the view propounded by Andrew +Knight, that this variability may be partly connected with excess of food. It +seems clear that organic beings must be exposed during several generations to +new conditions to cause any great amount of variation; and that, when the +organisation has once begun to vary, it generally continues varying for many +generations. No case is on record of a variable organism ceasing to vary under +cultivation. Our oldest cultivated plants, such as wheat, still yield new +varieties: our oldest domesticated animals are still capable of rapid +improvement or modification. +</p> + +<p> +As far as I am able to judge, after long attending to the subject, the +conditions of life appear to act in two ways—directly on the whole +organisation or on certain parts alone and in directly by affecting the +reproductive system. With respect to the direct +<a name="Page06"></a> +action, we must bear in mind that in every case, as Professor Weismann has +lately insisted, and as I have incidently shown in my work on “Variation +under Domestication,” there are two factors: namely, the nature of the +organism and the nature of the conditions. The former seems to be much the more +important; for nearly similar variations sometimes arise under, as far as we +can judge, dissimilar conditions; and, on the other hand, dissimilar variations +arise under conditions which appear to be nearly uniform. The effects on the +offspring are either definite or in definite. They may be considered as +definite when all or nearly all the offspring of individuals exposed to certain +conditions during several generations are modified in the same manner. It is +extremely difficult to come to any conclusion in regard to the extent of the +changes which have been thus definitely induced. There can, however, be little +doubt about many slight changes, such as size from the amount of food, colour +from the nature of the food, thickness of the skin and hair from climate, +&c. Each of the endless variations which we see in the plumage of our fowls +must have had some efficient cause; and if the same cause were to act uniformly +during a long series of generations on many individuals, all probably would be +modified in the same manner. Such facts as the complex and extraordinary out +growths which variably follow from the insertion of a minute drop of poison by +a gall-producing insect, shows us what singular modifications might result in +the case of plants from a chemical change in the nature of the sap. +</p> + +<p> +In definite variability is a much more common result of changed conditions than +definite variability, and has probably played a more important part in the +formation of our domestic races. We see in definite variability in the endless +slight peculiarities which distinguish the individuals of the same species, and +which cannot be accounted for by inheritance from either parent or from some +more remote ancestor. Even strongly-marked differences occasionally appear in +the young of the same litter, and in seedlings from the same seed-capsule. At +long intervals of time, out of millions of individuals reared in the same +country and fed on nearly the same food, deviations of structure so strongly +pronounced as to deserve to be called monstrosities arise; but monstrosities +cannot be separated by any distinct line from slighter variations. All such +changes of structure, whether extremely slight or strongly marked, which appear +among many individuals living together, may be considered as the in definite +effects of the conditions of life on each individual organism, in nearly the +same manner as the chill effects different men in an in definite manner, +according to their state +<a name="Page07"></a> +of body or constitution, causing coughs or colds, rheumatism, or inflammation +of various organs. +</p> + +<p> +With respect to what I have called the in direct action of changed conditions, +namely, through the reproductive system of being affected, we may infer that +variability is thus induced, partly from the fact of this system being +extremely sensitive to any change in the conditions, and partly from the +similarity, as Kölreuter and others have remarked, between the variability +which follows from the crossing of distinct species, and that which may be +observed with plants and animals when reared under new or unnatural conditions. +Many facts clearly show how eminently susceptible the reproductive system is to +very slight changes in the surrounding conditions. Nothing is more easy than to +tame an animal, and few things more difficult than to get it to breed freely +under confinement, even when the male and female unite. How many animals there +are which will not breed, though kept in an almost free state in their native +country! This is generally, but erroneously attributed to vitiated instincts. +Many cultivated plants display the utmost vigour, and yet rarely or never seed! +In some few cases it has been discovered that a very trifling change, such as a +little more or less water at some particular period of growth, will determine +whether or not a plant will produce seeds. I cannot here give the details which +I have collected and elsewhere published on this curious subject; but to show +how singular the laws are which determine the reproduction of animals under +confinement, I may 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, which seldom produce young; whereas, +carnivorous birds, with the rarest exception, hardly ever lay fertile eggs. +Many exotic plants have pollen utterly worthless, in the same condition as in +the most sterile hybrids. When, on the one hand, we see domesticated animals +and plants, though often weak and sickly, breeding 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 to act, we need not be surprised at this +system, when it does act under confinement, acting irregularly, and producing +offspring somewhat unlike their parents. I may add that as some organisms breed +freely under the most unnatural conditions—for instance, rabbits and +ferrets kept in hutches—showing that their reproductive organs are not +easily affected; so will some animals and plants withstand domestication or +<a name="Page08"></a> +cultivation, and vary very slightly—perhaps hardly more than in a state +of nature. +</p> + +<p> +Some naturalists have maintained that all variations are connected with the act +of sexual reproduction; but this is certainly an error; for I have given in +another work a long list of “sporting plants;” as they are called +by gardeners; that is, of plants which have suddenly produced a single bud with +a new and sometimes widely different character from that of the other buds on +the same plant. These bud variations, as they may be named, can be propagated +by grafts, offsets, &c., and sometimes by seed. They occur rarely under +nature, but are far from rare under culture. As a single bud out of many +thousands produced year after year on the same tree under uniform conditions, +has been known suddenly to assume a new character; and as buds on distinct +trees, growing under different conditions, have sometimes yielded nearly the +same variety—for instance, buds on peach-trees producing nectarines, and +buds on common roses producing moss-roses—we clearly see that the nature +of the conditions is of subordinate importance in comparison with the nature of +the organism in determining each particular form of variation; perhaps of not +more importance than the nature of the spark, by which a mass of combustible +matter is ignited, has in determining the nature of the flames. +</p> + +<p class="center"> +<i>Effects of Habit and of the Use or Disuse of Parts; Correlated Variation; +Inheritance.</i> +</p> + +<p> +Changed habits produce an inherited effect as in the period of the flowering of +plants when transported from one climate to another. With animals the increased +use or disuse of parts has had a more marked influence; thus 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 this change may be safely attributed to the domestic duck flying much +less, and walking more, than its wild parents. The great and inherited +development of the udders in cows and goats in countries where they are +habitually milked, in comparison with these organs in other countries, is +probably another instance of the effects of use. Not one of our domestic +animals can be named which has not in some country drooping ears; and the view +which has been suggested that the drooping is due to disuse of the muscles of +the ear, from the animals being seldom much alarmed, seems probable. +</p> + +<p> +Many laws regulate variation, some few of which can be dimly +<a name="Page09"></a> +seen, and will hereafter be briefly discussed. I will here only allude to what +may be called correlated variation. Important changes in the embryo or larva +will probably 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 which are +entirely white and have blue eyes are generally deaf; but it has been lately +stated by Mr. Tait that this is confined to the males. Colour and +constitutional peculiarities go together, of which many remarkable cases could +be given among animals and plants. From facts collected by Heusinger, it +appears that white sheep and pigs are injured by certain plants, while +dark-coloured individuals escape: Professor Wyman has recently communicated to +me a good illustration of this fact; on asking some farmers in Virginia how it +was that all their pigs were black, they informed him that the pigs ate the +paint-root (Lachnanthes), which coloured their bones pink, and which caused the +hoofs of all but the black varieties to drop off; and one of the +“crackers” (<i>i.e.</i> Virginia squatters) added, “we select +the black members of a litter for raising, as they alone have a good chance of +living.” 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 modify unintentionally other parts of the structure, owing to the +mysterious laws of correlation. +</p> + +<p> +The results of the various, unknown, or but dimly understood laws of variation +are infinitely complex and diversified. It is well worth while carefully to +study the several treatises on some of our old cultivated plants, as on the +hyacinth, potato, even the dahlia, &c.; and it is really surprising to note +the endless points of structure and constitution in which the varieties and +sub-varieties differ slightly from each other. The whole organisation seems to +have become plastic, and departs in a slight degree from that of the parental +type. +</p> + +<p> +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, are endless. Dr. Prosper +Lucas’ treatise, in two large volumes, is the fullest and the best on +this subject. No breeder +<a name="Page10"></a> +doubts how strong is the tendency to inheritance; that like produces like is +his fundamental belief: doubts have been thrown on this principle only by +theoretical writers. When any deviation of structure often appears, and we see +it in the father and child, we cannot tell whether it may not be due to the +same cause having acted on both; but when among individuals, apparently exposed +to the same conditions, any very rare deviation, due to some extraordinary +combination of circumstances, appears in the parent—say, once among +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, &c., 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. +</p> + +<p> +The laws governing inheritance are for the most part unknown; no one can say +why the same peculiarity in different individuals of the same species, or in +different species, is sometimes inherited and sometimes not so; why the child +often reverts in certain characteristics to its grandfather or grandmother or +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 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 the 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 reappear 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 silk-worm 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 <i>appearance</i> of the +peculiarity, and not to the primary cause +<a name="Page11"></a> +which may have acted on the ovules or on the male element; in nearly the same +manner as the increased length of the horns in the offspring from a +short-horned cow by a long-horned bull, though appearing late in life, is +clearly due to the male element. +</p> + +<p> +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 invariably 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 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 <i>definite</i> 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, while kept under the same conditions and while kept +in a considerable body, so that free intercrossing might check, by blending +together, any slight deviations in their 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 unlimited number of +generations, would be opposed to all experience. +</p> + +<p class="center"> +<a name="Page12"></a> +<i>Character of Domestic Varieties; difficulty of distinguishing between +Varieties and Species; origin of Domestic Varieties from one or more +Species.</i> +</p> + +<p> +When we look to the hereditary varieties or races of our domestic animals and +plants, and compare them with closely allied species, we generally perceive in +each domestic race, as already remarked, less uniformity of character than in +true species. Domestic races often have a somewhat monstrous character; by +which I mean, that, although differing from each other and from 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 the species under 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 do the +closely allied species of the same genus in a state of nature, but the +differences in most cases are less in degree. This must be admitted as true, +for the domestic races of many animals and plants have been ranked by some +competent judges as the descendants of aboriginally distinct species, and by +other competent judges as mere varieties. If any well marked distinction +existed between a domestic race and a species, this source of doubt would not +so perpetually recur. It has often been stated that domestic races do not +differ from each other in characters of generic value. It can be shown that +this statement is not correct; but naturalists differ much in determining what +characters are of generic value; all such valuations being at present +empirical. When it is explained how genera originate under nature, it will be +seen that we have no right to expect often to find a generic amount of +difference in our domesticated races. +</p> + +<p> +In attempting to estimate the amount of structural difference between allied +domestic races, we are soon involved in doubt, from not knowing whether they +are 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 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 closely allied natural species—for instance, of the many +foxes—inhabiting the different quarters of the world. I do not believe, +as we shall presently see, that the whole +<a name="Page13"></a> +amount of difference between the several breeds of the dog has been produced +under domestication; I believe that a small part of the difference is due to +their being descended from distinct species. In the case of strongly marked +races of some other domesticated species, there is presumptive or even strong +evidence that all are descended from a single wild stock. +</p> + +<p> +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 and goose, or the small power of endurance of +warmth by the reindeer, 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 on an average vary +as largely as the parent species of our existing domesticated productions have +varied. +</p> + +<p> +In the case of most of our anciently domesticated animals and plants, it is not +possible to come to any definite conclusion, whether they are descended from +one or several wild 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 times, on the monuments of Egypt, and in the lake-habitations of +Switzerland, much diversity in the breeds; and that some of these ancient +breeds closely resemble, or are even identical with, those still existing. But +this only throws far backward the history of civilisation, and shows that +animals were domesticated at a much earlier period than has hitherto been +supposed. The lake-inhabitants of Switzerland cultivated several kinds of wheat +and barley, the pea, the poppy for oil and flax; and they possessed several +domesticated animals. They also carried on commerce with other nations. All +this clearly shows, as Heer has remarked, that they had at this early age +progressed considerably in civilisation; and this again implies a long +continued previous period of less advanced civilisation, during which the +domesticated animals, kept by different tribes in different districts, might +have varied and given rise to distinct races. Since the discovery of flint +tools in the superficial formations of many parts of the world, all geologists +believe that barbarian men +<a name="Page14"></a> +existed at an enormously remote period; and we know that at the present day +there is hardly a tribe so barbarous as not to have domesticated at least the +dog. +</p> + +<p> +The origin of most of our domestic animals will probably forever remain vague. +But I may here state that, looking to the domestic dogs of the whole world, I +have, after a laborious collection of all known facts, come to the conclusion +that several wild species of Canidæ have been tamed, and that their blood, in +some cases mingled together, flows in the veins of our domestic breeds. In +regard to sheep and goats I can form no decided opinion. From facts +communicated to me by Mr. Blyth, on the habits, voice, constitution and +structure of the humped Indian cattle, it is almost certain that they are +descended from a different aboriginal stock from our European cattle; and some +competent judges believe that these latter have had two or three wild +progenitors, whether or not these deserve to be called species. This +conclusion, as well as that of the specific distinction between the humped and +common cattle, may, indeed, be looked upon as established by the admirable +researches of Professor Rütimeyer. With respect to horses, from reasons which I +cannot here give, I am doubtfully inclined to believe, in opposition to several +authors, that all the races belong to the same species. Having kept nearly all +the English breeds of the fowl alive, having bred and crossed them, and +examined their skeletons, it appears to me almost certain that all are the +descendants of the wild Indian fowl, Gallus bankiva; and this is the conclusion +of Mr. Blyth, and of others who have studied this bird in India. In regard to +ducks and rabbits, some breeds of which differ much from each other, the +evidence is clear that they are all descended from the common duck and wild +rabbit. +</p> + +<p> +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 eleven wild species of sheep peculiar to +Great Britain! When we bear in mind that Britain has now not one peculiar +mammal, and France but few distinct from those of Germany, and so with Hungary, +Spain, &c., but that each of these kingdoms possesses several peculiar +breeds of cattle, sheep, &c., we must admit that many domestic breeds must +have originated in Europe; for whence otherwise could they have been derived? +So it is +<a name="Page15"></a> +in India. Even in the case of the breeds of the domestic dog throughout the +world, which I admit are descended from several wild species, it cannot be +doubted that there has been an immense amount of inherited variation; for who +will believe that animals closely resembling the Italian greyhound, the +bloodhound, the bull-dog, pug-dog, or Blenheim spaniel, &c.—so unlike +all wild Canidæ—ever existed 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 only get 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, &c., in the wild +state. Moreover, the possibility of making distinct races by crossing has been +greatly exaggerated. Many cases are on record showing that a race may be +modified by occasional crosses if aided by the careful selection of the +individuals which present the desired character; but to obtain a race +intermediate between two quite distinct races would be very difficult. Sir J. +Sebright expressly experimented with this object and failed. The offspring from +the first cross between two pure breeds is tolerably and sometimes (as I have +found with pigeons) quite uniform in character, and every thing seems simple +enough; but when these mongrels are crossed one with another for several +generations, hardly two of them are alike, and then the difficulty of the task +becomes manifest. +</p> + +<p class="center"> +<i>Breeds of the Domestic Pigeon, their Differences and Origin.</i> +</p> + +<p> +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 Hon. W. Elliot from India, and by +the Hon. 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, +<a name="Page16"></a> +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 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 long beak, has a very short and 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 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 œsophagus. 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, 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 the members of the great pigeon family: +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 be specified. +</p> + +<p> +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 caudal and sacral vertebræ vary in number; 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 œsophagus; the development and abortion +of the oil-gland; the number of the primary wing and caudal feathers; the +relative length of the wing and tail to each other and to the body; the +relative length of the leg and foot; 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 +<a name="Page17"></a> +of the eggs vary. The manner of flight, and in some breeds the voice and +disposition, differ remarkably. Lastly, in certain breeds, the males and +females have come to differ in a slight degree from each other. +</p> + +<p> +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 be +ranked by him as well-defined species. Moreover, I do not believe that any +ornithologist would in this case 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 would call them, could be shown him. +</p> + +<p> +Great as are the differences between the breeds of the pigeon, I am fully +convinced that the common opinion of naturalists is correct, namely, that all +are 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, they did not breed or willingly perch 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 improbable; or they must have become extinct in +the wild state. But birds breeding on precipices, and good flyers, 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 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 +<a name="Page18"></a> +again into their native country; but not one has 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 difficult to get wild animals 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. +</p> + +<p> +An argument of great weight, and applicable in several other cases, is, that +the above-specified breeds, though agreeing generally with the wild rock-pigeon +in constitution, habits, voice, colouring, and in most parts of their +structure, yet are certainly highly abnormal in other parts; we may look in +vain through 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 are improbable in the highest degree. +</p> + +<p> +Some facts in regard to the colouring of pigeons well deserve consideration. +The rock-pigeon is of a slaty-blue, with white loins; but the Indian +sub-species, C. intermedia of Strickland, has this part bluish. The tail has a +terminal dark bar, with the outer feathers externally edged at the base with +white. The wings have two black bars. Some semi-domestic breeds, and some 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 birds +belonging to two or more distinct breeds are crossed, none of which are blue or +have any of the above-specified marks, the mongrel offspring are very apt +suddenly to acquire these characters. To give one instance out of several which +I have observed: I crossed some white fantails, which breed very true, with +some black barbs—and it so happens that blue varieties of barbs are so +rare that I never heard of an instance in England; and the mongrels were black, +brown and mottled. I also crossed a barb with a spot, which is a white bird +with a +<a name="Page19"></a> +red tail and red spot on the forehead, and which notoriously breeds very true; +the mongrels were dusky and mottled. I then crossed one of the mongrel +barb-fantails with a mongrel barb-spot, and they produced a bird of as +beautiful a blue colour, with the white loins, 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 are descended from the rock-pigeon. But +if we deny this, we must make one of the two following highly improbable +suppositions. Either, first, 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 no instance is known of crossed descendants reverting +to an ancestor of foreign blood, removed by a greater number of generations. In +a breed which has been crossed only once the tendency to revert to any +character derived from such a 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, and there is a tendency in the breed to revert to a +character which was 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 of reversion are +often confounded together by those who have written on inheritance. +</p> + +<p> +Lastly, the hybrids or mongrels from between all the breeds of the pigeon are +perfectly fertile, as I can state from my own observations, purposely made, on +the most distinct breeds. Now, hardly any cases have been ascertained with +certainty of hybrids from two quite distinct species of animals being perfectly +fertile. Some authors believe that long-continued domestication eliminates this +strong tendency to sterility in species. From the history of the dog, and of +some other domestic animals, this conclusion is probably quite correct, if +applied to species closely related to each other. But to extend it so far as to +suppose that species, aboriginally as distinct as carriers, tumblers, pouters, +and fantails now are, should yield offspring perfectly fertile, <i>inter +se</i>, seems to me rash in the extreme. +</p> + +<p> +From these several reasons, namely, the improbability of man having formerly +made seven or eight supposed species of pigeons to +<a name="Page20"></a> +breed freely under domestication—these supposed species being quite +unknown in a wild state, and their not having become anywhere feral—these +species presenting certain very abnormal characters, as compared with all other +Columbidæ, though so like the rock-pigeon in most other respects—the +occasional reappearance of the blue colour and various black marks in all the +breeds, both when kept pure and when crossed—and lastly, the mongrel +offspring being perfectly fertile—from these several reasons, taken +together, we may safely conclude that all our domestic breeds are descended +from the rock-pigeon or Columba livia with its geographical sub-species. +</p> + +<p> +In favour of this view, I may add, firstly, that the wild C. livia 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, that although an English carrier or a short-faced tumbler +differs immensely in certain characters from the rock-pigeon, yet that by +comparing the several sub-breeds of these two races, more especially those +brought from distant countries, we can make, between them and the rock-pigeon, +an almost perfect series; so we can in some other cases, but not with all the +breeds. Thirdly, those characters which are mainly distinctive of each breed +are in each eminently variable, 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; and the explanation of this fact will be obvious +when we 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 Ægyptian 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 likewise be obvious when +<a name="Page21"></a> +we treat of Selection. We shall then, also, see how it is that the several +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. +</p> + +<p> +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, well knowing how truly they breed, I felt fully as much difficulty in +believing that since they had been domesticated they had all proceeded from a +common parent, as any naturalist could in coming to a similar conclusion in +regard to the many species of finches, or other groups of birds, in nature. One +circumstance has struck me much; namely, that nearly all the breeders of the +various domestic animals and the cultivators of plants, with whom I have +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, +or both from a common parent-stock, 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 are 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? +</p> + +<p class="center"> +<a name="Page22"></a> +<i>Principles of Selection anciently followed, and their Effects.</i> +</p> + +<p> +Let us now briefly consider the steps by which domestic races have been +produced, either from one or from several allied species. Some effect may be +attributed to the direct and definite action of the external conditions of +life, and some to habit; but he would be a bold man who would account by such +agencies for the differences between 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 teasel, with its +hooks, which can not 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 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 can not +suppose that all the breeds were suddenly produced as perfect and as useful as +we now see them; indeed, in many 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 have made for himself useful breeds. +</p> + +<p> +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 their 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 +plastic, which they can model +<a name="Page23"></a> +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 animals, 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.” 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. +</p> + +<p> +What English breeders have actually effected is proved by the enormous prices +given for animals with a good pedigree; and these have 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 among 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. +</p> + +<p> +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 several cases in which exact records have been kept; thus, to +give a +<a name="Page24"></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, likewise followed; for hardly any one is so careless as +to breed from his worst animals. +</p> + +<p> +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—I speak after careful observation—perhaps never, the case. The +law of correlated variation, the importance of which should never be +overlooked, will ensure some differences; but, as a general rule, it cannot be +doubted 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. +</p> + +<p> +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 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 works of high antiquity, +in which the full importance of the principle is acknowledged. 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 +<a name="Page25"></a> +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 states that good domestic breeds are highly valued by the negroes +in 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. +</p> + +<p class="center"> +<i>Unconscious Selection.</i> +</p> + +<p> +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 of the kind in the country. But, for our purpose, a form 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 we may infer that this process, +continued during centuries, would improve and modify any breed, in the same way +as Bakewell, Collins, &c., by this very same process, only carried on more +methodically, did greatly modify, even during their 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 have been made long ago, which may serve for comparison. In some +cases, however, unchanged, or but little changed, individuals of the same breed +exist in less civilised districts, where the breed has been less improved. +There is reason to believe that King Charles’ 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 foxhound; +<a name="Page26"></a> +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. +</p> + +<p> +By a similar process of selection, and by careful training, English race-horses +have come to surpass in fleetness and size the parent Arabs, so that the +latter, by the regulations for the Goodwood Races, are favoured in the weights +which 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 various old +treatises of the former and present state of carrier and tumbler pigeons in +Britain, India, and Persia, we can trace the stages through which they have +insensibly passed, and come to differ so greatly from the rock-pigeon. +</p> + +<p> +Youatt gives an excellent illustration of the effects of a course of selection +which may be considered as unconscious, in so far that the breeders could never +have expected, or even wished, to produce the result which ensued—namely, +the production of the 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.” +</p> + +<p> +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. +</p> + +<p> +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 +<a name="Page27"></a> +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 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 chanced to appear, selecting it, and so onwards. But the +gardeners of the classical period, who cultivated the best pears which 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. +</p> + +<p> +A large amount of change, thus slowly and unconsciously accumulated, explains, +as I believe, the well-known fact, that in a 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 acquired by the plants in countries anciently civilised. +</p> + +<p> +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 +<a name="Page28"></a> +explains why the varieties kept by savages, as has been remarked by some +authors, have more of the character of true species than the varieties kept in +civilised countries. +</p> + +<p> +On the view here given of the 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 œsophagus—a habit which is disregarded by all +fanciers, as it is not one of the points of the breed. +</p> + +<p> +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 have been set on any slight +differences in the individuals of the same species, be judged of by the value +which is now set on them, after several breeds have fairly been established. It +is known that with pigeons many slight variations now occasionally appear, but +these are rejected as faults or deviations from the standard of perfection in +each breed. The common goose has not given rise to any marked varieties; hence +the Toulouse and the common breed, which differ only in colour, that most +<a name="Page29"></a> +fleeting of characters, have lately been exhibited as distinct at our +poultry-shows. +</p> + +<p> +These views appear to explain what has sometimes been noticed, namely, that we +know hardly anything 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 +a distinct 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 animals slowly spread in +the immediate neighbourhood. But they will as yet hardly have a distinct name, +and from being only slightly valued, their history will have been disregarded. +When further improved by the same slow and gradual process, they will spread +more widely, and will be 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 of a new sub-breed +will be a slow process. As soon as the points of value are once 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. +</p> + +<p class="center"> +<i>Circumstances favourable to Man’s Power of Selection.</i> +</p> + +<p> +I will 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. Hence number is of the highest +importance for success. On this principle Marshall formerly remarked, with +respect to the sheep of part of Yorkshire, “As they generally belong to +poor people, and are mostly <i>in small lots</i>, they never can be +improved.” On the other hand, nurserymen, from keeping large stocks of +the same plant, are generally far more successful than amateurs in raising new +and valuable varieties. A large number of individuals of an animal or plant can +<a name="Page30"></a> +be reared only where the conditions for its propagation are favourable. When +the individuals are scanty all will be allowed to breed, whatever their quality +may be, and this will effectually prevent selection. But probably the most +important element is that the animal or plant should be so highly valued by +man, that the closest attention is paid to even the slightest deviations in its +qualities or structure. 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 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 (with some aid +by crossing distinct species) those many admirable varieties of the strawberry +were raised which have appeared during the last half-century. +</p> + +<p> +With animals, facility in preventing crosses is an important element 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 improved and kept true, +though mingled in the same aviary; and this circumstance must have largely +favoured the 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, can not be easily matched, and, although so +much valued by women and children, we rarely see a distinct breed long kept up; +such breeds as we do sometimes see are almost always imported from some other +country. 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, &c., 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; for recently in certain parts of Spain and of the United +States this animal has been surprisingly modified and improved by careful +selection; 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 +<a name="Page31"></a> +especially from no pleasure having been felt in the display of distinct breeds; +but the goose, under the conditions to which it is exposed when domesticated, +seems to have a singularly inflexible organisation, though it has varied to a +slight extent, as I have elsewhere described. +</p> + +<p> +Some authors have maintained that the amount of variation in our domestic +productions is soon reached, and can never afterward be exceeded. It would be +somewhat rash to assert that the limit has been attained in any one case; for +almost all our animals and plants have been greatly improved in many ways +within a recent period; and this implies variation. It would be equally rash to +assert that characters now increased to their utmost limit, could not, after +remaining fixed for many centuries, again vary under new conditions of life. No +doubt, as Mr. Wallace has remarked with much truth, a limit will be at last +reached. For instance, there must be a limit to the fleetness of any +terrestrial animal, as this will be determined by the friction to be overcome, +the weight of the body to be carried, and the power of contraction in the +muscular fibres. But what concerns us is that the domestic varieties of the +same species differ from each other in almost every character, which man has +attended to and selected, more than do the distinct species of the same genera. +Isidore Geoffroy St. Hilaire has proved this in regard to size, and so it is +with colour, and probably with the length of hair. With respect to fleetness, +which depends on many bodily characters, Eclipse was far fleeter, and a +dray-horse is comparably stronger, than any two natural species belonging to +the same genus. So with plants, the seeds of the different varieties of the +bean or maize probably differ more in size than do the seeds of the distinct +species in any one genus in the same two families. The same remark holds good +in regard to the fruit of the several varieties of the plum, and still more +strongly with the melon, as well as in many other analogous cases. +</p> + +<p> +To sum up on the origin of our domestic races of animals and plants. Changed +conditions of life are of the highest importance in causing variability, both +by acting directly on the organisation, and indirectly by affecting the +reproductive system. It is not probable that variability is an inherent and +necessary contingent, under all circumstances. The greater or less force of +inheritance and reversion determine whether variations shall endure. +Variability is governed by many unknown laws, of which correlated growth is +probably the most important. Something, but how much we do not know, may be +attributed to the definite action of the conditions of life. Some, perhaps a +great, effect may be attributed to the +<a name="Page32"></a> +increased use or disuse of parts. The final result is thus rendered infinitely +complex. In some cases the intercrossing of aboriginally distinct species +appears to have played an important part in the origin of our breeds. When +several breeds have once been formed in any country, their occasional +intercrossing, with the aid of selection, has, no doubt, largely aided in the +formation of new sub-breeds; but the importance of crossing has been much +exaggerated, both in regard to animals and to those plants which are propagated +by seed. With plants which are temporarily propagated by cuttings, buds, +&c., the importance of crossing is immense; for the cultivator may here +disregard the extreme variability both of hybrids and of mongrels, and the +sterility of hybrids; but plants not propagated by seed are of little +importance to us, for their endurance is only temporary. Over all these causes +of change, the accumulative action of selection, whether applied methodically +and quickly, or unconsciously and slowly, but more efficiently, seems to have +been the predominant power. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page33"></a><a name="link2HCH0002" id="link2HCH0002"></a> +CHAPTER II.<br/> +VARIATION UNDER NATURE.</h2> + +<p class="letter"> +Variability—Individual differences—Doubtful species—Wide +ranging, much diffused, and common species, vary most—Species of the +larger genera in each country vary more frequently 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. +</p> + +<p> +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 properly, a long catalogue of dry facts +ought to be given; but these I shall reserve for a future work. Nor shall I +here discuss the various definitions which have been given of the term species. +No one definition has 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, +generally injurious, or not useful to the species. 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 a few generations? And in +this case I presume that the form would be called a variety. +</p> + +<p> +It may be doubted whether sudden and considerable deviations of structure, such +as we occasionally see in our domestic productions, more especially with +plants, are ever permanently propagated in a state of nature. Almost every part +of every organic being is so beautifully related to its complex conditions of +life that it seems as +<a name="Page34"></a> +improbable that any part should have been suddenly produced perfect, as that a +complex machine should have been invented by man in a perfect state. Under +domestication monstrosities sometimes occur which resemble normal structures in +widely different animals. Thus pigs have occasionally been born with a sort of +proboscis, and if any wild species of the same genus had naturally possessed a +proboscis, it might have been argued that this had appeared as a monstrosity; +but I have as yet failed to find, after diligent search, cases of monstrosities +resembling normal structures in nearly allied forms, and these alone bear on +the question. If monstrous forms of this kind ever do appear in a state of +nature and are capable of reproduction (which is not always the case), as they +occur rarely and singly, their preservation would depend on unusually +favourable circumstances. They would, also, during the first and succeeding +generations cross with the ordinary form, and thus their abnormal character +would almost inevitably be lost. But I shall have to return in a future chapter +to the preservation and perpetuation of single or occasional variations. +</p> + +<p class="center"> +<i>Individual Differences.</i> +</p> + +<p> +The many slight differences which appear in the offspring from the same +parents, or which it may be presumed have thus arisen, from being observed in +the individuals of the same species inhabiting the same confined locality, may +be called individual differences. No one supposes that all the individuals of +the same species are cast in the same actual mould. These individual +differences are of the highest importance for us, for they are often inherited, +as must be familiar to every one; and they thus afford materials for natural +selection to act on and accumulate, in the same manner as man accumulates 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 being 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 +<a name="Page35"></a> +the same species. It would never have been 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; it might have been thought that changes of this +nature could have been effected only by slow degrees; yet Sir J. 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 shown that the muscles in the +larvæ of certain insects are far from uniform. Authors sometimes argue in a +circle when they state that important organs never vary; for these same authors +practically rank those parts as important (as some few naturalists have +honestly confessed) which do not vary; and, under this point of view, no +instance will ever be found of an important part varying; but under any other +point of view many instances assuredly can be given. +</p> + +<p> +There is one point connected with individual differences which is extremely +perplexing: I refer to those genera which have been called +“protean” or “polymorphic,” in which species present an +inordinate amount of variation. With respect to many of these forms, hardly two +naturalists agree whether to rank them as species or as varieties. We may +instance Rubus, Rosa, and Hieracium among plants, several genera of insects, +and 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 a few exceptions, polymorphic in other countries, and likewise, +judging from Brachiopod shells, at former periods of time. These facts are 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, at least in +some of these polymorphic genera, variations 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 to be explained. +</p> + +<p> +Individuals of the same species often present, as is known to every one, great +differences of structure, independently of variation, as in the two sexes of +various animals, in the two or three castes of sterile females or workers among +insects, and in the immature and larval states of many of the lower animals. +There are, also, cases of dimorphism and trimorphism, both with animals and +plants. Thus, Mr. Wallace, who has lately called attention to the subject, has +shown that the females of certain species of butterflies, in the Malayan +Archipelago, regularly appear under two or even three conspicuously distinct +forms, not connected by intermediate varieties. Fritz Müller has described +analogous but more extraordinary cases +<a name="Page36"></a> +with the males of certain Brazilian Crustaceans: thus, the male of a Tanais +regularly occurs under two distinct forms; one of these has strong and +differently shaped pincers, and the other has antennæ much more abundantly +furnished with smelling-hairs. Although in most of these cases, the two or +three forms, both with animals and plants, are not now connected by +intermediate gradations, it is possible that they were once thus connected. Mr. +Wallace, for instance, describes a certain butterfly which presents in the same +island a great range of varieties connected by intermediate links, and the +extreme links of the chain closely resemble the two forms of an allied +dimorphic species inhabiting another part of the Malay Archipelago. Thus also +with ants, the several worker-castes are generally quite distinct; but in some +cases, as we shall hereafter see, the castes are connected together by finely +graduated varieties. So it is, as I have myself observed, with some dimorphic +plants. It certainly at first appears a highly remarkable fact that the same +female butterfly should have the power of producing at the same time three +distinct female forms and a male; and that an hermaphrodite plant should +produce from the same seed-capsule three distinct hermaphrodite forms, bearing +three different kinds of females and three or even six different kinds of +males. Nevertheless these cases are only exaggerations of the common fact that +the female produces offspring of two sexes which sometimes differ from each +other in a wonderful manner. +</p> + +<p class="center"> +<i>Doubtful Species.</i> +</p> + +<p> +The forms which possess in some considerable degree the character of species, +but which are so closely similar to 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 for a long time; for as long, as far +as we know, as have good and true species. Practically, when a naturalist can +unite by means of intermediate links any two forms, 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 arise 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 +<a name="Page37"></a> +the intermediate forms 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. +</p> + +<p> +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. +</p> + +<p> +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! Among 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 the +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, geographical races! +Mr. Wallace, in several valuable papers on the various animals, especially on +the Lepidoptera, inhabiting the islands of the great Malayan Archipelago, shows +that they may be classed under four heads, namely, as variable forms, as local +forms, as geographical races or sub-species, and as true representative +species. The first or variable forms vary much within the limits of the same +island. The local forms are moderately constant and distinct in each separate +island; but when all from the several islands are compared together, the +<a name="Page38"></a> +differences are seen to be so slight and graduated that it is impossible to +define or describe them, though at the same time the extreme forms are +sufficiently distinct. The geographical races or sub-species are local forms +completely fixed and isolated; but as they do not differ from each other by +strongly marked and important characters, “There is no possible test but +individual opinion to determine which of them shall be considered as species +and which as varieties.” Lastly, representative species fill the same +place in the natural economy of each island as do the local forms and +sub-species; but as they are distinguished from each other by a greater amount +of difference than that between the local forms and sub-species, they are +almost universally ranked by naturalists as true species. Nevertheless, no +certain criterion can possibly be given by which variable forms, local forms, +sub species and representative species can be recognised. +</p> + +<p> +Many years ago, when comparing, and seeing others compare, the birds from the +closely neighbouring islands of the Galapagos Archipelago, 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 would certainly 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 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 them as distinct species; but what distance, it has been +well asked, will suffice if that between America and Europe is ample, will that +between Europe and the Azores, or Madeira, or the Canaries, or between the +several islets of these small archipelagos, be sufficient? +</p> + +<p> +Mr. B.D. Walsh, a distinguished entomologist of the United States, has +described what he calls Phytophagic varieties and Phytophagic species. Most +vegetable-feeding insects live on one kind of plant or on one group of plants; +some feed indiscriminately on many kinds, but do not in consequence vary. In +several cases, however, insects found living on different plants, have been +observed by Mr. Walsh to present in their larval or mature state, or in both +states, slight, though constant differences in colour, size, or in the +<a name="Page39"></a> +nature of their secretions. In some instances the males alone, in other +instances, both males and females, have been observed thus to differ in a +slight degree. When the differences are rather more strongly marked, and when +both sexes and all ages are affected, the forms are ranked by all entomologists +as good species. But no observer can determine for another, even if he can do +so for himself, which of these Phytophagic forms ought to be called species and +which varieties. Mr. Walsh ranks the forms which it may be supposed would +freely intercross, as varieties; and those which appear to have lost this +power, as species. As the differences depend on the insects having long fed on +distinct plants, it cannot be expected that intermediate links connecting the +several forms should now be found. The naturalist thus loses his best guide in +determining whether to rank doubtful forms as varieties or species. This +likewise necessarily occurs with closely allied organisms, which inhabit +distinct continents or islands. When, on the other hand, an animal or plant +ranges over the same continent, or inhabits many islands in the same +archipelago, and presents different forms in the different areas, there is +always a good chance that intermediate forms will be discovered which will link +together the extreme states; and these are then degraded to the rank of +varieties. +</p> + +<p> +Some few naturalists maintain that animals never present varieties; but then +these same naturalists rank the slightest difference as of specific value; and +when the same identical form is met with in two distant countries, or in two +geological formations, they believe that two distinct species are hidden under +the same dress. The term species thus comes to be a mere useless abstraction, +implying and assuming a separate act of creation. It is certain that many +forms, considered by highly competent judges to be varieties, resemble species +so completely in character that they have been thus ranked by other highly +competent judges. But to discuss whether they ought to be called species or +varieties, before any definition of these terms has been generally accepted, is +vainly to beat the air. +</p> + +<p> +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, &c., have been brought to +bear in the attempt to determine their rank; but space does not here permit me +to discuss them. Close investigation, in many cases, will no doubt bring +naturalists to agree 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 them. 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 +<a name="Page40"></a> +attracts his attention, varieties of it will almost universally be found +recorded. These varieties, moreover, will often be 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 almost +universally considered by other botanists to be 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. +</p> + +<p> +I may here allude to a remarkable memoir lately published by A. de Candolle, on +the oaks of the whole world. No one ever had more ample materials for the +discrimination of the species, or could have worked on them with more zeal and +sagacity. He first gives in detail all the many points of structure which vary +in the several species, and estimates numerically the relative frequency of the +variations. He specifies above a dozen characters which may be found varying +even on the same branch, sometimes according to age or development, sometimes +without any assignable reason. Such characters are not of course of specific +value, but they are, as Asa Gray has remarked in commenting on this memoir, +such as generally enter into specific definitions. De Candolle then goes on to +say that he gives the rank of species to the forms that differ by characters +never varying on the same tree, and never found connected by intermediate +states. After this discussion, the result of so much labour, he emphatically +remarks: “They are mistaken, who repeat that the greater part of our +species are clearly limited, and that the doubtful species are in a feeble +minority. This seemed to be true, so long as a genus was imperfectly known, and +its species were founded upon a few specimens, that is to say, were +provisional. Just as we come to know them better, intermediate forms flow in, +and doubts as to specific limits augment.” He also adds that it is the +best known species which present the greatest number of spontaneous varieties +and sub-varieties. Thus Quercus robur has twenty-eight varieties, all of which, +excepting six, are clustered round three sub-species, namely Q. pedunculata, +sessiliflora and pubescens. The forms which connect these three sub-species are +comparatively rare; and, as Asa Gray again remarks, if these connecting forms +which are now rare were to become totally extinct the three sub-species would +hold exactly the same relation to each other as do the four or five +provisionally admitted species which closely surround the typical Quercus +robur. Finally, De Candolle admits that out of the 300 species, which will be +enumerated in his Prodromus as belonging to the oak family, at least two-thirds +are +<a name="Page41"></a> +provisional species, that is, are not known strictly to fulfil the definition +above given of a true species. It should be added that De Candolle no longer +believes that species are immutable creations, but concludes that the +derivative theory is the most natural one, “and the most accordant with +the known facts in palæontology, geographical botany and zoology, of anatomical +structure and classification.” +</p> + +<p> +When a young naturalist commences the study of a group of organisms quite +unknown to him he is at first much perplexed in determining what differences to +consider as specific and what as varietal; 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 able to +make up his own mind; 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 he comes to study allied forms brought from countries not now +continuous, in which case he cannot hope to find intermediate links, he will be +compelled to trust almost entirely to analogy, and his difficulties will rise +to a climax. +</p> + +<p> +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 +by an insensible series; and a series impresses the mind with the idea of an +actual passage. +</p> + +<p> +Hence I look at individual differences, though of small interest to the +systematist, as of the highest 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 towards +<a name="Page42"></a> +more strongly marked and permanent varieties; and at the latter, as leading to +sub-species, and then to species. The passage from one stage of difference to +another may, in many cases, be the simple result of the nature of the organism +and of the different physical conditions to which it has long been exposed; but +with respect to the more important and adaptive characters, the passage from +one stage of difference to another may be safely attributed to the cumulative +action of natural selection, hereafter to be explained, and to the effects of +the increased use or disuse of parts. A well-marked variety may therefore be +called an incipient species; but whether this belief is justifiable must be +judged by the weight of the various facts and considerations to be given +throughout this work. +</p> + +<p> +It need not be supposed that all varieties or incipient species attain the rank +of species. They may 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, and with plants by Gaston +de Saporta. 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 return +to this subject. +</p> + +<p> +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, for convenience sake. +</p> + +<p class="center"> +<i>Wide-ranging, much-diffused, and common Species vary most.</i> +</p> + +<p> +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 a future work the discussion of these +difficulties, and the tables of the proportional numbers of the varying +species. Dr. Hooker permits me to add that after having carefully read my +manuscript, and examined the +<a name="Page43"></a> +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. +</p> + +<p> +Alphonse de Candolle and others have shown that plants which have very wide +ranges generally present varieties; and this might have been expected, as they +are 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 the 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), oftenest 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, 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, still inherit those advantages +that enabled their parents to become dominant over their compatriots. In these +remarks on predominence, it should be understood that reference is made only to +the forms which come into competition with each other, and more especially to +the members of the same genus or class having nearly similar habits of life. +With respect to the number of individuals or commonness of species, the +comparison of course relates only to the members of the same group. One of the +higher plants may be said to be dominant if it be more numerous in individuals +and more widely diffused than the other plants of the same country, which live +under nearly the same conditions. A plant of this kind is not the less dominant +because some conferva inhabiting the water or some parasitic fungus is +infinitely more numerous in individuals, and more widely diffused. But if the +conferva or parasitic fungus exceeds its allies in the above respects, it will +then be dominant within its own class. +</p> + +<p class="center"> +<a name="Page44"></a> +<i>Species of the Larger Genera in each Country vary more Frequently than the +Species of the Smaller Genera.</i> +</p> + +<p> +If the plants inhabiting a country as described in any Flora, be divided into +two equal masses, all those in the larger genera (<i>i.e.</i>, those including +many species) being placed on one side, and all those in the smaller genera on +the other side, the former will be found to include a somewhat larger number of +the very common and much diffused or dominant species. This 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 +larger 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 generally have 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. +</p> + +<p> +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>i.e.</i>, 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 still be 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. +</p> + +<p> +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 +<a name="Page45"></a> +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 presented 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 least genera, with from only one to four species, are altogether +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 holds true 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 certainly +holds good. +</p> + +<p class="center"> +<i>Many of the Species included within the Larger Genera resemble Varieties in +being very closely, but unequally, related to each other, and in having +restricted ranges.</i> +</p> + +<p> +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 when 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 +<a name="Page46"></a> +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 confirm the view. I have also consulted some sagacious and +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 the +usual amount of difference. +</p> + +<p> +Moreover, the species of the larger 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 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 tend to increase into the greater +differences between species. +</p> + +<p> +There is one other point which is 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 would be reversed. But there is reason to +believe that the 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) sixty-three 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 sixty-three 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, +fifty-three acknowledged varieties are recorded, and these range over 7.7 +provinces; +<a name="Page47"></a> +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 the 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. +</p> + +<p class="center"> +<i>Summary.</i> +</p> + +<p> +Finally, varieties cannot be distinguished from species—except, first, by +the discovery of intermediate linking forms; and, secondly, by a certain +indefinite amount of difference between them; for two forms, if differing very +little, are generally ranked as varieties, notwithstanding that they cannot be +closely connected; but the amount of difference considered necessary to give to +any two forms the rank of species cannot be defined. 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 +other species. Species very closely allied to other species apparently have +restricted ranges. In all these respects the species of large genera present a +strong analogy with varieties. And we can clearly understand these analogies, +if species once existed as varieties, and thus originated; whereas, these +analogies are utterly inexplicable if species are independent creations. +</p> + +<p> +We have also seen that it is the most flourishing or dominant species of the +larger genera within each class which on an average yield the greatest number +of varieties, and varieties, as we shall hereafter see, tend to become +converted into new and distinct species. Thus the larger genera 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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page48"></a><a name="link2HCH0003" id="link2HCH0003"></a> +CHAPTER III.<br/> +STRUGGLE FOR EXISTENCE.</h2> + +<p class="letter"> +Its bearing on natural selection—The term used in a wide +sense—Geometrical ratio 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. +</p> + +<p> +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 among 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 organic being to another being, been perfected? +We see these beautiful co-adaptations most plainly in the woodpecker and the +mistletoe; 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. +</p> + +<p> +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 +<a name="Page49"></a> +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 from the struggle for life. Owing to this struggle, variations, +however slight and from whatever cause proceeding, if they be in any degree +profitable to the individuals of a species, in their infinitely complex +relations to other organic beings and to their physical conditions of life, +will tend to the preservation of such individuals, and will generally be +inherited by the 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 +natural selection, in order to mark its relation to man’s power of +selection. But the expression often used by Mr. Herbert Spencer, of the +Survival of the Fittest, is more accurate, and is sometimes equally convenient. +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, +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. +</p> + +<p> +We will now discuss in a little more detail the struggle for existence. In my +future work this subject will be treated, as it well deserves, at 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 found it so—than constantly +to bear this conclusion in mind. Yet unless it be thoroughly engrained in the +mind, 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. +</p> + +<p class="center"> +<a name="Page50"></a> +<i>The Term, Struggle for Existence, used in a large sense.</i> +</p> + +<p> +I should premise that I use this term 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 only one of an average 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 mistletoe 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 languishes +and dies. But several seedling mistletoes, growing close together on the same +branch, may more truly be said to struggle with each other. As the mistletoe is +disseminated by birds, its existence depends on them; and it may metaphorically +be said to struggle with other fruit-bearing plants, in tempting the birds to +devour and thus disseminate its seeds. In these several senses, which pass into +each other, I use for convenience sake the general term of Struggle for +Existence. +</p> + +<p class="center"> +<i>Geometrical Ratio of Increase.</i> +</p> + +<p> +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. +</p> + +<p> +There is no exception to the rule that every organic being +<a name="Page51"></a> +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 less than a 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 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 safest to assume that it begins breeding when thirty years old, and goes on +breeding till ninety years old, bringing forth six young in the interval, and +surviving till one hundred years old; if this be so, after a period of from 740 +to 750 years there would be nearly nineteen million elephants alive descended +from the first pair. +</p> + +<p> +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 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, such +as the cardoon and a tall thistle, which are now the commonest over the wide +plains of La Plata, clothing square leagues of surface almost to the exclusion +of every other plant, 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 others could be given, no one supposes that the fertility of +the animals or plants has been suddenly and temporarily increased in any +sensible degree. The obvious explanation is that the conditions of life have +been highly 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. +Their geometrical ratio of increase, the result of which never fails to be +surprising, simply explains their extraordinarily rapid increase and wide +diffusion in their new homes. +</p> + +<p> +<a name="Page52"></a> +In a state of nature almost every full-grown plant annually produces seed, and +among 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 rapidly stock every station in which +they could any how exist, and that this 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 do not keep in mind that thousands are +annually slaughtered for food, and that in a state of nature an equal number +would have somehow to be disposed of. +</p> + +<p> +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 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. +</p> + +<p> +In looking at Nature, it is most necessary to keep the foregoing considerations +always in mind—never to forget that every single organic being 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 +<a name="Page53"></a> +check, mitigate the destruction ever so little, and the number of the species +will almost instantaneously increase to any amount. +</p> + +<p class="center"> +<i>Nature of the Checks to Increase.</i> +</p> + +<p> +The causes which check the natural tendency of each species to increase are +most obscure. Look at the most vigorous species; by as much as it swarms in +numbers, by so much will it tend to increase still further. We know not exactly +what the checks are even in a single instance. Nor will this surprise any one +who reflects how ignorant we are on this head, even in regard to mankind, +although so incomparably better known than any other animal. This subject of +the checks to increase has been ably treated by several authors, and I hope in +a future work to discuss it 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 it appears that the seedlings 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 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 grown on a little plot of mown turf (three feet by +four) nine species perished, from the other species being allowed to grow up +freely. +</p> + +<p> +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 number 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 shot. On the other hand, in some cases, as +with the elephant, none are +<a name="Page54"></a> +destroyed by beasts of prey; for even the tiger in India most rarely dares to +attack a young elephant protected by its dam. +</p> + +<p> +Climate plays an important part in determining the average numbers of a +species, and periodical seasons of extreme cold or drought seem to be the most +effective of all checks. I estimated (chiefly from the greatly reduced numbers +of nests in the spring) that the winter of 1854-5 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 individuals, or those which have got least food through the advancing +winter, which will suffer the 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 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 must 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 +northward; hence in going northward, or in ascending a mountain, we far oftener +meet with stunted forms, due to the <i>directly</i> injurious action of +climate, than we do in proceeding southward 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. +</p> + +<p> +That climate acts in main part indirectly by favouring other species we clearly +see in the prodigious number of plants which in our gardens 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. +</p> + +<p> +<a name="Page55"></a> +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 among the crowded +animals, been disproportionally favoured: and here comes in a sort of struggle +between the parasite and its prey. +</p> + +<p> +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, +&c., 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 the 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 +exist; and that of some social plants being social, that is abounding in +individuals, even on the extreme verge 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 the species +from utter destruction. I should add that the good effects of intercrossing, +and the ill effects of close interbreeding, no doubt come into play in many of +these cases; but I will not here enlarge on this subject. +</p> + +<p class="center"> +<i>Complex Relations of all Animals and Plants to each other in the Struggle +for Existence.</i> +</p> + +<p> +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, +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 +<a name="Page56"></a> +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 of the land having 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, with twenty-six rings of growth, had, during many 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. +</p> + +<p> +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 +other parasitic insects. Hence, if certain +<a name="Page57"></a> +insectivorous birds were to decrease in Paraguay, the parasitic insects would +probably increase; and this would lessen the number of the navel-frequenting +flies—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 have +just seen in Staffordshire, the insectivorous birds, and so onwards in +ever-increasing circles of complexity. Not that under nature the relations will +ever be as simple as this. Battle within battle must be continually recurring +with varying success; and yet in the long-run the forces are so nicely balanced +that the face of nature remains for long periods of time uniform, though +assuredly the merest trifle would 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! +</p> + +<p class="p2"> I am tempted to give one more instance showing how plants and +animals, 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 is never visited in my garden by insects, and consequently, from its +peculiar structure, never sets a seed. Nearly all our orchidaceous plants +absolutely require the visits of insects to remove their pollen-masses and thus +to fertilise them. I find from experiments that humble-bees are almost +indispensable to the fertilisation of the heartsease (Viola tricolor), for +other bees do not visit this flower. I have also found that the visits of bees +are necessary for the fertilisation of some kinds of clover; for instance +twenty heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but twenty +other heads, protected from bees, produced not one. Again, 100 heads of red +clover (T. pratense) produced 2,700 seeds, but the same number of protected +heads produced not a single seed. Humble bees alone visit red clover, as other +bees cannot reach the nectar. It has been suggested that moths may fertilise +the clovers; but I doubt whether they could do so in the case of the red +clover, from their weight not being sufficient to depress the wing petals. +Hence we may infer as highly probable 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 measure upon the number of field-mice, which +destroy their combs and nests; and Colonel Newman, who has long attended to the +habits of humble-bees, +<a name="Page58"></a> +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 Colonel 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! +</p> + +<p> +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 will +concur 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 ancient Indian ruins in the Southern United +States, which must formerly have been cleared of trees, now display the same +beautiful diversity and proportion of kinds as in the surrounding virgin +forests. What a struggle must have gone on during long centuries between the +several kinds of trees, 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, all +feeding on each other, or on the trees, 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 fall to the ground according to +definite laws; but how simple is the problem where each shall fall compared to +that of 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! +</p> + +<p> +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 likewise +sometimes 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 will almost invariably be most severe between the individuals +of the same species, for they frequent the same districts, require the same +<a name="Page59"></a> +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 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 number 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 of our +domestic plants or animals have so exactly the same strength, habits, and +constitution, that the original proportions of a mixed stock (crossing being +prevented) could be kept up for half-a-dozen generations, if they were allowed +to struggle together, in the same manner as beings in a state of nature, and if +the seed or young were not annually preserved in due proportion. +</p> + +<p class="center"> +<i>Struggle for Life most severe between Individuals and Varieties of the same +Species.</i> +</p> + +<p> +As the species of the same genus usually have, though by no means invariably, +much similarity in habits and constitution, and always in structure, the +struggle will generally be more severe between them, if they come into +competition with each other, than between the 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. In Australia the +imported hive-bee is rapidly exterminating the small, stingless native bee. One +species of charlock has been known to supplant another species; 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. +</p> + +<p> +<a name="Page60"></a> +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 the plumed seeds no doubt stands in the +closest relation to the land being already thickly clothed with 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. +</p> + +<p> +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, it may be suspected that the chief use of the nutriment in +the seed is to favour the growth of the seedlings, whilst struggling with other +plants growing vigorously all around. +</p> + +<p> +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 +wish in imagination to give the plant the power of increasing in numbers, we +should have to give it some advantage over its competitors, or over the animals +which prey 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 exclusively by the rigour of the climate. 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. +</p> + +<p> +Hence we can see that when a plant or animal is placed in a new country, among +new competitors, the conditions of its life will generally be changed in an +essential manner, although the climate +<a name="Page61"></a> +may be exactly the same as in its former home. If its average numbers are to +increase in its new home, we should have to modify it in a different way to +what we should have had to do in its native country; for we should have to give +it some advantage over a different set of competitors or enemies. +</p> + +<p> +It is good thus to try in imagination to give any one species an advantage over +another. Probably in no single instance should we know what to do. This ought +to convince us of our ignorance on the mutual relations of all organic beings; +a conviction as necessary, as it is difficult to acquire. All that we can do is +to keep steadily in mind that each organic being is striving to increase in 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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page62"></a><a name="link2HCH0004" id="link2HCH0004"></a> +CHAPTER IV.<br/> +NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.</h2> + +<p class="letter"> +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 the results of 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—Advance in organisation—Low forms +preserved—Convergence of character—Indefinite multiplication of +species—Summary. +</p> + +<p> +How will the struggle for existence, briefly discussed 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 under nature? I think we shall see that it +can act most efficiently. Let the endless number of slight variations and +individual differences occurring in our domestic productions, and, in a lesser +degree, in those under nature, be borne in mind; as well as the strength of the +hereditary tendency. Under domestication, it may truly be said that the whole +organisation becomes in some degree plastic. But the variability, which we +almost universally meet with in our domestic productions is not directly +produced, as Hooker and Asa Gray have well remarked, by man; he can neither +originate varieties nor prevent their occurrence; he can only preserve and +accumulate such as do occur. Unintentionally he exposes organic beings to new +and changing conditions of life, and variability ensues; but similar changes of +conditions might and do occur under nature. Let it also 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; and consequently +what infinitely varied diversities of structure might be of use to each being +under changing conditions of life. Can it then be thought improbable, seeing +that variations useful to man have +<a name="Page63"></a> +undoubtedly occurred, that other variations useful in some way to each being in +the great and complex battle of life, should occur in the course of many +successive 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 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 individual differences and variations, and the +destruction of those which are injurious, I have called Natural Selection, or +the Survival of the Fittest. Variations neither useful nor injurious would not +be affected by natural selection, and would be left either a fluctuating +element, as perhaps we see in certain polymorphic species, or would ultimately +become fixed, owing to the nature of the organism and the nature of the +conditions. +</p> + +<p> +Several writers have misapprehended or objected to the term Natural Selection. +Some have even imagined that natural selection induces variability, whereas it +implies only the preservation of such variations as arise and are beneficial to +the being under its conditions of life. No one objects to agriculturists +speaking of the potent effects of man’s selection; and in this case the +individual differences given by nature, which man for some object selects, must +of necessity first occur. Others have objected that the term selection implies +conscious choice in the animals which become modified; and it has even been +urged that, as plants have no volition, natural selection is not applicable to +them! In the literal sense of the word, no doubt, natural selection is a false +term; but who ever objected to chemists speaking of the elective affinities of +the various elements?—and yet an acid cannot strictly be said to elect +the base with which it in preference combines. It has been said that I speak of +natural selection as an active power or Deity; but who objects to an author +speaking of the attraction of gravity as ruling the movements of the planets? +Every one knows what is meant and is implied by such metaphorical expressions; +and they are almost necessary for brevity. So again it is difficult to avoid +personifying the word Nature; but I mean by nature, only the aggregate action +and product of many natural laws, and by laws the sequence of events as +ascertained by us. With a little familiarity such superficial objections will +be forgotten. +</p> + +<p> +We shall best understand the probable course of natural selection by taking the +case of a country undergoing some slight physical change, for instance, of +climate. The proportional numbers of its +<a name="Page64"></a> +inhabitants will almost immediately undergo a change, and some species will +probably 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 the inhabitants, +independently of the change of climate itself, would seriously affect the +others. If the country were open on its borders, new forms would certainly +immigrate, and this would likewise 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 cases, slight modifications, which in any way favoured the individuals of +any species, by better adapting them to their altered conditions, would tend to +be preserved; and natural selection would have free scope for the work of +improvement. +</p> + +<p> +We have good reason to believe, as shown in the first chapter, that changes in +the conditions of life give a tendency to increased variability; and in the +foregoing cases the conditions the changed, and this would manifestly be +favourable to natural selection, by affording a better chance of the occurrence +of profitable variations. Unless such occur, natural selection can do nothing. +Under the term of “variations,” it must never be forgotten that +mere individual differences are included. As man can produce a great result +with his domestic animals and plants by adding up in any given direction +individual differences, so could natural selection, but far more easily from +having incomparably longer time for action. Nor do I believe that any great +physical change, as of climate, or any unusual degree of isolation, to check +immigration, is necessary in order that new and unoccupied places should be +left for natural selection to fill up by 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 species would often give it an advantage over others; and still +further modifications of the same kind would often still further increase the +advantage, as long as the species continued under the same conditions of life +and profited by similar means of subsistence and defence. No country can be +named in which all +<a name="Page65"></a> +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 be still +better adapted or improved; for in all countries, the natives have been so far +conquered by naturalised productions that they have allowed some foreigners to +take firm possession of the land. And as foreigners have thus in every country +beaten some of the natives, we may safely conclude that the natives might have +been modified with advantage, so as to have better resisted the intruders. +</p> + +<p> +As man can produce, and certainly has produced, a great result by his +methodical and unconscious means of selection, what may not natural selection +effect? Man can act only on external and visible characters: Nature, if I may +be allowed to personify the natural preservation or survival of the fittest, +cares nothing for appearances, except in so far as they are 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, as is implied by the fact of their selection. 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; 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 the eye or to be plainly +useful to him. Under nature, the slightest differences 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 be his results, 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? +</p> + +<p> +It may metaphorically be said that natural selection is daily and hourly +scrutinising, throughout the world, the slightest variations; rejecting those +that are bad, preserving and adding up all that are +<a name="Page66"></a> +good; silently and insensibly working, <i>whenever and wherever opportunity +offers</i>, 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 see only that the +forms of life are now different from what they formerly were. +</p> + +<p> +In order that any great amount of modification should be effected in a species, +a variety, when once formed must again, perhaps after a long interval of time, +vary or present individual differences of the same favourable nature as before; +and these must again be preserved, and so onward, step by step. Seeing that +individual differences of the same kind perpetually recur, this can hardly be +considered as an unwarrantable assumption. But whether it is true, we can judge +only by seeing how far the hypothesis accords with and explains the general +phenomena of nature. On the other hand, the ordinary belief that the amount of +possible variation is a strictly limited quantity, is likewise a simple +assumption. +</p> + +<p> +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, 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 natural +selection might be 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 a lamb with the faintest trace of +black. We have seen how the colour of hogs, which feed on the +“paint-root” in Virginia, determines whether they shall live or +die. 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 +<a name="Page67"></a> +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 a purple-fleshed fruit, should succeed. +</p> + +<p> +In looking at many small points of difference between species, which, as far as +our ignorance permits us to judge, seem quite unimportant, we must not forget +that climate, food, &c., have no doubt produced some direct effect. It is +also necessary to bear in mind that, owing to the law of correlation, when one +part varies and the variations are accumulated through natural selection, other +modifications, often of the most unexpected nature, will ensue. +</p> + +<p> +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 shape, size and flavour of 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 variations +profitable 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; and these modifications may affect, through +correlation, the structure of the adult. So, conversely, modifications in the +adult may affect the structure of the larva; but in all cases natural selection +will ensure that they shall not be injurious: for if they were so, the species +would become extinct. +</p> + +<p> +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 whole community; +if the community 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 +<a name="Page68"></a> +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 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, used exclusively for opening the +cocoon—or the hard tip to the beak of unhatched birds, used for breaking +the eggs. It has been asserted, that of the best short-beaked tumbler-pigeons a +greater number 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 all 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. +</p> + +<p> +It may be well here to remark that with all beings there must be much +fortuitous destruction, which can have little or no influence on the course of +natural selection. For instance, a vast number of eggs or seeds are annually +devoured, and these could be modified through natural selection only if they +varied in some manner which protected them from their enemies. Yet many of +these eggs or seeds would perhaps, if not destroyed, have yielded individuals +better adapted to their conditions of life than any of those which happened to +survive. So again a vast number of mature animals and plants, whether or not +they be the best adapted to their conditions, must be annually destroyed by +accidental causes, which would not be in the least degree mitigated by certain +changes of structure or constitution which would in other ways be beneficial to +the species. But let the destruction of the adults be ever so heavy, if the +number which can exist in any district be not wholly kept down by such +causes—or again let the destruction of eggs or seeds be so great that +only a hundredth or a thousandth part are developed—yet of those which do +survive, the best adapted individuals, supposing that there is any variability +in a favourable direction, will tend to propagate their kind in larger numbers +than the less well adapted. If the numbers be wholly kept down by the causes +just indicated, as will often have been the case, natural selection will be +powerless in certain beneficial directions; but this is no valid objection to +its efficiency at other times and in other ways; for we are far from having any +reason to suppose that many species ever +<a name="Page69"></a> +undergo modification and improvement at the same time in the same area. +</p> + +<p class="center"> +<i>Sexual Selection.</i> +</p> + +<p> +Inasmuch as peculiarities often appear under domestication in one sex and +become hereditarily attached to that sex, so no doubt it will be under nature. +Thus it is rendered possible for the two sexes to be modified through natural +selection in relation to different habits of life, as is sometimes the case; or +for one sex to be modified in relation to the other sex, as commonly occurs. +This leads me to say a few words on what I have called sexual selection. This +form of selection depends, not on a struggle for existence in relation to other +organic beings or to external conditions, but on a struggle between the +individuals of one sex, generally the males, for the possession of the other +sex. 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 +depends not so much 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 numerous offspring. Sexual selection, by always allowing the victor to +breed, might surely give indomitable courage, length of spur, and strength to +the wing to strike in the spurred leg, in nearly the same manner as does the +brutal cockfighter by the careful selection of his best cocks. How low in the +scale of nature the 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 observed +fighting all day long; male stag-beetles sometimes bear wounds from the huge +mandibles of other males; the males of certain hymenopterous insects have been +frequently seen by that inimitable observer M. Fabre, fighting for a particular +female who sits by, an apparently unconcerned beholder of the struggle, and +then retires with the conqueror. 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 of the lion, and the hooked jaw to the male +salmon; for the shield may be as important for victory as the sword or spear. +</p> + +<p> +Among birds, the contest is often of a more peaceful character. +<a name="Page70"></a> +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 with the most elaborate care, and show off in the best +manner, their gorgeous plumage; they likewise 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 a pied peacock was eminently attractive to all his hen birds. I +cannot here enter on the necessary details; but if man can in a short time give +beauty and an elegant carriage 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. Some +well-known laws, with respect to the plumage of male and female birds, in +comparison with the plumage of the young, can partly be explained through the +action of sexual selection on variations occurring at different ages, and +transmitted to the males alone or to both sexes at corresponding ages; but I +have not space here to enter on this subject. +</p> + +<p> +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, by +individual males having had, in successive generations, some slight advantage +over other males, in their weapons, means of defence, or charms; which they +have transmitted to their male offspring alone. Yet, I would not wish to +attribute all sexual differences to this agency: for we see in our domestic +animals peculiarities arising and becoming attached to the male sex, which +apparently have not been augmented through selection by man. The tuft of hair +on the breast of the wild turkey-cock cannot be of any use, and it is doubtful +whether it can be ornamental in the eyes of the female bird; indeed, had the +tuft appeared under domestication it would have been called a monstrosity. +</p> + +<p class="center"> +<i>Illustrations of the Action of Natural Selection, or the Survival of the +Fittest.</i> +</p> + +<p> +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; +<a name="Page71"></a> +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 was hardest pressed for +food. Under such circumstances the swiftest and slimmest wolves 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 some other period of the +year, when they were compelled to prey on other animals. I can see no more +reason to doubt that this would be the result, than that man should be able to +improve the fleetness of his greyhounds by careful and methodical selection, or +by that kind of unconscious selection which follows from each man trying to +keep the best dogs without any thought of modifying the breed. 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. +</p> + +<p> +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. +</p> + +<p> +It should be observed that in the above illustration, I speak of the slimmest +individual wolves, and not of any single strongly marked variation having been +preserved. In former editions of this work I sometimes spoke as if this latter +alternative had frequently occurred. I saw the great importance of individual +differences, and this led me fully to discuss the results of unconscious +selection by man, which depends on the preservation of all the more or less +valuable individuals, and on the destruction of the worst. I saw, also, that +the preservation in a state of nature of any occasional deviation of structure, +such as a monstrosity, would be a rare event; and that, if at first preserved, +it would generally be lost by subsequent intercrossing with ordinary +individuals. Nevertheless, until reading an able and valuable article in the +“North British Review” (1867), I did not appreciate how rarely +single variations, whether slight or strongly marked, could be perpetuated. The +author takes the case of a pair of animals, producing during their lifetime two +hundred offspring, of which, from various causes of destruction, only two on an +average survive to pro-create their kind. This is rather an extreme estimate +for most of the higher animals, but by no means so for many of the lower +organisms. He then shows that if a single individual were born, which varied in +some manner, giving it twice as good a chance of life as that of the other +individuals, yet the chances would be strongly against its survival. Supposing +it to survive and to breed, and that half its young inherited the favourable +variation; still, as the Reviewer goes onto show, the young +<a name="Page72"></a> +would have only a slightly better chance of surviving and breeding; and this +chance would go on decreasing in the succeeding generations. The justice of +these remarks cannot, I think, be disputed. If, for instance, a bird of some +kind could procure its food more easily by having its beak curved, and if one +were born with its beak strongly curved, and which consequently flourished, +nevertheless there would be a very poor chance of this one individual +perpetuating its kind to the exclusion of the common form; but there can hardly +be a doubt, judging by what we see taking place under domestication, that this +result would follow from the preservation during many generations of a large +number of individuals with more or less strongly curved beaks, and from the +destruction of a still larger number with the straightest beaks. +</p> + +<p> +It should not, however, be overlooked that certain rather strongly marked +variations, which no one would rank as mere individual differences, frequently +recur owing to a similar organisation being similarly acted on—of which +fact numerous instances could be given with our domestic productions. In such +cases, if the varying individual did not actually transmit to its offspring its +newly-acquired character, it would undoubtedly transmit to them, as long as the +existing conditions remained the same, a still stronger tendency to vary in the +same manner. There can also be little doubt that the tendency to vary in the +same manner has often been so strong that all the individuals of the same +species have been similarly modified without the aid of any form of selection. +Or only a third, fifth, or tenth part of the individuals may have been thus +affected, of which fact several instances could be given. Thus Graba estimates +that about one-fifth of the guillemots in the Faroe Islands consist of a +variety so well marked, that it was formerly ranked as a distinct species under +the name of Uria lacrymans. In cases of this kind, if the variation were of a +beneficial nature, the original form would soon be supplanted by the modified +form, through the survival of the fittest. +</p> + +<p> +To the effects of intercrossing in eliminating variations of all kinds, I shall +have to recur; but it may be here remarked that most animals and plants keep to +their proper homes, and do not needlessly wander about; we see this even with +migratory birds, which almost always return to the same spot. Consequently each +newly-formed variety would generally be at first local, as seems to be the +common rule with varieties in a state of nature; so that similarly modified +individuals would soon exist in a small body together, and would often breed +together. If the new variety were successful in its battle for life, it would +slowly spread from a central +<a name="Page73"></a> +district, competing with and conquering the unchanged individuals on the +margins of an ever-increasing circle. +</p> + +<p> +It may be worth while to give another and more complex illustration of the +action of natural selection. Certain plants excrete sweet juice, apparently for +the sake of eliminating something injurious from the sap: this is effected, for +instance, by glands at the base of the stipules in some Leguminosæ, and at the +backs of the leaves of the common laurel. This juice, though small in quantity, +is greedily sought by insects; but their visits do not in any way benefit the +plant. Now, let us suppose that the juice or nectar was excreted from the +inside of the flowers of a certain number of plants of any species. Insects in +seeking the nectar would get dusted with pollen, and would often transport it +from one flower to another. The flowers of two distinct individuals of the same +species would thus get crossed; and the act of crossing, as can be fully +proved, gives rise to vigorous seedlings, which consequently would have the +best chance of flourishing and surviving. The plants which produced flowers +with the largest glands or nectaries, excreting most nectar, would oftenest be +visited by insects, and would oftenest be crossed; and so in the long-run would +gain the upper hand and form a local variety. The flowers, also, which had +their stamens and pistils placed, in relation to the size and habits of the +particular insect which visited them, so as to favour in any degree the +transportal of the pollen, would likewise be favoured. 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 purpose of fertilisation, its +destruction appears to be 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 to be thus robbed; and the individuals which produced more +and more pollen, and had larger anthers, would be selected. +</p> + +<p> +When our plant, by the above process long continued, had been rendered highly +attractive to insects, they would, unintentionally on their part, regularly +carry pollen from flower to flower; and that they do this effectually I could +easily show by many striking facts. I will give only one, as likewise +illustrating one step in the separation of the sexes of plants. Some +holly-trees bear only male flowers, which have four stamens producing a rather +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. +<a name="Page74"></a> +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 a few pollen-grains, and on some a +profusion. 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, which had +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 might be effected. It would take up too much +space to show the various steps, through dimorphism and other means, by which +the separation of the sexes in plants of various kinds is apparently now in +progress; but I may add that some of the species of holly in North America are, +according to Asa Gray, in an exactly intermediate condition, or, as he +expresses it, are more or less dioeciously polygamous. +</p> + +<p> +Let us now turn to the nectar-feeding insects; 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 with a very little more trouble they can enter by the +mouth. Bearing such facts in mind, it may be believed that under certain +circumstances individual differences in the curvature or length of the +proboscis, &c., too slight to be appreciated by us, might profit a bee or +other insect, so that certain individuals would be able to obtain their food +more quickly +<a name="Page75"></a> +than others; and thus the communities to which they belonged would flourish and +throw off many swarms inheriting the same peculiarities. The tubes of the +corolla of the common red or 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. That this nectar is much liked by the hive-bee is certain; for I +have repeatedly seen, but only in the autumn, many hive-bees sucking the +flowers through holes bitten in the base of the tube by humble bees. The +difference in the length of the corolla in the two kinds of clover, which +determines the visits of the hive-bee, must be very trifling; for I have been +assured that when red clover has been mown, the flowers of the second crop are +somewhat smaller, and that these are visited by many hive-bees. I do not know +whether this statement is accurate; nor whether another published statement can +be trusted, namely, that the Ligurian bee, which is generally considered a mere +variety of the common hive-bee, and which freely crosses with it, is able to +reach and suck the nectar of the red clover. Thus, in a country where this kind +of clover abounded, it might be a great advantage to the hive-bee to have a +slightly longer or differently constructed proboscis. On the other hand, as the +fertility of this clover absolutely depends on bees visiting the flowers, if +humble-bees were to become rare in any country, it might be a great advantage +to the plant to have a shorter or more deeply divided corolla, so that the +hive-bees should be enabled to suck 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 to each other in the most perfect manner, by the +continued preservation of all the individuals which presented slight deviations +of structure mutually favourable to each other. +</p> + +<p> +I am well aware that this doctrine of natural selection, exemplified in the +above imaginary instances, is open to the same objections which were first +urged against Sir Charles Lyell’s noble views on “the modern +changes of the earth, as illustrative of geology;” but we now seldom hear +the agencies which we see still at work, spoken of as trifling and +insignificant, when used in explaining the excavation of the deepest valleys or +the formation of long lines of inland cliffs. Natural selection acts only by +the preservation and accumulation of small inherited modifications, each +profitable to the preserved being; and as modern geology has almost banished +such +<a name="Page76"></a> +views as the excavation of a great valley by a single diluvial wave, so will +natural selection banish the belief of the continued creation of new organic +beings, or of any great and sudden modification in their structure. +</p> + +<p class="center"> +<i>On the Intercrossing of Individuals.</i> +</p> + +<p> +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 +(with the exception of the curious and not well understood cases of +parthenogenesis) unite for each birth; but in the case of hermaphrodites this +is far from obvious. Nevertheless there is reason to believe that with all +hermaphrodites two individuals, either occasionally or habitually, concur for +the reproduction of their kind. This view was long ago doubtfully suggested by +Sprengel, Knight and Kölreuter. 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. +</p> + +<p> +In the first place, I have collected so large a body of facts, and made so many +experiments, 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 <i>close</i> +interbreeding diminishes vigour and fertility; that these facts alone incline +me to believe that it is a general law of nature that no organic being +fertilises itself for a perpetuity of generations; but that a cross with +another individual is occasionally—perhaps at long intervals of +time—indispensable. +</p> + +<p> +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 +<a name="Page77"></a> +what a multitude of flowers have their anthers and stigmas fully exposed to the +weather! If an occasional cross be indispensable, notwithstanding that the +plant’s own anthers and pistil stand so near each other as almost to +ensure self-fertilisation, the fullest freedom for the entrance of pollen from +another individual will explain the above state of exposure of the organs. Many +flowers, on the other hand, have their organs of fructification closely +enclosed, as in the great papilionaceous or pea-family; but these almost +invariably present beautiful and curious adaptations in relation to the visits +of insects. So necessary are the visits of bees to many papilionaceous flowers, +that their fertility is greatly diminished if these visits be prevented. Now, +it is scarcely possible for insects to fly from flower to flower, and not to +carry pollen from one to the other, to the great good of the plant. Insects act +like a camel-hair pencil, and it is sufficient, to ensure fertilisation, just +to touch with the same brush the anthers of one flower and then the stigma of +another; but it must not be supposed that bees would thus produce a multitude +of hybrids between distinct species; for if a plant’s own pollen and that +from another species are placed on the same stigma, the former is so prepotent +that it invariably and completely destroys, as has been shown by Gärtner, the +influence of the foreign pollen. +</p> + +<p> +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 in this very genus, which seems +to have a special contrivance for self-fertilisation, it is well known that, if +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 +numerous other cases, far from self-fertilisation being favoured, there are +special contrivances which effectually prevent the stigma receiving pollen from +its own flower, as I could show from the works of Sprengel and others, as well +as from my own observations: for instance, in Lobelia fulgens, there is a +really beautiful and elaborate contrivance by which all 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 +raise plenty of seedlings. Another species of Lobelia, which is visited by +bees, seeds +<a name="Page78"></a> +freely in my garden. In very many other cases, though there is no special +mechanical contrivance to prevent the stigma receiving pollen from the same +flower, yet, as Sprengel, and more recently Hildebrand and others have 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 so-named dichogamous plants have in fact separated sexes, +and must habitually be crossed. So it is with the reciprocally dimorphic and +trimorphic plants previously alluded to. 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 be +in so many cases 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! +</p> + +<p> +If several varieties of the cabbage, radish, onion, and of some other plants, +be allowed to seed near each other, a large majority of the seedlings thus +raised turn out, as I found, 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; +and the pollen of each flower readily gets on its stigma without insect agency; +for I have found that plants carefully protected from insects produce the full +number of pods. How, then, comes it that such a vast number of the seedlings +are mongrelized? It must arise from the pollen of a distinct <i>variety</i> +having a prepotent effect over the 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 <i>species</i> are +crossed the case is reversed, for a plant’s own pollen is always +prepotent over foreign pollen; but to this subject we shall return in a future +chapter. +</p> + +<p> +In the case of a large 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 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, pollen must be regularly carried from +<a name="Page79"></a> +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 informs me that the rule does not +hold good in Australia: but if most of the Australian trees are dichogamous, +the same result would follow as if they bore flowers with separated sexes. I +have made these few remarks on trees simply to call attention to the subject. +</p> + +<p> +Turning for a brief space to animals: various terrestrial species are +hermaphrodites, such as the land-mollusca and earth-worms; but these all pair. +As yet I have not found a single terrestrial animal which can fertilise itself. +This remarkable fact, which offers so strong a contrast with terrestrial +plants, is intelligible on the view of an occasional cross being indispensable; +for owing to the nature of the fertilising element there are no means, +analogous to the action of insects and of the wind with 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 the currents of water offer an +obvious means for an occasional cross. 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 hermaphrodite animal with the organs of +reproduction so perfectly enclosed 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, under this point of view, a case of +great difficulty; but I have been enabled, by a fortunate chance, to prove that +two individuals, though both are self-fertilising hermaphrodites, do sometimes +cross. +</p> + +<p> +It must have struck most naturalists as a strange anomaly that, both with +animals and plants, some species of the same family and even of the same genus, +though agreeing closely with each other in their whole organisation, are +hermaphrodites, and some unisexual. But if, in fact, all hermaphrodites do +occasionally intercross, the difference between them and unisexual species is, +as far as function is concerned, very small. +</p> + +<p> +From these several considerations and from the many special facts which I have +collected, but which I am unable here to give, it appears that with animals and +plants an occasional intercross between distinct individuals is a very general, +if not universal, law of nature. +</p> + +<p class="center"> +<a name="Page80"></a> +<i>Circumstances favourable for the production of new forms through Natural +Selection.</i> +</p> + +<p> +This is an extremely intricate subject. A great amount of variability, under +which term individual differences are always included, will evidently be +favourable. A large number of individuals, by giving a better chance within any +given period for the appearance of profitable variations, will compensate for a +lesser amount of variability in each individual, and is, I believe, a highly +important element of success. Though nature grants long periods of time for the +work of natural selection, she does not grant an indefinite period; for as all +organic beings are striving 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 be exterminated. Unless favourable variations be +inherited by some at least of the offspring, nothing can be effected by natural +selection. The tendency to reversion may often check or prevent the work; but +as this tendency has not prevented man from forming by selection numerous +domestic races, why should it prevail against natural selection? +</p> + +<p> +In the case of methodical selection, a breeder selects for some definite +object, and if the individuals be allowed freely to intercross, his work will +completely fail. But when many men, without intending to alter the breed, have +a nearly common standard of perfection, and all try to procure and breed from +the best animals, improvement surely but slowly follows from this unconscious +process of selection, notwithstanding that there is no separation of selected +individuals. Thus it will be under nature; for within a confined area, with +some place in the natural polity not perfectly occupied, all the individuals +varying in the right direction, though in different degrees, will tend to be +preserved. But if the area be large, its several districts will almost +certainly present different conditions of life; and then, if the same species +undergoes modification in different districts, the newly formed varieties will +intercross on the confines of each. But we shall see in the sixth chapter that +intermediate varieties, inhabiting intermediate districts, will in the long run +generally be supplanted by one of the adjoining varieties. Intercrossing will +chiefly affect those animals which unite for each birth and wander much, and +which do not breed at a very quick rate. Hence with animals of this nature, for +instance birds, varieties will generally be confined to separated countries; +and this I find to be the case. With hermaphrodite organisms which cross only +occasionally, and likewise with animals which unite for each +<a name="Page81"></a> +birth, but which wander little and can increase at a rapid rate, a new and +improved variety might be quickly formed on any one spot, and might there +maintain itself in a body and afterward spread, so that the individuals of the +new variety would chiefly cross together. On this principle nurserymen always +prefer saving seed from a large body of plants, as the chance of intercrossing +is thus lessened. +</p> + +<p> +Even with animals which unite for each birth, and which do not propagate +rapidly, we must not assume that free intercrossing would always eliminate the +effects of natural selection; for I can bring forward a considerable body of +facts showing that within the same area two varieties of the same animal may +long remain distinct, from haunting different stations, from breeding at +slightly different seasons, or from the individuals of each variety preferring +to pair together. +</p> + +<p> +Intercrossing plays a very important part in nature by 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, as already stated, we have reason to believe that occasional +intercrosses take place with all animals and plants. Even if these take place +only at long intervals of time, 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 crosses, even at rare intervals, will +be great. With respect to organic beings extremely low in the scale, which do +not propagate sexually, nor conjugate, and which cannot possibly intercross, +uniformity of character can be retained by them under the same conditions of +life, only through the principle of inheritance, and through natural selection +which will destroy any individuals departing from the proper type. If the +conditions of life change and the form undergoes modification, uniformity of +character can be given to the modified offspring, solely by natural selection +preserving similar favourable variations. +</p> + +<p> +Isolation also is an important element in the modification of species through +natural selection. In a confined or isolated area, if not very large, the +organic and inorganic conditions of life will generally be almost uniform; so +that natural selection will tend to modify all the varying individuals of the +same species in the same manner. Intercrossing with the inhabitants of the +surrounding districts, will also be thus prevented. Moritz Wagner has lately +published an interesting essay on this subject, and has shown that the service +rendered by isolation in preventing crosses between +<a name="Page82"></a> +newly-formed varieties is probably greater even than I supposed. But from +reasons already assigned I can by no means agree with this naturalist, that +migration and isolation are necessary elements for the formation of new +species. The importance of isolation is likewise great in preventing, after any +physical change in the conditions, such as of climate, elevation of the land, +&c., the immigration of better adapted organisms; and thus new places in +the natural economy of the district will be left open to be filled up by the +modification of the old inhabitants. Lastly, isolation will give time for a new +variety to be improved at a slow rate; and this may sometimes be of much +importance. 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 inhabitants will be small; and this will retard the +production of new species through natural selection, by decreasing the chances +of favourable variations arising. +</p> + +<p> +The mere lapse of time by itself does nothing, either for or against natural +selection. I state this because it has been erroneously asserted that the +element of time has been assumed by me to play an all-important part in +modifying species, as if all the forms of life were necessarily undergoing +change through some innate law. Lapse of time is only so far important, and its +importance in this respect is great, that it gives a better chance of +beneficial variations arising and of their being selected, accumulated, and +fixed. It likewise tends to increase the direct action of the physical +conditions of life, in relation to the constitution of each organism. +</p> + +<p> +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 number of the species +inhabiting it is 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 in the world. +Hence an oceanic island at first sight seems to have been highly favourable for +the production of new species. But we may thus 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. +</p> + +<p> +Although isolation is of great importance in the production of new species, on +the whole I am inclined to believe that largeness of area is still more +important, especially for the production of species which shall prove capable +of enduring for a long period, and of spreading widely. Throughout a great and +open area, not only will there be a +<a name="Page83"></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 +much more 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 +other forms. Moreover, great areas, though now continuous, will often, owing to +former oscillations of level, have 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 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, and will give rise to the greatest number of new varieties +and species. They will thus play a more important part in the changing history +of the organic world. +</p> + +<p> +In accordance with this view, we can, perhaps, understand some facts which will +be again alluded to in our chapter on Geographical Distribution; for instance, +the fact of the productions of the smaller continent of Australia 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, we can +understand how it is that the flora of Madeira, according to Oswald Heer, +resembles to a certain extent 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. Consequently, the competition between fresh water productions +will have been less severe than elsewhere; new forms will have been more slowly +produced, and old forms more slowly exterminated. And it is in fresh water +basins 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 at present widely separated in the +natural scale. These anomalous forms may be called living fossils; they have +endured to the present day, from having inhabited a confined +<a name="Page84"></a> +area, and from having been exposed to less varied, and therefore less severe, +competition. +</p> + +<p> +To sum up, as far as the extreme intricacy of the subject permits, the +circumstances favourable and unfavourable for the production of new species +through natural selection. I conclude that for terrestrial productions a large +continental area, which has undergone many oscillations of level, will have +been the most favourable for the production of many new forms of life, fitted +to endure for a long time and to spread widely. While the area existed as a +continent the inhabitants will have been numerous in individuals and kinds, and +will have been subjected to severe competition. When converted by subsidence +into large separate islands there will still have existed many individuals of +the same species on each island: intercrossing on the confines of the range of +each new species will have been checked: after physical changes of any kind +immigration will have been prevented, so that new places in the polity of each +island will have had to be filled up by the modification of the old +inhabitants; and time will have been allowed for the varieties in each to +become well modified and perfected. When, by renewed elevation, the islands +were reconverted into a continental area, there will again have been very +severe competition; the most favoured or improved varieties will have been +enabled to spread; there will have been much extinction of the less improved +forms, and the relative proportional numbers of the various inhabitants of the +reunited continent will again have been changed; and again there will have been +a fair field for natural selection to improve still further the inhabitants, +and thus to produce new species. +</p> + +<p> +That natural selection generally act with extreme slowness I fully admit. It +can act only when there are places in the natural polity of a district which +can be better occupied by the modification of some of its existing inhabitants. +The occurrence of such places will often depend on physical changes, which +generally take place very slowly, and on the immigration of better adapted +forms being prevented. As some few of the old inhabitants become modified the +mutual relations of others will often be disturbed; and this will create new +places, ready to be filled up by better adapted forms; but all this will take +place very slowly. Although all the individuals of the same species differ in +some slight degree from each other, it would often be long before differences +of the right nature in various parts of the organisation might occur. The +result would often be greatly retarded by free intercrossing. Many will exclaim +that these several causes are amply sufficient to neutralise the power of +natural selection. I do not believe so. But I do believe that +<a name="Page85"></a> +natural selection will generally act very slowly, only at long intervals of +time, and only on a few of the inhabitants of the same region. I further +believe that these slow, intermittent results accord well with what geology +tells us of the rate and manner at which the inhabitants of the world have +changed. +</p> + +<p> +Slow though the process of selection may be, if feeble man can do much by +artificial selection, I can see no limit to the amount of change, to the beauty +and complexity of the coadaptations between all organic beings, one with +another and with their physical conditions of life, which may have been +effected in the long course of time through nature’s power of selection, +that is by the survival of the fittest. +</p> + +<p class="center"> +<i>Extinction caused by Natural Selection.</i> +</p> + +<p> +This subject will be more fully discussed in our chapter on Geology; but it +must here be 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. Owing to the high geometrical rate +of increase of all organic beings, each area is already fully stocked with +inhabitants, and it follows from this, that as the favoured forms increase in +number, so, generally, will the less favoured decrease and become rare. Rarity, +as geology tells us, is the precursor to extinction. We can see that any form +which is represented by few individuals will run a good chance of utter +extinction, during great fluctuations in the nature or the seasons, or from a +temporary increase in the number of its enemies. But we may go further than +this; for as new forms are produced, unless we admit that specific forms can go +on indefinitely increasing in number, many old forms must become extinct. That +the number of specific forms has not indefinitely increased, geology plainly +tells us; and we shall presently attempt to show why it is that the number of +species throughout the world has not become immeasurably great. +</p> + +<p> +We have seen that the species which are most numerous in individuals have the +best chance of producing favourable variations within any given period. We have +evidence of this, in the facts stated in the second chapter, showing that it is +the common and diffused or dominant species which offer the greatest number of +recorded varieties. Hence, rare species will be less quickly modified or +improved within any given period; they will consequently be beaten in the race +for life by the modified and improved descendants of the commoner species. +</p> + +<p> +From these several considerations I think it inevitably follows, +<a name="Page86"></a> +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 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 among 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.” +</p> + +<p class="center"> +<i>Divergence of Character.</i> +</p> + +<p> +The principle, which I have designated by this term, is of high importance, 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 far less from each other 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 a degree of difference as that between +the species of the same genus. +</p> + +<p> +As has always been my practice, I have sought light on this +<a name="Page87"></a> +head from our domestic productions. We shall here find something analogous. It +will be admitted that the production of races so different as short-horn and +Hereford cattle, race and cart horses, the several breeds of pigeons, &c., +could never have been effected by the mere chance accumulation of similar +variations during many successive generations. In practice, a fancier is, for +instance, 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 the +sub-breeds of the tumbler-pigeon) 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 of history, the men of one nation or district required +swifter horses, while those of another required stronger and bulkier horses. +The early differences would be very slight; but, in the course of time, from +the continued selection of swifter horses in the one case, and of stronger ones +in the other, the differences would become greater, and would be noted as +forming two sub-breeds. Ultimately after the lapse of centuries, these +sub-breeds would become converted into two well-established and distinct +breeds. As the differences became greater, the inferior animals with +intermediate characters, being neither very swift nor very strong, would not +have been used for breeding, and will thus 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. +</p> + +<p> +But how, it may be asked, can any analogous principle apply in nature? I +believe it can and does apply most efficiently (though it was a long time +before I saw how), 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. +</p> + +<p> +We can clearly discern 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 power of +increase be allowed to act, it can succeed in increasing (the country not +undergoing any change in conditions) only by its varying descendants seizing on +places at present occupied by other animals: some of them, for +<a name="Page88"></a> +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 animals become, the more places they will 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 effect 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 be raised in the latter than +in the former case. The same has been found to hold good when one variety and +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 the varieties +were continually selected which differed from each other in the same manner, +though in a very slight degree, as do the distinct species and genera of +grasses, a greater number of individual plants of this species, including its +modified descendants, would succeed in living on the same piece of ground. And +we know that each species and each variety of grass is annually sowing almost +countless seeds; and is thus striving, as it may be said, to the utmost to +increase in number. Consequently, in the course of many thousand generations, +the most distinct varieties of any one species of grass would 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. +</p> + +<p> +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 very 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: also in small ponds of fresh water. Farmers find that they +can raise more 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 +<a name="Page89"></a> +piece of ground, could live on it (supposing its nature not to be in any way +peculiar), 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, 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. +</p> + +<p> +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 +would succeed 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 naturalised genera, no less than +100 genera are not there indigenous, and thus a large proportional addition is +made to the genera now living in the United States. +</p> + +<p> +By considering the nature of the plants or animals which have in any country +struggled successfully with the indigenes, and have there become naturalised, +we may gain some crude idea in what manner some of the natives would have had +to be modified in order to gain an advantage over their compatriots; and we may +at least infer that diversification of structure, amounting to new generic +differences, would be profitable to them. +</p> + +<p> +The advantage of diversification of structure 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 name="Page90"></a> +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-developed orders. In the +Australian mammals, we see the process of diversification in an early and +incomplete stage of development. +</p> + +<p class="center"> +<i>The Probable Effects of the Action of Natural Selection through Divergence +of Character and Extinction, on the Descendants of a Common Ancestor.</i> +</p> + +<p> +After the foregoing discussion, which has been much compressed, we may assume +that the modified descendants of any one species will succeed 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 benefit being derived from divergence of character, combined with the +principles of natural selection and of extinction, tends to act. +</p> + +<p> +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 as we +saw in the second chapter, on an average more species vary in large genera than +in 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 most widely-diffused, vary more than do the rare and restricted +species. Let (A) be a common, widely-diffused, and varying species, belonging +to a genus large in its own country. The branching and 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 derived +from divergence of character comes in; for this will generally lead to the most +different or divergent variations +<a name="Page91"></a> +(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 form it into a fairly well-marked variety, +such as would be thought worthy of record in a systematic work. +</p> + +<div class="fig" style="width:100%;"> +<a href="images/diagram.jpg"> +<img src="images/diagram.jpg" width="700" height="411" alt="[Illustration]" /></a> +</div> + +<p> +The intervals between the horizontal lines in the diagram, may represent each a +thousand or more generations. After a thousand generations, species (A) is +supposed to have produced two fairly well-marked varieties, namely +<i>a</i><sup>1</sup> and <i>m</i><sup>1</sup>. These two varieties will +generally still be exposed to the same conditions which made their parents +variable, and the tendency to variability is in itself hereditary; consequently +they will likewise tend to vary, and commonly in nearly the same manner as did +their parents. Moreover, these two varieties, being only slightly modified +forms, will tend to inherit those advantages which made their parent (A) more +numerous than most of the other inhabitants of the same country; they will also +partake of those more general advantages which made the genus to which the +parent-species belonged, a large genus in its own country. And all these +circumstances are favourable to the production of new varieties. +</p> + +<p> +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 a1 is supposed in the diagram to have produced +variety <i>a</i><sup>2</sup>, which will, owing to the principle of divergence, +differ more from (A) than did variety <i>a</i><sup>1</sup>. Variety +<i>m</i><sup>1</sup> is supposed to have produced two varieties, namely +<i>m</i><sup>2</sup> and <i>s</i><sup>2</sup>, 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 of 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. +</p> + +<p> +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, nor that it goes on continuously; it is far more probable that each +form remains for long periods unaltered, and then again undergoes modification. +Nor do I suppose +<a name="Page92"></a> +that the most divergent varieties are invariably preserved: 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 increase. 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 allow the accumulation of a considerable amount of +divergent variation. +</p> + +<p> +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 no doubt the +process of modification will be confined to a single line of descent, and the +number of modified descendants will not be increased; although the amount of +divergent modification may have been augmented. This case would be represented +in the diagram, if all the lines proceeding from (A) were removed, excepting +that from <i>a</i><sup>1</sup> to <i>a</i><sup>10</sup>. In the same way the +English racehorse 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. +</p> + +<p> +After ten thousand generations, species (A) is supposed to have produced three +forms, <i>a</i><sup>10</sup>, <i>f</i><sup>10</sup>, and <i>m</i><sup>10</sup>, +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; 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 +<a name="Page93"></a> +into doubtful or at least 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 <i>a</i><sup>14</sup> and <i>m</i><sup>14</sup>, all descended +from (A). Thus, as I believe, species are multiplied and genera are formed. +</p> + +<p> +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 +(<i>w</i><sup>10</sup> and <i>z</i><sup>10</sup>) 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 +<i>n</i><sup>14</sup> to <i>z</i><sup>14</sup>, are supposed to have been +produced. In any genus, the species which are already very different in +character from each other, will generally tend to produce the greatest number +of modified descendants; for these will have the best chance of seizing on 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 long but unequal periods continue to transmit unaltered +descendants; and this is shown in the diagram by the dotted lines unequally +prolonged upwards. +</p> + +<p> +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 progenitor. 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 states of a the same 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. If, however, the modified offspring of a species get into some +distinct country, or become quickly adapted to some +<a name="Page94"></a> +quite new station, in which offspring and progenitor do not come into +competition, both may continue to exist. +</p> + +<p> +If, then, our diagram be assumed to represent a considerable amount of +modification, species (A) and all the earlier varieties will have become +extinct, being replaced by eight new species (<i>a</i><sup>14</sup> to +<i>m</i><sup>14</sup>); and species (I) will be replaced by six +(<i>n</i><sup>14</sup> to <i>z</i><sup>14</sup>) new species. +</p> + +<p> +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, 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 (E and F) which were least +closely related to the other nine original species, has transmitted descendants +to this late stage of descent. +</p> + +<p> +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 +<i>a</i><sup>14</sup> and <i>z</i><sup>14</sup> will be much greater than that +between the most distinct 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 <i>a</i><sup>14</sup>, +<i>q</i><sup>14</sup>, <i>p</i><sup>14</sup>, will be nearly related from +having recently branched off from <i>a</i><sup>10</sup>; <i>b</i><sup>14</sup> +and <i>f</i><sup>14</sup>, from having diverged at an earlier period from +<i>a</i><sup>5</sup>, will be in some degree distinct from the three +first-named species; and lastly, <i>o</i><sup>14</sup>, <i>e</i><sup>14</sup>, +and <i>m</i><sup>14</sup>, 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 a distinct genus. +</p> + +<p> +The six descendants from (I) will form two sub-genera or genera. But as the +original species (I) differed largely from (A), standing +<a name="Page95"></a> +nearly at the extreme end of the original genus, the six descendants from (I) +will, owing to inheritance alone, 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, except (F), extinct, and have left no descendants. Hence +the six new species descended from (I), and the eight descendants from (A), +will have to be ranked as very distinct genera, or even as distinct +sub-families. +</p> + +<p> +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 be 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 represents a species, the supposed progenitor of our several new +sub-genera and genera. +</p> + +<p> +It is worth while to reflect for a moment on the character of the new species +<small>F</small><sup>14</sup>, 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. Being descended from a form +that stood between the 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 two species. But as these two groups have gone +on diverging in character from the type of their parents, the new species +(<small>F</small><sup>14</sup>) will not be directly intermediate between them, +but rather between types of the two groups; and every naturalist will be able +to call such cases before his mind. +</p> + +<p> +In the diagram each horizontal line has hitherto been supposed to represent a +thousand generations, but each may represent a million or more generations; it +may also represent 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, 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 various remote epochs when the branching lines of descent had diverged less. +</p> + +<p> +<a name="Page96"></a> +I see no reason to limit the process of modification, as now explained, to the +formation of genera alone. If, in the diagram, we suppose the amount of change +represented by each successive group of diverging dotted lines to be great, the +forms marked <i>a</i><sup>14</sup> to <i>p</i><sup>14</sup>, those marked +<i>b</i><sup>14</sup> and <i>f</i><sup>14</sup>, and those marked +<i>o</i><sup>14</sup> to <i>m</i><sup>14</sup>, will form three very distinct +genera. We shall also have two very distinct genera descended from (I), +differing widely from the descendants of (A). These two groups of genera will +thus form two distinct families, or orders, according to the amount of +divergent modification supposed to be represented in the diagram. And the two +new families, or orders, are descended from two species of the original genus; +and these are supposed to be descended from some still more ancient and unknown +form. +</p> + +<p> +We have seen that in each country it is the species belonging to 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 +number, 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 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 +have as yet suffered least extinction, will, for a long period, continue to +increase. But which groups will ultimately prevail, no man can predict; for we +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 as, according to +this view, extremely few of the +<a name="Page97"></a> +more ancient species have transmitted descendants to the present day, and, as +all the descendants of the same species form a class, we can understand how it +is that there exist so few classes in each main division of the animal and +vegetable kingdoms. Although few of the most ancient species have left modified +descendants, yet, at remote geological periods, the earth may have been almost +as well peopled with species of many genera, families, orders and classes, as +at the present day. +</p> + +<p class="center"> +<i>On the Degree to which Organisation tends to advance.</i> +</p> + +<p> +Natural selection acts exclusively by the preservation and accumulation of +variations, which are beneficial under the organic and inorganic conditions to +which each creature is exposed at all periods of life. The ultimate result is +that each creature tends to become more and more improved in relation to its +conditions. This improvement inevitably leads to the gradual advancement of the +organisation of the greater number of living beings throughout the world. But +here we enter on a very intricate subject, for naturalists have not defined to +each other’s satisfaction what is meant by an advance in organisation. +Among the vertebrata the degree of intellect and an approach in structure to +man clearly come into play. It might be thought that the amount of change which +the various parts and organs pass through in their development from embryo to +maturity would suffice as a standard of comparison; but there are cases, as +with certain parasitic crustaceans, in which several parts of the structure +become less perfect, so that the mature animal cannot be called higher than its +larva. Von Baer’s standard seems the most widely applicable and the best, +namely, the amount of differentiation of the parts of the same organic being, +in the adult state, as I should be inclined to add, and their specialisation +for different functions; or, as Milne Edwards would express it, the +completeness of the division of physiological labour. But we shall see how +obscure this subject is if we look, for instance, to fishes, among which some +naturalists rank those as highest which, like the sharks, approach nearest to +amphibians; while other naturalists rank the common bony or teleostean fishes +as the highest, inasmuch as they are most strictly fish-like, and differ most +from the other vertebrate classes. We see still more plainly the obscurity of +the subject by turning to plants, among which the standard of intellect is of +course quite excluded; and here some botanists rank those plants as highest +which have every organ, as sepals, petals, stamens and pistils, fully developed +in each flower; whereas other botanists, +<a name="Page98"></a> +probably with more truth, look at the plants which have their several organs +much modified and reduced in number as the highest. +</p> + +<p> +If we take as the standard of high organisation, the amount of differentiation +and specialisation of the several organs in each being when adult (and this +will include the advancement of the brain for intellectual purposes), natural +selection clearly leads towards this standard: for all physiologists admit that +the specialisation of organs, inasmuch as in this state they perform their +functions better, is an advantage to each being; and hence the accumulation of +variations tending towards specialisation is within the scope of natural +selection. On the other hand, we can see, bearing in mind that all organic +beings are striving to increase at a high ratio and to seize on every +unoccupied or less well occupied place in the economy of nature, that it is +quite possible for natural selection gradually to fit a being to a situation in +which several organs would be superfluous or useless: in such cases there would +be retrogression in the scale of organisation. Whether organisation on the +whole has actually advanced from the remotest geological periods to the present +day will be more conveniently discussed in our chapter on Geological +Succession. +</p> + +<p> +But it may be objected that if all organic beings thus tend to rise in the +scale, how is it that throughout the world a multitude of the lowest forms +still exist; and how is it that in each great class some forms are far more +highly developed than others? Why have not the more highly developed forms +every where supplanted and exterminated the lower? Lamarck, who believed in an +innate and inevitable tendency towards perfection in all organic beings, seems +to have felt this difficulty so strongly that he was led to suppose that new +and simple forms are continually being produced by spontaneous generation. +Science has not as yet proved the truth of this belief, whatever the future may +reveal. On our theory the continued existence of lowly organisms offers no +difficulty; for natural selection, or the survival of the fittest, does not +necessarily include progressive development—it only takes advantage of +such variations as arise and are beneficial to each creature under its complex +relations of life. And it may be asked what advantage, as far as we can see, +would it be to an infusorian animalcule—to an intestinal worm—or +even to an earth-worm, to be highly organised. If it were no advantage, these +forms would be left, by natural selection, unimproved or but little improved, +and might remain for indefinite ages in their present lowly condition. And +geology tells us that some of the lowest forms, as the infusoria and rhizopods, +<a name="Page99"></a> +have remained for an enormous period in nearly their present state. But to +suppose that most of the many now existing low forms have not in the least +advanced since the first dawn of life would be extremely rash; for every +naturalist who has dissected some of the beings now ranked as very low in the +scale, must have been struck with their really wondrous and beautiful +organisation. +</p> + +<p> +Nearly the same remarks are applicable, if we look to the different grades of +organisation within the same great group; for instance, in the vertebrata, to +the co-existence of mammals and fish—among mammalia, to the co-existence +of man and the ornithorhynchus—among fishes, to the co-existence of the +shark and the lancelet (Amphioxus), which latter fish in the extreme simplicity +of its structure approaches the invertebrate classes. But mammals and fish +hardly come into competition with each other; the advancement of the whole +class of mammals, or of certain members in this class, to the highest grade +would not lead to their taking the place of fishes. Physiologists believe that +the brain must be bathed by warm blood to be highly active, and this requires +aërial respiration; so that warm-blooded mammals when inhabiting the water lie +under a disadvantage in having to come continually to the surface to breathe. +With fishes, members of the shark family would not tend to supplant the +lancelet; for the lancelet, as I hear from Fritz Müller, has as sole companion +and competitor on the barren sandy shore of South Brazil, an anomalous annelid. +The three lowest orders of mammals, namely, marsupials, edentata, and rodents, +co-exist in South America in the same region with numerous monkeys, and +probably interfere little with each other. Although organisation, on the whole, +may have advanced and be still advancing throughout the world, yet the scale +will always present many degrees of perfection; for the high advancement of +certain whole classes, or of certain members of each class, does not at all +necessarily lead to the extinction of those groups with which they do not enter +into close competition. In some cases, as we shall hereafter see, lowly +organised forms appear to have been preserved to the present day, from +inhabiting confined or peculiar stations, where they have been subjected to +less severe competition, and where their scanty numbers have retarded the +chance of favourable variations arising. +</p> + +<p> +Finally, I believe that many lowly organised forms now exist throughout the +world, from various causes. In some cases variations or individual differences +of a favourable nature may never have arisen for natural selection to act on +and accumulate. In no case, probably, has time sufficed for the utmost possible +amount of +<a name="Page100"></a> +development. In some few cases there has been what we must call +retrogression or organisation. But the main cause lies in the fact that under +very simple conditions of life a high organisation would be of no +service—possibly would be of actual disservice, as being of a more +delicate nature, and more liable to be put out of order and injured. +</p> + +<p> +Looking to the first dawn of life, when all organic beings, as we may believe, +presented the simplest structure, how, it has been asked, could the first step +in the advancement or differentiation of parts have arisen? Mr. Herbert Spencer +would probably answer that, as soon as simple unicellular organisms came by +growth or division to be compounded of several cells, or became attached to any +supporting surface, his law “that homologous units of any order become +differentiated in proportion as their relations to incident forces become +different” would come into action. But as we have no facts to guide us, +speculation on the subject is almost useless. It is, however, an error to +suppose that there would be no struggle for existence, and, consequently, no +natural selection, until many forms had been produced: variations in a single +species inhabiting an isolated station might be beneficial, and thus the whole +mass of individuals might be modified, or two distinct forms might arise. But, +as I remarked towards the close of the introduction, no one ought to feel +surprise at much remaining as yet unexplained on the origin of species, if we +make due allowance for our profound ignorance on the mutual relations of the +inhabitants of the world at the present time, and still more so during past +ages. +</p> + +<p class="center"> +<i>Convergence of Character.</i> +</p> + +<p> +Mr. H.C. Watson thinks that I have overrated the importance of divergence of +character (in which, however, he apparently believes), and that convergence, as +it may be called, has likewise played a part. If two species belonging to two +distinct though allied genera, had both produced a large number of new and +divergent forms, it is conceivable that these might approach each other so +closely that they would have all to be classed under the same genus; and thus +the descendants of two distinct genera would converge into one. But it would in +most cases be extremely rash to attribute to convergence a close and general +similarity of structure in the modified descendants of widely distinct forms. +The shape of a crystal is determined solely by the molecular forces, and it is +not surprising that dissimilar substances should sometimes assume the same +form; but with organic beings we should bear in mind that the form of each +<a name="Page101"></a> +depends on an infinitude of complex relations, namely on the variations which +have arisen, these being due to causes far too intricate to be followed +out—on the nature of the variations which have been preserved or +selected, and this depends on the surrounding physical conditions, and in a +still higher degree on the surrounding organisms with which each being has come +into competition—and lastly, on inheritance (in itself a fluctuating +element) from innumerable progenitors, all of which have had their forms +determined through equally complex relations. It is incredible that the +descendants of two organisms, which had originally differed in a marked manner, +should ever afterwards converge so closely as to lead to a near approach to +identity throughout their whole organisation. If this had occurred, we should +meet with the same form, independently of genetic connection, recurring in +widely separated geological formations; and the balance of evidence is opposed +to any such an admission. +</p> + +<p> +Mr. Watson has also objected that the continued action of natural selection, +together with divergence of character, would tend to make an indefinite number +of specific forms. As far as mere inorganic conditions are concerned, it seems +probable that a sufficient number of species would soon become adapted to all +considerable diversities of heat, moisture, &c.; but I fully admit that the +mutual relations of organic beings are more important; and as the number of +species in any country goes on increasing, the organic conditions of life must +become more and more complex. Consequently there seems at first no limit to the +amount of profitable diversification of structure, and therefore no limit to +the number of species which might be produced. We do not know that even the +most prolific area is fully stocked with specific forms: at the Cape of Good +Hope and in Australia, which support such an astonishing number of species, +many European plants have become naturalised. But geology shows us, that from +an early part of the tertiary period the number of species of shells, and that +from the middle part of this same period, the number of mammals has not greatly +or at all increased. What then checks an indefinite increase in the number of +species? The amount of life (I do not mean the number of specific forms) +supported on an area must have a limit, depending so largely as it does on +physical conditions; therefore, if an area be inhabited by very many species, +each or nearly each species will be represented by few individuals; and such +species will be liable to extermination from accidental fluctuations in the +nature of the seasons or in the number of their enemies. The process of +extermination in such cases would be rapid, whereas the production of new +<a name="Page102"></a> +species must always be slow. Imagine the extreme case of as many species as +individuals in England, and the first severe winter or very dry summer would +exterminate thousands on thousands of species. Rare species, and each species +will become rare if the number of species in any country becomes indefinitely +increased, will, on the principal often explained, present within a given +period few favourable variations; consequently, the process of giving birth to +new specific forms would thus be retarded. When any species becomes very rare, +close interbreeding will help to exterminate it; authors have thought that this +comes into play in accounting for the deterioration of the aurochs in +Lithuania, of red deer in Scotland and of bears in Norway, &c. Lastly, and +this I am inclined to think is the most important element, a dominant species, +which has already beaten many competitors in its own home, will tend to spread +and supplant many others. Alph. de Candolle has shown that those species which +spread widely tend generally to spread <i>very</i> widely, consequently they +will tend to supplant and exterminate several species in several areas, and +thus check the inordinate increase of specific forms throughout the world. Dr. +Hooker has recently shown that in the southeast corner of Australia, where, +apparently, there are many invaders from different quarters of the globe, the +endemic Australian species have been greatly reduced in number. How much weight +to attribute to these several considerations I will not pretend to say; but +conjointly they must limit in each country the tendency to an indefinite +augmentation of specific forms. +</p> + +<p class="center"> +<i>Summary of Chapter.</i> +</p> + +<p> +If under changing conditions of life organic beings present individual +differences in almost every part of their structure, and this cannot be +disputed; if there be, owing to their geometrical rate of increase, a severe +struggle for life at some age, season or year, 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 life, causing an +infinite diversity in structure, constitution, and habits, to be advantageous +to them, it would be a most extraordinary fact if no variations had ever +occurred useful to each being’s own welfare, in the same manner as so +many variations have occurred useful to man. But if variations useful to any +organic being ever 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, these will tend to produce offspring similarly +characterised. This principle of +<a name="Page103"></a> +preservation, or the survival of the fittest, I have called Natural Selection. +It leads to the improvement of each creature in relation to its organic and +inorganic conditions of life; and consequently, in most cases, to what must be +regarded as an advance in organisation. Nevertheless, low and simple forms will +long endure if well fitted for their simple conditions of life. +</p> + +<p> +Natural selection, on the principle of qualities being inherited at +corresponding ages, can modify the egg, seed, or young as easily as the adult. +Among many animals sexual selection will have given 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 or rivalry with other males; and these +characters will be transmitted to one sex or to both sexes, according to the +form of inheritance which prevails. +</p> + +<p> +Whether natural selection has really thus acted in adapting the various forms +of life to their several conditions and stations, must be judged by the general +tenour and balance of evidence given in the following chapters. But we have +already seen 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 the more organic beings diverge in +structure, habits and constitution, by so much the more can a large number be +supported on the area, of which we see proof by looking to the inhabitants of +any small spot, and to the productions naturalised in foreign lands. 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 +the descendants become, the better will be their chance of success in the +battle for life. Thus the small differences distinguishing varieties of the +same species, steadily tend to increase, till they equal the greater +differences between species of the same genus, or even of distinct genera. +</p> + +<p> +We have seen that it is the common, the widely diffused, and widely ranging +species, belonging to the larger genera within each class, which vary most; and +these 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, the nature of the +affinities, and the generally well defined distinctions between the innumerable +organic beings in each class throughout +<a name="Page104"></a> +the world, 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 groups, +subordinate to groups, in the manner which we everywhere behold—namely, +varieties of the same species most closely related, species of the same genus +less closely and unequally related, 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 clustered round points, and these round other points, and so on in almost +endless cycles. If species had been independently created, no explanation would +have been possible of this kind of classification; but 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. +</p> + +<p> +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 former years 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 at all times overmastered 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 young, 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 the other branches; so with the species +which lived during long-past geological periods, very few have left living and +modified descendants. From the first growth of the tree, many a limb and branch +has decayed and dropped off; and these fallen 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 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 +<a name="Page105"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page106"></a><a name="link2HCH0005" id="link2HCH0005"></a> +CHAPTER V.<br/> +LAWS OF VARIATION.</h2> + +<p class="letter"> +Effects of changed conditions—Use and disuse, combined with natural +selection; organs of flight and of +vision—Acclimatisation—Correlated variation—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. +</p> + +<p> +I have hitherto sometimes spoken as if the variations—so common and +multiform with organic beings under domestication, and in a lesser degree with +those under nature—were 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 +slight deviations of structure, as to make the child like its parents. But the +fact of variations and monstrosities occurring much more frequently under +domestication than under nature, and the greater variability of species having +wide ranges than of those with restricted ranges, lead to the conclusion that +variability is generally related to the conditions of life to which each +species has been exposed during several successive generations. In the first +chapter I attempted to show that changed conditions act in two ways, directly +on the whole organisation or on certain parts alone, and indirectly through the +reproductive system. In all cases there are two factors, the nature of the +organism, which is much the most important of the two, and the nature of the +conditions. The direct action of changed conditions leads to definite or +indefinite results. In the latter case the organisation seems to become +plastic, and we have much fluctuating variability. In the former case the +nature of the organism is such that it yields readily, when subjected to +certain conditions, and all, or nearly all, the individuals become modified in +the same way. +</p> + +<p> +It is very difficult to decide how far changed conditions, such as of climate, +food, &c., have acted in a definite manner. There is +<a name="Page107"></a> +reason to believe that in the course of time the effects have been greater than +can be proved by clear evidence. But we may safely conclude that the +innumerable complex co-adaptations of structure, which we see throughout nature +between various organic beings, cannot be attributed simply to such action. In +the following cases the conditions seem to have produced some slight definite +effect: E. Forbes asserts that shells at their southern limit, and when living +in shallow water, are more brightly coloured than those of the same species +from further north or from a greater depth; but this certainly does not always +hold good. Mr. Gould believes that birds of the same species are more brightly +coloured under a clear atmosphere, than when living near the coast or on +islands; and Wollaston is convinced that residence near the sea affects the +colours of insects. 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. These slightly varying organisms are interesting in as far as +they present characters analogous to those possessed by the species which are +confined to similar conditions. +</p> + +<p> +When a variation is of the slightest use to any being, we cannot tell how much +to attribute to the accumulative action of natural selection, and how much to +the definite action of the conditions of life. Thus, it is well known to +furriers that animals of the same species have thicker and better fur the +further north they live; 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 action of the severe climate? For it +would appear that climate has some direct action on the hair of our domestic +quadrupeds. +</p> + +<p> +Instances could be given of similar varieties being produced from the same +species under external conditions of life as different as can well be +conceived; and, on the other hand, of dissimilar varieties being produced under +apparently the same external conditions. 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 less weight on the direct action of the surrounding conditions, than +on a tendency to vary, due to causes of which we are quite ignorant. +</p> + +<p> +In one sense the conditions of life may be said, not only to cause variability, +either directly or indirectly, but likewise to include natural selection, for +the conditions determine whether this or that variety shall survive. But when +man is the selecting agent, we clearly see that the two elements of change are +distinct; variability +<a name="Page108"></a> +is in some manner excited, but it is the will of man which accumulates the +variations in certain direction; and it is this latter agency which answers to +the survival of the fittest under nature. +</p> + +<p class="center"> +<i>Effects of the increased Use and Disuse of Parts, as controlled by Natural +Selection.</i> +</p> + +<p> +From the facts alluded to in the first chapter, I think there can be no doubt +that use in our domestic animals has strengthened and enlarged certain parts, +and disuse diminished them; and that such modifications are inherited. Under +free nature we 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 possess structures which can be best 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: it is a remarkable fact that the young birds, according to Mr. +Cunningham, can fly, while the adults have lost this power. As the larger +ground-feeding birds seldom take flight except to escape danger, it is probable +that the nearly wingless condition of several birds, now inhabiting or which +lately inhabited several oceanic islands, tenanted by no beasts 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 it can defend itself, by +kicking its enemies, as efficiently as many quadrupeds. We may believe that the +progenitor of the ostrich genus had habits like those of the bustard, and that, +as the size and weight of its body were increased during successive +generations, its legs were used more and its wings less, until they became +incapable of flight. +</p> + +<p> +Kirby has remarked (and I have observed the same fact) that the anterior tarsi, +or feet, of many male dung-feeding beetles are 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. The evidence that accidental mutilations can be +inherited is at present not decisive; but the remarkable cases observed by +Brown-Sequard in guinea-pigs, of the inherited effects of operations, should +make us cautious in denying +<a name="Page109"></a> +this tendency. Hence, it will perhaps be safest to look at the entire absence +of the anterior tarsi in Ateuchus, and their rudimentary condition in some +other genera, not as cases of inherited mutilations, but as due to the effects +of long-continued disuse; for as many dung-feeding beetles are generally found +with their tarsi lost, this must happen early in life; therefore the tarsi +cannot be of much importance or be much used by these insects. +</p> + +<p> +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 (but +more are now known) 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 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 Desertas than in Madeira itself; and +especially the extraordinary fact, so strongly insisted on by Mr. Wollaston, +that certain large groups of beetles, elsewhere excessively numerous, which +absolutely require the use of their wings, are here almost entirely absent. +These several considerations make me believe that the wingless condition of so +many Madeira beetles is mainly due to the action of natural selection, combined +probably with disuse. For during many 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 would oftenest have been blown to +sea, and thus destroyed. +</p> + +<p> +The insects in Madeira which are not ground-feeders, and which, as certain +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 +<a name="Page110"></a> +swimmers if they had not been able to swim at all and had stuck to the wreck. +</p> + +<p> +The eyes of moles and of some burrowing rodents are rudimentary in size, and in +some cases are quite covered 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 necessary to animals having 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 aid the +effects of disuse. +</p> + +<p> +It is well known that several animals, belonging to the most different classes, +which inhabit the caves of Carniola and 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, their loss may be attributed to +disuse. In one of the blind animals, namely, the cave-rat (Neotoma), two of +which were captured by Professor Silliman at above half a mile distance from +the mouth of the cave, and therefore not in the profoundest depths, the eyes +were lustrous and of large size; and these animals, as I am informed by +Professor Silliman, after having been exposed for about a month to a graduated +light, acquired a dim perception of objects. +</p> + +<p> +It is difficult to imagine conditions of life more similar than deep limestone +caverns under a nearly similar climate; so that, in accordance with the old +view of the blind animals having been separately created for the American and +European caverns, very close similarity in their organisation and affinities +might have been expected. This is certainly not the case if we look at the two +whole faunas; with respect to the insects alone, Schiödte has remarked: +“We are accordingly prevented from considering the entire phenomenon in +any other light than something purely local, and the similarity which is +exhibited in a few forms between the Mammoth Cave (in Kentucky) and the caves +in Carniola, otherwise than as a very plain expression of that analogy which +subsists +<a name="Page111"></a> +generally between the fauna of Europe and of North America.” On my view +we must suppose that American animals, having in most cases 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: “We accordingly look upon the subterranean faunas as +small ramifications which have penetrated into the earth from the +geographically limited faunas of the adjacent tracts, and which, as they +extended themselves into darkness, have been accommodated to surrounding +circumstances. 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, and whose +formation is quite peculiar.” These remarks of Schiödte’s it should +be understood, apply not to the same, but to distinct species. 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 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. As a blind species of Bathyscia is found in abundance on +shady rocks far from caves, the loss of vision in the cave species of this one +genus has probably had no relation to its dark habitation; for it is natural +that an insect already deprived of vision should readily become adapted to dark +caverns. Another blind genus (Anophthalmus) offers this remarkable peculiarity, +that the species, as Mr. Murray observes, have not as yet been found anywhere +except in caves; yet those which inhabit the several caves of Europe and +America are distinct; but it is possible that the progenitors of these several +species, while they were furnished with eyes, may formerly +<a name="Page112"></a> +have ranged over both continents, and then have become extinct, excepting in +their present secluded abodes. Far from feeling 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 scanty inhabitants of these dark abodes will have been exposed. +</p> + +<p class="center"> +<i>Acclimatisation.</i> +</p> + +<p> +Habit is hereditary with plants, as in the period of flowering, in the time of +sleep, in the amount of rain requisite for seeds to germinate, &c., and +this leads me to say a few words on acclimatisation. As it is extremely common +for distinct species belonging to the same genus to inhabit hot and cold +countries, if it be true that all the species of the same genus are descended +from a single parent-form, acclimatisation must be readily effected during a +long course of 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 different +countries which are here perfectly healthy. We have reason to believe that +species in a state of nature are closely 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 this adaptation is in +most cases very close, we have evidence with some few plants, of their +becoming, to a certain extent, naturally habituated to different temperatures; +that is, they become acclimatised: thus the pines and rhododendrons, raised +from seed collected by Dr. Hooker from the same species growing at different +heights on the Himalayas, were found to possess in this country different +constitutional powers of resisting cold. Mr. Thwaites informs me that he has +observed similar facts in Ceylon; analogous observations have been made by Mr. +H.C. Watson on European species of plants brought from the Azores to England; +and I could give other cases. In regard to animals, several authentic instances +could be adduced of species having largely extended, within historical times, +their range from warmer to colder latitudes, +<a name="Page113"></a> +and conversely; but we do not positively know that these animals were strictly +adapted to their native climate, though in all ordinary cases we assume such to +be the case; nor do we know that they have subsequently become specially +acclimatised to their new homes, so as to be better fitted for them than they +were at first. +</p> + +<p> +As we may infer that our domestic animals were originally chosen by uncivilised +man because they were useful, and because they bred readily under confinement, +and not because they were subsequently found capable of far-extended +transportation, 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 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; for they live under the cold climate of Faroe in the +north and of the Falklands in the south, and on many an island in the torrid +zones. Hence adaptation to any special climate may be looked at as a quality +readily grafted on an innate wide flexibility of constitution, common to most +animals. On this view, the capacity of enduring the most different climates by +man himself and by his domestic animals, and the fact of the extinct elephant +and rhinoceros having formerly endured 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 as examples of a very common flexibility of +constitution, brought, under peculiar circumstances, into action. +</p> + +<p> +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 an 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 transporting animals +from one district to another. And as 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, natural +<a name="Page114"></a> +selection would inevitably tend to preserve those individuals which were born +with constitutions best adapted to any country which they inhabited. In +treatises on many kinds of cultivated plants, certain varieties are said to +withstand certain climates better than others; this is 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 in England by seed, and of which, consequently, new +varieties have not been produced, has even been advanced, as proving that +acclimatisation cannot be effected, for it is now as tender as ever it was! 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 differences in the constitution of seedling +kidney-beans never appear, for an account has been published how much more +hardy some seedlings are than others; and of this fact I have myself observed +striking instances. +</p> + +<p> +On the whole, we may conclude that habit, or use and disuse, have, in some +cases, played a considerable part in the modification of the constitution and +structure; but that the effects have often been largely combined with, and +sometimes overmastered by, the natural selection of innate variations. +</p> + +<p class="center"> +<i>Correlated Variation</i> +</p> + +<p> +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, and no +doubt wholly different classes of facts may be here easily confounded together. +We shall presently see that simple inheritance often gives the false appearance +of correlation. One of the most obvious real cases is, that variations of +structure arising in the young or larvæ naturally tend to affect the structure +of the mature animal. The several parts which are homologous, and which, at an +early embryonic period, are identical in structure, and which are +<a name="Page115"></a> +necessarily exposed to similar conditions, seem eminently liable to vary in a +like 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 by some anatomists 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. +</p> + +<p> +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 in the union of the petals into a +tube. Hard parts seem to affect the form of adjoining soft parts; it is +believed by some authors that with birds the diversity in the shape of the +pelvis causes the remarkable diversity in the shape of the 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 and form of +several of the most important viscera. +</p> + +<p> +The nature of the bond is frequently quite obscure. M. Is. Geoffroy St. Hilaire +has forcibly remarked that certain malconformations frequently, and that others +rarely, coexist without our being able to assign any reason. What can be more +singular than the relation in cats between complete whiteness and blue eyes +with deafness, or between the tortoise-shell colour and the female sex; or in +pigeons, between their feathered feet and skin betwixt the outer toes, or +between the presence of more or less down on the young pigeon when first +hatched, with the future colour of its plumage; or, again, the relation between +the hair and the teeth in the naked Turkish dog, though here no doubt homology +comes into play? With respect to this latter case of correlation, I think it +can hardly be accidental that the two orders of mammals which are most abnormal +in their dermal covering, viz., Cetacea (whales) and Edentata (armadilloes, +scaly ant-eaters, &c.), are likewise on the whole the most abnormal in +their teeth, but there are so many exceptions to this rule, as Mr. Mivart has +remarked, that it has little value. +</p> + +<p> +I know of no case better adapted to show the importance of the laws of +correlation and variation, independently of utility, and therefore of natural +selection, than that of the difference between +<a name="Page116"></a> +the outer and inner flowers in some Compositous and Umbelliferous plants. +Everyone is familiar with the difference between the ray and central florets +of, for instance, the daisy, and this difference is often accompanied with the +partial or complete abortion of the reproductive organs. But in some of these +plants the seeds also differ in shape and sculpture. These differences have +sometimes been attributed to the pressure of the involucra on the florets, or +to their mutual pressure, and the shape of the seeds in the ray-florets of some +Compositæ countenances this idea; but with the Umbelliferæ it is by no means, +as Dr. Hooker informs me, the species with the densest heads which most +frequently differ in their inner and outer flowers. It might have been thought +that the development of the ray-petals, by drawing nourishment from the +reproductive organs causes their abortion; but this can hardly be the sole +case, for in some Compositæ the seeds of the outer and inner florets differ, +without any difference in the corolla. Possibly these several differences may +be connected with the different flow of nutriment towards the central and +external flowers. We know, at least, that with irregular flowers those nearest +to the axis are most subject to peloria, that is to become abnormally +symmetrical. I may add, as an instance of this fact, and as a striking case of +correlation, that in many pelargoniums the two upper petals in the central +flower of the truss often lose their patches of darker colour; and when this +occurs, the adherent nectary is quite aborted, the central flower thus becoming +peloric or regular. When the colour is absent from only one of the two upper +petals, the nectary is not quite aborted but is much shortened. +</p> + +<p> +With respect to the development of the corolla, Sprengel’s idea that the +ray-florets serve to attract insects, whose agency is highly advantageous, or +necessary for the fertilisation of these plants, is highly probable; and if so, +natural selection may have come into play. But with respect to the seeds, it +seems impossible that their differences in shape, which are not always +correlated with any difference in the corolla, can be in any way beneficial; +yet in the Umbelliferæ these differences are of such apparent +importance—the seeds being sometimes orthospermous in the exterior +flowers and cœlospermous in the central flowers—that the elder De +Candolle founded his main divisions in the order on such characters. Hence +modifications of structure, viewed by systematists as of high value, may be +wholly due to the laws of variation and correlation, without being, as far as +we can judge, of the slightest service to the species. +</p> + +<p> +We may often falsely attribute to correlated variation structures which are +common to whole groups of species, and which in truth +<a name="Page117"></a> +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 in some necessary manner +correlated. Some other correlations are apparently due to the manner in which +natural selection can alone act. For instance, Alph. De Candolle has remarked +that winged seeds are never found in fruits which do not open; I should explain +this rule by the impossibility of seeds gradually becoming winged through +natural selection, unless the capsules were open; for in this case alone could +the seeds, which were a little better adapted to be wafted by the wind, gain an +advantage over others less well fitted for wide dispersal. +</p> + +<p class="center"> +<i>Compensation and Economy of Growth.</i> +</p> + +<p> +The elder Geoffroy and Goethe propounded, at about the same time, 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 the 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. +</p> + +<p> +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 +<a name="Page118"></a> +less useful, its diminution will be favoured, for it will profit the individual +not to have its nutriment wasted in building up a 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 cirripede 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, 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 +would have a better chance of supporting itself, by less nutriment being +wasted. +</p> + +<p> +Thus, as I believe, natural selection will tend in the long run to reduce any +part of the organisation, as soon as it becomes, through changed habits, +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 an organ without requiring as a +necessary compensation the reduction of some adjoining part. +</p> + +<p class="center"> +<i>Multiple, Rudimentary, and Lowly-organised Structures are Variable.</i> +</p> + +<p> +It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both with +varieties and species, that when any part or organ is repeated many times in +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 as well as some +botanists, have further remarked that multiple parts are extremely liable to +vary in structure. As “vegetative repetition,” to use Professor +Owen’s expression, is a sign of low organisation; the foregoing +statements accord with the common opinion of naturalists, that beings which +stand low in the scale of nature are more variable than those which are higher. +I presume that lowness here 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, +<a name="Page119"></a> +why natural selection should not have preserved or rejected each little +deviation of form so carefully as when the part has to serve for some one +special purpose. 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 purpose +must be of some particular shape. Natural selection, it should never be +forgotten, can act solely through and for the advantage of each being. +</p> + +<p> +Rudimentary parts, as is generally admitted, are apt to be highly variable. We +shall have to recur to this subject; and I will here only add that their +variability seems to result from their uselessness, and consequently from +natural selection having had no power to check deviations in their structure. +</p> + +<p class="center"> +<i>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.</i> +</p> + +<p> +Several years ago I was much struck by a remark to the above effect made by Mr. +Waterhouse. Professor Owen, also, seems to have come to a nearly similar +conclusion. It is hopeless to attempt to convince any one of the truth of the +above 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 allowances 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 one species or in a few species +in comparison with the same part in many closely allied species. Thus, the wing +of the bat is a most abnormal structure in the class of mammals; but the rule +would not apply here, because the whole group of bats possesses wings; it would +apply only if some one species had wings developed in a 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, relates +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 more +rarely to females, as they seldom offer remarkable secondary sexual characters. +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 +<a name="Page120"></a> +hermaphrodite cirripedes; I particularly attended to Mr. Waterhouse’s +remark, whilst investigating this Order, and I am fully convinced that the rule +almost always holds good. I shall, in a future work, give a list of all the +more remarkable cases. I will here give only 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 distinct 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 the +same species is so great that it is no exaggeration to state that the varieties +of the same species differ more from each other in the characters derived from +these important organs, than do the species belonging to other distinct genera. +</p> + +<p> +As with birds the individuals of the same species, inhabiting the same country, +vary extremely little, I have particularly attended to them; and the rule +certainly seems to hold good in this class. I cannot make out that it applies +to plants, and this would have seriously shaken my belief in its truth, had not +the great variability in plants made it particularly difficult to compare their +relative degrees of variability. +</p> + +<p> +When we see any part or organ developed in a remarkable degree or manner in a +species, the fair presumption is that it is of high importance to that species: +nevertheless it is in this case 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 are descended from some other species, and have been modified +through natural selection, I think we can obtain some light. First let me make +some preliminary remarks. If, in our domestic animals, 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 uniform +character: and the breed may be said to be degenerating. 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 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 +particularly concerns us is, that those points in our domestic animals, which +at the present time are undergoing rapid change by continued selection, are +also eminently liable to variation. Look at +<a name="Page121"></a> +the individuals of the same breed of the pigeon; and see what a prodigious +amount of difference there is in the beak of tumblers, in the beak and wattle +of carriers, in the carriage and tail of fantails, &c., these being the +points now mainly attended to by English fanciers. Even in the same sub-breed, +as in that of the short-faced tumbler, it is notoriously difficult to breed +nearly perfect birds, many departing widely from the standard. There may truly +be said to be a constant struggle going on between, on the one hand, the +tendency to reversion to a less perfect state, as well as an innate tendency to +new variations, 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 completely as to breed a bird as coarse as a common tumbler pigeon +from a good short-faced strain. But as long as selection is rapidly going on, +much variability in the parts undergoing modification may always be expected. +</p> + +<p> +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 several species branched off from the +common progenitor of the genus. This period will seldom be remote in any +extreme degree, as species 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, still +expect 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 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 our theory, for an immense period in nearly +the same state; and thus it has come not to be 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 +<i>generative variability</i>, as it may be called, still present in a high +degree. For +<a name="Page122"></a> +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. +</p> + +<p class="center"> +<i>Specific Characters more Variable than Generic Characters.</i> +</p> + +<p> +The principle discussed under the last heading may be applied to our present +subject. It is notorious that specific characters are more variable than +generic. To explain by a simple example what is meant: if in a large genus of +plants some species 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 the explanation which +most naturalists would advance is not here applicable, 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 point in the chapter on Classification. It +would be almost superfluous to adduce evidence in support of the statement, +that ordinary specific characters are more variable than generic; but with +respect to important characters, I have repeatedly noticed in works on natural +history, that when an author remarks with surprise that some important organ or +part, which is generally very constant throughout a large group of species, +<i>differs</i> considerably in closely-allied species, it is often +<i>variable</i> in the individuals of the same 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 apparently +entertains no doubt, that the more an organ normally differs in the different +species of the same group, the more subject it is to anomalies in the +individuals. +</p> + +<p> +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 that species are only strongly marked +and fixed varieties, we might expect often to find them still continuing +<a name="Page123"></a> +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 allied genera, are called generic +characters; and these characters may be attributed to inheritance from a common +progenitor, for it can rarely have happened that natural selection will have +modified several distinct 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 before the period when the several 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 +since the period when the species branched off 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. +</p> + +<p> +<i>Secondary Sexual Characters Variable.</i>—I think it will be admitted +by naturalists, without my entering on details, that secondary sexual +characters are highly variable. It will also 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 the females. The cause of the original variability of these +characters is not manifest; but we can see why they should not have been +rendered as constant and uniform as others, for they are 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 have succeeded in giving to the species of the +same group a greater amount of difference in these than in other respects. +</p> + +<p> +It is a remarkable fact, that the secondary differences between the two sexes +of the same species are generally displayed in the very same parts of the +organisation in which the species of the same genus differ from each other. Of +this fact I will give in +<a name="Page124"></a> +illustration the first two instances 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 +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 the fossorial hymenoptera, the 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. Sir J. Lubbock has recently +remarked, that several minute crustaceans offer excellent illustrations of this +law. “In Pontella, for instance, the sexual characters are afforded +mainly by the anterior antennæ and by the fifth pair of legs: the specific +differences also are principally given by these organs.” This relation +has a clear meaning on my view: 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 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 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 to struggle with other males for the possession of the females. +</p> + +<p class="p2"> +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 are possessed by all the species; 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 slight 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 their great difference in 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 being the +descendants of 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 +<a name="Page125"></a> +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 having been adapted for secondary +sexual, and for ordinary purposes. +</p> + +<p> +<i>Distinct Species present analogous Variations, so that a Variety of one +Species often assumes a Character Proper to an allied Species, or reverts to +some of the Characters of an early Progenitor.</i>—These propositions +will be most readily understood by looking to our domestic races. The most +distinct breeds of the pigeon, in countries widely apart, present sub-varieties +with reversed feathers on the head, and with 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 as commonly called roots, 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. Many similar cases of analogous +variation have been observed by Naudin in the great gourd family, and by +various authors in our cereals. Similar cases occurring with insects under +natural conditions have lately been discussed with much ability by Mr. Walsh, +who has grouped them under his law of equable variability. +</p> + +<p> +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, white +loins, 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 +<a name="Page126"></a> +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. +</p> + +<p> +No doubt it is a very surprising fact that characters should reappear after +having been lost for many, probably for hundreds of generations. But when a +breed has been crossed only once by some other breed, the offspring +occasionally show for many generations a tendency to revert in character to the +foreign breed—some say, for a dozen or even a score of generations. After +twelve generations, the proportion of blood, to use a common expression, from +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 remnant of foreign blood. In a +breed which has not been crossed, but in which <i>both</i> parents have lost +some character which their progenitor possessed, the tendency, whether strong +or weak, to reproduce the lost character might, as was formerly remarked, for +all that we can see to the contrary, be 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 one +individual suddenly takes after an ancestor removed by some hundred +generations, but that in each successive generation the character in question +has been lying latent, and at last, under unknown favourable conditions, is +developed. With the barb-pigeon, for instance, which very rarely produces a +blue bird, it is probable that there is a latent tendency in each generation to +produce blue plumage. The abstract improbability of such a tendency being +transmitted through a vast number of generations, is not greater than that of +quite useless or rudimentary organs being similarly transmitted. A mere +tendency to produce a rudiment is indeed sometimes thus inherited. +</p> + +<p> +As all the species of the same genus are supposed to be descended from a common +progenitor, it might be expected that they would occasionally vary in an +analogous manner; so that the varieties of two or more species would resemble +each other, or that a variety of one species would resemble in certain +characters another and distinct species, this other species being, according to +our view, only a well-marked and permanent variety. But characters +<a name="Page127"></a> +exclusively due to analogous variation would probably be of an unimportant +nature, for the preservation of all functionally important characters will have +been determined through natural selection, in accordance with the different +habits of the species. It might further be expected that the species of the +same genus would occasionally exhibit reversions to long-lost characters. As, +however, we do not know the common ancestor of any natural group, we cannot +distinguish between reversionary and analogous characters. If, for instance, we +did not know that the parent rock-pigeon was not feather-footed or +turn-crowned, we could not have told, whether such characters in our domestic +breeds were reversions or only analogous variations; but we might have inferred +that the blue colour was a case of reversion from the number of the markings, +which are correlated with this tint, and which would not probably have all +appeared together from simple variation. More especially we might have inferred +this from the blue colour and the several marks so often appearing when +differently coloured breeds are crossed. Hence, although under nature it must +generally be left doubtful, what cases are reversions to formerly existing +characters, and what are new but analogous variations, yet we ought, on our +theory, sometimes to find the varying offspring of a species assuming +characters which are already present in other members of the same group. And +this undoubtedly is the case. +</p> + +<p> +The difficulty in distinguishing variable species is largely due to the +varieties mocking, as it were, other species of the same genus. A considerable +catalogue, also, could be given of forms intermediate between two other forms, +which themselves can only doubtfully be ranked as species; and this shows, +unless all these closely allied forms be considered as independently created +species, that they have in varying assumed some of the characters of the +others. But the best evidence of analogous variations is afforded by parts or +organs which are generally constant in character, but which occasionally vary +so as to resemble, in some degree, 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 +the great disadvantage of not being able to give them. I can only repeat that +such cases certainly occur, and seem to me very remarkable. +</p> + +<p> +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 almost +certainly of reversion. The ass sometimes has very distinct transverse bars on +its legs, like those on the legs of a zebra. It has been asserted that these +are plainest +<a name="Page128"></a> +in the foal, and from inquiries which I have made, I believe this to be true. +The stripe on the shoulder is sometimes double, and is very variable in length +and outline. A white ass, but <i>not</i> 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. Mr. Blyth has seen a specimen of +the hemionus with a distinct shoulder-stripe, though it properly has none; and +I have been informed by Colonel Poole that 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. +</p> + +<p> +With respect to the horse, I have collected cases in England of the spinal +stripe in horses of the most distinct breeds, and of <i>all</i> 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. I have myself seen a dun Devonshire pony, and a small dun +Welsh pony has been carefully described to me, both with <i>three</i> parallel +stripes on each shoulder. +</p> + +<p> +In the northwest part of India the Kattywar breed of horses is so generally +striped, that, as I hear from Colonel Poole, who examined this 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 often +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. I have myself recently bred a foal from a bay +mare (offspring of a Turkoman horse and a Flemish mare) by a bay English +race-horse. This foal, when a week old, was marked on its hinder quarters and +on its forehead with numerous very narrow, dark, zebra-like bars, and its legs +were feebly striped. All the stripes soon disappeared completely. 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 +<a name="Page129"></a> +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. +</p> + +<p> +I am aware that Colonel Hamilton Smith, who has written on this subject, +believes that the several breeds of the horse are 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 this view may be safely rejected, for it is highly improbable that the +heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the lanky Kattywar +race, &c., inhabiting the most distant parts of the world, should have all +have been crossed with one supposed aboriginal stock. +</p> + +<p> +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; according to Mr. Gosse, in certain +parts of the United States, about nine out of ten mules have striped legs. I +once saw a mule with its legs so much striped that any one might have thought +that it was a hybrid 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 Morton’s famous hybrid, from a chestnut +mare and male quagga, the hybrid and even the pure offspring subsequently +produced from the same 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 only occasionally has stripes on his 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 +Devonshire and Welsh ponies, 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 is commonly called chance, 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 occurred in the +eminently striped Kattywar breed of horses, and was, as we have seen, answered +in the affirmative. +</p> + +<p> +<a name="Page130"></a> +What now are we to say to these several facts? We see several 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 <i>tendency</i> 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. +</p> + +<p> +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 the 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, +<a name="Page131"></a> +but had been created in stone so as to mock the shells now living on the +sea-shore. +</p> + +<p> +<i>Summary.</i>—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 has varied. 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. Changed conditions generally induce mere +fluctuating variability, but sometimes they cause direct and definite effects; +and these may become strongly marked in the course of time, though we have not +sufficient evidence on this head. Habit in producing constitutional +peculiarities, and use in strengthening, and disuse in weakening and +diminishing organs, appear in many cases to have been potent in their effects. +Homologous parts tend to vary in the same manner, 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 will be saved. Changes of structure at an +early age may affect parts subsequently developed; and many cases of correlated +variation, the nature of which we are unable to understand, undoubtedly occur. +Multiple parts are variable in number and in structure, perhaps arising from +such parts not having been closely specialised for any particular function, so +that their modifications have not been closely checked by natural selection. It +follows probably from this same cause, that organic beings low in the scale are +more variable than those standing higher in the scale, and which have their +whole organisation more specialised. Rudimentary organs, from being useless, +are not regulated by natural selection, and hence 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 a 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—in that district and among these +species, +<a name="Page132"></a> +we now find, on an average, most varieties. 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 two 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 an extraordinarily +developed organ has become the parent of many modified descendants—which +on our view must be a very slow process, requiring a long lapse of +time—in this case, natural selection has succeeded in giving a fixed +character to the organ, in however extraordinary a manner it may have been +developed. Species inheriting nearly the same constitution from a common +parent, and exposed to similar influences, naturally tend to present analogous +variations, or 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. +</p> + +<p> +Whatever the cause may be of each slight difference between the offspring and +their parents—and a cause for each must exist—we have reason to +believe that it is the steady accumulation of beneficial differences which has +given rise to all the more important modifications of structure in relation to +the habits of each species. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page133"></a><a name="link2HCH0006" id="link2HCH0006"></a> +CHAPTER VI.<br/> +DIFFICULTIES OF THE THEORY.</h2> + +<p class="letter"> +Difficulties of the theory of descent with modification—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—Modes 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. +</p> + +<p> +Long before the reader has arrived at this part of my work, a crowd of +difficulties will have occurred to him. Some of them are so serious that to +this day I can hardly reflect on them without being in some degree 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 the theory. +</p> + +<p> +These difficulties and objections may be classed under the following heads: +First, why, if species have descended from other species by 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? +</p> + +<p> +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 other +animal with widely different habits and structure? Can we believe that natural +selection could produce, on the one hand, an organ of trifling importance, such +as the tail of a giraffe, which serves as a fly-flapper, and, on the other +hand, an organ so wonderful as the eye? +</p> + +<p> +Thirdly, can instincts be acquired and modified through natural selection? What +shall we say to the instinct which leads the bee to make cells, and which has +practically anticipated the discoveries of profound mathematicians? +</p> + +<p> +Fourthly, how can we account for species, when crossed, being sterile and +producing sterile offspring, whereas, when varieties are crossed, their +fertility is unimpaired? +</p> + +<p> +The two first heads will be here discussed; some miscellaneous +<a name="Page134"></a> +objections in the following chapter; Instinct and Hybridism in the two +succeeding chapters. +</p> + +<p> +<i>On the Absence or Rarity of Transitional Varieties.</i>—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-form and other less-favoured forms +with which it comes into competition. Thus extinction and natural selection go +hand in hand. Hence, if we look at each species as descended from some unknown +form, both the parent and all the transitional varieties will generally have +been exterminated by the very process of the formation and perfection of the +new form. +</p> + +<p> +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 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 crust of the earth is a vast museum; but the +natural collections have been imperfectly made, and only at long intervals of +time. +</p> + +<p> +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 are 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-form +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 +<a name="Page135"></a> +quite confounded me. But I think it can be in large part explained. +</p> + +<p> +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 most continents have 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. +</p> + +<p> +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 E. 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 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 lives, or by which it +is destroyed, or with which it comes into competition; and as these species are +already defined objects, 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 +<a name="Page136"></a> +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 nature of the seasons, be +extremely liable to utter extermination; and thus its geographical range will +come to be still more sharply defined. +</p> + +<p> +As allied or representative species, when inhabiting a continuous area, are +generally distributed in such a manner 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 take a +varying species inhabiting 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 conclude that varieties linking two other +varieties together generally have existed in lesser numbers than the forms +which they connect, then 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. +</p> + +<p> +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 it is a far +more important consideration, that during the process of further modification, +by which two varieties are supposed 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 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 +<a name="Page137"></a> +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 the 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. +</p> + +<p> +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: first, because new varieties are very slowly formed, for +variation is a slow process, and natural selection can do nothing until +favourable individual differences or variations 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 to see only a few species presenting slight modifications of structure in +some degree permanent; and this assuredly we do see. +</p> + +<p> +Secondly, areas now continuous must often have existed within the recent period +as isolated portions, in which many forms, more especially among 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 within each isolated portion of the +land, but these links during the process of natural selection will have been +supplanted and exterminated, so that they will no longer be found in a living +state. +</p> + +<p> +Thirdly, when two or more varieties have been formed in different +<a name="Page138"></a> +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 +varieties, and thus be further improved through natural selection and gain +further advantages. +</p> + +<p> +Lastly, looking not to any one time, but at all time, if my theory be true, +numberless intermediate varieties, linking closely together all the species of +the same group, 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 among fossil remains, which are preserved, as we +shall attempt to show in a future chapter, in an extremely imperfect and +intermittent record. +</p> + +<p> +<i>On the Origin and Transition of Organic Beings with peculiar Habits and +Structure.</i>—It has been asked by the opponents of such views as I +hold, how, for instance, could a land carnivorous animal 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 there now exist carnivorous +animals presenting close intermediate grades from strictly terrestrial to +aquatic habits; and as each exists by a struggle for life, it is clear that +each must be well adapted 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 to +answer. Yet I think such difficulties have little weight. +</p> + +<p> +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 +<a name="Page139"></a> +allied species; 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. +</p> + +<p> +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, to lessen 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 possible 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 become 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. +</p> + +<p> +Now look at the Galeopithecus or so-called flying lemur, which was formerly +ranked among bats, but is now believed to belong to the Insectivora. An +extremely wide flank-membrane stretches from the corners of the jaw to the +tail, and includes the limbs with the elongated fingers. This flank-membrane is +furnished with an extensor muscle. Although no graduated links of structure, +fitted for gliding through the air, now connect the Galeopithecus with the +other Insectivora, yet there is no difficulty in supposing that such links +formerly existed, and that each was developed in the same manner as with the +less perfectly gliding squirrels; each grade of structure having 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 have been greatly lengthened by natural selection; and this, as far as +the +<a name="Page140"></a> +organs of flight are concerned, would have converted the animal into a bat. In +certain bats in which the wing-membrane extends from the top of the shoulder to +the tail and includes the hind-legs, we perhaps see traces of an apparatus +originally fitted for gliding through the air rather than for flight. +</p> + +<p> +If about a dozen genera of birds were to become extinct, who would have +ventured to surmise 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 as 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 be the result of disuse, indicate the steps by which birds actually +acquired their perfect power of flight; but they serve to show what diversified +means of transition are at least possible. +</p> + +<p> +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, so far as we know, to escape being devoured by other fish? +</p> + +<p> +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 have survived to the +present day, for they will have been supplanted by their successors, which were +gradually rendered more perfect through natural selection. Furthermore, we may +conclude that transitional states 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 +<a name="Page141"></a> +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. +</p> + +<p> +I will now give two or three instances, both of diversified and of changed +habits, in the individuals of the same species. In either case it would be easy +for natural selection to adapt the structure of the animal to its changed +habits, or exclusively to one of its several habits. It is, however, difficult +to decide 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 occurring 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 into +it 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 sometimes, 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, almost like a whale, insects +in the water. +</p> + +<p> +As we sometimes see individuals following habits different from those proper to +their species and to the other species of the same genus, we might expect that +such individuals would occasionally give rise to new species, having anomalous +habits, and with their structure either slightly or considerably modified from +that of their type. And such instances occur in nature. Can a more striking +instance of adaptation be given than that of a woodpecker for climbing trees +and 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. On the plains of La Plata, where hardly a tree +grows, there is a woodpecker (Colaptes campestris) which has two toes before +and two behind, a long-pointed tongue, pointed tail-feathers, sufficiently +stiff to support the bird in a vertical position on a post, but not so +<a name="Page142"></a> +stiff as in the typical wood-peckers, and a straight, strong beak. The beak, +however, is not so straight or so strong as in the typical woodpeckers but it +is strong enough to bore into wood. Hence this Colaptes, in all the essential +parts of its structure, is a woodpecker. Even in such trifling characters as +the colouring, the harsh tone of the voice, and undulatory flight, its close +blood-relationship to our common woodpecker is plainly declared; yet, as I can +assert, not only from my own observations, but from those of the accurate +Azara, in certain large districts it does not climb trees, and it makes its +nest in holes in banks! In certain other districts, however, this same +woodpecker, as Mr. Hudson states, frequents trees, and bores holes in the trunk +for its nest. I may mention as another illustration of the varied habits of +this genus, that a Mexican Colaptes has been described by De Saussure as boring +holes into hard wood in order to lay up a store of acorns. +</p> + +<p> +Petrels are the most aërial and oceanic of birds, but, in the quiet sounds of +Tierra del Fuego, the Puffinuria berardi, in its general habits, in its +astonishing power of diving, in its manner of swimming and of flying when made +to take flight, would be mistaken by any one for an auk or a grebe; +nevertheless, it is essentially a petrel, but with many parts of its +organisation profoundly modified in relation to its new habits of life; whereas +the woodpecker of La Plata has had its structure only slightly modified. In the +case of the water-ouzel, the acutest observer, by examining its dead body, +would never have suspected its sub-aquatic habits; yet this bird, which is +allied to the thrush family, subsists by diving,—using its wings under +water and grasping stones with its feet. All the members of the great order of +Hymenopterous insects are terrestrial, excepting the genus Proctotrupes, which +Sir John Lubbock has discovered to be aquatic in its habits; it often enters +the water and dives about by the use not of its legs but of its wings, and +remains as long as four hours beneath the surface; yet it exhibits no +modification in structure in accordance with its abnormal habits. +</p> + +<p> +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 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 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 ocean. 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, not furnished with membrane, +<a name="Page143"></a> +of the Grallatores, are formed for walking over swamps and floating plants. The +water-hen and landrail are members of this order, yet the first is nearly as +aquatic as the coot, and the second is 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 almost 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. +</p> + +<p> +He who believes in separate and innumerable acts of creation may say, that in +these cases it has pleased the Creator to cause a being of one type to take the +place of one belonging to 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 varies ever so little, either in habits or structure, and thus gains an +advantage over some other inhabitant of the same country, it will seize on the +place of that inhabitant, however different that may be from its own place. +Hence it will cause him no surprise that there should be geese and +frigate-birds with webbed feet, living on the dry land and 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 hardly a tree grows; that +there should be diving thrushes and diving Hymenoptera, and petrels with the +habits of auks. +</p> + +<p class="center"> +<i>Organs of extreme Perfection and Complication.</i> +</p> + +<p> +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 degree. When +it was first said that the sun stood still and the world turned round, the +common sense of mankind declared the doctrine false; but the old saying of +<i>Vox populi, vox Dei</i>, as every philosopher knows, cannot be trusted in +science. Reason tells me, that if numerous gradations from a simple and +imperfect eye to one complex and perfect can be shown to exist, each grade +being useful to its possessor, as is certainly the case; if further, the eye +ever varies and the variations be inherited, as is likewise certainly the case; +and if such variations should be useful to any animal under changing conditions +of life, then the difficulty of believing that a perfect and complex eye could +<a name="Page144"></a> +be formed by natural selection, though insuperable by our imagination, should +not be considered as subversive of the theory. How a nerve comes to be +sensitive to light, hardly concerns us more than how life itself originated; +but I may remark that, as some of the lowest organisms in which nerves cannot +be detected, are capable of perceiving light, it does not seem impossible that +certain sensitive elements in their sarcode should become aggregated and +developed into nerves, endowed with this special sensibility. +</p> + +<p> +In searching for the gradations through which an organ in any species has been +perfected, we ought to look exclusively to its lineal progenitors; but this is +scarcely ever possible, and we are forced to look to other species and genera +of the same group, that is to the collateral descendants from the same +parent-form, in order to see what gradations are possible, and for the chance +of some gradations having been transmitted in an unaltered or little altered +condition. But the state of the same organ in distinct classes may incidentally +throw light on the steps by which it has been perfected. +</p> + +<p> +The simplest organ which can be called an eye consists of an optic nerve, +surrounded by pigment-cells and covered by translucent skin, but without any +lens or other refractive body. We may, however, according to M. Jourdain, +descend even a step lower and find aggregates of pigment-cells, apparently +serving as organs of vision, without any nerves, and resting merely on sarcodic +tissue. Eyes of the above simple nature are not capable of distinct vision, and +serve only to distinguish light from darkness. In certain star-fishes, small +depressions in the layer of pigment which surrounds the nerve are filled, as +described by the author just quoted, with transparent gelatinous matter, +projecting with a convex surface, like the cornea in the higher animals. He +suggests that this serves not to form an image, but only to concentrate the +luminous rays and render their perception more easy. In this concentration of +the rays we gain the first and by far the most important step towards the +formation of a true, picture-forming eye; for we have only to place the naked +extremity of the optic nerve, which in some of the lower animals lies deeply +buried in the body, and in some near the surface, at the right distance from +the concentrating apparatus, and an image will be formed on it. +</p> + +<p> +In the great class of the Articulata, we may start from an optic nerve simply +coated with pigment, the latter sometimes forming a sort of pupil, but +destitute of lens or other optical contrivance. With insects it is now known +that the numerous facets on the cornea of their great compound eyes form true +lenses, and that the cones include curiously modified nervous filaments. But +these +<a name="Page145"></a> +organs in the Articulata are so much diversified that Müller formerly made +three main classes with seven subdivisions, besides a fourth main class of +aggregated simple eyes. +</p> + +<p> +When we reflect on these facts, here given much too briefly, with respect to +the wide, diversified, and graduated range of structure in the eyes of the +lower animals; and when we bear in mind how small the number of all living +forms must be in comparison with those which have become extinct, the +difficulty ceases to be very great in believing that natural selection may have +converted the simple apparatus of an optic nerve, coated with pigment and +invested by transparent membrane, into an optical instrument as perfect as is +possessed by any member of the Articulata class. +</p> + +<p> +He who will go thus far, ought not to hesitate to go one step further, if he +finds on finishing this volume that large bodies of facts, otherwise +inexplicable, can be explained by the theory of modification through natural +selection; he ought to admit that a structure even as perfect as an +eagle’s eye might thus be formed, although in this case he does not know +the transitional states. It has been objected that in order to modify the eye +and still preserve it as a perfect instrument, many changes would have to be +effected simultaneously, which, it is assumed, could not be done through +natural selection; but as I have attempted to show in my work on the variation +of domestic animals, it is not necessary to suppose that the modifications were +all simultaneous, if they were extremely slight and gradual. Different kinds of +modification would, also, serve for the same general purpose: as Mr. Wallace +has remarked, “If a lens has too short or too long a focus, it may be +amended either by an alteration of curvature, or an alteration of density; if +the curvature be irregular, and the rays do not converge to a point, then any +increased regularity of curvature will be an improvement. So the contraction of +the iris and the muscular movements of the eye are neither of them essential to +vision, but only improvements which might have been added and perfected at any +stage of the construction of the instrument.” Within the highest division +of the animal kingdom, namely, the Vertebrata, we can start from an eye so +simple, that it consists, as in the lancelet, of a little sack of transparent +skin, furnished with a nerve and lined with pigment, but destitute of any other +apparatus. In fishes and reptiles, as Owen has remarked, “The range of +gradation of dioptric structures is very great.” It is a significant fact +that even in man, according to the high authority of Virchow, the beautiful +crystalline lens is formed in the embryo by an accumulation of epidermic cells, +lying in a sack-like fold of the skin; and the vitreous body is formed +<a name="Page146"></a> +from embryonic subcutaneous tissue. To arrive, however, at a just conclusion +regarding the formation of the eye, with all its marvellous yet not absolutely +perfect characters, it is indispensable that the reason should conquer the +imagination; but I have felt the difficulty far to keenly to be surprised at +others hesitating to extend the principle of natural selection to so startling +a length. +</p> + +<p> +It is scarcely possible to avoid comparing the eye with 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 spaces filled +with fluid, and 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, represented by +natural selection or the survival of the fittest, always intently watching each +slight alteration in the transparent layers; and carefully preserving each +which, under varied circumstances, in any way or degree, tends to produce a +distincter image. We must suppose each new state of the instrument to be +multiplied by the million; each to be preserved until a better is produced, and +then the old ones to be all destroyed. In living bodies, variation will cause +the slight alteration, generation will multiply them almost infinitely, and +natural selection will pick out with unerring skill each improvement. Let this +process go on for 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? +</p> + +<p class="center"> +<i>Modes of Transition.</i> +</p> + +<p> +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, around which, according to the +theory, there has been much extinction. Or again, if we take +<a name="Page147"></a> +an organ common to all the members of a class, for in this latter case the +organ must have been originally formed at a 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. +</p> + +<p> +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 +among the lower animals of the same organ performing at the same time wholly +distinct functions; thus in the larva of the dragon-fly and in the fish Cobites +the alimentary canal respires, digests, and excretes. 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 specialise, if any +advantage were thus gained, the whole or part of an organ, which had previously +performed two functions, for one function alone, and thus by insensible steps +greatly change its nature. Many plants are known which regularly produce at the +same time differently constructed flowers; and if such plants were to produce +one kind alone, a great change would be effected with comparative suddenness in +the character of the species. It is, however, probable that the two sorts of +flowers borne by the same plant were originally differentiated by finely +graduated steps, which may still be followed in some few cases. +</p> + +<p> +Again, two distinct organs, or the same organ under two very different forms, +may simultaneously perform in the same individual the same function, and this +is an extremely important means of transition: 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 swim-bladders, this latter +organ being divided by highly vascular partitions and having a ductus +pneumaticus for the supply of air. To give another instance from the vegetable +kingdom: plants climb by three distinct means, by spirally twining, by clasping +a support with their sensitive tendrils, and by the emission of aërial +rootlets; these three means are usually found in distinct groups, but some few +species exhibit two of the means, or even all three, combined in the same +individual. In all such cases one of the two organs might readily be modified +and perfected so as to perform all the work, being aided during the progress of +modification by the other organ; and then this other organ might be modified +for some other and quite distinct purpose, or be wholly obliterated. +</p> + +<p> +The illustration of the swim-bladder in fishes is a good one, +<a name="Page148"></a> +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 +widely different purpose, namely respiration. The swim-bladder has, also, been +worked in as an accessory to the auditory organs of certain fishes. All +physiologists admit that the swim-bladder is homologous, or “ideally +similar” in position and structure with the lungs of the higher +vertebrate animals: hence there is no reason to doubt that the swim-bladder has +actually been converted into lungs, or an organ used exclusively for +respiration. +</p> + +<p> +According to this view it may be inferred that all vertebrate animals with true +lungs are descended by ordinary generation from an ancient and unknown +prototype which was furnished with a floating apparatus or swim-bladder. 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—but in the embryo the slits on the sides of the neck and the +loop-like course of the arteries still mark their former position. But it is +conceivable that the now utterly lost branchiæ might have been gradually +worked in by natural selection for some distinct purpose: for instance, Landois +has shown that the wings of insects are developed from the trachea; it is +therefore highly probable that in this great class organs which once served for +respiration have been actually converted into organs for flight. +</p> + +<p> +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 +another 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 of the sack, +together with 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, within the well-enclosed shell; but they have, +in the same relative position with the frena, large, much-folded membranes, +which freely communicate with the circulatory lacunæ of the sack and body, and +which have been considered by all naturalists to act as 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, +<a name="Page149"></a> +they graduate into each other. Therefore it need not be doubted that the two +little folds of skin, which originally served as ovigerous frena, but which, +likewise, very slightly aided in 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 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? +</p> + +<p> +There is another possible mode of transition, namely, through the acceleration +or retardation of the period of reproduction. This has lately been insisted on +by Professor Cope and others in the United States. It is now known that some +animals are capable of reproduction at a very early age, before they have +acquired their perfect characters; and if this power became thoroughly well +developed in a species, it seems probable that the adult stage of development +would sooner or later be lost; and in this case, especially if the larva +differed much from the mature form, the character of the species would be +greatly changed and degraded. Again, not a few animals, after arriving at +maturity, go on changing in character during nearly their whole lives. With +mammals, for instance, the form of the skull is often much altered with age, of +which Dr. Murie has given some striking instances with seals. Every one knows +how the horns of stags become more and more branched, and the plumes of some +birds become more finely developed, as they grow older. Professor Cope states +that the teeth of certain lizards change much in shape with advancing years. +With crustaceans not only many trivial, but some important parts assume a new +character, as recorded by Fritz Müller, after maturity. In all such +cases—and many could be given—if the age for reproduction were +retarded, the character of the species, at least in its adult state, would be +modified; nor is it improbable that the previous and earlier stages of +development would in some cases be hurried through and finally lost. Whether +species have often or ever been modified through this comparatively sudden mode +of transition, I can form no opinion; but if this has occurred, it is probable +that the differences between the young and the mature, and between the mature +and the old, were primordially acquired by graduated steps. +</p> + +<p class="center"> +<a name="Page150"></a> +<i>Special Diffculties of the Theory of Natural Selection.</i> +</p> + +<p> +Although we must be extremely cautious in concluding that any organ could not +have been produced by successive, small, transitional gradations, yet +undoubtedly serious cases of difficulty occur. +</p> + +<p> +One of the most serious is that of neuter insects, which are often 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; for it is impossible to conceive by what steps +these wondrous organs have been produced. But this is not surprising, for we do +not even know of what use they are. In the gymnotus and torpedo they no doubt +serve as powerful means of defence, and perhaps for securing prey; yet in the +ray, as observed by Matteucci, an analogous organ in the tail manifests but +little electricity, even when the animal is greatly irritated; so little that +it can hardly be of any use for the above purposes. Moreover, in the ray, +besides the organ just referred to, there is, as Dr. R. McDonnell has shown, +another organ near the head, not known to be electrical, but which appears to +be the real homologue of the electric battery in the torpedo. It is generally +admitted that there exists between these organs and ordinary muscle a close +analogy, in intimate structure, in the distribution of the nerves, and in the +manner in which they are acted on by various reagents. It should, also, be +especially observed that muscular contraction is accompanied by an electrical +discharge; and, as Dr. Radcliffe insists, “in the electrical apparatus of +the torpedo during rest, there would seem to be a charge in every respect like +that which is met with in muscle and nerve during the rest, and the discharge +of the torpedo, instead of being peculiar, may be only another form of the +discharge which attends upon the action of muscle and motor nerve.” +Beyond this we cannot at present go in the way of explanation; but as we know +so little about the uses of these organs, and as we know nothing about the +habits and structure of the progenitors of the existing electric fishes, it +would be extremely bold to maintain that no serviceable transitions are +possible by which these organs might have been gradually developed. +</p> + +<p> +These organs appear at first to offer another and far more serious difficulty; +for they occur in about a dozen kinds of fish, of which several are widely +remote in their affinities. When the same organ is found in several members of +the same class, especially if in members having very different habits of life, +we may generally attribute its presence to inheritance from a common ancestor; +and +<a name="Page151"></a> +its absence in some of the members to loss through disuse or natural selection. +So that, if the electric organs had been inherited from some one ancient +progenitor, we might have expected that all electric fishes would have been +specially related to each other; but this is far from the case. Nor does +geology at all lead to the belief that most fishes formerly possessed electric +organs, which their modified descendants have now lost. But when we look at the +subject more closely, we find in the several fishes provided with electric +organs, that these are situated in different parts of the body, that they +differ in construction, as in the arrangement of the plates, and, according to +Pacini, in the process or means by which the electricity is excited—and +lastly, in being supplied with nerves proceeding from different sources, and +this is perhaps the most important of all the differences. Hence in the several +fishes furnished with electric organs, these cannot be considered as +homologous, but only as analogous in function. Consequently there is no reason +to suppose that they have been inherited from a common progenitor; for had this +been the case they would have closely resembled each other in all respects. +Thus the difficulty of an organ, apparently the same, arising in several +remotely allied species, disappears, leaving only the lesser yet still great +difficulty: namely, by what graduated steps these organs have been developed in +each separate group of fishes. +</p> + +<p> +The luminous organs which occur in a few insects, belonging to widely different +families, and which are situated in different parts of the body, offer, under +our present state of ignorance, a difficulty almost exactly parallel with that +of the electric organs. Other similar cases could be given; for instance in +plants, the very curious contrivance of a mass of pollen-grains, borne on a +foot-stalk with an adhesive gland, is apparently the same in Orchis and +Asclepias, genera almost as remote as is possible among flowering plants; but +here again the parts are not homologous. In all cases of beings, far removed +from each other in the scale of organisation, which are furnished with similar +and peculiar organs, it will be found that although the general appearance and +function of the organs may be the same, yet fundamental differences between +them can always be detected. For instance, the eyes of Cephalopods or +cuttle-fish and of vertebrate animals appear wonderfully alike; and in such +widely sundered groups no part of this resemblance can be due to inheritance +from a common progenitor. Mr. Mivart has advanced this case as one of special +difficulty, but I am unable to see the force of his argument. An organ for +vision must be formed of transparent tissue, and must include some sort of lens +for +<a name="Page152"></a> +throwing an image at the back of a darkened chamber. Beyond this superficial +resemblance, there is hardly any real similarity between the eyes of +cuttle-fish and vertebrates, as may be seen by consulting Hensen’s +admirable memoir on these organs in the Cephalopoda. It is impossible for me +here to enter on details, but I may specify a few of the points of difference. +The crystalline lens in the higher cuttle-fish consists of two parts, placed +one behind the other like two lenses, both having a very different structure +and disposition to what occurs in the vertebrata. The retina is wholly +different, with an actual inversion of the elemental parts, and with a large +nervous ganglion included within the membranes of the eye. The relations of the +muscles are as different as it is possible to conceive, and so in other points. +Hence it is not a little difficult to decide how far even the same terms ought +to be employed in describing the eyes of the Cephalopoda and Vertebrata. It is, +of course, open to any one to deny that the eye in either case could have been +developed through the natural selection of successive slight variations; but if +this be admitted in the one case it is clearly possible in the other; and +fundamental differences of structure in the visual organs of two groups might +have been anticipated, in accordance with this view of their manner of +formation. As two men have sometimes independently hit on the same invention, +so in the several foregoing cases it appears that natural selection, working +for the good of each being, and taking advantage of all favourable variations, +has produced similar organs, as far as function is concerned, in distinct +organic beings, which owe none of their structure in common to inheritance from +a common progenitor. +</p> + +<p> +Fritz Müller, in order to test the conclusions arrived at in this volume, has +followed out with much care a nearly similar line of argument. Several families +of crustaceans include a few species, possessing an air-breathing apparatus and +fitted to live out of the water. In two of these families, which were more +especially examined by Müller, and which are nearly related to each other, the +species agree most closely in all important characters: namely in their sense +organs, circulating systems, in the position of the tufts of hair within their +complex stomachs, and lastly in the whole structure of the water-breathing +branchiæ, even to the microscopical hooks by which they are cleansed. Hence it +might have been expected that in the few species belonging to both families +which live on the land, the equally important air-breathing apparatus would +have been the same; for why should this one apparatus, given for the same +purpose, have been made to differ, +<a name="Page153"></a> +whilst all the other important organs were closely similar, or rather, +identical. +</p> + +<p> +Fritz Müller argues that this close similarity in so many points of structure +must, in accordance with the views advanced by me, be accounted for by +inheritance from a common progenitor. But as the vast majority of the species +in the above two families, as well as most other crustaceans, are aquatic in +their habits, it is improbable in the highest degree that their common +progenitor should have been adapted for breathing air. Müller was thus led +carefully to examine the apparatus in the air-breathing species; and he found +it to differ in each in several important points, as in the position of the +orifices, in the manner in which they are opened and closed, and in some +accessory details. Now such differences are intelligible, and might even have +been expected, on the supposition that species belonging to distinct families +had slowly become adapted to live more and more out of water, and to breathe +the air. For these species, from belonging to distinct families, would have +differed to a certain extent, and in accordance with the principle that the +nature of each variation depends on two factors, viz., the nature of the +organism and that of the surrounding conditions, their variability assuredly +would not have been exactly the same. Consequently natural selection would have +had different materials or variations to work on, in order to arrive at the +same functional result; and the structures thus acquired would almost +necessarily have differed. On the hypothesis of separate acts of creation the +whole case remains unintelligible. This line of argument seems to have had +great weight in leading Fritz Müller to accept the views maintained by me in +this volume. +</p> + +<p> +Another distinguished zoologist, the late Professor Claparède, has argued in +the same manner, and has arrived at the same result. He shows that there are +parasitic mites (Acaridæ), belonging to distinct sub-families and families, +which are furnished with hair-claspers. These organs must have been +independently developed, as they could not have been inherited from a common +progenitor; and in the several groups they are formed by the modification of +the fore legs, of the hind legs, of the maxillæ or lips, and of appendages on +the under side of the hind part of the body. +</p> + +<p class="p2"> +In the foregoing cases, we see the same end gained and the same function +performed, in beings not at all or only remotely allied, by organs in +appearance, though not in development, closely similar. On the other hand, it +is a common rule throughout nature that the same end should be gained, even +sometimes in the case of closely related beings, by the most diversified means. +How differently +<a name="Page154"></a> +constructed is the feathered wing of a bird and the membrane-covered wing of a +bat; and still more so the four wings of a butterfly, the two wings of a fly, +and the two wings with the elytra of a beetle. Bivalve shells are made to open +and shut, but on what a number of patterns is the hinge constructed, from the +long row of neatly interlocking teeth in a Nucula to the simple ligament of a +Mussel! Seeds are disseminated by their minuteness, by their capsule being +converted into a light balloon-like envelope, by being embedded in pulp or +flesh, formed of the most diverse parts, and rendered nutritious, as well as +conspicuously coloured, so as to attract and be devoured by birds, by having +hooks and grapnels of many kinds and serrated awns, so as to adhere to the fur +of quadrupeds, and by being furnished with wings and plumes, as different in +shape as they are elegant in structure, so as to be wafted by every breeze. I +will give one other instance: for this subject of the same end being gained by +the most diversified means well deserves attention. Some authors maintain that +organic beings have been formed in many ways for the sake of mere variety, +almost like toys in a shop, but such a view of nature is incredible. With +plants having separated sexes, and with those in which, though hermaphrodites, +the pollen does not spontaneously fall on the stigma, some aid is necessary for +their fertilisation. With several kinds this is effected by the pollen-grains, +which are light and incoherent, being blown by the wind through mere chance on +to the stigma; and this is the simplest plan which can well be conceived. An +almost equally simple, though very different plan occurs in many plants in +which a symmetrical flower secretes a few drops of nectar, and is consequently +visited by insects; and these carry the pollen from the anthers to the stigma. +</p> + +<p> +From this simple stage we may pass through an inexhaustible number of +contrivances, all for the same purpose and effected in essentially the same +manner, but entailing changes in every part of the flower. The nectar may be +stored in variously shaped receptacles, with the stamens and pistils modified +in many ways, sometimes forming trap-like contrivances, and sometimes capable +of neatly adapted movements through irritability or elasticity. From such +structures we may advance till we come to such a case of extraordinary +adaptation as that lately described by Dr. Crüger in the Coryanthes. This +orchid has part of its labellum or lower lip hollowed out into a great bucket, +into which drops of almost pure water continually fall from two secreting horns +which stand above it; and when the bucket is half-full, the water overflows by +a spout on one side. The basal part of the labellum stands over the bucket, and +is itself hollowed out into a sort of chamber with two +<a name="Page155"></a> +lateral entrances; within this chamber there are curious fleshy ridges. The +most ingenious man, if he had not witnessed what takes place, could never have +imagined what purpose all these parts serve. But Dr. Crüger saw crowds of large +humble-bees visiting the gigantic flowers of this orchid, not in order to suck +nectar, but to gnaw off the ridges within the chamber above the bucket; in +doing this they frequently pushed each other into the bucket, and their wings +being thus wetted they could not fly away, but were compelled to crawl out +through the passage formed by the spout or overflow. Dr. Crüger saw a +“continual procession” of bees thus crawling out of their +involuntary bath. The passage is narrow, and is roofed over by the column, so +that a bee, in forcing its way out, first rubs its back against the viscid +stigma and then against the viscid glands of the pollen-masses. The +pollen-masses are thus glued to the back of the bee which first happens to +crawl out through the passage of a lately expanded flower, and are thus carried +away. Dr. Crüger sent me a flower in spirits of wine, with a bee which he had +killed before it had quite crawled out, with a pollen-mass still fastened to +its back. When the bee, thus provided, flies to another flower, or to the same +flower a second time, and is pushed by its comrades into the bucket and then +crawls out by the passage, the pollen-mass necessarily comes first into contact +with the viscid stigma, and adheres to it, and the flower is fertilised. Now at +last we see the full use of every part of the flower, of the water-secreting +horns of the bucket half-full of water, which prevents the bees from flying +away, and forces them to crawl out through the spout, and rub against the +properly placed viscid pollen-masses and the viscid stigma. +</p> + +<p> +The construction of the flower in another closely allied orchid, namely, the +Catasetum, is widely different, though serving the same end; and is equally +curious. Bees visit these flowers, like those of the Coryanthes, in order to +gnaw the labellum; in doing this they inevitably touch a long, tapering, +sensitive projection, or, as I have called it, the antenna. This antenna, when +touched, transmits a sensation or vibration to a certain membrane which is +instantly ruptured; this sets free a spring by which the pollen-mass is shot +forth, like an arrow, in the right direction, and adheres by its viscid +extremity to the back of the bee. The pollen-mass of the male plant (for the +sexes are separate in this orchid) is thus carried to the flower of the female +plant, where it is brought into contact with the stigma, which is viscid enough +to break certain elastic threads, and retain the pollen, thus effecting +fertilisation. +</p> + +<p> +How, it may be asked, in the foregoing and in innumerable other +<a name="Page156"></a> +instances, can we understand the graduated scale of complexity and the +multifarious means for gaining the same end. The answer no doubt is, as already +remarked, that when two forms vary, which already differ from each other in +some slight degree, the variability will not be of the same exact nature, and +consequently the results obtained through natural selection for the same +general purpose will not be the same. We should also bear in mind that every +highly developed organism has passed through many changes; and that each +modified structure tends to be inherited, so that each modification will not +readily be quite lost, but may be again and again further altered. Hence, the +structure of each part of each species, for whatever purpose it may serve, is +the sum of many inherited changes, through which the species has passed during +its successive adaptations to changed habits and conditions of life. +</p> + +<p> +Finally, then, although in many cases it is most difficult even to conjecture +by what transitions organs could have arrived at their present state; yet, +considering how small the proportion of living and known forms is to the +extinct and unknown, I have been astonished how rarely an organ can be named, +towards which no transitional grade is known to lead. It is certainly true, +that new organs appearing as if created for some special purpose rarely or +never appear in any being; as indeed is shown by that old, but somewhat +exaggerated, 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 there be so much variety and so little real novelty? Why +should all the parts and organs of many independent beings, each supposed to +have been separately created for its own proper place in nature, be so commonly +linked together by graduated steps? Why should not Nature take a sudden leap +from structure to structure? On the theory of natural selection, we can clearly +understand why she should not; for natural selection acts only by taking +advantage of slight successive variations; she can never take a great and +sudden leap, but must advance by the short and sure, though slow steps. +</p> + +<p class="center"> +<i>Organs of little apparent Importance, as affected by Natural Selection.</i> +</p> + +<p> +As natural selection acts by life and death, by the survival of the fittest, +and by the destruction of the less well-fitted individuals, I have sometimes +felt great difficulty in understanding the origin or formation of parts of +little importance; almost as +<a name="Page157"></a> +great, though of a very different kind, as in the case of the most perfect and +complex organs. +</p> + +<p> +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 very trifling characters, +such as the down on fruit and the colour of its flesh, the colour of the skin +and hair of quadrupeds, which, from being correlated with constitutional +differences, or from determining the attacks of insects, 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 fitted, for so trifling an object as to +drive 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 depend 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 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. +</p> + +<p> +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 to existing species in nearly the same +state, although now of very slight use; but any actually injurious deviations +in their structure would of course 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 in the case of the dog, though the aid in this latter respect must +be slight, for the hare, with hardly any tail, can double still more quickly. +</p> + +<p> +In the second place, we may easily err in attributing importance to characters, +and in believing that they have been developed +<a name="Page158"></a> +through natural selection. We must by no means overlook the effects of the +definite action of changed conditions of life, of so-called spontaneous +variations, which seem to depend in a quite subordinate degree on the nature of +the conditions, of the tendency to reversion to long-lost characters, of the +complex laws of growth, such as of correlation, comprehension, of the pressure +of one part on another, &c., and finally of sexual selection, by which +characters of use to one sex are often gained and then transmitted more or less +perfectly to the other sex, though of no use to the sex. But structures thus +indirectly gained, although at first of no advantage to a species, may +subsequently have been taken advantage of by its modified descendants, under +new conditions of life and newly acquired habits. +</p> + +<p> +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 conceal this tree-frequenting bird +from its enemies; and consequently that it was a character of importance, and +had been acquired through natural selection; as it is, the colour is probably +in chief part due to sexual selection. A trailing palm 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, and which, as there is reason to believe from the +distribution of the thorn-bearing species in Africa and South America, serve as +a defence against browsing quadrupeds, so the spikes on the palm may at first +have been developed for this object, and subsequently have been improved and +taken advantage of by the plant, as it underwent further modification and +became a climber. The naked skin on the head of a vulture is generally +considered 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. +</p> + +<p> +We are profoundly ignorant of the cause of each slight variation +<a name="Page159"></a> +or individual difference; and we are immediately made conscious of this by +reflecting on the differences between the breeds of our domesticated animals in +different countries, more especially in the less civilized countries, where +there has been but little methodical selection. Animals kept by savages in +different countries often have to struggle for their own subsistence, and are +exposed to a certain extent to natural selection, and individuals with slightly +different constitutions would succeed best under different climates. With +cattle susceptibility to the attacks of flies is correlated with colour, as is +the liability to be poisoned by certain plants; so that even colour would be +thus subjected to the action of natural selection. Some 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 the head would probably be affected. +The shape, also, of the pelvis might affect by pressure the shape of certain +parts of the young in the womb. The laborious breathing necessary in high +regions tends, as we have good reason to believe, to increase the size of the +chest; and again correlation would come into play. The effects of lessened +exercise, together with abundant food, on the whole organisation is probably +still more important, and this, as H. von Nathusius has lately shown in his +excellent Treatise, is apparently one chief cause of the great modification +which the breeds of swine have undergone. But we are far too ignorant to +speculate on the relative importance of the several known and unknown causes of +variation; and I have made these remarks only to show that, if we are unable to +account for the characteristic differences of our several domestic breeds, +which nevertheless are generally admitted to have arisen through ordinary +generation from one or a few parent-stocks, we ought not to lay too much stress +on our ignorance of the precise cause of the slight analogous differences +between true species. +</p> + +<p class="center"> +<i>Utilitarian Doctrine, how far true: Beauty, how acquired.</i> +</p> + +<p> +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 +many structures have been created for the sake of beauty, to delight man or the +Creator (but this latter point is beyond the scope of scientific discussion), +or for the +<a name="Page160"></a> +sake of mere variety, a view already discussed. Such doctrines, if true, would +be absolutely fatal to my theory. I fully admit that many structures are now of +no direct use to their possessors, and may never have been of any use to their +progenitors; but this does not prove that they were formed solely for beauty or +variety. No doubt the definite action of changed conditions, and the various +causes of modifications, lately specified, have all produced an effect, +probably a great effect, independently of any advantage thus gained. But a +still more important consideration is that the chief part of the organisation +of every living creature is due to inheritance; and consequently, though each +being assuredly is well fitted for its place in nature, many structures have +now no very close and direct relation to present habits of life. 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 +similar 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 webbed +feet no doubt were as useful to the progenitor of the upland goose and of the +frigate-bird, as they now are to the most aquatic of living birds. So we may +believe that the progenitor of the seal did not possess 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, were +originally developed, on the principle of utility, probably through the +reduction of more numerous bones in the fin of some ancient fish-like +progenitor of the whole class. It is scarcely possible to decide how much +allowance ought to be made for such causes of change, as the definite action of +external conditions, so-called spontaneous variations, and the complex laws of +growth; but with these important exceptions, we may conclude that the structure +of every living creature either now is, or was formerly, of some direct or +indirect use to its possessor. +</p> + +<p> +With respect to the belief that organic beings have been created beautiful for +the delight of man—a belief which it has been pronounced is subversive of +my whole theory—I may first remark that the sense of beauty obviously +depends on the nature of the mind, irrespective of any real quality in the +admired object; and that the idea of what is beautiful, is not innate or +unalterable. We see this, for instance, in the men of different races admiring +an entirely different standard of beauty in their women. If beautiful objects +had been created solely for man’s gratification, it ought to +<a name="Page161"></a> +be shown that before man appeared there was less beauty on the face of the +earth than since he came on the stage. Were the beautiful volute and cone +shells of the Eocene epoch, and the gracefully sculptured ammonites of the +Secondary period, created that man might ages afterwards admire them in his +cabinet? Few objects are more beautiful than the minute siliceous cases of the +diatomaceæ: were these created that they might be examined and admired under +the higher powers of the microscope? The beauty in this latter case, and in +many others, is apparently wholly due to symmetry of growth. Flowers rank among +the most beautiful productions of nature; but they have been rendered +conspicuous in contrast with the green leaves, and in consequence at the same +time beautiful, so that they may be easily observed by insects. I have come to +this conclusion from finding it an invariable rule that when a flower is +fertilised by the wind it never has a gaily-coloured corolla. Several plants +habitually produce two kinds of flowers; one kind open and coloured so as to +attract insects; the other closed, not coloured, destitute of nectar, and never +visited by insects. Hence, we may conclude that, if insects had not been +developed on the face of the earth, our plants would not have been decked with +beautiful flowers, but would have produced only such poor flowers as we see on +our fir, oak, nut and ash trees, on grasses, spinach, docks and nettles, which +are all fertilised through the agency of the wind. A similar line of argument +holds good with fruits; that a ripe strawberry or cherry is as pleasing to the +eye as to the palate—that the gaily-coloured fruit of the spindle-wood +tree and the scarlet berries of the holly are beautiful objects—will be +admitted by everyone. But this beauty serves merely as a guide to birds and +beasts, in order that the fruit may be devoured and the matured seeds +disseminated. I infer that this is the case from having as yet found no +exception to the rule that seeds are always thus disseminated when embedded +within a fruit of any kind (that is within a fleshy or pulpy envelope), if it +be coloured of any brilliant tint, or rendered conspicuous by being white or +black. +</p> + +<p> +On the other hand, I willingly admit that a great number of male animals, as +all our most gorgeous birds, some fishes, reptiles, and mammals, and a host of +magnificently coloured butterflies, have been rendered beautiful for +beauty’s sake. But this has been effected through sexual selection, that +is, by the more beautiful males having been continually preferred by the +females, and not for the delight of man. So it is with the music of birds. We +may infer from all this that a nearly similar taste for beautiful colours and +for musical sounds runs through a large part of the animal +<a name="Page162"></a> +kingdom. When the female is as beautifully coloured as the male, which is not +rarely the case with birds and butterflies, the cause apparently lies in the +colours acquired through sexual selection having been transmitted to both +sexes, instead of to the males alone. How the sense of beauty in its simplest +form—that is, the reception of a peculiar kind of pleasure from certain +colours, forms and sounds—was first developed in the mind of man and of +the lower animals, is a very obscure subject. The same sort of difficulty is +presented if we enquire how it is that certain flavours and odours give +pleasure, and others displeasure. Habit in all these cases appears to have come +to a certain extent into play; but there must be some fundamental cause in the +constitution of the nervous system in each species. +</p> + +<p class="p2"> +Natural selection cannot possibly produce any modification in a species +exclusively for the good of another species; though throughout nature one +species incessantly takes advantage of, and profits by the structures of +others. But natural selection can and does often produce structures for the +direct injury of other animals, 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 +it is furnished with a rattle for its own injury, namely, to warn its prey. 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. It is a much more +probable view that the rattlesnake uses its rattle, the cobra expands its frill +and the puff-adder swells while hissing so loudly and harshly, in order to +alarm the many birds and beasts which are known to attack even the most +venomous species. Snakes act on the same principle which makes the hen ruffle +her feathers and expand her wings when a dog approaches her chickens. But I +have not space here to enlarge on the many ways by which animals endeavour to +frighten away their enemies. +</p> + +<p> +Natural selection will never produce in a being any structure more injurious +than beneficial to that being, for natural selection +<a name="Page163"></a> +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. +</p> + +<p> +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 comes into competition. And we see that this is the standard 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 by Müller not to be perfect even in that +most perfect organ, the human eye. Helmholtz, whose judgment no one will +dispute, after describing in the strongest terms the wonderful powers of the +human eye, adds these remarkable words: “That which we have discovered in +the way of inexactness and imperfection in the optical machine and in the image +on the retina, is as nothing in comparison with the incongruities which we have +just come across in the domain of the sensations. One might say that nature has +taken delight in accumulating contradictions in order to remove all foundation +from the theory of a pre-existing harmony between the external and internal +worlds.” 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 bee as perfect, which, when used against many kinds +of enemies, cannot be withdrawn, owing to the backward serratures, and thus +inevitably causes the death of the insect by tearing out its viscera? +</p> + +<p> +If we look at the sting of the bee, as having existed in a remote progenitor, +as a boring and serrated instrument, like that in so many members of the same +great order, and that it has since been modified but not perfected for its +present purpose, with the poison originally adapted for some other object, such +as to produce galls, since 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 social community, +<a name="Page164"></a> +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 purpose, 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 to destroy the young queens, her daughters, as soon as they are 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 principles of natural +selection. If we admire the several ingenious contrivances by which orchids and +many other plants are fertilised through insect agency, can we consider as +equally perfect the elaboration of dense clouds of pollen by our fir-trees, so +that a few granules may be wafted by chance on to the ovules? +</p> + +<p class="center"> +<i>Summary: the Law of Unity of Type and of the Conditions of Existence +embraced by the Theory of Natural Selection.</i> +</p> + +<p> +<a name="Page165"></a> +We have in this chapter discussed some of the difficulties and objections which +may be urged against the theory. Many of them are serious; but I think that in +the discussion light has been thrown on several facts, which on the belief 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 is always very slow, and at any one time acts only on a few forms; +and partly because the very process of natural selection 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. +</p> + +<p> +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 only glided through the air. +</p> + +<p> +We have seen that a species under new conditions of life may change its habits, +or it may have diversified habits, with some 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. +</p> + +<p> +Although the belief that an organ so perfect as the eye could have been formed +by natural selection, is 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 extremely cautious in concluding that none +can have existed, for the metamorphoses of many organs show what wonderful +changes 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 in part +or in whole specialised for one function; and two 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. +</p> + +<p> +We have seen that in two beings widely remote from each other in the natural +scale, organs serving for the same purpose and in external appearance closely +similar may have been separately and independently formed; but when such organs +are closely examined, essential differences in their structure can almost +always be detected; and this naturally follows from the principle of natural +selection. On the other hand, the common rule throughout nature is infinite +diversity of structure for gaining the same end; and this again naturally +follows from the same great principle. +</p> + +<p> +In many cases we are far too ignorant to be enabled to assert that a 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. +In many other cases, +<a name="Page166"></a> +modifications are probably the direct result of the laws of variation or of +growth, independently of any good having been thus gained. But even such +structures have often, as we may feel assured, been subsequently taken +advantage of, and still further modified, for the good of species under new +conditions of life. We may, also, believe that a part formerly of high +importance has frequently 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 means of natural +selection. +</p> + +<p> +Natural selection can 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 possessor. In each well-stocked +country natural selection acts through the competition of the inhabitants and +consequently leads to success in the battle for life, only in accordance with +the standard of that particular country. Hence the inhabitants of one country, +generally the smaller one, often yield to the inhabitants of another and +generally the 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 lead to absolute perfection; +nor, as far as we can judge by our limited faculties, can absolute perfection +be everywhere predicated. +</p> + +<p> +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 to the present inhabitants alone of the world, is not +strictly correct; but if we include all those of past times, whether known or +unknown, it must on this theory be strictly true. +</p> + +<p> +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 +<a name="Page167"></a> +having adapted them during past periods of time: the adaptations being aided in +many cases by the increased use or disuse of parts, being affected by the +direct action of external conditions of life, and subjected in all cases to the +several laws of growth and variation. Hence, in fact, the law of the Conditions +of Existence is the higher law; as it includes, through the inheritance of +former variations and adaptations, that of Unity of Type. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page168"></a><a name="link2HCH0007" id="link2HCH0007"></a> +CHAPTER VII.<br/> +MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.</h2> + +<p class="letter"> +Longevity—Modifications not necessarily simultaneous—Modifications +apparently of no direct service—Progressive development—Characters +of small functional importance, the most constant—Supposed incompetence +of natural selection to account for the incipient stages of useful +structures—Causes which interfere with the acquisition through natural +selection of useful structures—Gradations of structure with changed +functions—Widely different organs in members of the same class, developed +from one and the same source—Reasons for disbelieving in great and abrupt +modifications. +</p> + +<p> +I will devote this chapter to the consideration of various miscellaneous +objections which have been advanced against my views, as some of the previous +discussions may thus be made clearer; but it would be useless to discuss all of +them, as many have been made by writers who have not taken the trouble to +understand the subject. Thus a distinguished German naturalist has asserted +that the weakest part of my theory is, that I consider all organic beings as +imperfect: what I have really said is, that all are not as perfect as they +might have been in relation to their conditions; and this is shown to be the +case by so many native forms in many quarters of the world having yielded their +places to intruding foreigners. Nor can organic beings, even if they were at +any one time perfectly adapted to their conditions of life, have remained so, +when their conditions changed, unless they themselves likewise changed; and no +one will dispute that the physical conditions of each country, as well as the +number and kinds of its inhabitants, have undergone many mutations. +</p> + +<p> +A critic has lately insisted, with some parade of mathematical accuracy, that +longevity is a great advantage to all species, so that he who believes in +natural selection “must arrange his genealogical tree” in such a +manner that all the descendants have longer lives than their progenitors! +Cannot our critics conceive that a biennial plant or one of the lower animals +might range into a cold climate and perish there every winter; and yet, owing +to advantages +<a name="Page169"></a> +gained through natural selection, survive from year to year by means of its +seeds or ova? Mr. E. Ray Lankester has recently discussed this subject, and he +concludes, as far as its extreme complexity allows him to form a judgment, that +longevity is generally related to the standard of each species in the scale of +organisation, as well as to the amount of expenditure in reproduction and in +general activity. And these conditions have, it is probable, been largely +determined through natural selection. +</p> + +<p> +It has been argued that, as none of the animals and plants of Egypt, of which +we know anything, have changed during the last three or four thousand years, so +probably have none in any part of the world. But, as Mr. G.H. Lewes has +remarked, this line of argument proves too much, for the ancient domestic races +figured on the Egyptian monuments, or embalmed, are closely similar or even +identical with those now living; yet all naturalists admit that such races have +been produced through the modification of their original types. The many +animals which have remained unchanged since the commencement of the glacial +period, would have been an incomparably stronger case, for these have been +exposed to great changes of climate and have migrated over great distances; +whereas, in Egypt, during the last several thousand years, the conditions of +life, as far as we know, have remained absolutely uniform. The fact of little +or no modification having been effected since the glacial period, would have +been of some avail against those who believe in an innate and necessary law of +development, but is powerless against the doctrine of natural selection or the +survival of the fittest, which implies that when variations or individual +differences of a beneficial nature happen to arise, these will be preserved; +but this will be effected only under certain favourable circumstances. +</p> + +<p> +The celebrated palæontologist, Bronn, at the close of his German translation of +this work, asks how, on the principle of natural selection, can a variety live +side by side with the parent species? If both have become fitted for slightly +different habits of life or conditions, they might live together; and if we lay +on one side polymorphic species, in which the variability seems to be of a +peculiar nature, and all mere temporary variations, such as size, albinism, +&c., the more permanent varieties are generally found, as far as I can +discover, inhabiting distinct stations, such as high land or low land, dry or +moist districts. Moreover, in the case of animals which wander much about and +cross freely, their varieties seem to be generally confined to distinct +regions. +</p> + +<p> +Bronn also insists that distinct species never differ from each other +<a name="Page170"></a> +in single characters, but in many parts; and he asks, how it always comes that +many parts of the organisation should have been modified at the same time +through variation and natural selection? But there is no necessity for +supposing that all the parts of any being have been simultaneously modified. +The most striking modifications, excellently adapted for some purpose, might, +as was formerly remarked, be acquired by successive variations, if slight, +first in one part and then in another; and as they would be transmitted all +together, they would appear to us as if they had been simultaneously developed. +The best answer, however, to the above objection is afforded by those domestic +races which have been modified, chiefly through man’s power of selection, +for some special purpose. Look at the race and dray-horse, or at the greyhound +and mastiff. Their whole frames, and even their mental characteristics, have +been modified; but if we could trace each step in the history of their +transformation—and the latter steps can be traced—we should not see +great and simultaneous changes, but first one part and then another slightly +modified and improved. Even when selection has been applied by man to some one +character alone—of which our cultivated plants offer the best +instances—it will invariably be found that although this one part, +whether it be the flower, fruit, or leaves, has been greatly changed, almost +all the other parts have been slightly modified. This may be attributed partly +to the principle of correlated growth, and partly to so-called spontaneous +variation. +</p> + +<p> +A much more serious objection has been urged by Bronn, and recently by Broca, +namely, that many characters appear to be of no service whatever to their +possessors, and therefore cannot have been influenced through natural +selection. Bronn adduces the length of the ears and tails in the different +species of hares and mice—the complex folds of enamel in the teeth of +many animals, and a multitude of analogous cases. With respect to plants, this +subject has been discussed by Nägeli in an admirable essay. He admits that +natural selection has effected much, but he insists that the families of plants +differ chiefly from each other in morphological characters, which appear to be +quite unimportant for the welfare of the species. He consequently believes in +an innate tendency towards progressive and more perfect development. He +specifies the arrangement of the cells in the tissues, and of the leaves on the +axis, as cases in which natural selection could not have acted. To these may be +added the numerical divisions in the parts of the flower, the position of the +ovules, the shape of the seed, when not of any use for dissemination, &c. +</p> + +<p> +<a name="Page171"></a> +There is much force in the above objection. Nevertheless, we ought, in the +first place, to be extremely cautious in pretending to decide what structures +now are, or have formerly been, of use to each species. In the second place, it +should always be borne in mind that when one part is modified, so will be other +parts, through certain dimly seen causes, such as an increased or diminished +flow of nutriment to a part, mutual pressure, an early developed part affecting +one subsequently developed, and so forth—as well as through other causes +which lead to the many mysterious cases of correlation, which we do not in the +least understand. These agencies may be all grouped together, for the sake of +brevity, under the expression of the laws of growth. In the third place, we +have to allow for the direct and definite action of changed conditions of life, +and for so-called spontaneous variations, in which the nature of the conditions +apparently plays a quite subordinate part. Bud-variations, such as the +appearance of a moss-rose on a common rose, or of a nectarine on a peach-tree, +offer good instances of spontaneous variations; but even in these cases, if we +bear in mind the power of a minute drop of poison in producing complex galls, +we ought not to feel too sure that the above variations are not the effect of +some local change in the nature of the sap, due to some change in the +conditions. There must be some efficient cause for each slight individual +difference, as well as for more strongly marked variations which occasionally +arise; and if the unknown cause were to act persistently, it is almost certain +that all the individuals of the species would be similarly modified. +</p> + +<p> +In the earlier editions of this work I underrated, as it now seems probable, +the frequency and importance of modifications due to spontaneous variability. +But it is impossible to attribute to this cause the innumerable structures +which are so well adapted to the habits of life of each species. I can no more +believe in this than that the well-adapted form of a race-horse or greyhound, +which before the principle of selection by man was well understood, excited so +much surprise in the minds of the older naturalists, can thus be explained. +</p> + +<p> +It may be worth while to illustrate some of the foregoing remarks. With respect +to the assumed inutility of various parts and organs, it is hardly necessary to +observe that even in the higher and best-known animals many structures exist, +which are so highly developed that no one doubts that they are of importance, +yet their use has not been, or has only recently been, ascertained. As Bronn +gives the length of the ears and tail in the several species of mice as +instances, though trifling ones, of differences in structure which can +<a name="Page172"></a> +be of no special use, I may mention that, according to Dr. Schöbl, the external +ears of the common mouse are supplied in an extraordinary manner with nerves, +so that they no doubt serve as tactile organs; hence the length of the ears can +hardly be quite unimportant. We shall, also, presently see that the tail is a +highly useful prehensile organ to some of the species; and its use would be +much influence by its length. +</p> + +<p> +With respect to plants, to which on account of Nägeli’s essay I shall +confine myself in the following remarks, it will be admitted that the flowers +of the orchids present a multitude of curious structures, which a few years ago +would have been considered as mere morphological differences without any +special function; but they are now known to be of the highest importance for +the fertilisation of the species through the aid of insects, and have probably +been gained through natural selection. No one until lately would have imagined +that in dimorphic and trimorphic plants the different lengths of the stamens +and pistils, and their arrangement, could have been of any service, but now we +know this to be the case. +</p> + +<p> +In certain whole groups of plants the ovules stand erect, and in others they +are suspended; and within the same ovarium of some few plants, one ovule holds +the former and a second ovule the latter position. These positions seem at +first purely morphological, or of no physiological signification; but Dr. +Hooker informs me that within the same ovarium the upper ovules alone in some +cases, and in others the lower ones alone are fertilised; and he suggests that +this probably depends on the direction in which the pollen-tubes enter the +ovarium. If so, the position of the ovules, even when one is erect and the +other suspended within the same ovarium, would follow the selection of any +slight deviations in position which favoured their fertilisation, and the +production of seed. +</p> + +<p> +Several plants belonging to distinct orders habitually produce flowers of two +kinds—the one open, of the ordinary structure, the other closed and +imperfect. These two kinds of flowers sometimes differ wonderfully in +structure, yet may be seen to graduate into each other on the same plant. The +ordinary and open flowers can be intercrossed; and the benefits which certainly +are derived from this process are thus secured. The closed and imperfect +flowers are, however, manifestly of high importance, as they yield with the +utmost safety a large stock of seed, with the expenditure of wonderfully little +pollen. The two kinds of flowers often differ much, as just stated, in +structure. The petals in the imperfect flowers almost always consist of mere +rudiments, and the pollen-grains are reduced in diameter. In Ononis columnæ +five of the alternate stamens are +<a name="Page173"></a> +rudimentary; and in some species of Viola three stamens are in this state, two +retaining their proper function, but being of very small size. In six out of +thirty of the closed flowers in an Indian violet (name unknown, for the plants +have never produced with me perfect flowers), the sepals are reduced from the +normal number of five to three. In one section of the Malpighiaceæ the closed +flowers, according to A. de Jussieu, are still further modified, for the five +stamens which stand opposite to the sepals are all aborted, a sixth stamen +standing opposite to a petal being alone developed; and this stamen is not +present in the ordinary flowers of this species; the style is aborted; and the +ovaria are reduced from three to two. Now although natural selection may well +have had the power to prevent some of the flowers from expanding, and to reduce +the amount of pollen, when rendered by the closure of the flowers superfluous, +yet hardly any of the above special modifications can have been thus +determined, but must have followed from the laws of growth, including the +functional inactivity of parts, during the progress of the reduction of the +pollen and the closure of the flowers. +</p> + +<p> +It is so necessary to appreciate the important effects of the laws of growth, +that I will give some additional cases of another kind, namely of differences +in the same part or organ, due to differences in relative position on the same +plant. In the Spanish chestnut, and in certain fir-trees, the angles of +divergence of the leaves differ, according to Schacht, in the nearly horizontal +and in the upright branches. In the common rue and some other plants, one +flower, usually the central or terminal one, opens first, and has five sepals +and petals, and five divisions to the ovarium; while all the other flowers on +the plant are tetramerous. In the British Adoxa the uppermost flower generally +has two calyx-lobes with the other organs tetramerous, while the surrounding +flowers generally have three calyx-lobes with the other organs pentamerous. In +many Compositæ and Umbelliferæ (and in some other plants) the circumferential +flowers have their corollas much more developed than those of the centre; and +this seems often connected with the abortion of the reproductive organs. It is +a more curious fact, previously referred to, that the achenes or seeds of the +circumference and centre sometimes differ greatly in form, colour and other +characters. In Carthamus and some other Compositæ the central achenes alone are +furnished with a pappus; and in Hyoseris the same head yields achenes of three +different forms. In certain Umbelliferæ the exterior seeds, according to +Tausch, are orthospermous, and the central one cœlospermous, and this is a +character which was considered by De Candolle to be in other species of the +highest systematic +<a name="Page174"></a> +importance. Professor Braun mentions a Fumariaceous genus, in which the flowers +in the lower part of the spike bear oval, ribbed, one-seeded nutlets; and in +the upper part of the spike, lanceolate, two-valved and two-seeded siliques. In +these several cases, with the exception of that of the well-developed +ray-florets, which are of service in making the flowers conspicuous to insects, +natural selection cannot, as far as we can judge, have come into play, or only +in a quite subordinate manner. All these modifications follow from the relative +position and inter-action of the parts; and it can hardly be doubted that if +all the flowers and leaves on the same plant had been subjected to the same +external and internal condition, as are the flowers and leaves in certain +positions, all would have been modified in the same manner. +</p> + +<p> +In numerous other cases we find modifications of structure, which are +considered by botanists to be generally of a highly important nature, affecting +only some of the flowers on the same plant, or occurring on distinct plants, +which grow close together under the same conditions. As these variations seem +of no special use to the plants, they cannot have been influenced by natural +selection. Of their cause we are quite ignorant; we cannot even attribute them, +as in the last class of cases, to any proximate agency, such as relative +position. I will give only a few instances. It is so common to observe on the +same plant, flowers indifferently tetramerous, pentamerous, &c., that I +need not give examples; but as numerical variations are comparatively rare when +the parts are few, I may mention that, according to De Candolle, the flowers of +Papaver bracteatum offer either two sepals with four petals (which is the +common type with poppies), or three sepals with six petals. The manner in which +the petals are folded in the bud is in most groups a very constant +morphological character; but Professor Asa Gray states that with some species +of Mimulus, the æstivation is almost as frequently that of the Rhinanthideæ as +of the Antirrhinideæ, to which latter tribe the genus belongs. Aug. St. Hilaire +gives the following cases: the genus Zanthoxylon belongs to a division of the +Rutaceæ with a single ovary, but in some species flowers may be found on the +same plant, and even in the same panicle, with either one or two ovaries. In +Helianthemum the capsule has been described as unilocular or tri-locular; and +in H. mutabile, “Une lame <i>plus ou moins large</i>, s’étend entre +le pericarpe et le placenta.” In the flowers of Saponaria officinalis Dr. +Masters has observed instances of both marginal and free central placentation. +Lastly, St. Hilaire found towards the southern extreme of the range of Gomphia +oleæformis two forms which he did +<a name="Page175"></a> +not at first doubt were distinct species, but he subsequently saw them growing +on the same bush; and he then adds, “Voilà donc dans un même individu des +loges et un style qui se rattachent tantôt à un axe verticale et tantôt à un +gynobase.” +</p> + +<p> +We thus see that with plants many morphological changes may be attributed to +the laws of growth and the inter-action of parts, independently of natural +selection. But with respect to Nägeli’s doctrine of an innate tendency +towards perfection or progressive development, can it be said in the case of +these strongly pronounced variations, that the plants have been caught in the +act of progressing towards a higher state of development? On the contrary, I +should infer from the mere fact of the parts in question differing or varying +greatly on the same plant, that such modifications were of extremely small +importance to the plants themselves, of whatever importance they may generally +be to us for our classifications. The acquisition of a useless part can hardly +be said to raise an organism in the natural scale; and in the case of the +imperfect, closed flowers, above described, if any new principle has to be +invoked, it must be one of retrogression rather than of progression; and so it +must be with many parasitic and degraded animals. We are ignorant of the +exciting cause of the above specified modifications; but if the unknown cause +were to act almost uniformly for a length of time, we may infer that the result +would be almost uniform; and in this case all the individuals of the species +would be modified in the same manner. +</p> + +<p> +From the fact of the above characters being unimportant for the welfare of the +species, any slight variations which occurred in them would not have been +accumulated and augmented through natural selection. A structure which has been +developed through long-continued selection, when it ceases to be of service to +a species, generally becomes variable, as we see with rudimentary organs; for +it will no longer be regulated by this same power of selection. But when, from +the nature of the organism and of the conditions, modifications have been +induced which are unimportant for the welfare of the species, they may be, and +apparently often have been, transmitted in nearly the same state to numerous, +otherwise modified, descendants. It cannot have been of much importance to the +greater number of mammals, birds, or reptiles, whether they were clothed with +hair, feathers or scales; yet hair has been transmitted to almost all mammals, +feathers to all birds, and scales to all true reptiles. A structure, whatever +it may be, which is common to many allied forms, is ranked by us as of high +systematic importance, and consequently is often assumed to be of high vital +<a name="Page176"></a> +importance to the species. Thus, as I am inclined to believe, morphological +differences, which we consider as important—such as the arrangement of +the leaves, the divisions of the flower or of the ovarium, the position of the +ovules, &c., first appeared in many cases as fluctuating variations, which +sooner or later became constant through the nature of the organism and of the +surrounding conditions, as well as through the intercrossing of distinct +individuals, but not through natural selection; for as these morphological +characters do not affect the welfare of the species, any slight deviations in +them could not have been governed or accumulated through this latter agency. It +is a strange result which we thus arrive at, namely, that characters of slight +vital importance to the species, are the most important to the systematist; +but, as we shall hereafter see when we treat of the genetic principle of +classification, this is by no means so paradoxical as it may at first appear. +</p> + +<p> +Although we have no good evidence of the existence in organic beings of an +innate tendency towards progressive development, yet this necessarily follows, +as I have attempted to show in the fourth chapter, through the continued action +of natural selection. For the best definition which has ever been given of a +high standard of organisation, is the degree to which the parts have been +specialised or differentiated; and natural selection tends towards this end, +inasmuch as the parts are thus enabled to perform their functions more +efficiently. +</p> + +<p class="p2"> +A distinguished zoologist, Mr. St. George Mivart, has recently collected all +the objections which have ever been advanced by myself and others against the +theory of natural selection, as propounded by Mr. Wallace and myself, and has +illustrated them with admirable art and force. When thus marshalled, they make +a formidable array; and as it forms no part of Mr. Mivart’s plan to give +the various facts and considerations opposed to his conclusions, no slight +effort of reason and memory is left to the reader, who may wish to weigh the +evidence on both sides. When discussing special cases, Mr. Mivart passes over +the effects of the increased use and disuse of parts, which I have always +maintained to be highly important, and have treated in my “Variation +under Domestication” at greater length than, as I believe, any other +writer. He likewise often assumes that I attribute nothing to variation, +independently of natural selection, whereas in the work just referred to I have +collected a greater number of well-established cases than can be found in any +other work known to me. My judgment may not be trustworthy, but after reading +with care Mr. Mivart’s book, and +<a name="Page177"></a> +comparing each section with what I have said on the same head, I never before +felt so strongly convinced of the general truth of the conclusions here arrived +at, subject, of course, in so intricate a subject, to much partial error. +</p> + +<p> +All Mr. Mivart’s objections will be, or have been, considered in the +present volume. The one new point which appears to have struck many readers is, +“That natural selection is incompetent to account for the incipient +stages of useful structures.” This subject is intimately connected with +that of the gradation of the characters, often accompanied by a change of +function, for instance, the conversion of a swim-bladder into lungs, points +which were discussed in the last chapter under two headings. Nevertheless, I +will here consider in some detail several of the cases advanced by Mr. Mivart, +selecting those which are the most illustrative, as want of space prevents me +from considering all. +</p> + +<p> +The giraffe, by its lofty stature, much elongated neck, fore legs, head and +tongue, has its whole frame beautifully adapted for browsing on the higher +branches of trees. It can thus obtain food beyond the reach of the other +Ungulata or hoofed animals inhabiting the same country; and this must be a +great advantage to it during dearths. The Niata cattle in South America show us +how small a difference in structure may make, during such periods, a great +difference in preserving an animal’s life. These cattle can browse as +well as others on grass, but from the projection of the lower jaw they cannot, +during the often recurrent droughts, browse on the twigs of trees, reeds, +&c., to which food the common cattle and horses are then driven; so that at +these times the Niatas perish, if not fed by their owners. Before coming to Mr. +Mivart’s objections, it may be well to explain once again how natural +selection will act in all ordinary cases. Man has modified some of his animals, +without necessarily having attended to special points of structure, by simply +preserving and breeding from the fleetest individuals, as with the race-horse +and greyhound, or as with the game-cock, by breeding from the victorious birds. +So under nature with the nascent giraffe, the individuals which were the +highest browsers and were able during dearths to reach even an inch or two +above the others, will often have been preserved; for they will have roamed +over the whole country in search of food. That the individuals of the same +species often differ slightly in the relative lengths of all their parts may be +seen in many works of natural history, in which careful measurements are given. +These slight proportional differences, due to the laws of growth and variation, +are not of the slightest use or importance to most species. But it will have +been otherwise with +<a name="Page178"></a> +the nascent giraffe, considering its probable habits of life; for those +individuals which had some one part or several parts of their bodies rather +more elongated than usual, would generally have survived. These will have +intercrossed and left offspring, either inheriting the same bodily +peculiarities, or with a tendency to vary again in the same manner; while the +individuals less favoured in the same respects will have been the most liable +to perish. +</p> + +<p> +We here see that there is no need to separate single pairs, as man does, when +he methodically improves a breed: natural selection will preserve and thus +separate all the superior individuals, allowing them freely to intercross, and +will destroy all the inferior individuals. By this process long-continued, +which exactly corresponds with what I have called unconscious selection by man, +combined, no doubt, in a most important manner with the inherited effects of +the increased use of parts, it seems to me almost certain that an ordinary +hoofed quadruped might be converted into a giraffe. +</p> + +<p> +To this conclusion Mr. Mivart brings forward two objections. One is that the +increased size of the body would obviously require an increased supply of food, +and he considers it as “very problematical whether the disadvantages +thence arising would not, in times of scarcity, more than counterbalance the +advantages.” But as the giraffe does actually exist in large numbers in +Africa, and as some of the largest antelopes in the world, taller than an ox, +abound there, why should we doubt that, as far as size is concerned, +intermediate gradations could formerly have existed there, subjected as now to +severe dearths. Assuredly the being able to reach, at each stage of increased +size, to a supply of food, left untouched by the other hoofed quadrupeds of the +country, would have been of some advantage to the nascent giraffe. Nor must we +overlook the fact, that increased bulk would act as a protection against almost +all beasts of prey excepting the lion; and against this animal, its tall +neck—and the taller the better—would, as Mr. Chauncey Wright has +remarked, serve as a watch-tower. It is from this cause, as Sir S. Baker +remarks, that no animal is more difficult to stalk than the giraffe. This +animal also uses its long neck as a means of offence or defence, by violently +swinging its head armed with stump-like horns. The preservation of each species +can rarely be determined by any one advantage, but by the union of all, great +and small. +</p> + +<p> +Mr. Mivart then asks (and this is his second objection), if natural selection +be so potent, and if high browsing be so great an advantage, why has not any +other hoofed quadruped acquired a long neck and lofty stature, besides the +giraffe, and, in a lesser degree, the camel, guanaco and macrauchenia? Or, +again, why has not any +<a name="Page179"></a> +member of the group acquired a long proboscis? With respect to South Africa, +which was formerly inhabited by numerous herds of the giraffe, the answer is +not difficult, and can best be given by an illustration. In every meadow in +England, in which trees grow, we see the lower branches trimmed or planed to an +exact level by the browsing of the horses or cattle; and what advantage would +it be, for instance, to sheep, if kept there, to acquire slightly longer necks? +In every district some one kind of animal will almost certainly be able to +browse higher than the others; and it is almost equally certain that this one +kind alone could have its neck elongated for this purpose, through natural +selection and the effects of increased use. In South Africa the competition for +browsing on the higher branches of the acacias and other trees must be between +giraffe and giraffe, and not with the other ungulate animals. +</p> + +<p> +Why, in other quarters of the world, various animals belonging to this same +order have not acquired either an elongated neck or a proboscis, cannot be +distinctly answered; but it is as unreasonable to expect a distinct answer to +such a question as why some event in the history of mankind did not occur in +one country while it did in another. We are ignorant with respect to the +conditions which determine the numbers and range of each species, and we cannot +even conjecture what changes of structure would be favourable to its increase +in some new country. We can, however, see in a general manner that various +causes might have interfered with the development of a long neck or proboscis. +To reach the foliage at a considerable height (without climbing, for which +hoofed animals are singularly ill-constructed) implies greatly increased bulk +of body; and we know that some areas support singularly few large quadrupeds, +for instance South America, though it is so luxuriant, while South Africa +abounds with them to an unparalleled degree. Why this should be so we do not +know; nor why the later tertiary periods should have been much more favourable +for their existence than the present time. Whatever the causes may have been, +we can see that certain districts and times would have been much more +favourable than others for the development of so large a quadruped as the +giraffe. +</p> + +<p> +In order that an animal should acquire some structure specially and largely +developed, it is almost indispensable that several other parts should be +modified and coadapted. Although every part of the body varies slightly, it +does not follow that the necessary parts should always vary in the right +direction and to the right degree. With the different species of our +domesticated animals we know that the parts vary in a different manner and +degree, and that +<a name="Page180"></a> +some species are much more variable than others. Even if the fitting variations +did arise, it does not follow that natural selection would be able to act on +them and produce a structure which apparently would be beneficial to the +species. For instance, if the number of individuals existing in a country is +determined chiefly through destruction by beasts of prey—by external or +internal parasites, &c.—as seems often to be the case, then natural +selection will be able to do little, or will be greatly retarded, in modifying +any particular structure for obtaining food. Lastly, natural selection is a +slow process, and the same favourable conditions must long endure in order that +any marked effect should thus be produced. Except by assigning such general and +vague reasons, we cannot explain why, in many quarters of the world, hoofed +quadrupeds have not acquired much elongated necks or other means for browsing +on the higher branches of trees. +</p> + +<p> +Objections of the same nature as the foregoing have been advanced by many +writers. In each case various causes, besides the general ones just indicated, +have probably interfered with the acquisition through natural selection of +structures, which it is thought would be beneficial to certain species. One +writer asks, why has not the ostrich acquired the power of flight? But a +moment’s reflection will show what an enormous supply of food would be +necessary to give to this bird of the desert force to move its huge body +through the air. Oceanic islands are inhabited by bats and seals, but by no +terrestrial mammals; yet as some of these bats are peculiar species, they must +have long inhabited their present homes. Therefore Sir C. Lyell asks, and +assigns certain reasons in answer, why have not seals and bats given birth on +such islands to forms fitted to live on the land? But seals would necessarily +be first converted into terrestrial carnivorous animals of considerable size, +and bats into terrestrial insectivorous animals; for the former there would be +no prey; for the bats ground-insects would serve as food, but these would +already be largely preyed on by the reptiles or birds, which first colonise and +abound on most oceanic islands. Gradations of structure, with each stage +beneficial to a changing species, will be favoured only under certain peculiar +conditions. A strictly terrestrial animal, by occasionally hunting for food in +shallow water, then in streams or lakes, might at last be converted into an +animal so thoroughly aquatic as to brave the open ocean. But seals would not +find on oceanic islands the conditions favourable to their gradual reconversion +into a terrestrial form. Bats, as formerly shown, probably acquired their wings +by at first gliding through the air from tree to tree, like the so-called +flying-squirrels, +<a name="Page181"></a> +for the sake of escaping from their enemies, or for avoiding falls; but when +the power of true flight had once been acquired, it would never be reconverted +back, at least for the above purposes, into the less efficient power of gliding +through the air. Bats, might, indeed, like many birds, have had their wings +greatly reduced in size, or completely lost, through disuse; but in this case +it would be necessary that they should first have acquired the power of running +quickly on the ground, by the aid of their hind legs alone, so as to compete +with birds or other ground animals; and for such a change a bat seems +singularly ill-fitted. These conjectural remarks have been made merely to show +that a transition of structure, with each step beneficial, is a highly complex +affair; and that there is nothing strange in a transition not having occurred +in any particular case. +</p> + +<p> +Lastly, more than one writer has asked why have some animals had their mental +powers more highly developed than others, as such development would be +advantageous to all? Why have not apes acquired the intellectual powers of man? +Various causes could be assigned; but as they are conjectural, and their +relative probability cannot be weighed, it would be useless to give them. A +definite answer to the latter question ought not to be expected, seeing that no +one can solve the simpler problem, why, of two races of savages, one has risen +higher in the scale of civilisation than the other; and this apparently implies +increased brain power. +</p> + +<p class="p2"> +We will return to Mr. Mivart’s other objections. Insects often resemble +for the sake of protection various objects, such as green or decayed leaves, +dead twigs, bits of lichen, flowers, spines, excrement of birds, and living +insects; but to this latter point I shall hereafter recur. The resemblance is +often wonderfully close, and is not confined to colour, but extends to form, +and even to the manner in which the insects hold themselves. The caterpillars +which project motionless like dead twigs from the bushes on which they feed, +offer an excellent instance of a resemblance of this kind. The cases of the +imitation of such objects as the excrement of birds, are rare and exceptional. +On this head, Mr. Mivart remarks, “As, according to Mr. Darwin’s +theory, there is a constant tendency to indefinite variation, and as the minute +incipient variations will be in <i>all directions</i>, they must tend to +neutralize each other, and at first to form such unstable modifications that it +is difficult, if not impossible, to see how such indefinite oscillations of +infinitesimal beginnings can ever build up a sufficiently appreciable +resemblance to a leaf, bamboo, or other object, for natural selection to seize +upon and perpetuate.” +</p> + +<p> +<a name="Page182"></a> +But in all the foregoing cases the insects in their original state no doubt +presented some rude and accidental resemblance to an object commonly found in +the stations frequented by them. Nor is this at all improbable, considering the +almost infinite number of surrounding objects and the diversity in form and +colour of the hosts of insects which exist. As some rude resemblance is +necessary for the first start, we can understand how it is that the larger and +higher animals do not (with the exception, as far as I know, of one fish) +resemble for the sake of protection special objects, but only the surface which +commonly surrounds them, and this chiefly in colour. Assuming that an insect +originally happened to resemble in some degree a dead twig or a decayed leaf, +and that it varied slightly in many ways, then all the variations which +rendered the insect at all more like any such object, and thus favoured its +escape, would be preserved, while other variations would be neglected and +ultimately lost; or, if they rendered the insect at all less like the imitated +object, they would be eliminated. There would indeed be force in Mr. +Mivart’s objection, if we were to attempt to account for the above +resemblances, independently of natural selection, through mere fluctuating +variability; but as the case stands there is none. +</p> + +<p> +Nor can I see any force in Mr. Mivart’s difficulty with respect to +“the last touches of perfection in the mimicry;” as in the case +given by Mr. Wallace, of a walking-stick insect (Ceroxylus laceratus), which +resembles “a stick grown over by a creeping moss or jungermannia.” +So close was this resemblance, that a native Dyak maintained that the +foliaceous excrescences were really moss. Insects are preyed on by birds and +other enemies whose sight is probably sharper than ours, and every grade in +resemblance which aided an insect to escape notice or detection, would tend +towards its preservation; and the more perfect the resemblance so much the +better for the insect. Considering the nature of the differences between the +species in the group which includes the above Ceroxylus, there is nothing +improbable in this insect having varied in the irregularities on its surface, +and in these having become more or less green-coloured; for in every group the +characters which differ in the several species are the most apt to vary, while +the generic characters, or those common to all the species, are the most +constant. +</p> + +<p class="p2"> +The Greenland whale is one of the most wonderful animals in the world, and the +baleen, or whalebone, one of its greatest peculiarities. The baleen consists of +a row, on each side of the upper +<a name="Page183"></a> +jaw, of about 300 plates or laminæ, which stand close together transversely to +the longer axis of the mouth. Within the main row there are some subsidiary +rows. The extremities and inner margins of all the plates are frayed into stiff +bristles, which clothe the whole gigantic palate, and serve to strain or sift +the water, and thus to secure the minute prey on which these great animals +subsist. The middle and longest lamina in the Greenland whale is ten, twelve, +or even fifteen feet in length; but in the different species of Cetaceans there +are gradations in length; the middle lamina being in one species, according to +Scoresby, four feet, in another three, in another eighteen inches, and in the +Balænoptera rostrata only about nine inches in length. The quality of the +whalebone also differs in the different species. +</p> + +<p> +With respect to the baleen, Mr. Mivart remarks that if it “had once +attained such a size and development as to be at all useful, then its +preservation and augmentation within serviceable limits would be promoted by +natural selection alone. But how to obtain the beginning of such useful +development?” In answer, it may be asked, why should not the early +progenitors of the whales with baleen have possessed a mouth constructed +something like the lamellated beak of a duck? Ducks, like whales, subsist by +sifting the mud and water; and the family has sometimes been called +<i>Criblatores</i>, or sifters. I hope that I may not be misconstrued into +saying that the progenitors of whales did actually possess mouths lamellated +like the beak of a duck. I wish only to show that this is not incredible, and +that the immense plates of baleen in the Greenland whale might have been +developed from such lamellæ by finely graduated steps, each of service to its +possessor. +</p> + +<p> +The beak of a shoveller-duck (Spatula clypeata) is a more beautiful and complex +structure than the mouth of a whale. The upper mandible is furnished on each +side (in the specimen examined by me) with a row or comb formed of 188 thin, +elastic lamellæ, obliquely bevelled so as to be pointed, and placed +transversely to the longer axis of the mouth. They arise from the palate, and +are attached by flexible membrane to the sides of the mandible. Those standing +towards the middle are the longest, being about one-third of an inch in length, +and they project fourteen one-hundredths of an inch beneath the edge. At their +bases there is a short subsidiary row of obliquely transverse lamellæ. In these +several respects they resemble the plates of baleen in the mouth of a whale. +But towards the extremity of the beak they differ much, as they project inward, +instead of straight downward. The entire head of the shoveller, though +incomparably less bulky, is about one-eighteenth of the +<a name="Page184"></a> +length of the head of a moderately large Balænoptera rostrata, in which species +the baleen is only nine inches long; so that if we were to make the head of the +shoveller as long as that of the Balænoptera, the lamellæ would be six inches +in length, that is, two-thirds of the length of the baleen in this species of +whale. The lower mandible of the shoveller-duck is furnished with lamellæ of +equal length with these above, but finer; and in being thus furnished it +differs conspicuously from the lower jaw of a whale, which is destitute of +baleen. On the other hand, the extremities of these lower lamellæ are frayed +into fine bristly points, so that they thus curiously resemble the plates of +baleen. In the genus Prion, a member of the distinct family of the Petrels, the +upper mandible alone is furnished with lamellæ, which are well developed and +project beneath the margin; so that the beak of this bird resembles in this +respect the mouth of a whale. +</p> + +<p> +From the highly developed structure of the shoveller’s beak we may +proceed (as I have learned from information and specimens sent to me by Mr. +Salvin), without any great break, as far as fitness for sifting is concerned, +through the beak of the Merganetta armata, and in some respects through that of +the Aix sponsa, to the beak of the common duck. In this latter species the +lamellæ are much coarser than in the shoveller, and are firmly attached to the +sides of the mandible; they are only about fifty in number on each side, and do +not project at all beneath the margin. They are square-topped, and are edged +with translucent, hardish tissue, as if for crushing food. The edges of the +lower mandible are crossed by numerous fine ridges, which project very little. +Although the beak is thus very inferior as a sifter to that of a shoveller, yet +this bird, as every one knows, constantly uses it for this purpose. There are +other species, as I hear from Mr. Salvin, in which the lamellæ are considerably +less developed than in the common duck; but I do not know whether they use +their beaks for sifting the water. +</p> + +<p> +Turning to another group of the same family. In the Egyptian goose (Chenalopex) +the beak closely resembles that of the common duck; but the lamellæ are not so +numerous, nor so distinct from each other, nor do they project so much inward; +yet this goose, as I am informed by Mr. E. Bartlett, “uses its bill like +a duck by throwing the water out at the corners.” Its chief food, +however, is grass, which it crops like the common goose. In this latter bird +the lamellæ of the upper mandible are much coarser than in the common duck, +almost confluent, about twenty-seven in number on each side, and terminating +upward in teeth-like knobs. The palate is also covered with hard rounded knobs. +The edges of the lower +<a name="Page185"></a> +mandible are serrated with teeth much more prominent, coarser and sharper than +in the duck. The common goose does not sift the water, but uses its beak +exclusively for tearing or cutting herbage, for which purpose it is so well +fitted that it can crop grass closer than almost any other animal. There are +other species of geese, as I hear from Mr. Bartlett, in which the lamellæ are +less developed than in the common goose. +</p> + +<p> +We thus see that a member of the duck family, with a beak constructed like that +of a common goose and adapted solely for grazing, or even a member with a beak +having less well-developed lamellæ, might be converted by small changes into a +species like the Egyptian goose—this into one like the common +duck—and, lastly, into one like the shoveller, provided with a beak +almost exclusively adapted for sifting the water; for this bird could hardly +use any part of its beak, except the hooked tip, for seizing or tearing solid +food. The beak of a goose, as I may add, might also be converted by small +changes into one provided with prominent, recurved teeth, like those of the +Merganser (a member of the same family), serving for the widely different +purpose of securing live fish. +</p> + +<p> +Returning to the whales. The Hyperoodon bidens is destitute of true teeth in an +efficient condition, but its palate is roughened, according to Lacepede, with +small unequal, hard points of horn. There is, therefore, nothing improbable in +supposing that some early Cetacean form was provided with similar points of +horn on the palate, but rather more regularly placed, and which, like the knobs +on the beak of the goose, aided it in seizing or tearing its food. If so, it +will hardly be denied that the points might have been converted through +variation and natural selection into lamellæ as well-developed as those of the +Egyptian goose, in which case they would have been used both for seizing +objects and for sifting the water; then into lamellæ like those of the domestic +duck; and so onward, until they became as well constructed as those of the +shoveller, in which case they would have served exclusively as a sifting +apparatus. From this stage, in which the lamellæ would be two-thirds of the +length of the plates of baleen in the Balænoptera rostrata, gradations, which +may be observed in still-existing Cetaceans, lead us onward to the enormous +plates of baleen in the Greenland whale. Nor is there the least reason to doubt +that each step in this scale might have been as serviceable to certain ancient +Cetaceans, with the functions of the parts slowly changing during the progress +of development, as are the gradations in the beaks of the different existing +members of the duck-family. We should bear in mind that each species of duck is +subjected to a severe +<a name="Page186"></a> +struggle for existence, and that the structure of every part of its frame must +be well adapted to its conditions of life. +</p> + +<p class="p2"> +The Pleuronectidæ, or Flat-fish, are remarkable for their asymmetrical bodies. +They rest on one side—in the greater number of species on the left, but +in some on the right side; and occasionally reversed adult specimens occur. The +lower, or resting-surface, resembles at first sight the ventral surface of an +ordinary fish; it is of a white colour, less developed in many ways than the +upper side, with the lateral fins often of smaller size. But the eyes offer the +most remarkable peculiarity; for they are both placed on the upper side of the +head. During early youth, however, they stand opposite to each other, and the +whole body is then symmetrical, with both sides equally coloured. Soon the eye +proper to the lower side begins to glide slowly round the head to the upper +side; but does not pass right through the skull, as was formerly thought to be +the case. It is obvious that unless the lower eye did thus travel round, it +could not be used by the fish while lying in its habitual position on one side. +The lower eye would, also, have been liable to be abraded by the sandy bottom. +That the Pleuronectidæ are admirably adapted by their flattened and +asymmetrical structure for their habits of life, is manifest from several +species, such as soles, flounders, &c., being extremely common. The chief +advantages thus gained seem to be protection from their enemies, and facility +for feeding on the ground. The different members, however, of the family +present, as Schiödte remarks, “a long series of forms exhibiting a +gradual transition from Hippoglossus pinguis, which does not in any +considerable degree alter the shape in which it leaves the ovum, to the soles, +which are entirely thrown to one side.” +</p> + +<p> +Mr. Mivart has taken up this case, and remarks that a sudden spontaneous +transformation in the position of the eyes is hardly conceivable, in which I +quite agree with him. He then adds: “If the transit was gradual, then how +such transit of one eye a minute fraction of the journey towards the other side +of the head could benefit the individual is, indeed, far from clear. It seems, +even, that such an incipient transformation must rather have been +injurious.” But he might have found an answer to this objection in the +excellent observations published in 1867 by Malm. The Pleuronectidæ, while very +young and still symmetrical, with their eyes standing on opposite sides of the +head, cannot long retain a vertical position, owing to the excessive depth of +their bodies, the small size of their lateral fins, and to their being +destitute of a +<a name="Page187"></a> +swimbladder. Hence, soon growing tired, they fall to the bottom on one side. +While thus at rest they often twist, as Malm observed, the lower eye upward, to +see above them; and they do this so vigorously that the eye is pressed hard +against the upper part of the orbit. The forehead between the eyes consequently +becomes, as could be plainly seen, temporarily contracted in breadth. On one +occasion Malm saw a young fish raise and depress the lower eye through an +angular distance of about seventy degrees. +</p> + +<p> +We should remember that the skull at this early age is cartilaginous and +flexible, so that it readily yields to muscular action. It is also known with +the higher animals, even after early youth, that the skull yields and is +altered in shape, if the skin or muscles be permanently contracted through +disease or some accident. With long-eared rabbits, if one ear flops forward and +downward, its weight drags forward all the bones of the skull on the same side, +of which I have given a figure. Malm states that the newly-hatched young of +perches, salmon, and several other symmetrical fishes, have the habit of +occasionally resting on one side at the bottom; and he has observed that they +often then strain their lower eyes so as to look upward; and their skulls are +thus rendered rather crooked. These fishes, however, are soon able to hold +themselves in a vertical position, and no permanent effect is thus produced. +With the Pleuronectidæ, on the other hand, the older they grow the more +habitually they rest on one side, owing to the increasing flatness of their +bodies, and a permanent effect is thus produced on the form of the head, and on +the position of the eyes. Judging from analogy, the tendency to distortion +would no doubt be increased through the principle of inheritance. Schiödte +believes, in opposition to some other naturalists, that the Pleuronectidæ are +not quite symmetrical even in the embryo; and if this be so, we could +understand how it is that certain species, while young, habitually fall over +and rest on the left side, and other species on the right side. Malm adds, in +confirmation of the above view, that the adult Trachypterus arcticus, which is +not a member of the Pleuronectidæ, rests on its left side at the bottom, and +swims diagonally through the water; and in this fish, the two sides of the head +are said to be somewhat dissimilar. Our great authority on Fishes, Dr. Günther, +concludes his abstract of Malm’s paper, by remarking that “the +author gives a very simple explanation of the abnormal condition of the +Pleuronectoids.” +</p> + +<p> +We thus see that the first stages of the transit of the eye from one side of +the head to the other, which Mr. Mivart considers would +<a name="Page188"></a> +be injurious, may be attributed to the habit, no doubt beneficial to the +individual and to the species, of endeavouring to look upward with both eyes, +while resting on one side at the bottom. We may also attribute to the inherited +effects of use the fact of the mouth in several kinds of flat-fish being bent +towards the lower surface, with the jaw bones stronger and more effective on +this, the eyeless side of the head, than on the other, for the sake, as Dr. +Traquair supposes, of feeding with ease on the ground. Disuse, on the other +hand, will account for the less developed condition of the whole inferior half +of the body, including the lateral fins; though Yarrel thinks that the reduced +size of these fins is advantageous to the fish, as “there is so much less +room for their action than with the larger fins above.” Perhaps the +lesser number of teeth in the proportion of four to seven in the upper halves +of the two jaws of the plaice, to twenty-five to thirty in the lower halves, +may likewise be accounted for by disuse. From the colourless state of the +ventral surface of most fishes and of many other animals, we may reasonably +suppose that the absence of colour in flat-fish on the side, whether it be the +right or left, which is under-most, is due to the exclusion of light. But it +cannot be supposed that the peculiar speckled appearance of the upper side of +the sole, so like the sandy bed of the sea, or the power in some species, as +recently shown by Pouchet, of changing their colour in accordance with the +surrounding surface, or the presence of bony tubercles on the upper side of the +turbot, are due to the action of the light. Here natural selection has probably +come into play, as well as in adapting the general shape of the body of these +fishes, and many other peculiarities, to their habits of life. We should keep +in mind, as I have before insisted, that the inherited effects of the increased +use of parts, and perhaps of their disuse, will be strengthened by natural +selection. For all spontaneous variations in the right direction will thus be +preserved; as will those individuals which inherit in the highest degree the +effects of the increased and beneficial use of any part. How much to attribute +in each particular case to the effects of use, and how much to natural +selection, it seems impossible to decide. +</p> + +<p> +I may give another instance of a structure which apparently owes its origin +exclusively to use or habit. The extremity of the tail in some American monkeys +has been converted into a wonderfully perfect prehensile organ, and serves as a +fifth hand. A reviewer, who agrees with Mr. Mivart in every detail, remarks on +this structure: “It is impossible to believe that in any number of ages +the first slight incipient tendency to grasp could preserve the lives of +<a name="Page189"></a> +the individuals possessing it, or favour their chance of having and of rearing +offspring.” But there is no necessity for any such belief. Habit, and +this almost implies that some benefit great or small is thus derived, would in +all probability suffice for the work. Brehm saw the young of an African monkey +(Cercopithecus) clinging to the under surface of their mother by their hands, +and at the same time they hooked their little tails round that of their mother. +Professor Henslow kept in confinement some harvest mice (Mus messorius) which +do not possess a structurally prehensive tail; but he frequently observed that +they curled their tails round the branches of a bush placed in the cage, and +thus aided themselves in climbing. I have received an analogous account from +Dr. Günther, who has seen a mouse thus suspend itself. If the harvest mouse had +been more strictly arboreal, it would perhaps have had its tail rendered +structurally prehensile, as is the case with some members of the same order. +Why Cercopithecus, considering its habits while young, has not become thus +provided, it would be difficult to say. It is, however, possible that the long +tail of this monkey may be of more service to it as a balancing organ in making +its prodigious leaps, than as a prehensile organ. +</p> + +<p class="p2"> +The mammary glands are common to the whole class of mammals, and are +indispensable for their existence; they must, therefore, have been developed at +an extremely remote period, and we can know nothing positively about their +manner of development. Mr. Mivart asks: “Is it conceivable that the young +of any animal was ever saved from destruction by accidentally sucking a drop of +scarcely nutritious fluid from an accidentally hypertrophied cutaneous gland of +its mother? And even if one was so, what chance was there of the perpetuation +of such a variation?” But the case is not here put fairly. It is admitted +by most evolutionists that mammals are descended from a marsupial form; and if +so, the mammary glands will have been at first developed within the marsupial +sack. In the case of the fish (Hippocampus) the eggs are hatched, and the young +are reared for a time, within a sack of this nature; and an American +naturalist, Mr. Lockwood, believes from what he has seen of the development of +the young, that they are nourished by a secretion from the cutaneous glands of +the sack. Now, with the early progenitors of mammals, almost before they +deserved to be thus designated, is it not at least possible that the young +might have been similarly nourished? And in this case, the individuals which +secreted a fluid, in some degree or manner the most nutritious, so as to +partake of the nature of milk, would +<a name="Page190"></a> +in the long run have reared a larger number of well-nourished offspring, than +would the individuals which secreted a poorer fluid; and thus the cutaneous +glands, which are the homologues of the mammary glands, would have been +improved or rendered more effective. It accords with the widely extended +principle of specialisation, that the glands over a certain space of the sack +should have become more highly developed than the remainder; and they would +then have formed a breast, but at first without a nipple, as we see in the +Ornithorhyncus, at the base of the mammalian series. Through what agency the +glands over a certain space became more highly specialised than the others, I +will not pretend to decide, whether in part through compensation of growth, the +effects of use, or of natural selection. +</p> + +<p> +The development of the mammary glands would have been of no service, and could +not have been affected through natural selection, unless the young at the same +time were able to partake of the secretion. There is no greater difficulty in +understanding how young mammals have instinctively learned to suck the breast, +than in understanding how unhatched chickens have learned to break the +egg-shell by tapping against it with their specially adapted beaks; or how a +few hours after leaving the shell they have learned to pick up grains of food. +In such cases the most probable solution seems to be, that the habit was at +first acquired by practice at a more advanced age, and afterwards transmitted +to the offspring at an earlier age. But the young kangaroo is said not to suck, +only to cling to the nipple of its mother, who has the power of injecting milk +into the mouth of her helpless, half-formed offspring. On this head Mr. Mivart +remarks: “Did no special provision exist, the young one must infallibly +be choked by the intrusion of the milk into the wind-pipe. But there <i>is</i> +a special provision. The larynx is so elongated that it rises up into the +posterior end of the nasal passage, and is thus enabled to give free entrance +to the air for the lungs, while the milk passes harmlessly on each side of this +elongated larynx, and so safely attains the gullet behind it.” Mr. Mivart +then asks how did natural selection remove in the adult kangaroo (and in most +other mammals, on the assumption that they are descended from a marsupial +form), “this at least perfectly innocent and harmless structure?” +It may be suggested in answer that the voice, which is certainly of high +importance to many animals, could hardly have been used with full force as long +as the larynx entered the nasal passage; and Professor Flower has suggested to +me that this structure would have greatly interfered with an animal swallowing +solid food. +</p> + +<p> +<a name="Page191"></a> +We will now turn for a short space to the lower divisions of the animal +kingdom. The Echinodermata (star-fishes, sea-urchins, &c.) are furnished +with remarkable organs, called pedicellariæ, which consist, when well +developed, of a tridactyle forceps—that is, of one formed of three +serrated arms, neatly fitting together and placed on the summit of a flexible +stem, moved by muscles. These forceps can seize firmly hold of any object; and +Alexander Agassiz has seen an Echinus or sea-urchin rapidly passing particles +of excrement from forceps to forceps down certain lines of its body, in order +that its shell should not be fouled. But there is no doubt that besides +removing dirt of all kinds, they subserve other functions; and one of these +apparently is defence. +</p> + +<p> +With respect to these organs, Mr. Mivart, as on so many previous occasions, +asks: “What would be the utility of the <i>first rudimentary +beginnings</i> of such structures, and how could such insipient buddings have +ever preserved the life of a single Echinus?” He adds, “not even +the <i>sudden</i> development of the snapping action would have been beneficial +without the freely movable stalk, nor could the latter have been efficient +without the snapping jaws, yet no minute, nearly indefinite variations could +simultaneously evolve these complex co-ordinations of structure; to deny this +seems to do no less than to affirm a startling paradox.” Paradoxical as +this may appear to Mr. Mivart, tridactyle forcepses, immovably fixed at the +base, but capable of a snapping action, certainly exist on some star-fishes; +and this is intelligible if they serve, at least in part, as a means of +defence. Mr. Agassiz, to whose great kindness I am indebted for much +information on the subject, informs me that there are other star-fishes, in +which one of the three arms of the forceps is reduced to a support for the +other two; and again, other genera in which the third arm is completely lost. +In Echinoneus, the shell is described by M. Perrier as bearing two kinds of +pedicellariæ, one resembling those of Echinus, and the other those of +Spatangus; and such cases are always interesting as affording the means of +apparently sudden transitions, through the abortion of one of the two states of +an organ. +</p> + +<p> +With respect to the steps by which these curious organs have been evolved, Mr. +Agassiz infers from his own researches and those of Mr. Müller, that both in +star-fishes and sea-urchins the pedicellariæ must undoubtedly be looked at as +modified spines. This may be inferred from their manner of development in the +individual, as well as from a long and perfect series of gradations in +different species and genera, from simple granules to ordinary spines, to +perfect tridactyle pedicellariæ. The gradation extends even to +<a name="Page192"></a> +the manner in which ordinary spines and the pedicellariæ, with their supporting +calcareous rods, are articulated to the shell. In certain genera of +star-fishes, “the very combinations needed to show that the pedicellariæ +are only modified branching spines” may be found. Thus we have fixed +spines, with three equi-distant, serrated, movable branches, articulated to +near their bases; and higher up, on the same spine, three other movable +branches. Now when the latter arise from the summit of a spine they form, in +fact, a rude tridactyle pedicellariæ, and such may be seen on the same spine +together with the three lower branches. In this case the identity in nature +between the arms of the pedicellariæ and the movable branches of a spine, is +unmistakable. It is generally admitted that the ordinary spines serve as a +protection; and if so, there can be no reason to doubt that those furnished +with serrated and movable branches likewise serve for the same purpose; and +they would thus serve still more effectively as soon as by meeting together +they acted as a prehensile or snapping apparatus. Thus every gradation, from an +ordinary fixed spine to a fixed pedicellariæ, would be of service. +</p> + +<p> +In certain genera of star-fishes these organs, instead of being fixed or borne +on an immovable support, are placed on the summit of a flexible and muscular, +though short, stem; and in this case they probably subserve some additional +function besides defence. In the sea-urchins the steps can be followed by which +a fixed spine becomes articulated to the shell, and is thus rendered movable. I +wish I had space here to give a fuller abstract of Mr. Agassiz’s +interesting observations on the development of the pedicellariæ. All possible +gradations, as he adds, may likewise be found between the pedicellariæ of the +star-fishes and the hooks of the Ophiurians, another group of the +Echinodermata; and again between the pedicellariæ of sea-urchins and the +anchors of the Holothuriæ, also belonging to the same great class. +</p> + +<p class="p2"> +Certain compound animals, or zoophytes, as they have been termed, namely the +Polyzoa, are provided with curious organs called avicularia. These differ much +in structure in the different species. In their most perfect condition they +curiously resemble the head and beak of a vulture in miniature, seated on a +neck and capable of movement, as is likewise the lower jaw or mandible. In one +species observed by me, all the avicularia on the same branch often moved +simultaneously backwards and forwards, with the lower jaw widely open, through +an angle of about 90 degrees, in the course of five seconds; and their movement +caused the whole polyzoary to +<a name="Page193"></a> +tremble. When the jaws are touched with a needle they seize it so firmly that +the branch can thus be shaken. +</p> + +<p> +Mr. Mivart adduces this case, chiefly on account of the supposed difficulty of +organs, namely the avicularia of the Polyzoa and the pedicellariæ of the +Echinodermata, which he considers as “essentially similar,” having +been developed through natural selection in widely distinct divisions of the +animal kingdom. But, as far as structure is concerned, I can see no similarity +between tridactyle pedicellariæ and avicularia. The latter resembles somewhat +more closely the chelæ or pincers of Crustaceans; and Mr. Mivart might have +adduced with equal appropriateness this resemblance as a special difficulty, or +even their resemblance to the head and beak of a bird. The avicularia are +believed by Mr. Busk, Dr. Smitt and Dr. Nitsche—naturalists who have +carefully studied this group—to be homologous with the zooids and their +cells which compose the zoophyte, the movable lip or lid of the cell +corresponding with the lower and movable mandible of the avicularium. Mr. Busk, +however, does not know of any gradations now existing between a zooid and an +avicularium. It is therefore impossible to conjecture by what serviceable +gradations the one could have been converted into the other, but it by no means +follows from this that such gradations have not existed. +</p> + +<p> +As the chelæ of Crustaceans resemble in some degree the avicularia of Polyzoa, +both serving as pincers, it may be worth while to show that with the former a +long series of serviceable gradations still exists. In the first and simplest +stage, the terminal segment of a limb shuts down either on the square summit of +the broad penultimate segment, or against one whole side, and is thus enabled +to catch hold of an object, but the limb still serves as an organ of +locomotion. We next find one corner of the broad penultimate segment slightly +prominent, sometimes furnished with irregular teeth, and against these the +terminal segment shuts down. By an increase in the size of this projection, +with its shape, as well as that of the terminal segment, slightly modified and +improved, the pincers are rendered more and more perfect, until we have at last +an instrument as efficient as the chelæ of a lobster. And all these gradations +can be actually traced. +</p> + +<p> +Besides the avicularia, the polyzoa possess curious organs called vibracula. +These generally consist of long bristles, capable of movement and easily +excited. In one species examined by me the vibracula were slightly curved and +serrated along the outer margin, and all of them on the same polyzoary often +moved simultaneously; so that, acting like long oars, they swept a branch +rapidly +<a name="Page194"></a> +across the object-glass of my microscope. When a branch was placed on its face, +the vibracula became entangled, and they made violent efforts to free +themselves. They are supposed to serve as a defence, and may be seen, as Mr. +Busk remarks, “to sweep slowly and carefully over the surface of the +polyzoary, removing what might be noxious to the delicate inhabitants of the +cells when their tentacula are protruded.” The avicularia, like the +vibracula, probably serve for defence, but they also catch and kill small +living animals, which, it is believed, are afterwards swept by the currents +within reach of the tentacula of the zooids. Some species are provided with +avicularia and vibracula, some with avicularia alone and a few with vibracula +alone. +</p> + +<p> +It is not easy to imagine two objects more widely different in appearance than +a bristle or vibraculum, and an avicularium like the head of a bird; yet they +are almost certainly homologous and have been developed from the same common +source, namely a zooid with its cell. Hence, we can understand how it is that +these organs graduate in some cases, as I am informed by Mr. Busk, into each +other. Thus, with the avicularia of several species of Lepralia, the movable +mandible is so much produced and is so like a bristle that the presence of the +upper or fixed beak alone serves to determine its avicularian nature. The +vibracula may have been directly developed from the lips of the cells, without +having passed through the avicularian stage; but it seems more probable that +they have passed through this stage, as during the early stages of the +transformation, the other parts of the cell, with the included zooid, could +hardly have disappeared at once. In many cases the vibracula have a grooved +support at the base, which seems to represent the fixed beak; though this +support in some species is quite absent. This view of the development of the +vibracula, if trustworthy, is interesting; for supposing that all the species +provided with avicularia had become extinct, no one with the most vivid +imagination would ever have thought that the vibracula had originally existed +as part of an organ, resembling a bird’s head, or an irregular box or +hood. It is interesting to see two such widely different organs developed from +a common origin; and as the movable lip of the cell serves as a protection to +the zooid, there is no difficulty in believing that all the gradations, by +which the lip became converted first into the lower mandible of an avicularium, +and then into an elongated bristle, likewise served as a protection in +different ways and under different circumstances. +</p> + +<p class="p2"> +In the vegetable kingdom Mr. Mivart only alludes to two cases, +<a name="Page195"></a> +namely the structure of the flowers of orchids, and the movements of climbing +plants. With respect to the former, he says: “The explanation of their +<i>origin</i> is deemed thoroughly unsatisfactory—utterly insufficient to +explain the incipient, infinitesimal beginnings of structures which are of +utility only when they are considerably developed.” As I have fully +treated this subject in another work, I will here give only a few details on +one alone of the most striking peculiarities of the flowers of orchids, namely, +their pollinia. A pollinium, when highly developed, consists of a mass of +pollen-grains, affixed to an elastic foot-stalk or caudicle, and this to a +little mass of extremely viscid matter. The pollinia are by this means +transported by insects from one flower to the stigma of another. In some +orchids there is no caudicle to the pollen-masses, and the grains are merely +tied together by fine threads; but as these are not confined to orchids, they +need not here be considered; yet I may mention that at the base of the +orchidaceous series, in Cypripedium, we can see how the threads were probably +first developed. In other orchids the threads cohere at one end of the +pollen-masses; and this forms the first or nascent trace of a caudicle. That +this is the origin of the caudicle, even when of considerable length and highly +developed, we have good evidence in the aborted pollen-grains which can +sometimes be detected embedded within the central and solid parts. +</p> + +<p> +With respect to the second chief peculiarity, namely, the little mass of viscid +matter attached to the end of the caudicle, a long series of gradations can be +specified, each of plain service to the plant. In most flowers belonging to +other orders the stigma secretes a little viscid matter. Now, in certain +orchids similar viscid matter is secreted, but in much larger quantities by one +alone of the three stigmas; and this stigma, perhaps in consequence of the +copious secretion, is rendered sterile. When an insect visits a flower of this +kind, it rubs off some of the viscid matter, and thus at the same time drags +away some of the pollen-grains. From this simple condition, which differs but +little from that of a multitude of common flowers, there are endless +gradations—to species in which the pollen-mass terminates in a very +short, free caudicle—to others in which the caudicle becomes firmly +attached to the viscid matter, with the sterile stigma itself much modified. In +this latter case we have a pollinium in its most highly developed and perfect +condition. He who will carefully examine the flowers of orchids for himself +will not deny the existence of the above series of gradations—from a mass +of pollen-grains merely tied together by threads, with the stigma differing but +little from that of the ordinary flowers, +<a name="Page196"></a> +to a highly complex pollinium, admirably adapted for transportal by insects; +nor will he deny that all the gradations in the several species are admirably +adapted in relation to the general structure of each flower for its +fertilisation by different insects. In this, and in almost every other case, +the enquiry may be pushed further backwards; and it may be asked how did the +stigma of an ordinary flower become viscid, but as we do not know the full +history of any one group of beings, it is as useless to ask, as it is hopeless +to attempt answering, such questions. +</p> + +<p> +We will now turn to climbing plants. These can be arranged in a long series, +from those which simply twine round a support, to those which I have called +leaf-climbers, and to those provided with tendrils. In these two latter classes +the stems have generally, but not always, lost the power of twining, though +they retain the power of revolving, which the tendrils likewise possess. The +gradations from leaf-climbers to tendril bearers are wonderfully close, and +certain plants may be differently placed in either class. But in ascending the +series from simple twiners to leaf-climbers, an important quality is added, +namely sensitiveness to a touch, by which means the foot-stalks of the leaves +or flowers, or these modified and converted into tendrils, are excited to bend +round and clasp the touching object. He who will read my memoir on these plants +will, I think, admit that all the many gradations in function and structure +between simple twiners and tendril-bearers are in each case beneficial in a +high degree to the species. For instance, it is clearly a great advantage to a +twining plant to become a leaf-climber; and it is probable that every twiner +which possessed leaves with long foot-stalks would have been developed into a +leaf-climber, if the foot-stalks had possessed in any slight degree the +requisite sensitiveness to a touch. +</p> + +<p> +As twining is the simplest means of ascending a support, and forms the basis of +our series, it may naturally be asked how did plants acquire this power in an +incipient degree, afterwards to be improved and increased through natural +selection. The power of twining depends, firstly, on the stems while young +being extremely flexible (but this is a character common to many plants which +are not climbers); and, secondly, on their continually bending to all points of +the compass, one after the other in succession, in the same order. By this +movement the stems are inclined to all sides, and are made to move round and +round. As soon as the lower part of a stem strikes against any object and is +stopped, the upper part still goes on bending and revolving, and thus +necessarily twines round and up the support. The revolving movement ceases +after +<a name="Page197"></a> +the early growth of each shoot. As in many widely separated families of plants, +single species and single genera possess the power of revolving, and have thus +become twiners, they must have independently acquired it, and cannot have +inherited it from a common progenitor. Hence, I was led to predict that some +slight tendency to a movement of this kind would be found to be far from +uncommon with plants which did not climb; and that this had afforded the basis +for natural selection to work on and improve. When I made this prediction, I +knew of only one imperfect case, namely, of the young flower-peduncles of a +Maurandia which revolved slightly and irregularly, like the stems of twining +plants, but without making any use of this habit. Soon afterwards Fritz Müller +discovered that the young stems of an Alisma and of a Linum—plants which +do not climb and are widely separated in the natural system—revolved +plainly, though irregularly, and he states that he has reason to suspect that +this occurs with some other plants. These slight movements appear to be of no +service to the plants in question; anyhow, they are not of the least use in the +way of climbing, which is the point that concerns us. Nevertheless we can see +that if the stems of these plants had been flexible, and if under the +conditions to which they are exposed it had profited them to ascend to a +height, then the habit of slightly and irregularly revolving might have been +increased and utilised through natural selection, until they had become +converted into well-developed twining species. +</p> + +<p> +With respect to the sensitiveness of the foot-stalks of the leaves and flowers, +and of tendrils, nearly the same remarks are applicable as in the case of the +revolving movements of twining plants. As a vast number of species, belonging +to widely distinct groups, are endowed with this kind of sensitiveness, it +ought to be found in a nascent condition in many plants which have not become +climbers. This is the case: I observed that the young flower-peduncles of the +above Maurandia curved themselves a little towards the side which was touched. +Morren found in several species of Oxalis that the leaves and their foot-stalks +moved, especially after exposure to a hot sun, when they were gently and +repeatedly touched, or when the plant was shaken. I repeated these observations +on some other species of Oxalis with the same result; in some of them the +movement was distinct, but was best seen in the young leaves; in others it was +extremely slight. It is a more important fact that according to the high +authority of Hofmeister, the young shoots and leaves of all plants move after +being shaken; and with climbing plants it is, as we know, only during the early +stages of growth that the foot-stalks and tendrils are sensitive. +</p> + +<p> +<a name="Page198"></a> +It is scarcely possible that the above slight movements, due to a touch or +shake, in the young and growing organs of plants, can be of any functional +importance to them. But plants possess, in obedience to various stimuli, powers +of movement, which are of manifest importance to them; for instance, towards +and more rarely from the light—in opposition to, and more rarely in the +direction of, the attraction of gravity. When the nerves and muscles of an +animal are excited by galvanism or by the absorption of strychnine, the +consequent movements may be called an incidental result, for the nerves and +muscles have not been rendered specially sensitive to these stimuli. So with +plants it appears that, from having the power of movement in obedience to +certain stimuli, they are excited in an incidental manner by a touch, or by +being shaken. Hence there is no great difficulty in admitting that in the case +of leaf-climbers and tendril-bearers, it is this tendency which has been taken +advantage of and increased through natural selection. It is, however, probable, +from reasons which I have assigned in my memoir, that this will have occurred +only with plants which had already acquired the power of revolving, and had +thus become twiners. +</p> + +<p> +I have already endeavoured to explain how plants became twiners, namely, by the +increase of a tendency to slight and irregular revolving movements, which were +at first of no use to them; this movement, as well as that due to a touch or +shake, being the incidental result of the power of moving, gained for other and +beneficial purposes. Whether, during the gradual development of climbing +plants, natural selection has been aided by the inherited effects of use, I +will not pretend to decide; but we know that certain periodical movements, for +instance the so-called sleep of plants, are governed by habit. +</p> + +<p class="p2"> +I have now considered enough, perhaps more than enough, of the cases, selected +with care by a skilful naturalist, to prove that natural selection is +incompetent to account for the incipient stages of useful structures; and I +have shown, as I hope, that there is no great difficulty on this head. A good +opportunity has thus been afforded for enlarging a little on gradations of +structure, often associated with strange functions—an important subject, +which was not treated at sufficient length in the former editions of this work. +I will now briefly recapitulate the foregoing cases. +</p> + +<p> +With the giraffe, the continued preservation of the individuals of some extinct +high-reaching ruminant, which had the longest necks, legs, &c., and could +browse a little above the average height, and +<a name="Page199"></a> +the continued destruction of those which could not browse so high, would have +sufficed for the production of this remarkable quadruped; but the prolonged use +of all the parts, together with inheritance, will have aided in an important +manner in their co-ordination. With the many insects which imitate various +objects, there is no improbability in the belief that an accidental resemblance +to some common object was in each case the foundation for the work of natural +selection, since perfected through the occasional preservation of slight +variations which made the resemblance at all closer; and this will have been +carried on as long as the insect continued to vary, and as long as a more and +more perfect resemblance led to its escape from sharp-sighted enemies. In +certain species of whales there is a tendency to the formation of irregular +little points of horn on the palate; and it seems to be quite within the scope +of natural selection to preserve all favourable variations, until the points +were converted, first into lamellated knobs or teeth, like those on the beak of +a goose—then into short lamellæ, like those of the domestic +ducks—and then into lamellæ, as perfect as those of the +shoveller-duck—and finally into the gigantic plates of baleen, as in the +mouth of the Greenland whale. In the family of the ducks, the lamellæ are first +used as teeth, then partly as teeth and partly as a sifting apparatus, and at +last almost exclusively for this latter purpose. +</p> + +<p> +With such structures as the above lamellæ of horn or whalebone, habit or use +can have done little or nothing, as far as we can judge, towards their +development. On the other hand, the transportal of the lower eye of a flat-fish +to the upper side of the head, and the formation of a prehensile tail, may be +attributed almost wholly to continued use, together with inheritance. With +respect to the mammæ of the higher animals, the most probable conjecture is +that primordially the cutaneous glands over the whole surface of a marsupial +sack secreted a nutritious fluid; and that these glands were improved in +function through natural selection, and concentrated into a confined area, in +which case they would have formed a mamma. There is no more difficulty in +understanding how the branched spines of some ancient Echinoderm, which served +as a defence, became developed through natural selection into tridactyle +pedicellariæ, than in understanding the development of the pincers of +crustaceans, through slight, serviceable modifications in the ultimate and +penultimate segments of a limb, which was at first used solely for locomotion. +In the avicularia and vibracula of the Polyzoa we have organs widely different +in appearance developed from the same source; and with the vibracula we can +understand how the successive gradations might have been +<a name="Page200"></a> +of service. With the pollinia of orchids, the threads which originally served +to tie together the pollen-grains, can be traced cohering into caudicles; and +the steps can likewise be followed by which viscid matter, such as that +secreted by the stigmas of ordinary flowers, and still subserving nearly but +not quite the same purpose, became attached to the free ends of the +caudicles—all these gradations being of manifest benefit to the plants in +question. With respect to climbing plants, I need not repeat what has been so +lately said. +</p> + +<p> +It has often been asked, if natural selection be so potent, why has not this or +that structure been gained by certain species, to which it would apparently +have been advantageous? But it is unreasonable to expect a precise answer to +such questions, considering our ignorance of the past history of each species, +and of the conditions which at the present day determine its numbers and range. +In most cases only general reasons, but in some few cases special reasons, can +be assigned. Thus to adapt a species to new habits of life, many co-ordinated +modifications are almost indispensable, and it may often have happened that the +requisite parts did not vary in the right manner or to the right degree. Many +species must have been prevented from increasing in numbers through destructive +agencies, which stood in no relation to certain structures, which we imagine +would have been gained through natural selection from appearing to us +advantageous to the species. In this case, as the struggle for life did not +depend on such structures, they could not have been acquired through natural +selection. In many cases complex and long-enduring conditions, often of a +peculiar nature, are necessary for the development of a structure; and the +requisite conditions may seldom have concurred. The belief that any given +structure, which we think, often erroneously, would have been beneficial to a +species, would have been gained under all circumstances through natural +selection, is opposed to what we can understand of its manner of action. Mr. +Mivart does not deny that natural selection has effected something; but he +considers it as “demonstrably insufficient” to account for the +phenomena which I explain by its agency. His chief arguments have now been +considered, and the others will hereafter be considered. They seem to me to +partake little of the character of demonstration, and to have little weight in +comparison with those in favour of the power of natural selection, aided by the +other agencies often specified. I am bound to add, that some of the facts and +arguments here used by me, have been advanced for the same purpose in an able +article lately published in the “Medico-Chirurgical Review.” +</p> + +<p> +<a name="Page201"></a> +At the present day almost all naturalists admit evolution under some form. Mr. +Mivart believes that species change through “an internal force or +tendency,” about which it is not pretended that anything is known. That +species have a capacity for change will be admitted by all evolutionists; but +there is no need, as it seems to me, to invoke any internal force beyond the +tendency to ordinary variability, which through the aid of selection, by man +has given rise to many well-adapted domestic races, and which, through the aid +of natural selection, would equally well give rise by graduated steps to +natural races or species. The final result will generally have been, as already +explained, an advance, but in some few cases a retrogression, in organisation. +</p> + +<p> +Mr. Mivart is further inclined to believe, and some naturalists agree with him, +that new species manifest themselves “with suddenness and by +modifications appearing at once.” For instance, he supposes that the +differences between the extinct three-toed Hipparion and the horse arose +suddenly. He thinks it difficult to believe that the wing of a bird “was +developed in any other way than by a comparatively sudden modification of a +marked and important kind;” and apparently he would extend the same view +to the wings of bats and pterodactyles. This conclusion, which implies great +breaks or discontinuity in the series, appears to me improbable in the highest +degree. +</p> + +<p> +Everyone who believes in slow and gradual evolution, will of course admit that +specific changes may have been as abrupt and as great as any single variation +which we meet with under nature, or even under domestication. But as species +are more variable when domesticated or cultivated than under their natural +conditions, it is not probable that such great and abrupt variations have often +occurred under nature, as are known occasionally to arise under domestication. +Of these latter variations several may be attributed to reversion; and the +characters which thus reappear were, it is probable, in many cases at first +gained in a gradual manner. A still greater number must be called +monstrosities, such as six-fingered men, porcupine men, Ancon sheep, Niata +cattle, &c.; and as they are widely different in character from natural +species, they throw very little light on our subject. Excluding such cases of +abrupt variations, the few which remain would at best constitute, if found in a +state of nature, doubtful species, closely related to their parental types. +</p> + +<p> +My reasons for doubting whether natural species have changed as abruptly as +have occasionally domestic races, and for entirely disbelieving that they have +changed in the wonderful manner +<a name="Page202"></a> +indicated by Mr. Mivart, are as follows. According to our experience, abrupt +and strongly marked variations occur in our domesticated productions, singly +and at rather long intervals of time. If such occurred under nature, they would +be liable, as formerly explained, to be lost by accidental causes of +destruction and by subsequent intercrossing; and so it is known to be under +domestication, unless abrupt variations of this kind are specially preserved +and separated by the care of man. Hence, in order that a new species should +suddenly appear in the manner supposed by Mr. Mivart, it is almost necessary to +believe, in opposition to all analogy, that several wonderfully changed +individuals appeared simultaneously within the same district. This difficulty, +as in the case of unconscious selection by man, is avoided on the theory of +gradual evolution, through the preservation of a large number of individuals, +which varied more or less in any favourable direction, and of the destruction +of a large number which varied in an opposite manner. +</p> + +<p> +That many species have been evolved in an extremely gradual manner, there can +hardly be a doubt. The species and even the genera of many large natural +families are so closely allied together that it is difficult to distinguish not +a few of them. On every continent, in proceeding from north to south, from +lowland to upland, &c., we meet with a host of closely related or +representative species; as we likewise do on certain distinct continents, which +we have reason to believe were formerly connected. But in making these and the +following remarks, I am compelled to allude to subjects hereafter to be +discussed. Look at the many outlying islands round a continent, and see how +many of their inhabitants can be raised only to the rank of doubtful species. +So it is if we look to past times, and compare the species which have just +passed away with those still living within the same areas; or if we compare the +fossil species embedded in the sub-stages of the same geological formation. It +is indeed manifest that multitudes of species are related in the closest manner +to other species that still exist, or have lately existed; and it will hardly +be maintained that such species have been developed in an abrupt or sudden +manner. Nor should it be forgotten, when we look to the special parts of allied +species, instead of to distinct species, that numerous and wonderfully fine +gradations can be traced, connecting together widely different structures. +</p> + +<p> +Many large groups of facts are intelligible only on the principle that species +have been evolved by very small steps. For instance, the fact that the species +included in the larger genera are more closely related to each other, and +present a greater number of varieties than do the species in the smaller +genera. The former are also +<a name="Page203"></a> +grouped in little clusters, like varieties round species; and they present +other analogies with varieties, as was shown in our second chapter. On this +same principle we can understand how it is that specific characters are more +variable than generic characters; and how the parts which are developed in an +extraordinary degree or manner are more variable than other parts of the same +species. Many analogous facts, all pointing in the same direction, could be +added. +</p> + +<p> +Although very many species have almost certainly been produced by steps not +greater than those separating fine varieties; yet it may be maintained that +some have been developed in a different and abrupt manner. Such an admission, +however, ought not to be made without strong evidence being assigned. The vague +and in some respects false analogies, as they have been shown to be by Mr. +Chauncey Wright, which have been advanced in favour of this view, such as the +sudden crystallisation of inorganic substances, or the falling of a facetted +spheroid from one facet to another, hardly deserve consideration. One class of +facts, however, namely, the sudden appearance of new and distinct forms of life +in our geological formations supports at first sight the belief in abrupt +development. But the value of this evidence depends entirely on the perfection +of the geological record, in relation to periods remote in the history of the +world. If the record is as fragmentary as many geologists strenuously assert, +there is nothing strange in new forms appearing as if suddenly developed. +</p> + +<p> +Unless we admit transformations as prodigious as those advocated by Mr. Mivart, +such as the sudden development of the wings of birds or bats, or the sudden +conversion of a Hipparion into a horse, hardly any light is thrown by the +belief in abrupt modifications on the deficiency of connecting links in our +geological formations. But against the belief in such abrupt changes, +embryology enters a strong protest. It is notorious that the wings of birds and +bats, and the legs of horses or other quadrupeds, are undistinguishable at an +early embryonic period, and that they become differentiated by insensibly fine +steps. Embryological resemblances of all kinds can be accounted for, as we +shall hereafter see, by the progenitors of our existing species having varied +after early youth, and having transmitted their newly-acquired characters to +their offspring, at a corresponding age. The embryo is thus left almost +unaffected, and serves as a record of the past condition of the species. Hence +it is that existing species during the early stages of their development so +often resemble ancient and extinct forms belonging to the same class. On this +view of the meaning of embryological +<a name="Page204"></a> +resemblances, and indeed on any view, it is incredible that an animal should +have undergone such momentous and abrupt transformations as those above +indicated, and yet should not bear even a trace in its embryonic condition of +any sudden modification, every detail in its structure being developed by +insensibly fine steps. +</p> + +<p> +He who believes that some ancient form was transformed suddenly through an +internal force or tendency into, for instance, one furnished with wings, will +be almost compelled to assume, in opposition to all analogy, that many +individuals varied simultaneously. It cannot be denied that such abrupt and +great changes of structure are widely different from those which most species +apparently have undergone. He will further be compelled to believe that many +structures beautifully adapted to all the other parts of the same creature and +to the surrounding conditions, have been suddenly produced; and of such complex +and wonderful co-adaptations, he will not be able to assign a shadow of an +explanation. He will be forced to admit that these great and sudden +transformations have left no trace of their action on the embryo. To admit all +this is, as it seems to me, to enter into the realms of miracle, and to leave +those of science. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page205"></a><a name="link2HCH0008" id="link2HCH0008"></a> +CHAPTER VIII.<br/> +INSTINCT.</h2> + +<p class="letter"> +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, molothrus, +ostrich, and parasitic bees—Slave-making ants—Hive-bee, its +cell-making instinct—Changes of instinct and structure not necessarily +simultaneous—Difficulties of the theory of the Natural Selection of +instincts—Neuter or sterile insects—Summary. +</p> + +<p> +Many instincts are so wonderful that their development will probably appear to +the reader a difficulty sufficient to overthrow my whole theory. I may here +premise, that I have nothing to do with the origin of the 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 faculties in animals of the +same class. +</p> + +<p> +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 require experience to enable us to perform, when performed by an +animal, more especially by a very young one, without 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 are universal. A little dose of judgment or +reason, as Pierre Huber expresses it, often comes into play, even with animals +low in the scale of nature. +</p> + +<p> +Frederick Cuvier and several of the older metaphysicians have compared instinct +with habit. This comparison gives, I think, an accurate notion of the frame of +mind under which an instinctive action is performed, but not necessarily 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, with +certain periods of time and states of the body. When once acquired, they often +remain constant throughout life. +<a name="Page206"></a> +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 deriving any benefit from 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. +</p> + +<p> +If we suppose any habitual action to become inherited—and 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, be +might truly be said to have done so instinctively. But it would be a 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 acquired by habit. +</p> + +<p> +It will be universally admitted that instincts are as important as corporeal +structures 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 was 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 in many cases of subordinate importance +to the effects of the natural selection +<a name="Page207"></a> +of what may be called spontaneous variations of instincts;—that is of +variations produced by the same unknown causes which produce slight deviations +of bodily structure. +</p> + +<p> +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 among extinct species, how very generally gradations, leading to +the most complex instincts, can be discovered. 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, &c.; in which case either the 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. +</p> + +<p> +Again, as in the case of corporeal structure, and conformably to 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, +as was first observed by Huber, 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, as soon as it felt the antennæ, immediately lifted up its +abdomen and excreted a limpid drop of sweet juice, which was eagerly devoured +<a name="Page208"></a> +by the ant. Even the quite young aphides behaved in this manner, showing that +the action was instinctive, and not the result of experience. It is certain, +from the observations of Huber, that the aphides show no dislike to the ants: +if the latter be not present they are at last compelled to eject their +excretion. But as the excretion is extremely viscid, it is no doubt a +convenience to the aphides to have it removed; therefore probably they do not +excrete solely for the good of the ants. Although there is no evidence that any +animal performs an action for the exclusive good of another species, yet each +tries to take advantage of the instincts of others, as each takes advantage of +the weaker bodily structure of other species. So again 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. +</p> + +<p> +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 be 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 the nests of the same species in the +northern and southern United States. Why, it has been asked, if instinct be +variable, has it not granted to the bee “the ability to use some other +material when wax was deficient?” But what other natural material could +bees use? They will work, as I have seen, with wax hardened with vermilion or +softened with lard. Andrew Knight observed that his bees, instead of +laboriously collecting propolis, used a cement of wax and turpentine, with +which he had covered decorticated trees. It has lately been shown that bees, +instead of searching for pollen, will gladly use a very different substance, +namely, oatmeal. 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. The +fear of man is slowly acquired, as I have elsewhere shown, by the various +animals which inhabit desert islands; and we see an instance of this, even in +England, in the greater wildness of all our large birds in comparison with 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 +<a name="Page209"></a> +are not more fearful than small; and the magpie, so wary in England, is tame in +Norway, as is the hooded crow in Egypt. +</p> + +<p> +That the mental qualities of animals of the same kind, born in a state of +nature, vary much, could be shown by many facts. Several cases could also be +adduced of occasional and strange habits in wild animals, which, if +advantageous to the species, might have given rise, through natural selection, +to new instincts. But I am well aware that these general statements, without +the facts 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. +</p> + +<p class="center"> +<i>Inherited Changes of Habit or Instinct in Domesticated Animals.</i> +</p> + +<p> +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 be enabled to see the part which habit and the +selection of so-called spontaneous variations have played in modifying the +mental qualities of our domestic animals. It is notorious how much domestic +animals vary in their mental qualities. With cats, for instance, one naturally +takes to catching rats, and another mice, and these tendencies are known to be +inherited. One cat, according to Mr. St. John, always brought home game birds, +another hares or rabbits, and another hunted on marshy ground and almost +nightly caught woodcocks or snipes. A number of curious and authentic instances +could be given of various shades of disposition and taste, 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 breeds of dogs: it cannot be doubted +that young pointers (I have myself seen striking instances) 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 behold 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 +<a name="Page210"></a> +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 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. +</p> + +<p> +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. +</p> + +<p> +Domestic instincts are sometimes spoken of as actions which have become +inherited solely from long-continued and compulsory habit, but this 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: the act of pointing is probably, as many have thought, only +the exaggerated pause of an animal preparing to spring on its prey. When the +first tendency to point 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 in progress, as each +man tries to procure, without intending to improve the breed, dogs which stand +and hunt best. On the other hand, habit alone in some cases has sufficed; +hardly any animal is more +<a name="Page211"></a> +difficult to tame than the young of the wild rabbit; scarcely any animal is +tamer than the young of the tame rabbit; but I can hardly suppose that domestic +rabbits have often been selected for tameness alone; so that we must attribute +at least the greater part of the inherited change from extreme wildness to +extreme tameness, to habit and long-continued close confinement. +</p> + +<p> +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 largely and how permanently the minds of our +domestic animals have been modified. 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 and some degree of selection have 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, for I am informed by Captain Hutton that the young chickens of the +parent stock, the Gallus bankiva, when reared in India under a hen, are at +first excessively wild. So it is with young pheasants reared in England 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. +</p> + +<p> +Hence, we may conclude that under domestication 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 +<a name="Page212"></a> +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 habit and selection have probably concurred. +</p> + +<p class="center"> +<i>Special Instincts.</i> +</p> + +<p> +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, namely, the instinct which leads the cuckoo to lay her eggs in other +birds’ nests; the slave-making instinct of certain ants; and the +cell-making power of the hive-bee: these two latter instincts have generally +and justly been ranked by naturalists as the most wonderful of all known +instincts. +</p> + +<p> +<i>Instincts of the Cuckoo.</i>—It is supposed by some naturalists that +the more immediate cause of the instinct of the cuckoo 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 migrates 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 both asserted and denied that the American cuckoo occasionally lays +her eggs in other birds’ nests; but I have lately heard from Dr. Merrill, +of Iowa, that he once found in Illinois a young cuckoo, together with a young +jay in the nest of a blue jay (Garrulus cristatus); and as both were nearly +full feathered, there could be no mistake in their identification. I could also +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, and +that she occasionally laid an egg in another bird’s nest. If the old bird +profited by this occasional habit through being enabled to emigrate earlier or +through any other cause; or if the young were made more vigorous by advantage +being taken of the mistaken instinct of another species than when reared by +their own mother, encumbered as she could 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 us to +<a name="Page213"></a> +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 more successful in +rearing their young. By a continued process of this nature, I believe that the +strange instinct of our cuckoo has been generated. It has, also recently been +ascertained on sufficient evidence, by Adolf Müller, that the cuckoo +occasionally lays her eggs on the bare ground, sits on them and feeds her +young. This rare event is probably a case of reversion to the long-lost, +aboriginal instinct of nidification. +</p> + +<p> +It has been objected that I have not noticed other related instincts and +adaptations of structure in the cuckoo, which are spoken of as necessarily +co-ordinated. But in all cases, speculation on an instinct known to us only in +a single species, is useless, for we have hitherto had no facts to guide us. +Until recently the instincts of the European and of the non-parasitic American +cuckoo alone were known; now, owing to Mr. Ramsay’s observations, we have +learned something about three Australian species, which lay their eggs in other +birds’ nests. The chief points to be referred to are three: first, that +the common cuckoo, with rare exceptions, lays only one egg in a nest, so that +the large and voracious young bird receives ample food. Secondly, that the eggs +are remarkably small, not exceeding those of the skylark—a bird about +one-fourth as large as the cuckoo. That the small size of the egg is a real +case of adaptation we may infer from the fact of the mon-parasitic American +cuckoo laying full-sized eggs. Thirdly, that the young cuckoo, soon after +birth, has the instinct, the strength and a properly shaped back for ejecting +its foster-brothers, which then perish from cold and hunger. This has been +boldly called a beneficent arrangement, in order that the young cuckoo may get +sufficient food, and that its foster-brothers may perish before they had +acquired much feeling! +</p> + +<p> +Turning now to the Australian species: though these birds generally lay only +one egg in a nest, it is not rare to find two and even three eggs in the same +nest. In the bronze cuckoo the eggs vary greatly in size, from eight to ten +lines in length. Now, if it had been of an advantage to this species to have +laid eggs even smaller than those now laid, so as to have deceived certain +foster-parents, or, as is more probable, to have been hatched within a shorter +period (for it is asserted that there is a relation between the size of eggs +and the period of their incubation), then there is no difficulty in believing +that a race or species might have been formed which would have laid smaller and +smaller eggs; for these would have been more safely hatched and reared. Mr. +Ramsay remarks that +<a name="Page214"></a> +two of the Australian cuckoos, when they lay their eggs in an open nest, +manifest a decided preference for nests containing eggs similar in colour to +their own. The European species apparently manifests some tendency towards a +similar instinct, but not rarely departs from it, as is shown by her laying her +dull and pale-coloured eggs in the nest of the hedge-warbler with bright +greenish-blue eggs. Had our cuckoo invariably displayed the above instinct, it +would assuredly have been added to those which it is assumed must all have been +acquired together. The eggs of the Australian bronze cuckoo vary, according to +Mr. Ramsay, to an extraordinary degree in colour; so that in this respect, as +well as in size, natural selection might have secured and fixed any +advantageous variation. +</p> + +<p> +In the case of the European cuckoo, the offspring of the foster-parents are +commonly ejected from the nest within three days after the cuckoo is hatched; +and as the latter at this age is in a most helpless condition, Mr. Gould was +formerly inclined to believe that the act of ejection was performed by the +foster-parents themselves. But he has now received a trustworthy account of a +young cuckoo which was actually seen, while still blind and not able even to +hold up its own head, in the act of ejecting its foster-brothers. One of these +was replaced in the nest by the observer, and was again thrown out. With +respect to the means by which this strange and odious instinct was acquired, if +it were of great importance for the young cuckoo, as is probably the case, to +receive as much food as possible soon after birth, I can see no special +difficulty in its having gradually acquired, during successive generations, the +blind desire, the strength, and structure necessary for the work of ejection; +for those cuckoos which had such habits and structure best developed would be +the most securely reared. The first step towards the acquisition of the proper +instinct might have been mere unintentional restlessness on the part of the +young bird, when somewhat advanced in age and strength; the habit having been +afterwards improved, and transmitted to an earlier age. I can see no more +difficulty in this than in the unhatched young of other birds acquiring the +instinct to break through their own shells; or than in young snakes acquiring +in their upper jaws, as Owen has remarked, a transitory sharp tooth for cutting +through the tough egg-shell. For if each part is liable to individual +variations at all ages, and the variations tend to be inherited at a +corresponding or earlier age—propositions which cannot be +disputed—then the instincts and structure of the young could be slowly +modified as surely as those of the adult; and both cases must stand or fall +together with the whole theory of natural selection. +</p> + +<p> +<a name="Page215"></a> +Some species of Molothrus, a widely distinct genus of American birds, allied to +our starlings, have parasitic habits like those of the cuckoo; and the species +present an interesting gradation in the perfection of their instincts. The +sexes of Molothrus badius are stated by an excellent observer, Mr. Hudson, +sometimes to live promiscuously together in flocks, and sometimes to pair. They +either build a nest of their own or seize on one belonging to some other bird, +occasionally throwing out the nestlings of the stranger. They either lay their +eggs in the nest thus appropriated, or oddly enough build one for themselves on +the top of it. They usually sit on their own eggs and rear their own young; but +Mr. Hudson says it is probable that they are occasionally parasitic, for he has +seen the young of this species following old birds of a distinct kind and +clamouring to be fed by them. The parasitic habits of another species of +Molothrus, the M. bonariensis, are much more highly developed than those of the +last, but are still far from perfect. This bird, as far as it is known, +invariably lays its eggs in the nests of strangers; but it is remarkable that +several together sometimes commence to build an irregular untidy nest of their +own, placed in singular ill-adapted situations, as on the leaves of a large +thistle. They never, however, as far as Mr. Hudson has ascertained, complete a +nest for themselves. They often lay so many eggs—from fifteen to +twenty—in the same foster-nest, that few or none can possibly be hatched. +They have, moreover, the extraordinary habit of pecking holes in the eggs, +whether of their own species or of their foster parents, which they find in the +appropriated nests. They drop also many eggs on the bare ground, which are thus +wasted. A third species, the M. pecoris of North America, has acquired +instincts as perfect as those of the cuckoo, for it never lays more than one +egg in a foster-nest, so that the young bird is securely reared. Mr. Hudson is +a strong disbeliever in evolution, but he appears to have been so much struck +by the imperfect instincts of the Molothrus bonariensis that he quotes my +words, and asks, “Must we consider these habits, not as especially +endowed or created instincts, but as small consequences of one general law, +namely, transition?” +</p> + +<p> +Various birds, as has already been remarked, occasionally lay their eggs in the +nests of other birds. This habit is not very uncommon with the Gallinaceæ, and +throws some light on the singular instinct of the ostrich. In this family +several hen birds 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, +<a name="Page216"></a> +as with the cuckoo, at intervals of two or three days. The instinct, however, +of the American ostrich, as in the case of the Molothrus bonariensis, 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. +</p> + +<p> +Many bees are parasitic, and regularly lay their eggs in the nests of other +kinds of bees. This case is more remarkable than that of the cuckoo; for these +bees have not only had their instincts but their structure modified in +accordance with their parasitic habits; for they do not possess the +pollen-collecting apparatus which would have been indispensable if they had +stored up food for their own young. Some species of Sphegidæ (wasp-like +insects) are likewise parasitic; 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, 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 that +of the Molothrus or 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 feloniously appropriated, be not thus +exterminated. +</p> + +<p> +<i>Slave-making instinct.</i>—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 of any kind, and 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 +speculate how so wonderful an instinct could have been perfected. +</p> + +<p> +<a name="Page217"></a> +Another species, 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 existence of so extraordinary an instinct as that of +making slaves. Hence, I will give the observations which I 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 (F. fusca) 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 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 together 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 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 year 1860, 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, the +slaves in Switzerland 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 +<a name="Page218"></a> +on the slaves being captured in greater numbers in Switzerland than in England. +</p> + +<p> +One day I fortunately witnessed a migration of F. sanguinea from one nest to +another, and it was a most interesting spectacle to behold the masters +carefully carrying their slaves in their jaws instead of being carried by them, +as in the case of F. rufescens. 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. +</p> + +<p> +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 their 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. +</p> + +<p> +One evening I visited another community of F. sanguinea, and found a number of +these ants returning home and entering their nests, carrying the dead bodies of +F. fusca (showing that it was not a migration) and numerous pupæ. I traced a +long file of ants +<a name="Page219"></a> +burthened with booty, for about forty yards back, 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, an +image of despair over its ravaged home. +</p> + +<p> +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 continental 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. +</p> + +<p> +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 +such pupæ originally stored as food might become developed; and the foreign +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, natural selection might increase and modify the +<a name="Page220"></a> +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. +</p> + +<p> +<i>Cell-making instinct of the Hive-Bee.</i>—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 effected by a crowd of bees +working in a dark hive. Granting whatever instincts you please, 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 at first appears: all this beautiful work can be shown, +I think, to follow from a few simple instincts. +</p> + +<p> +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 an inverted pyramid, 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 +<a name="Page221"></a> +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 flat surfaces, according as the +cell adjoins two, three or more other cells. When one cell rests on 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 base 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, and what is more important, labour, 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. +</p> + +<p> +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 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:— +</p> + +<p> +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 +sqrt(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. But I hear from +Professor Wyman, who has made numerous careful measurements, that the accuracy +of the +<a name="Page222"></a> +workmanship of the bee has been greatly exaggerated; so much so, that whatever +the typical form of the cell may be, it is rarely, if ever, realised. +</p> + +<p> +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 have the power of forming 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 many insects make in wood, 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 to a certain extent; and then +she unites the points of intersection by perfectly flat surfaces. By such +modifications of instincts which in themselves are 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. +</p> + +<p> +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, +rectangular 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 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>i.e.</i> 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 scalloped edge of a smooth basin, instead of on the +straight edges of a three-sided pyramid as in the case of ordinary cells. +</p> + +<p> +I then put into the hive, instead of a thick, rectangular piece of +<a name="Page223"></a> +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 bases; and these flat bases, formed by thin little +plates of the vermilion wax left ungnawed, were situated, as far as the eye +could judge, exactly along the planes of imaginary intersection between the +basins on the opposite side of the ridge of wax. In some parts, only small +portions, in other parts, large portions of a rhombic plate were thus 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 in circularly gnawing away and deepening the basins on both sides of +the ridge of vermilion wax, in order to have thus succeeded in leaving flat +plates between the basins, by stopping work at the planes of intersection. +</p> + +<p> +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 <i>perfectly +flat:</i> it was absolutely impossible, from the extreme thinness of the little +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. +</p> + +<p> +From the experiment of the ridge of vermilion wax we can 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, +<a name="Page224"></a> +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 this away 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 that 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. +</p> + +<p> +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 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. These walls, as Professor Miller has kindly +ascertained for me, vary greatly in thickness; being, on an average of twelve +measurements made near the border of the comb, 1/353 of an +<a name="Page225"></a> +inch in thickness; whereas the basal rhomboidal plates are thicker, nearly in +the proportion of three to two, having a mean thickness, from twenty-one +measurements, of 1/229 of an inch. By the above singular manner of building, +strength is continually given to the comb, with the utmost ultimate economy of +wax. +</p> + +<p> +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 it 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 pull down and rebuild in different ways the same cell, sometimes +recurring to a shape which they had at first rejected. +</p> + +<p> +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 subversive of the foregoing theory; namely, that the cells on +the extreme margin of wasp-combs are sometimes strictly +<a name="Page226"></a> +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 were to 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. +</p> + +<p> +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: cells constructed like those of the bee or the +wasp gain in strength, and save much in labour and space, and in the materials +of which they are constructed. With respect to the formation of wax, 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 proved that from +twelve to fifteen pounds of dry sugar are consumed by a hive of bees for the +secretion of a 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, and the time +consumed in collecting the honey, must be an important element of success any +family of bees. Of course the success of the species may be dependent on the +number of its enemies, or parasites, or on quite distinct causes, and so be +altogether independent of the quantity of honey which the bees can collect. But +let us suppose that this latter circumstance determined, as it probably often +has determined, whether a bee allied to our humble-bees could exist in large +numbers in any country; and let us further suppose that the community lived +through 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 imaginary humble-bee +if a slight modification of her instincts 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 labour and wax. Hence, it would +continually be more and more +<a name="Page227"></a> +advantageous to our humble-bees, if they were to make their 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 the adjoining cells, and much labour and 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 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 labour and wax. +</p> + +<p> +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 the construction of cells of due +strength and of the proper size and shape for the larvæ, this being effected +with the greatest possible economy of labour and wax; that individual swarm +which thus made the best cells with least labour, and least waste of honey in +the secretion of wax, having succeeded best, and having transmitted their +newly-acquired economical instincts to new swarms, which in their turn will +have had the best chance of succeeding in the struggle for existence. +</p> + +<p class="center"> +<i>Objections to the Theory of Natural Selection as applied to Instincts: +Neuter and Sterile Insects.</i> +</p> + +<p> +It has been objected to the foregoing view of the origin of instincts that +“the variations of structure and of instinct must have been simultaneous +and accurately adjusted to each other, as a modification in the one without an +immediate corresponding change in the other would have been fatal.” The +force of this objection rests entirely on the assumption that the changes in +the instincts and structure are abrupt. To take as an illustration the case of +the larger titmouse, (Parus major) alluded to in a previous chapter; +<a name="Page228"></a> +this bird often holds the seeds of the yew between its feet on a branch, and +hammers with its beak till it gets at the kernel. Now what special difficulty +would there be in natural selection preserving all the slight individual +variations in the shape of the beak, which were better and better adapted to +break open the seeds, until a beak was formed, as well constructed for this +purpose as that of the nuthatch, at the same time that habit, or compulsion, or +spontaneous variations of taste, led the bird to become more and more of a +seed-eater? In this case the beak is supposed to be slowly modified by natural +selection, subsequently to, but in accordance with, slowly changing habits or +taste; but let the feet of the titmouse vary and grow larger from correlation +with the beak, or from any other unknown cause, and it is not improbable that +such larger feet would lead the bird to climb more and more until it acquired +the remarkable climbing instinct and power of the nuthatch. In this case a +gradual change of structure is supposed to lead to changed instinctive habits. +To take one more case: few instincts are more remarkable than that which leads +the swift of the Eastern Islands to make its nest wholly of inspissated saliva. +Some birds build their nests of mud, believed to be moistened with saliva; and +one of the swifts of North America makes its nest (as I have seen) of sticks +agglutinated with saliva, and even with flakes of this substance. Is it then +very improbable that the natural selection of individual swifts, which secreted +more and more saliva, should at last produce a species with instincts leading +it to neglect other materials and to make its nest exclusively of inspissated +saliva? And so in other cases. It must, however, be admitted that in many +instances we cannot conjecture whether it was instinct or structure which first +varied. +</p> + +<p> +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 have originated; cases, in which no intermediate gradations are known to +exist; cases of instincts of 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 progenitor, and consequently must +believe that they were independently acquired through 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 +the whole theory. I allude to the neuters or sterile females in insect +communities: for these neuters often differ widely in instinct and in structure +<a name="Page229"></a> +from both the males and fertile females, and yet, from being sterile, they +cannot propagate their kind. +</p> + +<p> +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 especial difficulty in this having been effected +through 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 wonderful difference in this respect between +the workers and the perfect females would have been better exemplified by the +hive-bee. If a working ant or other neuter insect had been an ordinary animal, +I should have unhesitatingly assumed that all its characters had been slowly +acquired through natural selection; namely, by individuals having been born +with slight profitable modifications, which were inherited by the offspring, +and that these again varied and again were 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 it is possible to reconcile this case with the theory of natural +selection? +</p> + +<p> +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 inherited structure which are correlated with certain ages and +with either sex. We have differences correlated not only with one sex, but with +that short period 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 the oxen of other breeds, relatively to the length of the +horns in both the bulls and cows of these same breeds. Hence, I can see no +great difficulty in any character becoming correlated with the sterile +condition of certain +<a name="Page230"></a> +members of insect communities; the difficulty lies in understanding how such +correlated modifications of structure could have been slowly accumulated by +natural selection. +</p> + +<p> +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. Breeders of +cattle wish the flesh and fat to be well marbled together. An animal thus +characterized has been slaughtered, but the breeder has gone with confidence to +the same stock and has succeeded. Such faith may be placed in the power of +selection that a breed of cattle, always yielding oxen with extraordinarily +long horns, could, it is probable, be formed by carefully watching which +individual bulls and cows, when matched, produced oxen with the longest horns; +and yet no one ox would ever have propagated its kind. Here is a better and +real illustration: According to M. Verlot, some varieties of the double annual +stock, from having been long and carefully selected to the right degree, always +produce a large proportion of seedlings bearing double and quite sterile +flowers, but they likewise yield some single and fertile plants. These latter, +by which alone the variety can be propagated, may be compared with the fertile +male and female ants, and the double sterile plants with the neuters of the +same community. As with the varieties of the stock, so with social insects, +selection has been applied to the family, and not to the individual, for the +sake of gaining a serviceable end. Hence, we may conclude that slight +modifications of structure or of instinct, correlated with the sterile +condition of certain members of the community, have proved advantageous; +consequently the fertile males and females have flourished, and transmitted to +their fertile offspring a tendency to produce sterile members with the same +modifications. This process must have been repeated many times, 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. +</p> + +<p> +But we have not as yet touched on the acme 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 +<a name="Page231"></a> +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 and imprison. +</p> + +<p> +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 the theory. In the simpler case of +neuter insects all of one caste, which, as I believe, have been rendered +different from the fertile males and females through natural selection, we may +conclude from the analogy of ordinary variations, that the successive, slight, +profitable modifications did not first arise in all the neuters in the same +nest, but in some few alone; and that by the survival of the communities with +females which produced most neuters having the advantageous modification, all +the neuters ultimately came to be thus characterized. According to this view we +ought occasionally to find in the same nest neuter-insects, presenting +gradations of structure; and this we do find, even not rarely, considering how +few neuter-insects out of Europe have been carefully examined. Mr. F. Smith has +shown that the neuters of several British ants differ surprisingly from each +other in size and sometimes in colour; and that the extreme forms can be linked +together by individuals taken out of the same nest: I have myself compared +perfect gradations of this kind. It sometimes happens that the larger or the +smaller sized workers are the most numerous; or that both large and small are +numerous, while those of an intermediate size are scanty in numbers. Formica +flava has larger and smaller workers, with some few 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 +proportionately 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 here we 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 +<a name="Page232"></a> +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 were in this condition; we should then have had a species +of ant with neuters in nearly the same condition as 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. +</p> + +<p> +I may give one other case: so confidently did I expect occasionally to find +gradations of important structures 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 in addition 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 Sir J. Lubbock made drawings for me, with the camera +lucida, of the jaws which I dissected from the workers of the several sizes. +Mr. Bates, in his interesting “Naturalist on the Amazons,” has +described analogous cases. +</p> + +<p> +With these facts before me, I believe that natural selection, by acting on the +fertile ants or parents, could form a species which should regularly produce +neuters, all of large size with one form of jaw, or all of small size with +widely different jaws; or lastly, and this is the greatest 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 first been formed, +as in the case of the driver ant, and then the extreme forms having been +produced in greater and greater numbers, through the survival of the parents +which generated them, until none with an intermediate structure were produced. +</p> + +<p> +An analogous explanation has been given by Mr. Wallace, of the equally complex +case, of certain Malayan butterflies regularly +<a name="Page233"></a> +appearing under two or even three distinct female forms; and by Fritz Müller, +of certain Brazilian crustaceans likewise appearing under two widely distinct +male forms. But this subject need not here be discussed. +</p> + +<p> +I have now explained how, 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 ants, on the +same principle that the division of labour is useful to civilised man. Ants, +however, work by inherited instincts and by inherited organs or tools, while +man works by acquired knowledge and manufactured instruments. But I must +confess, that, with all my faith in natural selection, I should never have +anticipated that this principle could have been efficient in so high a degree, +had not the case of these neuter insects led me to this conclusion. 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 may be effected by the accumulation of +numerous, slight, spontaneous variations, which are in any way profitable, +without exercise or habit having been brought into play. For peculiar habits, +confined to the workers of sterile females, however long they might be +followed, could not possibly affect the males and fertile females, which alone +leave descendants. I am surprised that no one has advanced this demonstrative +case of neuter insects, against the well-known doctrine of inherited habit, as +advanced by Lamarck. +</p> + +<p class="center"> +<i>Summary.</i> +</p> + +<p> +I have endeavoured in this chapter briefly 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, there is no real difficulty, under changing conditions +of life, in natural selection accumulating to any extent slight modifications +of instinct which are in any way useful. In many 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 +<a name="Page234"></a> +are not always absolutely perfect and are liable to mistakes;—that no +instinct can be shown to have been produced for the good of other animals, +though animals take 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. +</p> + +<p> +This theory is also strengthened by some few other facts in regard to +instincts; as by that common case of closely allied, but 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 tropical South America lines its nest with mud, in the same +peculiar manner as does our British thrush; how it is that the Hornbills of +Africa and India have the same extraordinary instinct of plastering up and +imprisoning the females in a hole in a tree, with only a small hole left in the +plaster through which the males feed them and their young when hatched; how it +is that the male wrens (Troglodytes) of North America, build +“cock-nests,” to roost in, like the males of our +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page235"></a><a name="link2HCH0009" id="link2HCH0009"></a> +CHAPTER IX.<br/> +HYBRIDISM.</h2> + +<p class="letter"> +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, not accumulated by natural selection—Causes of the sterility +of first crosses and of hybrids—Parallelism between the effects of +changed conditions of life and of crossing—Dimorphism and +trimorphism—Fertility of varieties when crossed and of their mongrel +offspring not universal—Hybrids and mongrels compared independently of +their fertility—Summary. +</p> + +<p> +The view commonly entertained by naturalists is that species, when +intercrossed, have been specially endowed with sterility, in order to prevent +their confusion. This view certainly seems at first highly probable, for +species living together could hardly have been kept distinct had they been +capable of freely crossing. The subject is in many ways important for us, more +especially as the sterility of species when first crossed, and that of their +hybrid offspring, cannot have been acquired, as I shall show, by the +preservation of successive profitable degrees of sterility. It is an incidental +result of differences in the reproductive systems of the parent-species. +</p> + +<p> +In treating this subject, two classes of facts, to a large extent fundamentally +different, have generally been confounded; namely, the sterility of species +when first crossed, and the sterility of the hybrids produced from them. +</p> + +<p> +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 formative 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 +<a name="Page236"></a> +two cases, has to be considered. The distinction probably has been slurred +over, owing to the sterility in both cases being looked on as a special +endowment, beyond the province of our reasoning powers. +</p> + +<p> +The fertility of varieties, that is of the forms known or believed to be +descended from common parents, when crossed, and likewise the fertility of +their mongrel offspring, is, with reference to my theory, of equal importance +with the sterility of species; for it seems to make a broad and clear +distinction between varieties and species. +</p> + +<p> +<i>Degrees of Sterility.</i>—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 first crossed, and the maximum produced by their hybrid offspring, +with the average number produced by both pure parent-species in a state of +nature. But causes of serious error here intervene: 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 experimented on by Gärtner were potted, and 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 repeatedly +crossed some forms, such as the common red and blue pimpernels (Anagallis +arvensis and coerulea), which the best botanists rank as varieties, and found +them absolutely sterile, we may doubt whether many species are really so +sterile, when intercrossed, as he believed. +</p> + +<p> +It is certain, on the one hand, that the sterility of various species when +crossed is so different in degree and graduates away so +<a name="Page237"></a> +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, +arrived at diametrically opposite conclusions in regard to some of the very +same forms. 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 observer from experiments made during different years. It can +thus be shown that neither sterility nor fertility affords any certain +distinction between species and varieties. 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. +</p> + +<p> +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 increases, but generally +decreases greatly and suddenly. With respect to this decrease, it may first be +noticed that when any deviation in structure or constitution is common to both +parents, this is often transmitted in an augmented degree to the offspring; and +both sexual elements in hybrid plants are already affected in some degree. But +I believe that their fertility has been diminished in nearly all these cases by +an independent cause, namely, by too close interbreeding. I have made so many +experiments and collected so many facts, showing on the one hand that an +occasional cross with a distinct individual or variety increases the vigour and +fertility of the offspring, and on the other hand that very close interbreeding +lessens their vigour and fertility, that I cannot doubt the correctness of this +conclusion. 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, if left to themselves, will generally be fertilised +during each generation by pollen from the same flower; and this would probably +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 +<a name="Page238"></a> +with hybrid pollen of the same kind, their fertility, notwithstanding the +frequent ill effects from manipulation, sometimes decidedly increases, and goes +on increasing. Now, in the process of 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 often 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 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 an increase of fertility in +the successive generations of <i>artificially fertilised</i> hybrids, in +contrast with those spontaneously self-fertilised, may, as I believe, be +accounted for by too close interbreeding having been avoided. +</p> + +<p> +Now let us turn to the results arrived at by a third most experienced +hybridiser, namely, the Hon. and Rev. 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 experimented 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 hot-houses 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 I +never saw to occur in a case of its natural fecundation.” So that here we +have perfect, or even more than commonly perfect fertility, in a first cross +between two distinct species. +</p> + +<p> +This case of the Crinum leads me to refer to a singular fact, namely, that +individual plants of certain species of Lobelia, Verbascum and Passiflora, can +easily be fertilised by the pollen from a distinct species, but not by pollen +from the same plant, though this pollen can be proved to be perfectly sound by +fertilising other plants or species. In the genus Hippeastrum, in Corydalis as +shown by Professor Hildebrand, in various orchids as shown by Mr. Scott and +Fritz Müller, all the individuals are in this peculiar condition. So that with +some species, certain abnormal individuals, and in other species all the +individuals, can actually be hybridised much more readily than they can be +fertilised by pollen from the same individual plant! To give one instance, a +bulb of Hippeastrum aulicum +<a name="Page239"></a> +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 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.” Mr. Herbert tried similar experiments during many years, and +always with the same result. These cases serve to show on what slight and +mysterious causes the lesser or greater fertility of a species sometimes +depends. +</p> + +<p> +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, &c., +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, “reproduces 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 Rhod. ponticum and catawbiense, and that this +hybrid “seeds as freely as it is possible to imagine.” Had hybrids, +when fairly treated, always gone on decreasing in fertility in each successive +generation, as Gärtner believed to be the case, the fact would have been +notorious to nurserymen. Horticulturists raise large beds of the same hybrid, +and such alone are fairly treated, for by insect agency the several individuals +are allowed to cross freely 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. +</p> + +<p> +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 distinct in the scale of nature can +be crossed more easily than in the case of plants; but the hybrids themselves +are, I think, more sterile. It should, however, be borne in mind that, owing to +few animals breeding freely under confinement, few +<a name="Page240"></a> +experiments have been fairly tried: for instance, the canary-bird has been +crossed with nine distinct species of finches, but, as not one of these 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. +</p> + +<p> +Although I know of hardly any thoroughly well-authenticated cases of perfectly +fertile hybrid animals, I have reason to believe that the hybrids from Cervulus +vaginalis and Reevesii, and from Phasianus colchicus with P. torquatus, are +perfectly fertile. M. Quatrefages states that the hybrids from two moths +(Bombyx cynthia and arrindia) were proved in Paris to be fertile <i>inter +se</i> for eight generations. It has lately been asserted that two such +distinct species as the hare and rabbit, when they can be got to breed +together, produce offspring, which are highly fertile when crossed with one of +the parent-species. 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 <i>inter se</i>. 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 Captain 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 or perfectly fertile. +</p> + +<p> +With our domesticated animals, the various races when crossed together are +quite fertile; yet in many cases they are descended from two or more wild +species. From this fact we must conclude either that the aboriginal +parent-species at first produced perfectly fertile hybrids, or that the hybrids +subsequently reared under domestication became quite fertile. This latter +alternative, which +<a name="Page241"></a> +was first propounded by Pallas, seems by far the most probable, and can, +indeed, hardly be doubted. It is, for instance, almost certain that our dogs +are descended from several wild stocks; yet, with perhaps the exception of +certain indigenous domestic dogs of South America, all are quite fertile +together; but 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 I have lately acquired decisive evidence that the +crossed offspring from the Indian humped and common cattle are inter se +perfectly fertile; and from the observations by Rütimeyer on their important +osteological differences, as well as from those by Mr. Blyth on their +differences in habits, voice, constitution, &c., these two forms must be +regarded as good and distinct species. The same remarks may be extended to the +two chief races of the pig. We must, therefore, either give up the belief of +the universal sterility of species when crossed; or we must look at this +sterility in animals, not as an indelible characteristic, but as one capable of +being removed by domestication. +</p> + +<p> +Finally, considering 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. +</p> + +<p class="center"> +<i>Laws governing the Sterility of first Crosses and of Hybrids.</i> +</p> + +<p> +We will now consider a little more in detail the laws governing the sterility +of first crosses and of hybrids. Our chief object will be to see whether or not +these laws indicate that species have been specially endowed with this quality, +in order to prevent their crossing and blending together in utter confusion. +The following conclusions are drawn up chiefly from Gärtner’s admirable +work on the hybridisation of plants. I have taken much pains to ascertain how +far they 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. +</p> + +<p> +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; 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 +<a name="Page242"></a> +fertility, the pollen of different species applied to the stigma of some one +species of the same genus, 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 produces. So in hybrids themselves, +there are some which never have produced, and probably never would produce, +even with the pollen of the pure parents, 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. +</p> + +<p> +The hybrids raised 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, as in the genus Verbascum, 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. +</p> + +<p> +The fertility, both of first crosses and of hybrids, is more easily affected by +unfavourable conditions, than is that of pure species. But the fertility of +first crosses is likewise innately variable; for it is not always the same in +degree when the same two species are crossed under the same circumstances; it +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 the same conditions. +</p> + +<p> +By the term systematic affinity is meant, the general resemblance between +species in structure and constitution. Now the fertility of first crosses, 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 +<a name="Page243"></a> +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. Many analogous +facts could be given. +</p> + +<p> +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. +</p> + +<p> +By a reciprocal cross between two species, I mean the case, for instance, of a +female-ass being first crossed by a stallion, and then a mare by a male-ass: +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, that is of any difference in their structure or constitution, +excepting in their reproductive systems. The diversity of the result in +reciprocal crosses between the same two species was long ago observed by +Kölreuter. To give an instance: Mirabilis jalapa 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. +jalapa, 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 +<a name="Page244"></a> +facility in making reciprocal crosses is extremely common in a lesser degree. +He has observed it even between closely related forms (as Matthiola annua and +glabra) which many botanists rank 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, though they rarely differ in external +characters, yet generally differ in fertility in a small, and occasionally in a +high degree. +</p> + +<p> +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 among +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 the fertility +of a hybrid may be independent of its external resemblance to either pure +parent. +</p> + +<p> +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 the difference in the result of 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, +<a name="Page245"></a> +in the facility of effecting an union. The hybrids, moreover, produced from +reciprocal crosses often differ in fertility. +</p> + +<p> +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 a strange arrangement. +</p> + +<p> +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 in their reproductive systems; +the differences being of so peculiar and limited a nature, that, in reciprocal +crosses between the same 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 unimportant for their welfare in a state of nature, I +presume that no one will suppose that this capacity is a <i>specially</i> +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, +&c.; 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 together trees belonging to quite +distinct families; and, on the other hand, closely allied species and varieties +<a name="Page246"></a> +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. +</p> + +<p> +As Gärtner found that there was sometimes an innate difference in different +<i>individuals</i> 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. +</p> + +<p> +We have seen that the sterility of hybrids which have their reproductive organs +in an imperfect condition, is a different case from the difficulty of uniting +two pure species, which have their reproductive organs perfect; yet these two +distinct classes of cases run to a large 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 a fourth 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 cases of Hippeastrum, Passiflora, &c., which seed +much more freely when fertilised with the pollen of a distinct species than +when fertilised with pollen from the same plant. +</p> + +<p> +We thus see that, although there is a clear and great 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 in their reproductive +systems. These differences in both cases follow, to a certain extent, as might +have been expected, +<a name="Page247"></a> +systematic affinity, by which term every kind of resemblance and dissimilarity +between organic beings is attempted to be expressed. The facts by no means seem +to indicate that the greater or lesser difficulty of either grafting or +crossing 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. +</p> + +<p class="center"> +<i>Origin and Causes of the Sterility of first Crosses and of Hybrids.</i> +</p> + +<p> +At one time it appeared to me probable, as it has to others, that the sterility +of first crosses and of hybrids might have been slowly acquired through the +natural selection of slightly lessened degrees of fertility, which, like any +other variation, spontaneously appeared in certain individuals of one variety +when crossed with those of another variety. For it would clearly be +advantageous to two varieties or incipient species if they could be kept from +blending, on the same principle that, when man is selecting at the same time +two varieties, it is necessary that he should keep them separate. In the first +place, it may be remarked that species inhabiting distinct regions are often +sterile when crossed; now it could clearly have been of no advantage to such +separated species to have been rendered mutually sterile, and consequently this +could not have been effected through natural selection; but it may perhaps be +argued, that, if a species was rendered sterile with some one compatriot, +sterility with other species would follow as a necessary contingency. In the +second place, it is almost as much opposed to the theory of natural selection +as to that of special creation, that in reciprocal crosses the male element of +one form should have been rendered utterly impotent on a second form, while at +the same time the male element of this second form is enabled freely to +fertilise the first form; for this peculiar state of the reproductive system +could hardly have been advantageous to either species. +</p> + +<p> +In considering the probability of natural selection having come into action, in +rendering species mutually sterile, the greatest difficulty will be found to +lie in the existence of many graduated steps, from slightly lessened fertility +to absolute sterility. It may be admitted that it would profit an incipient +species, if it were rendered in some slight degree sterile when crossed with +its parent form or with some other variety; for thus fewer bastardised and +deteriorated offspring would be produced to commingle their blood with the new +species in process of formation. But he who will take the trouble to reflect on +the steps by which this first degree of +<a name="Page248"></a> +sterility could be increased through natural selection to that high degree +which is common with so many species, and which is universal with species which +have been differentiated to a generic or family rank, will find the subject +extraordinarily complex. After mature reflection, it seems to me that this +could not have been effected through natural selection. Take the case of any +two species which, when crossed, produced few and sterile offspring; now, what +is there which could favour the survival of those individuals which happened to +be endowed in a slightly higher degree with mutual infertility, and which thus +approached by one small step towards absolute sterility? Yet an advance of this +kind, if the theory of natural selection be brought to bear, must have +incessantly occurred with many species, for a multitude are mutually quite +barren. With sterile neuter insects we have reason to believe that +modifications in their structure and fertility have been slowly accumulated by +natural selection, from an advantage having been thus indirectly given to the +community to which they belonged over other communities of the same species; +but an individual animal not belonging to a social community, if rendered +slightly sterile when crossed with some other variety, would not thus itself +gain any advantage or indirectly give any advantage to the other individuals of +the same variety, thus leading to their preservation. +</p> + +<p> +But it would be superfluous to discuss this question in detail: for with plants +we have conclusive evidence that the sterility of crossed species must be due +to some principle, quite independent of natural selection. Both Gärtner and +Kölreuter have proved that in genera including numerous species, a series can +be formed from species which when crossed yield fewer and fewer seeds, to +species which never produce a single seed, but yet are affected by the pollen +of certain other species, for the germen swells. It is here manifestly +impossible to select the more sterile individuals, which have already ceased to +yield seeds; so that this acme of sterility, when the germen alone is effected, +cannot have been gained through selection; and from the laws governing the +various grades of sterility being so uniform throughout the animal and +vegetable kingdoms, we may infer that the cause, whatever it may be, is the +same or nearly the same in all cases. +</p> + +<p class="p2"> +We will now look a little closer at the probable nature of the differences +between species which induce sterility in first crosses and in hybrids. In the +case of first crosses, the greater or less difficulty in effecting a union and +in obtaining offspring apparently depends on several distinct causes. There +must sometimes be a +<a name="Page249"></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 the 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 pheasants and +fowls, that the early death of the embryo is a very frequent cause of sterility +in first crosses. Mr. Salter has recently given the results of an examination +of about 500 eggs produced from various crosses between three species of Gallus +and their hybrids; the majority of these eggs had been fertilised; and in the +majority of the fertilised eggs, the embryos had either been partially +developed and had then perished, or had become nearly mature, but the young +chickens had been unable to break through the shell. Of the chickens which were +born, more than four-fifths died within the first few days, or at latest weeks, +“without any obvious cause, apparently from mere inability to +live;” so that from the 500 eggs only twelve chickens were reared. With +plants, hybridized embryos probably often perish in a like manner; at least it +is known that hybrids raised from very distinct species are sometimes weak and +dwarfed, and perish at an early age; of which fact Max Wichura has recently +given some striking cases with hybrid willows. It may be here worth noticing +that in some cases of parthenogenesis, the embryos within the eggs of silk +moths which had not been fertilised, pass through their early stages of +development and then perish like the embryos produced by a cross between +distinct species. Until becoming acquainted with these facts, I was unwilling +to believe in the frequent early death of hybrid embryos; for 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 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; it may therefore, +before birth, as long as it is nourished within its mother’s womb, or +within +<a name="Page250"></a> +the egg or seed produced by the mother, 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 are eminently sensitive to injurious or +unnatural conditions of life. But after all, the cause more probably lies in +some imperfection in the original act of impregnation, causing the embryo to be +imperfectly developed, rather than in the conditions to which it is +subsequently exposed. +</p> + +<p> +In regard to the sterility of hybrids, in which the sexual elements are +imperfectly developed, the case is somewhat different. I have more than once +alluded to a large body of facts 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 exotic 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 seems +to be partly due to their reproductive systems having been specially affected, +though in a lesser degree than when sterility ensues. So it is with hybrids, +for their offspring in successive generations are eminently liable to vary, as +every experimentalist has observed. +</p> + +<p> +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 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 +<a name="Page251"></a> +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 distinct +structures and constitutions, including of course the reproductive systems, +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 relations of the different +parts and organs one to another or to the conditions of life. When hybrids are +able to breed <i>inter se</i>, 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, does not +diminish; it is even apt to increase, this being generally the result, as +before explained, of too close interbreeding. The above view of the sterility +of hybrids being caused by two constitutions being compounded into one has been +strongly maintained by Max Wichura. +</p> + +<p> +It must, however, be owned that we cannot understand, on the above or any other +view, 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 being compounded into one. +</p> + +<p> +A similar parallelism holds good with an allied yet very different class of +facts. It is an old and almost universal belief, founded on a considerable body +of evidence, which I have elsewhere given, 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, &c., +from one soil or climate to another, and back again. During the convalescence +of animals, great benefit is derived from almost any change in their habits of +life. Again, both with plants and animals, there is the clearest evidence that +a cross between individuals of the same species, which differ to a certain +extent, gives vigour and fertility to the offspring; and that close +interbreeding continued during several generations between the nearest +relations, if these be kept under the same conditions of life, almost always +leads to decreased size, weakness, or sterility. +</p> + +<p> +<a name="Page252"></a> +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 been +subjected to slightly different conditions, or which have slightly varied, give +vigour and fertility to the offspring. But, as we have seen, organic beings +long habituated to certain uniform conditions under a state of nature, when +subjected, as under confinement, to a considerable change in their conditions, +very frequently are rendered more or less sterile; and we know that a cross +between two forms that have become widely or specifically different, produce +hybrids which are almost always in some degree sterile. I am fully persuaded +that this double parallelism is by no means an accident or an illusion. He who +is able to explain why the elephant, and a multitude of other animals, are +incapable of breeding when kept under only partial confinement in their native +country, will be able to explain the primary cause of hybrids being so +generally sterile. He will at the same time be able to explain how it is that +the races of some of our domesticated animals, which have often been subjected +to new and not uniform conditions, are quite fertile together, although they +are descended from distinct species, which would probably have been sterile if +aboriginally crossed. The above two parallel series of facts seem to be +connected together by some common but unknown bond, which is essentially +related to the principle of life; this principle, according to Mr. Herbert +Spencer, being that life depends on, or consists in, the incessant action and +reaction of various forces, which, as throughout nature, are always tending +towards an equilibrium; and when this tendency is slightly disturbed by any +change, the vital forces gain in power. +</p> + +<p class="center"> +<i>Reciprocal Dimorphism and Trimorphism.</i> +</p> + +<p> +This subject may be here briefly discussed, and will be found to throw some +light on hybridism. Several plants belonging to distinct orders present two +forms, which exist in about equal numbers and which differ in no respect except +in their reproductive organs; one form having a long pistil with short stamens, +the other a short pistil with long stamens; the two having differently sized +pollen-grains. With trimorphic plants there are three forms likewise differing +in the lengths of their pistils and stamens, in the size and colour of the +pollen-grains, and in some other respects; and as in each of the three forms +there are two sets of stamens, the three forms possess altogether six sets of +stamens and three kinds of pistils. These organs are so proportioned in length +to each other, +<a name="Page253"></a> +that half the stamens in two of the forms stand on a level with the stigma of +the third form. Now I have shown, and the result has been confirmed by other +observers, that in order to obtain full fertility with these plants, it is +necessary that the stigma of the one form should be fertilised by pollen taken +from the stamens of corresponding height in another form. So that with +dimorphic species two unions, which may be called legitimate, are fully +fertile; and two, which may be called illegitimate, are more or less infertile. +With trimorphic species six unions are legitimate, or fully fertile, and twelve +are illegitimate, or more or less infertile. +</p> + +<p> +The infertility which may be observed in various dimorphic and trimorphic +plants, when they are illegitimately fertilised, that is by pollen taken from +stamens not corresponding in height with the pistil, differs much in degree, up +to absolute and utter sterility; just in the same manner as occurs in crossing +distinct species. As the degree of sterility in the latter case depends in an +eminent degree on the conditions of life being more or less favourable, so I +have found it with illegitimate unions. It is well known that if pollen of a +distinct species be placed on the stigma of a flower, and its own pollen be +afterwards, even after a considerable interval of time, placed on the same +stigma, its action is so strongly prepotent that it generally annihilates the +effect of the foreign pollen; so it is with the pollen of the several forms of +the same species, for legitimate pollen is strongly prepotent over illegitimate +pollen, when both are placed on the same stigma. I ascertained this by +fertilising several flowers, first illegitimately, and twenty-four hours +afterwards legitimately, with pollen taken from a peculiarly coloured variety, +and all the seedlings were similarly coloured; this shows that the legitimate +pollen, though applied twenty-four hours subsequently, had wholly destroyed or +prevented the action of the previously applied illegitimate pollen. Again, as +in making reciprocal crosses between the same two species, there is +occasionally a great difference in the result, so the same thing occurs with +trimorphic plants; for instance, the mid-styled form of Lythrum salicaria was +illegitimately fertilised with the greatest ease by pollen from the longer +stamens of the short-styled form, and yielded many seeds; but the latter form +did not yield a single seed when fertilised by the longer stamens of the +mid-styled form. +</p> + +<p> +In all these respects, and in others which might be added, the forms of the +same undoubted species, when illegitimately united, behave in exactly the same +manner as do two distinct species when crossed. This led me carefully to +observe during four years many seedlings, raised from several illegitimate +unions. The chief result is +<a name="Page254"></a> +that these illegitimate plants, as they may be called, are not fully fertile. +It is possible to raise from dimorphic species, both long-styled and +short-styled illegitimate plants, and from trimorphic plants all three +illegitimate forms. These can then be properly united in a legitimate manner. +When this is done, there is no apparent reason why they should not yield as +many seeds as did their parents when legitimately fertilised. But such is not +the case. They are all infertile, in various degrees; some being so utterly and +incurably sterile that they did not yield during four seasons a single seed or +even seed-capsule. The sterility of these illegitimate plants, when united with +each other in a legitimate manner, may be strictly compared with that of +hybrids when crossed <i>inter se</i>. If, on the other hand, a hybrid is +crossed with either pure parent-species, the sterility is usually much +lessened: and so it is when an illegitimate plant is fertilised by a legitimate +plant. In the same manner as the sterility of hybrids does not always run +parallel with the difficulty of making the first cross between the two +parent-species, so that sterility of certain illegitimate plants was unusually +great, while the sterility of the union from which they were derived was by no +means great. With hybrids raised from the same seed-capsule the degree of +sterility is innately variable, so it is in a marked manner with illegitimate +plants. Lastly, many hybrids are profuse and persistent flowerers, while other +and more sterile hybrids produce few flowers, and are weak, miserable dwarfs; +exactly similar cases occur with the illegitimate offspring of various +dimorphic and trimorphic plants. +</p> + +<p> +Altogether there is the closest identity in character and behaviour between +illegitimate plants and hybrids. It is hardly an exaggeration to maintain that +illegitimate plants are hybrids, produced within the limits of the same species +by the improper union of certain forms, while ordinary hybrids are produced +from an improper union between so-called distinct species. We have also already +seen that there is the closest similarity in all respects between first +illegitimate unions and first crosses between distinct species. This will +perhaps be made more fully apparent by an illustration; we may suppose that a +botanist found two well-marked varieties (and such occur) of the long-styled +form of the trimorphic Lythrum salicaria, and that he determined to try by +crossing whether they were specifically distinct. He would find that they +yielded only about one-fifth of the proper number of seed, and that they +behaved in all the other above specified respects as if they had been two +distinct species. But to make the case sure, he would raise plants from his +supposed hybridised seed, and he would +<a name="Page255"></a> +find that the seedlings were miserably dwarfed and utterly sterile, and that +they behaved in all other respects like ordinary hybrids. He might then +maintain that he had actually proved, in accordance with the common view, that +his two varieties were as good and as distinct species as any in the world; but +he would be completely mistaken. +</p> + +<p> +The facts now given on dimorphic and trimorphic plants are important, because +they show us, first, that the physiological test of lessened fertility, both in +first crosses and in hybrids, is no safe criterion of specific distinction; +secondly, because we may conclude that there is some unknown bond which +connects the infertility of illegitimate unions with that of their illegitimate +offspring, and we are led to extend the same view to first crosses and hybrids; +thirdly, because we find, and this seems to me of especial importance, that two +or three forms of the same species may exist and may differ in no respect +whatever, either in structure or in constitution, relatively to external +conditions, and yet be sterile when united in certain ways. For we must +remember that it is the union of the sexual elements of individuals of the same +form, for instance, of two long-styled forms, which results in sterility; while +it is the union of the sexual elements proper to two distinct forms which is +fertile. Hence the case appears at first sight exactly the reverse of what +occurs, in the ordinary unions of the individuals of the same species and with +crosses between distinct species. It is, however, doubtful whether this is +really so; but I will not enlarge on this obscure subject. +</p> + +<p> +We may, however, infer as probable from the consideration of dimorphic and +trimorphic plants, that the sterility of distinct species when crossed and of +their hybrid progeny, depends exclusively on the nature of their sexual +elements, and not on any difference in their structure or general constitution. +We are also led to this same conclusion by considering reciprocal crosses, in +which the male of one species cannot be united, or can be united with great +difficulty, with the female of a second species, while the converse cross can +be effected with perfect facility. That excellent observer, Gärtner, likewise +concluded that species when crossed are sterile owing to differences confined +to their reproductive systems. +</p> + +<p class="center"> +<i>Fertility of Varieties when Crossed, and of their Mongrel Offspring, not +universal.</i> +</p> + +<p> +It may be urged as an overwhelming argument that there must be some essential +distinction between species and varieties inasmuch as the latter, however much +they may differ from each other in external appearance, cross with perfect +facility, and yield perfectly +<a name="Page256"></a> +fertile offspring. With some exceptions, presently to be given, I fully admit +that this is the rule. But the subject is surrounded by difficulties, for, +looking to varieties produced under nature, if two forms hitherto reputed to be +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, which +are considered by most botanists as varieties, are said by Gärtner to be quite +sterile 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. +</p> + +<p> +If we turn to varieties, produced, or supposed to have been produced, under +domestication, we are still involved in some doubt. For when it is stated, for +instance, that certain South American indigenous domestic dogs do not readily +unite with European dogs, the explanation which will occur to everyone, and +probably the true one, is that they are descended from aboriginally distinct +species. Nevertheless the perfect fertility of so many domestic races, +differing widely from each other in appearance, for instance, those 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. In the +first place, it may be observed that the amount of external difference between +two species is no sure guide to their degree of mutual sterility, so that +similar differences in the case of varieties would be no sure guide. It is +certain that with species the cause lies exclusively in differences in their +sexual constitution. Now the varying conditions to which domesticated animals +and cultivated plants have been subjected, have had so little tendency towards +modifying the reproductive system in a manner leading to mutual sterility, that +we have good grounds for admitting the directly opposite doctrine of Pallas, +namely, that such conditions generally eliminate this tendency; so that the +domesticated descendants of species, which in their natural state probably +would have been in some degree sterile when crossed, become perfectly fertile +together. With plants, so far is cultivation from giving a tendency towards +sterility between distinct species, that in several well-authenticated cases +already alluded to, certain plants have been affected in an opposite manner, +for they have become self-impotent, while still retaining the capacity of +fertilising, and being fertilised by, other species. If the Pallasian doctrine +of the elimination of sterility through long-continued domestication be +admitted, and it can hardly be rejected, +<a name="Page257"></a> +it becomes in the highest degree improbable that similar conditions +long-continued should likewise induce this tendency; though in certain cases, +with species having a peculiar constitution, sterility might occasionally be +thus caused. Thus, as I believe, we can understand why, with domesticated +animals, varieties have not been produced which are mutually sterile; and why +with plants only a few such cases, immediately to be given, have been observed. +</p> + +<p> +The real difficulty in our present subject is not, as it appears to me, why +domestic varieties have not become mutually infertile when crossed, but why +this has so generally occurred with natural varieties, as soon as they have +been permanently modified in a sufficient degree to take rank as species. We +are far from precisely knowing the cause; nor is this surprising, seeing how +profoundly ignorant we are in regard to the normal and abnormal action of the +reproductive system. But we can see that species, owing to their struggle for +existence with numerous competitors, will have been exposed during long periods +of time to more uniform conditions, than have domestic varieties; and this may +well make a wide difference in the result. For we know how commonly wild +animals and plants, when taken from their natural conditions and subjected to +captivity, are rendered sterile; and the reproductive functions of organic +beings which have always lived under natural conditions would probably in like +manner be eminently sensitive to the influence of an unnatural cross. +Domesticated productions, on the other hand, which, as shown by the mere fact +of their domestication, were not originally highly sensitive to changes in +their conditions of life, and which can now generally resist with undiminished +fertility repeated changes of conditions, might be expected to produce +varieties, which would be little liable to have their reproductive powers +injuriously affected by the act of crossing with other varieties which had +originated in a like manner. +</p> + +<p> +I have as yet spoken as if the varieties of the same species were invariably +fertile when intercrossed. But it is 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 kind with pollen of the +<a name="Page258"></a> +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 <i>perfectly</i> fertile; so that +even Gärtner did not venture to consider the two varieties as specifically +distinct. +</p> + +<p> +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 experimented on are ranked by +Sagaret, who mainly founds his classification by the test of infertility, as +varieties, and Naudin has come to the same conclusion. +</p> + +<p> +The following case is far more remarkable, and seems at first 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 the yellow and white varieties when crossed produce +less seed than the similarly coloured varieties of the same species. Moreover, +he asserts that, when yellow and white varieties of one species are crossed +with yellow and white varieties of a <i>distinct</i> species, more seed is +produced by the crosses between the similarly coloured flowers, than between +those which are differently coloured. Mr. Scott also has experimented on the +species and varieties of Verbascum; and although unable to confirm +Gärtner’s results on the crossing of the distinct species, he finds that +the dissimilarly coloured varieties of the same species yield fewer seeds, in +the proportion of eighty-six to 100, than the similarly coloured varieties. Yet +these varieties differ in no respect, except in the colour of their flowers; +and one variety can sometimes be raised from the seed of another. +</p> + +<p> +Kölreuter, whose accuracy has been confirmed by every subsequent observer, has +proved the remarkable fact that one particular variety of the common tobacco +was more fertile than the other varieties, when crossed with a widely distinct +species. He experimented 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 the 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 +<a name="Page259"></a> +of this one variety must have been in some manner and in some degree modified. +</p> + +<p> +From these facts it can no longer be maintained that varieties when crossed are +invariably quite fertile. From the great difficulty of ascertaining the +infertility of varieties in a state of nature, for a supposed variety, if +proved to be infertile in any degree, would almost universally be ranked as a +species; from man attending only to external characters in his domestic +varieties, and from such varieties not having been exposed for very long +periods to uniform conditions of life; from these several considerations we may +conclude that fertility does not constitute a fundamental distinction between +varieties and species when crossed. The general sterility of crossed species +may safely be looked at, not as a special acquirement or endowment, but as +incidental on changes of an unknown nature in their sexual elements. +</p> + +<p class="center"> +<i>Hybrids and Mongrels compared, independently of their fertility.</i> +</p> + +<p> +Independently of the question of fertility, the offspring of species and of +varieties when crossed may be compared in several other respects. Gärtner, +whose strong wish it was to draw a distinct line 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 many +important respects. +</p> + +<p> +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 the offspring in both +cases is notorious; but some few instances of both hybrids and mongrels long +retaining a uniform character could be given. The variability, however, in the +successive generations of mongrels is, perhaps, greater than in hybrids. +</p> + +<p> +This greater variability in mongrels than in hybrids does not seem at all +surprising. For the parents of mongrels are varieties, and mostly domestic +varieties (very few experiments +<a name="Page260"></a> +having been tried on natural varieties), and this implies that there has been +recent variability; which would often continue and would augment that arising +from the act of crossing. The slight variability of hybrids in the first +generation, in contrast with that in the succeeding generations, is a curious +fact and deserves attention. For it bears on the view which I have taken of one +of the causes of ordinary variability; namely, that the reproductive system, +from being eminently sensitive to changed conditions of life, fails under these +circumstances to perform its proper function of producing offspring closely +similar in all respects to 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. +</p> + +<p> +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. Moreover, +Gärtner expressly states that the hybrids from long cultivated plants are more +subject to reversion than hybrids from species in their natural state; and this +probably explains the singular difference in the results arrived at by +different observers. Thus Max Wichura doubts whether hybrids ever revert to +their parent forms, and he experimented on uncultivated species of willows, +while Naudin, on the other hand, insists in the strongest terms on the almost +universal tendency to reversion in hybrids, and he experimented chiefly on +cultivated plants. Gärtner further states 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. +</p> + +<p> +Such alone are the unimportant differences which Gärtner is able to point out +between hybrid and mongrel plants. On the other hand, the degrees and kinds of +resemblance in mongrels and in hybrids to their respective parents, more +especially in hybrids produced from nearly related species, follow, according +to Gärtner the same laws. When two species are crossed, one has sometimes a +prepotent power of impressing its likeness on the hybrid. So I believe it to be +with varieties of plants; and with animals, one variety certainly often has +this prepotent power over another +<a name="Page261"></a> +variety. Hybrid plants produced from a reciprocal cross generally resemble each +other closely, and so it is with mongrel plants 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. +</p> + +<p> +These several remarks are apparently applicable to animals; but the subject is +here much 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 resemble +more closely the ass than the horse; but that the prepotency runs more strongly +in the male than in the female ass, so that the mule, which is an 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. +</p> + +<p> +Much stress has been laid by some authors on the supposed fact, that it is only +with mongrels that the offspring are not intermediate in character, but closely +resemble one of their parents; but this does sometimes occur with hybrids, yet +I grant much less frequently 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 through selection. A tendency to +sudden reversions to the perfect character of either parent would, also, be +much 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 little or much from each other, namely, in the union of individuals of +the same variety, or of different varieties, or of distinct species. +</p> + +<p> +Independently of 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 +<a name="Page262"></a> +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. +</p> + +<p class="center"> +<i>Summary of Chapter.</i> +</p> + +<p> +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 most careful +experimentalists have arrived at diametrically opposite conclusions in ranking +forms by this test. The sterility is innately variable in individuals of the +same species, and is eminently susceptible to action 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 hybrids produced from this cross. +</p> + +<p> +In the same manner as in grafting trees, the capacity in one species or variety +to take on another, is incidental on differences, generally of an unknown +nature, 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 their 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 their inarching in our +forests. +</p> + +<p> +The sterility of first crosses and of their hybrid progeny has not been +acquired through natural selection. In the case of first crosses it seems to +depend on several circumstances; in some instances in chief part on the early +death of the embryo. In the case of hybrids, it apparently depends on their +whole organisation having been disturbed by being compounded from two distinct +forms; the sterility being closely allied to that which so frequently affects +pure species, when exposed to new and unnatural conditions of life. He who will +explain these latter cases will be able to explain the sterility of hybrids. +This view is strongly supported by a parallelism of another kind: namely, that, +firstly, slight changes in the conditions of life add to the vigour and +fertility of all organic beings; and secondly, that the crossing of forms, +which have been exposed to slightly different conditions of life, or which have +varied, favours the size, vigour and fertility of their offspring. The facts +given on the +<a name="Page263"></a> +sterility of the illegitimate unions of dimorphic and trimorphic plants and of +their illegitimate progeny, perhaps render it probable that some unknown bond +in all cases connects the degree of fertility of first unions with that of +their offspring. The consideration of these facts on dimorphism, as well as of +the results of reciprocal crosses, clearly leads to the conclusion that the +primary cause of the sterility of crossed species is confined to differences in +their sexual elements. But why, in the case of distinct species, the sexual +elements should so generally have become more or less modified, leading to +their mutual infertility, we do not know; but it seems to stand in some close +relation to species having been exposed for long periods of time to nearly +uniform conditions of life. +</p> + +<p> +It is not surprising that the difficulty in crossing any two species, and the +sterility of their hybrid offspring, should in most cases correspond, even if +due to distinct causes: for both depend on the amount of difference between the +species which are crossed. Nor is it surprising that the facility of effecting +a first cross, and the fertility of the hybrids thus 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 subjected to experiment; for +systematic affinity includes resemblances of all kinds. +</p> + +<p> +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, as is so often stated, invariably fertile. Nor is this almost universal +and perfect fertility surprising, when it is remembered 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 that they have +not been long exposed to uniform conditions of life. It should also be +especially kept in mind, that long-continued domestication tends to eliminate +sterility, and is therefore little likely to induce this same quality. +Independently of the question of fertility, in all other respects there is the +closest general resemblance between hybrids and mongrels, in their variability, +in their power of absorbing each other by repeated crosses, and in their +inheritance of characters from both parent-forms. Finally, then, although we +are as ignorant of the precise cause of the sterility of first crosses and of +hybrids as we are why animals and plants removed from their natural conditions +become sterile, yet the facts given in this chapter do not seem to me opposed +to the belief that species aboriginally existed as varieties. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page264"></a><a name="link2HCH0010" id="link2HCH0010"></a> +CHAPTER X.<br/> +ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.</h2> + +<p class="letter"> +On the absence of intermediate varieties at the present day—On the nature +of extinct intermediate varieties; on their number—On the lapse of time, +as inferred from the rate of denudation and of deposition number—On the +lapse of time as estimated by years—On the poorness of our +palæontological collections—On the intermittence of geological +formations—On the denudation of granitic areas—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—Antiquity of the habitable earth. +</p> + +<p> +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 supplant 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, be truly enormous. Why then is not every geological formation +and every stratum full of +<a name="Page265"></a> +such intermediate links? Geology assuredly does not reveal any such finely +graduated organic chain; and this, perhaps, is the most obvious and serious +objection which can be urged against my theory. The explanation lies, as I +believe, in the extreme imperfection of the geological record. +</p> + +<p> +In the first place, it should always be borne in mind what sort of intermediate +forms must, on the theory, have formerly existed. I have found it difficult, +when looking at any two species, to avoid picturing to myself forms +<i>directly</i> 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 are 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, C. livia, whether they had descended from this species or from +some other allied species, such as C. oenas. +</p> + +<p> +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 directly intermediate +between them ever existed, 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. +</p> + +<p> +It is just possible, by the theory, that one of two living forms might have +descended from the other; for instance, a horse from a tapir; and in this case +<i>direct</i> 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 +<a name="Page266"></a> +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 tend to supplant the +old and unimproved forms. +</p> + +<p> +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 natural and domestic 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 forms; 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 the earth. +</p> + +<p class="center"> +<i>On the Lapse of Time, as inferred from the rate of deposition and extent of +Denudation.</i> +</p> + +<p> +Independently of our not finding fossil remains of such infinitely numerous +connecting links, it may be objected that time cannot have sufficed for so +great an amount of organic change, all changes having been effected slowly. It +is hardly possible for me to recall to the reader who is not 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, and yet does not admit how 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 of each stratum. +We can best gain some idea of past time by knowing the agencies at work; and +learning how deeply the surface of the land has been denuded, and how much +sediment has been deposited. As Lyell has well remarked, the extent and +thickness of our sedimentary formations are the result and the measure of the +denudation which the earth’s crust has elsewhere undergone. Therefore a +man should examine for himself the great piles of superimposed strata, and +watch the rivulets bringing down mud, and the waves wearing away the +sea-cliffs, in order to comprehend something about the duration of past time, +the monuments of which we see all around us. +</p> + +<p> +<a name="Page267"></a> +It is good to wander along the 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 good evidence that pure water +effects 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 after being reduced in size they can be rolled +about by the waves, and then they 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. +</p> + +<p> +We have, however, recently learned from the observations of Ramsay, in the van +of many excellent observers—of Jukes, Geikie, Croll and others, that +subaërial degradation is a much more important agency than coast-action, or the +power of the waves. The whole surface of the land is exposed to the chemical +action of the air and of the rainwater, with its dissolved carbonic acid, and +in colder countries to frost; the disintegrated matter is carried down even +gentle slopes during heavy rain, and to a greater extent than might be +supposed, especially in arid districts, by the wind; it is then transported by +the streams and rivers, which, when rapid deepen their channels, and triturate +the fragments. On a rainy day, even in a gently undulating country, we see the +effects of subaërial degradation in the muddy rills which flow down every +slope. Messrs. Ramsay and Whitaker have shown, and the observation is a most +striking one, that the great lines of escarpment in the Wealden district and +those ranging across England, which formerly were looked at as ancient +sea-coasts, cannot have been thus formed, for each line is composed of one and +the same formation, while our sea-cliffs are everywhere formed by the +intersection of various formations. This being the case, we are compelled to +admit that the escarpments owe their origin in chief part to the rocks of which +they are composed, having resisted subaërial denudation better than the +surrounding surface; this surface consequently has been gradually lowered, with +the lines of harder rock +<a name="Page268"></a> +left projecting. Nothing impresses the mind with the vast duration of time, +according to our ideas of time, more forcibly than the conviction thus gained +that subaërial agencies, which apparently have so little power, and which seem +to work so slowly, have produced great results. +</p> + +<p> +When thus impressed with the slow rate at which the land is worn away through +subaërial and littoral action, it is good, in order to appreciate the past +duration of time, to consider, on the one hand, the masses of rock which have +been removed over many extensive areas, and on the other hand the thickness of +our sedimentary formations. I remember having been much struck 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 told still more plainly 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, and +it makes no great difference whether the upheaval was sudden, or, as most +geologists now believe, was slow and effected by many starts, the surface of +the land has been so completely planed down that no trace of these vast +dislocations is externally visible. The Craven fault, for instance, extends for +upward of thirty miles, and along this line the vertical displacement of the +strata varies from 600 to 3,000 feet. Professor Ramsay has published an account +of a downthrow in Anglesea of 2,300 feet; and he informs me that he fully +believes that there is one in Merionethshire of 12,000 feet; yet in these cases +there is nothing on the surface of the land to show such prodigious movements; +the pile of rocks on either side of the crack having been smoothly swept away. +</p> + +<p> +On the other hand, in all parts of the world the piles of sedimentary strata +are of wonderful thickness. In the Cordillera, I estimated one mass of +conglomerate at ten thousand feet; and although conglomerates have probably +been accumulated at a quicker rate than finer sediments, yet from being formed +of worn and rounded pebbles, each of which bears the stamp of time, they are +good to show how slowly the mass must have been heaped together. Professor +Ramsay has given me the maximum thickness, from actual measurement in most +cases, of the successive formations in <i>different</i> parts of Great Britain; +and this is the result:—<br /><br /><a name="Page269"></a> +</p> + +<table summary="" style="margin-left: 4em;"> + +<tr> +<td></td><td>Feet</td> +</tr> + +<tr> +<td>Palæozoic strata (not including igneous beds)</td><td>57,154.</td> +</tr> + +<tr> +<td>Secondary strata</td><td>13,190.</td> +</tr> + +<tr> +<td>Tertiary strata</td><td>2,240.</td> +</tr> + +</table> + +<p class="noindent"> +<br /> +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, blank periods of enormous length. +So that the lofty pile of sedimentary rocks in Britain gives but an inadequate +idea of the time which has elapsed during their accumulation. The consideration +of these various facts impresses the mind almost in the same manner as does the +vain endeavour to grapple with the idea of eternity. +</p> + +<p> +Nevertheless this impression is partly false. Mr. Croll, in an interesting +paper, remarks that we do not err “in forming too great a conception of +the length of geological periods,” but in estimating them by years. When +geologists look at large and complicated phenomena, and then at the figures +representing several million years, the two produce a totally different effect +on the mind, and the figures are at once pronounced too small. In regard to +subaërial denudation, Mr. Croll shows, by calculating the known amount of +sediment annually brought down by certain rivers, relatively to their areas of +drainage, that 1,000 feet of solid rock, as it became gradually disintegrated, +would thus be removed from the mean level of the whole area in the course of +six million years. This seems an astonishing result, and some considerations +lead to the suspicion that it may be too large, but if halved or quartered it +is still very surprising. Few of us, however, know what a million really means: +Mr. Croll gives the following illustration: Take a narrow strip of paper, +eighty-three feet four inches in length, and stretch it along the wall of a +large hall; then mark off at one end the tenth of an inch. This tenth of an +inch will represent one hundred years, and the entire strip a million years. +But let it be borne in mind, in relation to the subject of this work, what a +hundred years implies, represented as it is by a measure utterly insignificant +in a hall of the above dimensions. Several eminent breeders, during a single +lifetime, have so largely modified some of the higher animals, which propagate +their kind much more slowly than most of the lower animals, that they have +formed what well deserves to be called a new sub-breed. Few men have attended +with due care to any one strain for more than half a century, so that a hundred +years represents the work of two breeders in succession. It is not to be +<a name="Page270"></a> +supposed that species in a state of nature ever change so quickly as domestic +animals under the guidance of methodical selection. The comparison would be in +every way fairer with the effects which follow from unconscious selection, that +is, the preservation of the most useful or beautiful animals, with no intention +of modifying the breed; but by this process of unconscious selection, various +breeds have been sensibly changed in the course of two or three centuries. +</p> + +<p> +Species, however, probably change much more slowly, and within the same country +only a few change at the same time. This slowness follows from all the +inhabitants of the same country being already so well adapted to each other, +that new places in the polity of nature do not occur until after long +intervals, due to the occurrence of physical changes of some kind, or through +the immigration of new forms. Moreover, variations or individual differences of +the right nature, by which some of the inhabitants might be better fitted to +their new places under the altered circumstance, would not always occur at +once. Unfortunately we have no means of determining, according to the standard +of years, how long a period it takes to modify a species; but to the subject of +time we must return. +</p> + +<p class="center"> +<i>On the Poorness of Palæontological Collections.</i> +</p> + +<p> +Now let us turn to our richest museums, and what a paltry display we behold! +That our collections are imperfect is admitted by every one. The remark of that +admirable palæontologist, Edward Forbes, should never be forgotten, namely, +that very many 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 decay and +disappear when left on the bottom of the sea, where sediment is not +accumulating. We probably take a quite erroneous view, when we assume 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 immense 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 +<a name="Page271"></a> +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 charged with carbonic acid. Some of the many kinds of +animals which live on the beach between high and low water mark seem to be +rarely 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 this has been found fossil +in Sicily, whereas not one other species has hitherto been found in any +tertiary formation: yet it is known that the genus Chthamalus existed during +the Chalk period. Lastly, many great deposits, requiring a vast length of time +for their accumulation, are entirely destitute of organic remains, without our +being able to assign any reason: one of the most striking instances is that of +the Flysch formation, which consists of shale and sandstone, several thousand, +occasionally even six thousand feet in thickness, and extending for at least +300 miles from Vienna to Switzerland; and although this great mass has been +most carefully searched, no fossils, except a few vegetable remains, have been +found. +</p> + +<p> +With respect to the terrestrial productions which lived during the Secondary +and Palæozoic periods, it is superfluous to state that our evidence is +fragmentary in an extreme degree. For instance, until recently not a land-shell +was known belonging to either of these vast periods, with the exception of one +species discovered by Sir C. Lyell and Dr. Dawson in the carboniferous strata +of North America; but now land-shells have been found in the lias. In regard to +mammiferous remains, a glance at the historical table published in +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. +</p> + +<p> +But the imperfection in the geological record largely 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. This +doctrine has been emphatically admitted by many geologists and palæontologists, +who, like E. Forbes, entirely disbelieve in the change of species. 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 +<a name="Page272"></a> +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 confined +exclusively 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 every 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 was derived, accord with the belief of vast intervals of time having +elapsed between each formation. +</p> + +<p> +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 +poorly 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 the 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. +</p> + +<p> +We may, I think, 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, as well +as the subsequent subaërial degradation. Such thick and extensive accumulations +of sediment may be formed in two ways; either in profound depths of the sea, in +which case the bottom will not be inhabited by so many and such varied forms of +life as the more shallow seas; and the mass +<a name="Page273"></a> +when upraised will give an imperfect record of the organisms which existed in +the neighbourhood during the period of its accumulation. Or sediment may be +deposited 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 many and varied forms, and thus a rich fossiliferous formation, +thick enough, when upraised, to resist a large amount of denudation, may be +formed. +</p> + +<p> +I am convinced that nearly all our ancient formations, which are throughout the +greater part of their thickness <i>rich in fossils</i>, 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 deposited during a downward +oscillation of level, and thus gained considerable thickness. +</p> + +<p> +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, will 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, <i>thick</i> deposits cannot have been accumulated in the +shallow parts, which are the most favourable to life. Still less can this have +happened during the alternate periods of elevation; or, to speak more +accurately, the beds which were then accumulated will generally have been +destroyed by being upraised and brought within the limits of the coast-action. +</p> + +<p> +These remarks apply chiefly to littoral and sublittoral deposits. In the case +of an extensive and shallow sea, such as that within a large part of the Malay +Archipelago, where the depth varies from thirty or forty to sixty fathoms, a +widely extended formation might be formed during a period of elevation, and yet +not suffer excessively from denudation during its slow upheaval; but the +thickness of the formation could not be great, for owing to the elevatory +movement it would be less than the depth in which it was formed; nor would +<a name="Page274"></a> +the deposit be much consolidated, nor be capped by overlying formations, so +that it would run a good chance of being worn away by atmospheric degradation +and by the action of the sea during subsequent oscillations of level. It has, +however, been suggested by Mr. Hopkins, that if one part of the area, after +rising and before being denuded, subsided, the deposit formed during the rising +movement, though not thick, might afterwards become protected by fresh +accumulations, and thus be preserved for a long period. +</p> + +<p> +Mr. Hopkins also expresses his belief that sedimentary beds of considerable +horizontal extent have rarely been completely destroyed. But all geologists, +excepting the few who believe that our present metamorphic schists and plutonic +rocks once formed the primordial nucleus of the globe, will admit that these +latter rocks have been stripped of their covering to an enormous extent. For it +is scarcely possible that such rocks could have been solidified and +crystallised while uncovered; but if the metamorphic action occurred at +profound depths of the ocean, the former protecting mantle of rock may not have +been very thick. Admitting then that gneiss, mica-schist, granite, diorite, +&c., were once necessarily covered up, how can we account for the naked and +extensive areas of such rocks in many parts of the world, except on the belief +that they have subsequently been completely denuded of all overlying strata? +That such extensive areas do exist cannot be doubted: the granitic region of +Parime is described by Humboldt as being at least nineteen times as large as +Switzerland. South of the Amazon, Boue colours an area composed of rocks of +this nature as equal to that of Spain, France, Italy, part of Germany, and the +British Islands, all conjoined. This region has not been carefully explored, +but from the concurrent testimony of travellers, the granitic area is very +large: thus Von Eschwege gives a detailed section of these rocks, stretching +from Rio de Janeiro for 260 geographical miles inland in a straight line; and I +travelled for 150 miles in another direction, and saw nothing but granitic +rocks. Numerous specimens, collected along the whole coast, from near Rio de +Janeiro to the mouth of the Plata, a distance of 1,100 geographical miles, were +examined by me, and they all belonged to this class. Inland, along the whole +northern bank of the Plata, I saw, besides modern tertiary beds, only one small +patch of slightly metamorphosed rock, which alone could have formed a part of +the original capping of the granitic series. Turning to a well-known region, +namely, to the United States and Canada, as shown in Professor H.D. +Rogers’ beautiful map, I have estimated the areas by cutting out and +weighing the paper, and I find that the metamorphic (excluding the +“semi-metamorphic”) +<a name="Page275"></a> +and granite rocks exceed, in the proportion of 19 to 12.5, the whole of the +newer Palæozoic formations. In many regions the metamorphic and granite rocks +would be found much more widely extended than they appear to be, if all the +sedimentary beds were removed which rest unconformably on them, and which could +not have formed part of the original mantle under which they were crystallised. +Hence, it is probable that in some parts of the world whole formations have +been completely denuded, with not a wreck left behind. +</p> + +<p> +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 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 on the shores of a continent when first +broken up into an archipelago), and consequently during subsidence, though +there will be much extinction, few new varieties or species will be formed; and +it is during these very periods of subsidence that the deposits which are +richest in fossils have been accumulated. +</p> + +<p class="center"> +<i>On the Absence of Numerous Intermediate Varieties in any Single +Formation.</i> +</p> + +<p> +From these several 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 much 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. Several cases are on +record of the same species presenting varieties in the upper and lower parts of +the same formation. Thus Trautschold gives a number of instances with +Ammonites, and Hilgendorf has described a most curious case of ten graduated +forms of Planorbis multiformis in the successive beds of a fresh-water +formation in Switzerland. Although each formation has indisputably required a +vast number of years for its deposition, several reasons can be given why each +should not commonly include a graduated series of links between the species +which lived at its commencement and close, but I cannot assign due proportional +weight to the following considerations. +</p> + +<p> +Although each formation may mark a very long lapse of years, each probably 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 +<a name="Page276"></a> +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 from 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 +last layers have been deposited, it would be equally rash to suppose that it +then became 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. +</p> + +<p> +We may safely infer that with marine animals of all kinds there has been a +large amount of migration due to 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 appear 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, in 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 epoch, which forms only a part of one +whole geological period; and likewise to reflect on the changes of level, on +the extreme change of climate, and on the great lapse of time, all included +within this same glacial period. Yet it may be doubted whether, in any quarter +of the world, sedimentary deposits, <i>including fossil remains</i>, 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 best flourish: for we know that great 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 migrations of species and to geographical changes. And in the distant +future, a geologist, examining +<a name="Page277"></a> +these beds, would 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. +</p> + +<p> +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 continuously +accumulating during a long period, sufficient for the slow process of +modification; hence, the deposit must be a very thick one; and the species +undergoing change must have lived in the same district throughout the whole +time. But we have seen that a thick formation, fossiliferous throughout its +entire thickness, can accumulate only during a period of subsidence; and to +keep the depth approximately the same, which is necessary that the same marine +species may live on the same space, the supply of sediment must nearly +counterbalance the amount of subsidence. But this same movement of subsidence +will tend to submerge 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. +</p> + +<p> +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 widely different +mineralogical composition, we may reasonably suspect that the process of +deposition has been more or less interrupted. Nor will the closest inspection +of a formation give us any idea of the length of time which its deposition may +have consumed. Many instances could be given of beds, only a few feet in +thickness, representing formations which are 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 even 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 not have been +<a name="Page278"></a> +suspected, had not the trees been preserved: thus Sir C. Lyell and Dr. Dawson +found carboniferous beds 1,400 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 occurs at the bottom, middle, and top of a +formation, the probability is that it has not lived on the same spot during the +whole period of deposition, but has disappeared and reappeared, perhaps many +times, during the same geological period. Consequently if it were to undergo a +considerable amount of modification during the deposition of any one geological +formation, a section would not include all the fine intermediate gradations +which must on our theory have existed, but abrupt, though perhaps slight, +changes of form. +</p> + +<p> +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 the closest 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 +older and 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 closely connected by intermediate varieties with either one or +both forms. Nor should it be forgotten, as before explained, that A might be +the actual progenitor of B and C, and yet would not necessarily be strictly +intermediate between them in all respects. So that we might obtain the +parent-species and its several modified descendants from the lower and upper +beds of the same formation, and unless we obtained numerous transitional +gradations, we should not recognise their blood-relationship, and should +consequently rank them as distinct species. +</p> + +<p> +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 the theory we ought to find. Look again at the +later tertiary deposits, which include many shells believed by the majority of +naturalists to be identical with existing species; but some excellent +naturalists, as Agassiz and Pictet, maintain that all these tertiary species +are specifically distinct, though the distinction +<a name="Page279"></a> +is admitted to be very slight; so that here, unless we believe that these +eminent naturalists have been misled by their imaginations, and that these late +tertiary species really present no difference whatever from their living +representatives, or unless we admit, in opposition to the judgment of most +naturalists, that these tertiary species are all truly distinct from the +recent, we have evidence of the frequent occurrence of slight modifications of +the kind required. If we look to rather wider intervals of time, namely, to +distinct but consecutive stages of the same great formation, we find that the +embedded fossils, though universally ranked as specifically different, yet are +far more closely related to each other than are the species found in more +widely separated formations; so that here again we have undoubted evidence of +change in the direction required by the theory; but to this latter subject I +shall return in the following chapter. +</p> + +<p> +With animals and plants that propagate rapidly and do not wander much, 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-form 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 +probable that those which had the widest range, far exceeding the limits of the +known geological formations in Europe, 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. +</p> + +<p> +It is a more important consideration, leading to the same result, as lately +insisted on by Dr. Falconer, namely, that the period during which each species +underwent modification, though long as measured by years, was probably short in +comparison with that during which it remained without undergoing any change. +</p> + +<p> +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 are collected from +many places; and with fossil species this can rarely be done. We shall, +perhaps, best perceive the improbability of our being enabled to connect +species +<a name="Page280"></a> +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 are 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 by the future geologist only by +his discovering in a fossil state numerous intermediate gradations; and such +success is improbable in the highest degree. +</p> + +<p> +It has been asserted over and over again, by writers who believe in the +immutability of species, that geology yields no linking forms. This assertion, +as we shall see in the next chapter, is certainly erroneous. As Sir J. Lubbock +has remarked, “Every species is a link between other allied forms.” +If we take a genus having a score of species, recent and extinct, and destroy +four-fifths of them, no one doubts that the remainder will stand much more +distinct from each other. If the extreme forms in the genus happen to have been +thus destroyed, the genus itself will stand more distinct from other allied +genera. What geological research has not revealed, is the former existence of +infinitely numerous gradations, as fine as existing varieties, connecting +together nearly all existing and extinct species. But this ought not to be +expected; yet this has been repeatedly advanced as a most serious objection +against my views. +</p> + +<p> +It may be worth while to sum up the foregoing remarks on the causes of the +imperfection of the geological record under an imaginary illustration. The +Malay Archipelago is 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, while most of our formations were accumulating. The Malay +Archipelago is one of the richest regions 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! +</p> + +<p> +But we have every reason to believe that the terrestrial productions of the +archipelago would be preserved in an extremely imperfect manner in the +formations which we suppose to be there accumulating. Not many of the strictly +littoral animals, or of +<a name="Page281"></a> +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. +</p> + +<p> +Formations rich in fossils of many kinds, and of thickness sufficient to last +to an age as distant in futurity as the secondary formations lie in the past, +would generally be formed in the archipelago only during periods of subsidence. +These periods of subsidence would be separated from each other by immense +intervals of time, during which the area would be either stationary or rising; +whilst rising, the fossiliferous formations on the steeper shores would be +destroyed, almost as soon as accumulated, by the incessant coast-action, as we +now see on the shores of South America. Even throughout the extensive and +shallow seas within the archipelago, sedimentary beds could hardly be +accumulated of great thickness during the periods of elevation, or become +capped and protected by subsequent deposits, so as to have a good chance of +enduring to a very distant future. 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 less +perfect. +</p> + +<p> +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 <i>exceed</i> 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 all fully preserved, transitional varieties would merely +appear as so many new, though closely allied species. It is also probable that +each great period of subsidence would be interrupted by oscillations of level, +and that slight climatical changes would intervene during such lengthy periods; +and in these cases the inhabitants of the archipelago would migrate, and no +closely consecutive record of their modifications could be preserved in any one +formation. +</p> + +<p> +Very many of the marine inhabitants of the archipelago now range thousands of +miles beyond its confines; and analogy plainly leads to the belief that it +would be chiefly these far-ranging species, though only some of them, which +would oftenest produce new varieties; and the varieties would at first 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 +<a name="Page282"></a> +their ancient homes, as they would differ from their former state in a nearly +uniform, though perhaps extremely slight degree, and as they would be found +embedded in slightly different sub-stages of the same formation, they would, +according to the principles followed by many palæontologists, be ranked as new +and distinct species. +</p> + +<p> +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 our theory, 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, and such assuredly we do find—some +more distantly, some more closely, related to each other; and these links, let +them be ever so close, if found in different stages of the same formation, +would, by many palæontologists, be ranked as distinct species. But I do not +pretend that I should ever have suspected how poor was the record in the best +preserved geological sections, had not the absence of innumerable transitional +links between the species which lived at the commencement and close of each +formation, pressed so hardly on my theory. +</p> + +<p class="center"> +<i>On the sudden Appearance of whole Groups of allied Species.</i> +</p> + +<p> +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 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 at once, the fact would be fatal to the +theory of evolution through natural selection. For the development by this +means of a group of forms, all of which are descended from some one progenitor, +must have been an extremely slow process; and the progenitors must have lived +long before their modified descendants. But we continually overrate 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. In all cases positive palæontological evidence may be +implicitly trusted; negative evidence is worthless, as experience has so often +shown. 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 the +United States. We do not make due allowance for the enormous intervals of time +which have elapsed between our consecutive +formations,—<a name="Page283"></a>longer perhaps in many 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 parent-form: and in +the succeeding formation, such groups or species will appear as if suddenly +created. +</p> + +<p> +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; and consequently that the transitional +forms would often long remain confined to some one region; but that, when this +adaptation had once 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 spread rapidly and widely +throughout the world. Professor Pictet, in his excellent Review of this work, +in commenting on early transitional forms, and taking birds as an illustration, +cannot see how the successive modifications of the anterior limbs of a supposed +prototype could possibly have been of any advantage. But look at the penguins +of the Southern Ocean; have not these birds their front limbs in this precise +intermediate state of “neither true arms nor true wings?” Yet these +birds hold their place victoriously in the battle for life; for they exist in +infinite numbers and of many kinds. I do not suppose that we here see the real +transitional grades through which the wings of birds have passed; but what +special difficulty is there in believing that it might profit the modified +descendants of the penguin, first to become enabled to flap along the surface +of the sea like the logger-headed duck, and ultimately to rise from its surface +and glide through the air? +</p> + +<p> +I will now give a few examples to illustrate the foregoing remarks, and to show +how liable we are to error in supposing that whole groups of species have +suddenly been produced. Even in so short an interval as that between the first +and second editions of Pictet’s great work on Palæontology, published in +1844-46 and in 1853-57, the conclusions on the first appearance and +disappearance of several groups of animals have been considerably modified; and +a third edition would require still further changes. I may recall the +well-known fact that in geological treatises, published not many years ago, +mammals were 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 true mammals have +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 +<a name="Page284"></a> +tertiary stratum; but now extinct species have been discovered in India, South +America and in Europe, as far back as the miocene stage. Had it not been for +the rare accident of the preservation of footsteps in the new red sandstone of +the United States, who would have ventured to suppose that no less than at +least thirty different bird-like animals, some of gigantic size, existed during +that period? Not a fragment of bone has been discovered in these beds. Not long +ago, palæontologists maintained that the whole class of birds came suddenly +into existence during the eocene period; but now we know, on the authority of +Professor Owen, that a bird certainly lived during the deposition of the upper +greensand; and still more recently, that strange bird, the Archeopteryx, with a +long lizard-like tail, bearing a pair of feathers on each joint, and with its +wings furnished with two free claws, has been discovered in the oolitic slates +of Solenhofen. Hardly any recent discovery shows more forcibly than this how +little we as yet know of the former inhabitants of the world. +</p> + +<p> +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 stated that, from +the large 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 fifty 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 then 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 then 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 unmistakable sessile cirripede, which he had himself +extracted from the chalk of Belgium. And, as if to make the case as striking as +possible, this cirripede was a Chthamalus, a very common, large, and ubiquitous +genus, of which not one species has as yet been found even in any tertiary +stratum. Still more recently, a Pyrgoma, a member of a distinct subfamily of +sessile cirripedes, has been discovered by Mr. Woodward in the upper chalk; so +that we now +<a name="Page285"></a> +have abundant evidence of the existence of this group of animals during the +secondary period. +</p> + +<p> +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, according to Agassiz, in the Chalk period. This group +includes the large majority of existing species. But certain Jurassic and +Triassic forms are now commonly admitted to be teleostean; and even some +palæozoic forms have thus been classed by one high authority. If the +teleosteans had really appeared suddenly in the northern hemisphere at the +commencement of the chalk formation, the fact would have been highly +remarkable; but it would not have formed an insuperable difficulty, unless it +could likewise have been shown that at the same period the species were +suddenly and simultaneously developed in other quarters of the world. 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 fishes might +formerly have had a similarly confined range, and after having been largely +developed in some one sea, 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. +</p> + +<p> +From these considerations, 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 knowledge effected by the discoveries of the last dozen +years, it seems to me to be about as rash to dogmatize on the succession of +organic forms throughout the world, as it would be for a naturalist to land for +five minutes on a barren point in Australia, and then to discuss the number and +range of its productions. +</p> + +<p class="center"> +<i>On the sudden Appearance of Groups of allied Species in the lowest known +Fossiliferous Strata.</i> +</p> + +<p> +There is another and allied difficulty, which is much more serious. I allude to +the manner in which species belonging to several of the +<a name="Page286"></a> +main divisions of the animal kingdom 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 are descended from a single progenitor, +apply with equal force to the earliest known species. For instance, it cannot +be doubted that all the Cambrian and Silurian trilobites are descended from +some one crustacean, which must have lived long before the Cambrian age, and +which probably differed greatly from any known animal. Some of the most ancient +animals, as the Nautilus, Lingula, &c., do not differ much from living +species; and it cannot on our theory be supposed, that these old species were +the progenitors of all the species belonging to the same groups which have +subsequently appeared, for they are not in any degree intermediate in +character. +</p> + +<p> +Consequently, if the theory be true, it is indisputable that before the lowest +Cambrian stratum was deposited long periods elapsed, as long as, or probably +far longer than, the whole interval from the Cambrian age to the present day; +and that during these vast periods the world swarmed with living creatures. +Here we encounter a formidable objection; for it seems doubtful whether the +earth, in a fit state for the habitation of living creatures, has lasted long +enough. Sir W. Thompson concludes that the consolidation of the crust can +hardly have occurred less than twenty or more than four hundred million years +ago, but probably not less than ninety-eight or more than two hundred million +years. These very wide limits show how doubtful the data are; and other +elements may have hereafter to be introduced into the problem. Mr. Croll +estimates that about sixty million years have elapsed since the Cambrian +period, but this, judging from the small amount of organic change since the +commencement of the Glacial epoch, appears a very short time for the many and +great mutations of life, which have certainly occurred since the Cambrian +formation; and the previous one hundred and forty million years can hardly be +considered as sufficient for the development of the varied forms of life which +already existed during the Cambrian period. It is, however, probable, as Sir +William Thompson insists, that the world at a very early period was subjected +to more rapid and violent changes in its physical conditions than those now +occurring; and such changes would have tended to induce changes at a +corresponding rate in the organisms which then existed. +</p> + +<p> +To the question why we do not find rich fossiliferous deposits belonging to +these assumed earliest periods prior to the Cambrian system, I can give no +satisfactory answer. Several eminent geologists, with Sir R. Murchison at their +head, were until recently convinced that we beheld in the organic remains of +the lowest +<a name="Page287"></a> +Silurian stratum the first dawn of life. Other highly competent judges, as +Lyell and E. Forbes, have disputed this conclusion. We should not forget that +only a small portion of the world is known with accuracy. Not very long ago M. +Barrande added another and lower stage, abounding with new and peculiar +species, beneath the then known Silurian system; and now, still lower down in +the Lower Cambrian formation, Mr Hicks has found South Wales beds rich in +trilobites, and containing various molluscs and annelids. The presence of +phosphatic nodules and bituminous matter, even in some of the lowest azotic +rocks, probably indicates life at these periods; and the existence of the +Eozoon in the Laurentian formation of Canada is generally admitted. There are +three great series of strata beneath the Silurian system in Canada, in the +lowest of which the Eozoon is found. Sir W. Logan states that their +“united thickness may possibly far surpass that of all the succeeding +rocks, from the base of the palæozoic series to the present time. We are thus +carried back to a period so remote, that the appearance of the so-called +primordial fauna (of Barrande) may by some be considered as a comparatively +modern event.” The Eozoon belongs to the most lowly organised of all +classes of animals, but is highly organised for its class; it existed in +countless numbers, and, as Dr. Dawson has remarked, certainly preyed on other +minute organic beings, which must have lived in great numbers. Thus the words, +which I wrote in 1859, about the existence of living beings long before the +Cambrian period, and which are almost the same with those since used by Sir W. +Logan, have proved true. Nevertheless, the difficulty of assigning any good +reason for the absence of vast piles of strata rich in fossils beneath the +Cambrian system is very great. It does not seem probable that the most ancient +beds have been quite worn away by denudation, or that their fossils have been +wholly obliterated by metamorphic action, for if this had been the case we +should have found only small remnants of the formations next succeeding them in +age, and these would always have existed in a partially metamorphosed +condition. But the descriptions which we 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 invariably it has suffered extreme +denudation and metamorphism. +</p> + +<p> +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 +<a name="Page288"></a> +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 now existing continents of Europe +and North America. This same view has since been maintained by Agassiz and +others. But we do not know what was the state of things in the intervals +between the several 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 as the bed of an open and +unfathomable sea. +</p> + +<p> +Looking to the existing oceans, which are thrice as extensive as the land, we +see them studded with many islands; but hardly one truly oceanic island (with +the exception of New Zealand, if this can be called a truly 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, 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 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 Cambrian 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 we have no reason to assume that things +have thus remained from the beginning of the world. 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 long antecedent to the Cambrian 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 +<a name="Page289"></a> +we should there find sedimentary formations, in recognisable condition, older +than the Cambrian 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 naked 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 Cambrian epoch in +a completely metamorphosed and denuded condition. +</p> + +<p class="p2"> +The several difficulties here discussed, namely, that, though we find in our +geological formations many links between the species which now exist and which +formerly existed, we do not find infinitely numerous fine transitional forms +closely joining them all together. The sudden manner in which several groups of +species first appear in our European formations, the almost entire absence, as +at present known, of formations rich in fossils beneath the Cambrian strata, +are all undoubtedly of the most serious nature. We see this in the fact that +the most eminent palæontologists, namely, Cuvier, Agassiz, Barrande, Pictet, +Falconer, E. Forbes, &c., and all our greatest geologists, as Lyell, +Murchison, Sedgwick, &c., have unanimously, often vehemently, maintained +the immutability of species. But Sir Charles Lyell now gives the support of his +high authority to the opposite side, and most geologists and palæontologists +are much shaken in their former belief. Those who believe that the geological +record is in any degree perfect, will undoubtedly at once reject my theory. For +my part, following out Lyell’s metaphor, I look at the 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, more or less different in the successive chapters, +may represent the forms of life, which are entombed in our consecutive +formations, and which falsely appear to have been abruptly introduced. On this +view the difficulties above discussed are greatly diminished or even disappear. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page290"></a><a name="link2HCH0011" id="link2HCH0011"></a> +CHAPTER XI.<br/> +ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.</h2> + +<p class="letter"> +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. +</p> + +<p> +Let us now see whether the several facts and laws relating to the geological +succession of organic beings accord best with the common view of the +immutability of species, or with that of their slow and gradual modification, +through variation and natural selection. +</p> + +<p> +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 the stages, and to make the proportion +between the lost and existing 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 extinct, and only one or two are new, having appeared there for +the first time, either locally, or, as far as we know, on the face of the +earth. The secondary formations are more broken; but, as Bronn has remarked, +neither the appearance nor disappearance of the many species embedded in each +formation has been simultaneous. +</p> + +<p> +Species belonging to different genera and classes have not changed at the same +rate, or in the same degree. In the older 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, for an existing crocodile is associated +with many 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 +<a name="Page291"></a> +seem to have changed at a quicker rate than those of the sea, of which a +striking instance has been observed in Switzerland. There is some reason to +believe that organisms high in the scale, change more quickly than those that +are low: though there are exceptions to this rule. The amount of organic +change, as Pictet has remarked, is not the same in each successive so-called +formation. 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 no reason to believe that +the same identical form ever 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 satisfactory. +</p> + +<p> +These several facts accord well with our theory, which includes no fixed law of +development, causing all the inhabitants of an area to change abruptly, or +simultaneously, or to an equal degree. The process of modification must be +slow, and will generally affect only a few species at the same time; for the +variability of each species is independent of that of all others. Whether such +variations or individual differences as may arise will be accumulated through +natural selection in a greater or less degree, thus causing a greater or less +amount of permanent modification, will depend on many complex +contingencies—on the variations being of a beneficial nature, on the +freedom of intercrossing, on the slowly changing physical conditions of the +country, on the immigration of new colonists, and on the nature of the other +inhabitants with which the varying species come 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, should change in a less degree. We +find similar relations between the existing inhabitants of distinct countries; +for instance, the land-shells and coleopterous insects of Madeira have 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 any area have become modified and improved, we can understand, +on the principle of competition, and from the all-important relations of +<a name="Page292"></a> +organism to organism in the struggle for life, that any form which did not +become in some degree modified and improved, would be liable to extermination. +Hence, we see why all the species in the same region do at last, if we look to +long enough intervals of time, become modified; for otherwise they would become +extinct. +</p> + +<p> +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 enduring formations, rich in fossils, depends on great masses +of sediment being deposited on subsiding areas, our formations have been almost +necessarily accumulated at wide and irregularly intermittent intervals of time; +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 an ever slowly changing drama. +</p> + +<p> +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 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; and +organisms already differing would vary in a different manner. For instance, it +is possible, if all our fantail-pigeons were destroyed, that fanciers might +make a new breed hardly distinguishable from the present breed; but if the +parent rock-pigeon were likewise destroyed, and under nature we have every +reason to believe that parent forms are generally supplanted and exterminated +by their improved offspring, it is incredible that a fantail, identical with +the existing breed, could be raised from any other species of pigeon, or even +from any other well established race of the domestic pigeon, for the successive +variations would almost certainly be in some degree different, and the +newly-formed variety would probably inherit from its progenitor some +characteristic differences. +</p> + +<p> +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, when it has once +disappeared, never reappears; that is, 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 +<a name="Page293"></a> +Woodward (though all strongly opposed to such views as I maintain) admit its +truth; and the rule strictly accords with the theory. For all the species of +the same group, however long it may have lasted, are the modified descendants +one from the other, and all from a common progenitor. In the genus Lingula, for +instance, the species which have successively appeared at all ages must have +been connected by an unbroken series of generations, from the lowest Silurian +stratum to the present day. +</p> + +<p> +We have seen in the last chapter that whole groups of species sometimes falsely +appear to have been abruptly developed; and I have attempted to give an +explanation of this fact, which if true would be fatal to my views. But such +cases are certainly exceptional; the general rule being a gradual increase in +number, until the group reaches its maximum, and then, sooner or later, a +gradual decrease. If the number of the species included within a genus, or the +number of the genera within a family, be represented by a vertical line of +varying thickness, ascending through 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, often keeping of equal thickness for a space, 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 the theory; for the species of the same genus, and the genera +of the same family, can increase only slowly and progressively; the process of +modification and the production of a number of allied forms necessarily being a +slow and gradual process, one species first giving rise to two or three +varieties, these being slowly converted into species, which in their turn +produce by equally slow steps other varieties and species, and so on, like the +branching of a great tree from a single stem, till the group becomes large. +</p> + +<p class="center"> +<i>On Extinction.</i> +</p> + +<p> +We have as yet only spoken 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 by catastrophes at successive periods is very generally given up, even by +those geologists, as Elie de Beaumont, Murchison, Barrande, &c., whose +general views would naturally lead them to this conclusion. On the contrary, we +have every reason to believe, from the study of the +<a name="Page294"></a> +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. In some few cases, however, as by the breaking of an isthmus and the +consequent irruption of a multitude of new inhabitants into an adjoining sea, +or by the final subsidence of an island, the process of extinction may have +been rapid. Both single species and whole groups of species last for very +unequal periods; some groups, as we have seen, have endured from the earliest +known dawn of life to the present day; some have 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 extinction of a whole group of species is generally a slower +process than their production: if their appearance and disappearance 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 the +early increase in number of the species. In some cases, however, the +extermination of whole groups, as of ammonites, towards the close of the +secondary period, has been wonderfully sudden. +</p> + +<p> +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 can have marvelled more +than I have done at the extinction of species. 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 my astonishment was +groundless. 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 +<a name="Page295"></a> +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. +</p> + +<p> +It is most difficult always to remember that the increase of every living +creature is constantly being checked by unperceived hostile agencies; and that +these same unperceived agencies are amply sufficient to cause rarity, and +finally extinction. So little is this subject understood, that I have heard +surprise repeatedly expressed at such great monsters as the Mastodon and the +more ancient Dinosaurians having become extinct; as if mere bodily strength +gave victory in the battle of life. Mere size, on the contrary, would in some +cases determine, as has been remarked by Owen, quicker extermination, from the +greater amount of requisite food. Before man inhabited India or Africa, some +cause must have checked the continued increase of the existing elephant. A +highly capable judge, Dr. Falconer, believes that it is chiefly insects which, +from incessantly harassing and weakening the elephant in India, check its +increase; and this was Bruce’s conclusion with respect to the African +elephant in Abyssinia. It is certain that insects and blood-sucking bats +determine the existence of the larger naturalised quadrupeds in several parts +of South America. +</p> + +<p> +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 the +species 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 deed +of violence. +</p> + +<p> +The theory of natural selection is grounded on the belief that each new variety +and ultimately each new species, is produced and +<a name="Page296"></a> +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 those naturally and artificially produced, are +bound together. In flourishing groups, the number of new specific forms which +have been produced within a given time has at some periods probably been +greater than the number of the old specific forms which have been exterminated; +but we know that species have not gone on indefinitely increasing, at least +during the later geological epochs, 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. +</p> + +<p> +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>i.e.</i> 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 has seized on the place occupied by a species belonging to a +distinct group, and thus have caused its extermination. If many allied forms be +developed from the successful intruder, many will have to yield their places; +and it will generally be the 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 have yielded their places to other modified +and improved species, a few of the sufferers may often be preserved for a long +time, from being fitted to some peculiar line of life, or from inhabiting some +distant and isolated station, where they will have escaped severe competition. +For instance, some species of Trigonia, a great genus of shells in the +secondary formations, survive 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. +</p> + +<p> +<a name="Page297"></a> +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 an area, many of the older species will +have been exterminated in a correspondingly rapid manner; and the forms which +thus yield their places will commonly be allied, for they will partake of the +same inferiority in common. +</p> + +<p> +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 until then, we may justly feel surprise why we +cannot account for the extinction of any particular species or group of +species. +</p> + +<p class="center"> +<i>On the Forms of Life changing almost simultaneously throughout the +World.</i> +</p> + +<p> +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 regions, 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 unmistakable +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, +<a name="Page298"></a> +but which occur in the formations either above or below, occur in the same +order 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 European and North American tertiary deposits. Even if the +few fossil species which are common to the Old and New Worlds were kept wholly +out of view, the general parallelism in the successive forms of life, in the +palæozoic and tertiary stages, would still be manifest, and the several +formations could be easily correlated. +</p> + +<p> +These observations, however, relate to the marine inhabitants of the world: we +have not sufficient data to judge whether the productions of the land and of +fresh water at distant points change 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 +co-existed with sea-shells all still living; but as these anomalous monsters +co-existed with the Mastodon and Horse, it might at least have been inferred +that they had lived during one of the later tertiary stages. +</p> + +<p> +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 year, or even to the same century, or even that it has a very strict +geological sense; for if all the marine animals now living in Europe, and all +those that lived in Europe during the pleistocene period (a very remote period +as measured by years, including the whole glacial epoch) were compared with +those now existing in South America or in Australia, the most skilful +naturalist would hardly be able to say whether the present or the pleistocene +inhabitants of Europe resembled most closely those of the southern hemisphere. +So, again, several highly competent observers maintain that the existing +productions of the United States are more closely related to those which lived +in Europe during certain late tertiary stages, than to the present inhabitants +of Europe; and if this be so, it is evident that fossiliferous beds now +deposited 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 be little doubt that all the more modern <i>marine</i> +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 +<a name="Page299"></a> +forms which are found only in the older underlying deposits, would be correctly +ranked as simultaneous in a geological sense. +</p> + +<p> +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. +</p> + +<p> +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 having some advantage over older forms; and the forms, which are already +dominant, or have some advantage over the other forms in their own country, +give birth to the greatest number of new varieties or incipient species. We +have distinct evidence on this head, in the plants which are dominant, that is, +which are commonest and most widely diffused, producing the greatest number of +new varieties. It is also natural that the dominant, varying and far-spreading +species, which have already 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 other new varieties and +species. The process of diffusion would often be very slow, depending on +climatal and geographical changes, on strange accidents, and on the gradual +acclimatization of new species to the various climates through which they might +have to pass, but in the course of time the dominant forms would generally +succeed in spreading and would ultimately prevail. 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 do find, a less strict degree +<a name="Page300"></a> +of parallelism in the succession of the productions of the land than with those +of the sea. +</p> + +<p> +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 their having had some advantage over their +already dominant parents, as well as over other species; and again spreading, +varying, and producing new forms. The old 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 disappear from the +world; and the succession of forms everywhere tends to correspond both in their +first appearance and final disappearance. +</p> + +<p> +There is one other remark connected with this subject worth making. I have +given my reasons for believing that most of our great formations, rich in +fossils, were deposited during periods of subsidence; and that blank intervals +of vast duration, as far as fossils are concerned, 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 very 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. +</p> + +<p> +I suspect that cases of this nature occur 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 +<a name="Page301"></a> +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 forms 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. +</p> + +<p class="center"> +<i>On the Affinities of Extinct Species to each other, and to Living Forms.</i> +</p> + +<p> +Let us now look to the mutual affinities of extinct and living species. All +fall into a few grand classes; 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, extinct +species can all be classed either in still existing groups, or between them. +That the extinct forms of life help to fill up the intervals between existing +genera, families, and orders, is certainly true; but as this statement has +often been ignored or even denied, it may be well to make some remarks on this +subject, and to give some instances. If we confine our attention either to the +living or to the extinct species of the same class, the series is far less +perfect than if we combine both into one general system. In the writings of +Professor Owen we continually meet with the expression of generalised forms, as +applied to extinct animals; and in the writings of Agassiz, of prophetic or +synthetic types; and these terms imply that such forms are, in fact, +intermediate or connecting links. Another distinguished palæontologist, M. +Gaudry, has shown in the most striking manner that many of the fossil mammals +discovered by him in Attica serve to break +<a name="Page302"></a> +down the intervals between existing genera. Cuvier ranked the Ruminants and +Pachyderms as two of the most distinct orders of mammals; but so many fossil +links have been disentombed that Owen has had to alter the whole +classification, and has placed certain Pachyderms in the same sub-order with +ruminants; for example, he dissolves by gradations the apparently wide interval +between the pig and the camel. The Ungulata or hoofed quadrupeds are now +divided into the even-toed or odd-toed divisions; but the Macrauchenia of South +America connects to a certain extent these two grand divisions. No one will +deny that the Hipparion is intermediate between the existing horse and certain +other ungulate forms. What a wonderful connecting link in the chain of mammals +is the Typotherium from South America, as the name given to it by Professor +Gervais expresses, and which cannot be placed in any existing order. The +Sirenia form a very distinct group of the mammals, and one of the most +remarkable peculiarities in existing dugong and lamentin is the entire absence +of hind limbs, without even a rudiment being left; but the extinct Halitherium +had, according to Professor Flower, an ossified thigh-bone “articulated +to a well-defined acetabulum in the pelvis,” and it thus makes some +approach to ordinary hoofed quadrupeds, to which the Sirenia are in other +respects allied. The cetaceans or whales are widely different from all other +mammals, but the tertiary Zeuglodon and Squalodon, which have been placed by +some naturalists in an order by themselves, are considered by Professor Huxley +to be undoubtedly cetaceans, “and to constitute connecting links with the +aquatic carnivora.” +</p> + +<p> +Even the wide interval between birds and reptiles has been shown by the +naturalist just quoted to be partially bridged over in the most unexpected +manner, on the one hand, by the ostrich and extinct Archeopteryx, and on the +other hand by the Compsognathus, one of the Dinosaurians—that group which +includes the most gigantic of all terrestrial reptiles. Turning to the +Invertebrata, Barrande asserts, a higher authority could not be named, that he +is every day taught that, although palæozoic animals can certainly be classed +under existing groups, yet that at this ancient period the groups were not so +distinctly separated from each other as they now are. +</p> + +<p> +Some writers have objected to any extinct species, or group of species, being +considered as intermediate between any two living species, or groups of +species. If by this term it is meant that an extinct form is directly +intermediate in all its characters between two living forms or groups, the +objection is probably valid. +<a name="Page303"></a> +But in a natural classification many fossil species certainly 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 by a score of characters, +the ancient members are separated by a somewhat lesser number of characters, so +that the two groups formerly made a somewhat nearer approach to each other than +they now do. +</p> + +<p> +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 recent members of the same classes, we must admit that there is truth +in the remark. +</p> + +<p> +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 in italics 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, <i>a</i><sup>14</sup>, <i>q</i><sup>14</sup>, +<i>p</i><sup>14</sup>, will form a small family; <i>b</i><sup>14</sup> and +<i>f</i><sup>14</sup>, a closely allied family or subfamily; and +<i>o</i><sup>14</sup>, <i>e</i><sup>14</sup>, <i>m</i><sup>14</sup>, 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 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 +<a name="Page304"></a> +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 <small>F</small><sup>14</sup>. +</p> + +<p> +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. +</p> + +<p> +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 +<i>a</i><sup>1</sup>, <i>a</i><sup>5</sup>, <i>a</i><sup>10</sup>, +<i>f</i><sup>8</sup>, <i>m</i><sup>3</sup>, <i>m</i><sup>6</sup>, +<i>m</i><sup>9</sup>, 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 +certain pachyderms. Yet he who objected to consider as intermediate the extinct +genera, which thus link together the living genera of three families, would be +partly justified, for 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 No. VI.—but none from +beneath this line, then only two of the families (those on the left hand +<i>a</i><sup>14</sup>, &c., and <i>b</i><sup>14</sup>, &c.) would have +to be united into one; and there would remain two families which would be less +distinct from each other than they were before the discovery of the fossils. So +again, if the three families formed of eight genera (<i>a</i><sup>14</sup> to +<i>m</i><sup>14</sup>), on the uppermost line, be supposed to differ from each +other by half-a-dozen important characters, then the families which existed at +a period marked VI would certainly have differed from each other by a less +number of characters; for they would at this early stage of descent have +diverged in a less degree from their common progenitor. Thus it comes that +ancient and extinct genera are often in a greater or less degree intermediate +in character between their modified descendants, or between their collateral +relations. +</p> + +<p> +Under nature the process will be far more complicated than is +<a name="Page305"></a> +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 rare cases, to fill up the wide intervals in the natural +system, and thus to 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 is +frequently the case. +</p> + +<p> +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, are explained in a satisfactory manner. And they are wholly +inexplicable on any other view. +</p> + +<p> +On this same theory, it is evident that the fauna during any one 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 in any one region for the immigration of new forms from other +regions, 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 systems. But each fauna is not necessarily exactly +intermediate, as unequal intervals of time have elapsed between consecutive +formations. +</p> + +<p> +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 +<a name="Page306"></a> +offer exceptions to the rule. For instance, the species of mastodons and +elephants, when arranged by Dr. Falconer in two series—in the first place +according to their mutual affinities, and in the second place 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 complete, which is far from the case, we +have no reason to believe that forms successively produced necessarily endure +for corresponding lengths of time. A very ancient form may occasionally have +lasted 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 in serial affinity, this arrangement would not +closely accord with the order in time of their production, and even less with +the order of their disappearance; for the parent rock-pigeon still 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 respect. +</p> + +<p> +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 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 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 epoch, +and note how little the specific forms of the inhabitants of the sea have been +affected. +</p> + +<p> +<a name="Page307"></a> +On the theory of descent, the full meaning of the fossil remains from closely +consecutive formations, being closely related, though ranked as distinct +species, 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 in any 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 +assuredly we do find. We find, in short, such evidence of the slow and scarcely +sensible mutations of specific forms, as we have the right to expect. +</p> + +<p class="center"> +<i>On the State of Development of Ancient compared with Living Forms.</i> +</p> + +<p> +We have seen in the fourth chapter that the degree of differentiation and +specialisation of the parts in organic beings, when arrived at maturity, is the +best standard, as yet suggested, of their degree of perfection or highness. We +have also seen that, as the specialisation of parts is an advantage to each +being, so natural selection will tend to render the organisation of each being +more specialised and perfect, and in this sense higher; not but that it may +leave many creatures with simple and unimproved structures fitted for simple +conditions of life, and in some cases will even degrade or simplify the +organisation, yet leaving such degraded beings better fitted for their new +walks of life. In another and more general manner, new species become superior +to their predecessors; for they have to beat in the struggle for life all the +older forms, with which they come into close competition. We may therefore +conclude that if under a nearly similar climate the eocene inhabitants of the +world could be put into competition with the existing inhabitants, the former +would be beaten and exterminated by the latter, as would the secondary by the +eocene, and the palæozoic by the secondary forms. So that by this fundamental +test of victory in the battle for life, as well as by the standard of the +specialisation of organs, modern forms ought, on the theory of natural +selection, to stand higher than ancient forms. Is this the case? A large +majority of palæontologists would answer in the affirmative; and it seems that +this answer must be admitted as true, though difficult of proof. +</p> + +<p> +It is no valid objection to this conclusion, that certain Brachiopods +<a name="Page308"></a> +have been but slightly modified from an extremely remote geological epoch; and +that certain land and fresh-water shells have remained nearly the same, from +the time when, as far as is known, they first appeared. It is not an +insuperable difficulty that Foraminifera have not, as insisted on by Dr. +Carpenter, progressed in organisation since even the Laurentian epoch; for some +organisms would have to remain fitted for simple conditions of life, and what +could be better fitted for this end than these lowly organised Protozoa? Such +objections as the above would be fatal to my view, if it included advance in +organisation as a necessary contingent. They would likewise be fatal, if the +above Foraminifera, for instance, could be proved to have first come into +existence during the Laurentian epoch, or the above Brachiopods during the +Cambrian formation; for in this case, there would not have been time sufficient +for the development of these organisms up to the standard which they had then +reached. When advanced up to any given point, there is no necessity, on the +theory of natural selection, for their further continued process; though they +will, during each successive age, have to be slightly modified, so as to hold +their places in relation to slight changes in their conditions. The foregoing +objections hinge on the question whether we really know how old the world is, +and at what period the various forms of life first appeared; and this may well +be disputed. +</p> + +<p> +The problem whether organisation on the whole has advanced is in many ways +excessively intricate. The geological record, at all times imperfect, does not +extend far enough back to show with unmistakable clearness that within the +known history of the world organisation has largely advanced. Even at the +present day, looking to members of the same class, naturalists are not +unanimous which forms ought to be ranked as highest: thus, some look at the +selaceans or sharks, from their approach in some important points of structure +to reptiles, as the highest fish; others look at the teleosteans as the +highest. The ganoids stand intermediate between the selaceans and teleosteans; +the latter at the present day are largely preponderant in number; but formerly +selaceans and ganoids alone existed; and in this case, according to the +standard of highness chosen, so will it be said that fishes have advanced or +retrograded in organisation. To attempt to compare members of distinct types in +the scale of highness seems hopeless; who will decide whether a cuttle-fish be +higher than a bee—that insect which the great Von Baer believed to be +“in fact more highly organised than a fish, although upon another +type?” In the complex struggle for life it is quite credible that +crustaceans, not very high in their +<a name="Page309"></a> +own class, might beat cephalopods, the highest molluscs; and such crustaceans, +though not highly developed, would stand very high in the scale of invertebrate +animals, if judged by the most decisive of all trials—the law of battle. +Beside these inherent difficulties in deciding which forms are the most +advanced in organisation, we ought not solely to compare the highest members of +a class at any two periods—though undoubtedly this is one and perhaps the +most important element in striking a balance—but we ought to compare all +the members, high and low, at two periods. At an ancient epoch the highest and +lowest molluscoidal animals, namely, cephalopods and brachiopods, swarmed in +numbers; at the present time both groups are greatly reduced, while others, +intermediate in organisation, have largely increased; consequently some +naturalists maintain that molluscs were formerly more highly developed than at +present; but a stronger case can be made out on the opposite side, by +considering the vast reduction of brachiopods, and the fact that our existing +cephalopods, though few in number, are more highly organised than their ancient +representatives. We ought also to compare the relative proportional numbers, at +any two periods, of the high and low classes throughout the world: if, for +instance, at the present day fifty thousand kinds of vertebrate animals exist, +and if we knew that at some former period only ten thousand kinds existed, we +ought to look at this increase in number in the highest class, which implies a +great displacement of lower forms, as a decided advance in the organisation of +the world. We thus see how hopelessly difficult it is to compare with perfect +fairness, under such extremely complex relations, the standard of organisation +of the imperfectly-known faunas of successive periods. +</p> + +<p> +We shall appreciate this difficulty more clearly by looking to certain existing +faunas and floras. 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 by the indigenes, we must believe, that if all +the animals and plants of Great Britain were set free in New Zealand, a +multitude of British forms would in the course of time become thoroughly +naturalized there, and would exterminate many of the natives. On the other +hand, from the fact that hardly a single inhabitant of the southern hemisphere +has become wild in any part of Europe, we may well doubt whether, if all the +productions of New Zealand were set free in Great Britain, 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 stand +much higher in the scale than those of New Zealand. Yet the +<a name="Page310"></a> +most skilful naturalist, from an examination of the species of the two +countries, could not have foreseen this result. +</p> + +<p> +Agassiz and several other highly competent judges insist that ancient animals +resemble to a certain extent the embryos of recent animals belonging to the +same classes; and that the geological succession of extinct forms is nearly +parallel with the embryological development of existing forms. This view +accords admirably well with our theory. In a future chapter I shall attempt to +show that the adult differs from its embryo, owing to variations having +supervened at a not early age, and having been 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 former and less modified condition of the species. This view may +be true, and yet may never be capable of proof. Seeing, for instance, that the +oldest known mammals, reptiles, and fishes strictly belong to their 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 rich in fossils are discovered far +beneath the lowest Cambrian strata—a discovery of which the chance is +small. +</p> + +<p class="center"> +<i>On the Succession of the same Types within the same Areas, during the later +Tertiary periods.</i> +</p> + +<p> +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 +<a name="Page311"></a> +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 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. +</p> + +<p> +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 through dissimilar physical +conditions, for the dissimilarity of the inhabitants of these two continents; +and, on the other hand through similarity of conditions, for the uniformity of +the same types in each continent 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. +</p> + +<p> +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 intermigration, the feebler will yield to the more +dominant forms, and there will be nothing immutable in the distribution of +organic beings. +</p> + +<p> +It may be asked in ridicule whether I suppose that the megatherium and other +allied huge monsters, which formerly lived in +<a name="Page312"></a> +South America, have left behind them 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 all other characters to the species still living in South America; and +some of these fossils may have been the actual progenitors of the living +species. It must not be forgotten that, on our theory, all the species of the +same genus are the descendants of some one species; so that, if six genera, +each having eight species, be found in one geological formation, and in a +succeeding formation there be six other allied or representative genera, each +with the same number of species, then we may conclude that generally only one +species of each of the older genera has left modified descendants, which +constitute the new genera containing the several species; the other seven +species of each old genus having died out and left no progeny. Or, and this +will be a far commoner case, two or three species in two or three alone of the +six older genera will be the parents of the new genera: the other species and +the other old genera having become utterly extinct. In failing orders, with the +genera and species decreasing in numbers as is the case with the Edentata of +South America, still fewer genera and species will leave modified +blood-descendants. +</p> + +<p class="center"> +<i>Summary of the preceding and present Chapters.</i> +</p> + +<p> +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 number of +generations which must have passed away even during a single formation; that, +owing to subsidence being almost necessary for the accumulation of deposits +rich in fossil species of many kinds, and thick enough to outlast future +degradation, great intervals of time must have elapsed between most of our +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 probably 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; +<a name="Page313"></a> +that widely ranging species are those which have varied most frequently, and +have oftenest given rise to new species; that varieties have at first been +local; and lastly, although each species must have passed through numerous +transitional stages, it is probable that the periods, during which each +underwent modification, though many and long as measured by years, have been +short in comparison with the periods during which each remained in an unchanged +condition. These causes, taken conjointly, will to a large extent explain +why—though we do find many links—we do not find interminable +varieties, connecting together all extinct and existing forms by the finest +graduated steps. It should also be constantly borne in mind that any linking +variety between two forms, which might be found, would be ranked, unless the +whole chain could be perfectly restored, as a new and distinct species; for it +is not pretended that we have any sure criterion by which species and varieties +can be discriminated. +</p> + +<p> +He who rejects this view of the imperfection of the geological record, will +rightly reject the 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 successive stages of the same +great formation? He may disbelieve in the immense intervals of time which must +have elapsed between our consecutive formations; he may overlook how important +a part migration has played, when the formations of any one great region, as +those 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 Cambrian system was deposited? We now know that at least one animal +did then exist; but I can answer this last question only by supposing that +where our oceans now extend they have extended for an enormous period, and +where our oscillating continents now stand they have stood since the +commencement of the Cambrian system; but that, long before that epoch, the +world presented a widely different aspect; and that the older continents, +formed of formations older than any known to us, exist now only as remnants in +a metamorphosed condition, or lie still buried under the ocean. +</p> + +<p> +Passing from these difficulties, the other great leading facts in palæontology +agree admirably with the theory of descent with modification through variation +and 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 +<a name="Page314"></a> +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 belonging to large and dominant groups tend +to leave many modified descendants, which form new sub-groups and groups. 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 has sometimes been a 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. +</p> + +<p> +We can understand how it is that dominant forms which spread widely and yield +the greatest number of varieties tend to people the world with allied, but +modified, descendants; and these will generally succeed in displacing the +groups which are their inferiors in the struggle for existence. Hence, after +long intervals of time, the productions of the world appear to have changed +simultaneously. +</p> + +<p> +We can understand how it is that all the forms of life, ancient and recent, +make together a few grand classes. 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 bringing them only a +little closer together. The more ancient a form is, the more often it stands 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 other extinct and different forms. We +can clearly see why the organic remains of closely consecutive formations are +closely allied; for they are closely linked together by generation. We can +clearly see why the remains of an intermediate formation are intermediate in +character. +</p> + +<p> +<a name="Page315"></a> +The inhabitants of the world at each successive period in its history have +beaten their predecessors in the race for life, and are, in so far, higher in +the scale, and their structure has generally become more specialised; and this +may account for the common belief held by so many palæontologists, that +organisation on the whole has progressed. Extinct and ancient animals resemble +to a certain extent the embryos of the more recent animals belonging to the +same classes, and this wonderful fact receives a simple explanation according +to our views. The succession of the same types of structure within the same +areas during the later geological periods ceases to be mysterious, and is +intelligible on the principle of inheritance. +</p> + +<p> +If, then, the geological record be as imperfect as many believe, 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, +the products of variation and the survival of the fittest. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page316"></a><a name="link2HCH0012" id="link2HCH0012"></a> +CHAPTER XII.<br/> +GEOGRAPHICAL DISTRIBUTION.</h2> + +<p class="letter"> +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—Alternate Glacial periods +in the North and South. +</p> + +<p> +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 wholly accounted for +by 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 arctic and +northern temperate parts, 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; 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 so closely as the same species +generally require. No doubt small areas can be pointed out in the Old World +hotter than any in the New World; but these are not inhabited by a fauna +different from that of the surrounding districts; for it is rare to find a +group of organisms confined to a small area, of which the conditions are +peculiar in only a slight degree. Notwithstanding this general parallelism in +the conditions of Old and New Worlds, how widely different are their living +productions! +</p> + +<p> +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 +<a name="Page317"></a> +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 latitude 35° with those north of 25°, which consequently are +separated by a space of ten degrees of latitude, and are exposed to +considerably different conditions; yet they are 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. +</p> + +<p> +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 in 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 even of +large rivers, we find different productions; though as mountain chains, +deserts, &c., 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. +</p> + +<p> +Turning to the sea, we find the same law. The marine inhabitants of the eastern +and western shores of South America are very distinct, with extremely few +shells, crustacea, or echinodermata in common; but Dr. Günther has recently +shown that about thirty per cent of the fishes are the same on the opposite +sides of the isthmus of Panama; and this fact has led naturalists to believe +that the isthmus was formerly open. 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 three marine faunas range northward and southward in +parallel lines not far from each other, under corresponding climate; but from +being separated from each other by impassable barriers, either of land or open +sea, they are almost wholly distinct. On the other hand, proceeding still +farther westward from the eastern +<a name="Page318"></a> +islands of the tropical parts of the Pacific, we encounter no impassable +barriers, and we have innumerable islands as halting-places, or continuous +coasts, 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 so few marine animals are common to the above-named +three approximate faunas of Eastern and Western America and the eastern Pacific +islands, yet many fishes 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. +</p> + +<p> +A third great fact, partly included in the foregoing statement, is the affinity +of the productions of the same continent or of the same 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, though nearly 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 emu, like +those inhabiting 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 muskrat, but the coypu and +capybara, rodents of the South 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 are essentially American, +though they may be all peculiar species. We may look back to past ages, as +shown in the last chapter, and we find American types then prevailing on the +American continent and in the American seas. We see in these facts some deep +organic bond, throughout space and time, over the same areas of land and water, +independently of physical conditions. The naturalist must be dull who is not +led to inquire what this bond is. +</p> + +<p> +The bond is simply inheritance, that cause which alone, as far as +<a name="Page319"></a> +we positively know, produces organisms quite like each other, or, as we see in +the case of varieties, nearly alike. The dissimilarity of the inhabitants of +different regions may be attributed to modification through variation and +natural selection, and probably in a subordinate degree to the definite +influence of different physical conditions. The degrees of dissimilarity will +depend on the migration of the more dominant forms of life from one region into +another having been more or less effectually prevented, at periods more or less +remote—on the nature and number of the former immigrants—and on the +action of the inhabitants on each other in leading to the preservation of +different modifications; the relation of organism to organism in the struggle +for life 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 out 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. +</p> + +<p> +There is no evidence, as was remarked in the last chapter, of the existence of +any 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 each individual in its complex struggle for life, so the +amount of modification in different species will be no uniform quantity. If a +number of species, after having long competed with each other in their old +home, were to migrate in a body into a new and afterwards isolated country, +they would be little liable to modification; for neither migration nor +isolation in themselves effect 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 or at all modified. +</p> + +<p> +According to these views, it is obvious that the several species of the same +genus, though inhabiting the most distant quarters of the +<a name="Page320"></a> +world, must originally have proceeded from the same source, as they are +descended from the same progenitor. In the case of those species which have +undergone, during whole geological periods, little modification, there is not +much difficulty in believing that they have migrated from the same region; for +during the vast geographical and climatical changes which 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 has been explained, it is +incredible that individuals identically the same should have been produced from +parents specifically distinct. +</p> + +<p> +<i>Single Centres of supposed Creation.</i>—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 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 +that 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 have been easily +passed over by migration, the fact is given as something remarkable and +exceptional. The incapacity of migrating across a wide sea is more clear in the +case of terrestrial mammals than perhaps with any other organic beings; and, +accordingly, we find no inexplicable instances of the same mammals inhabiting +distant points of the world. No geologist feels any difficulty in Great Britain +possessing the same quadrupeds with the rest of Europe, for they were no doubt +once united. 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 +<a name="Page321"></a> +I believe, is, that mammals have not been able to migrate, whereas some plants, +from their varied means of dispersal, have migrated across the wide and broken +interspaces. The great and striking influence of barriers of all kinds, is +intelligible only on the view that the great majority of species have been +produced on one side, and have not been able to migrate to the opposite side. +Some few families, many subfamilies, very many genera, 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 +confined to the same country, or if they have a wide range that their range is +continuous. What a strange anomaly it would be if a directly opposite rule were +to prevail when we go down one step lower in the series, namely to the +individuals of the same species, and these had not been, at least at first, +confined to some one region! +</p> + +<p> +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 climatical changes which +have certainly occurred within recent geological times, must have 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 instances. 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 fresh water productions; and +thirdly, the occurrence of the same terrestrial species on islands and on the +nearest 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 +<a name="Page322"></a> +from a single birthplace; then, considering our ignorance with respect to +former climatical and geographical changes, and to the various occasional means +of transport, the belief that a single birthplace is the law seems to me +incomparably the safest. +</p> + +<p> +In discussing this subject we shall be enabled at the same time to consider a +point equally important for us, namely, whether the several species of a genus +which must on our theory all be descended from a common progenitor, can have +migrated, undergoing modification during their migration from some one area. +If, when most of the species inhabiting one region are different from those of +another region, though closely allied to them, it can be shown that migration +from the one region to the other has probably occurred at some former period, +our general view will be much strengthened; for the explanation is obvious on +the principle of descent with modification. 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 related by +inheritance to the inhabitants of that continent. Cases of this nature are +common, and are, as we shall hereafter see, inexplicable on the theory of +independent creation. This view of the relation of the species of one region to +those of another, does not differ much from that advanced by Mr. Wallace, who +concludes that “every species has come into existence coincident both in +space and time with a pre-existing closely allied species.” And it is now +well known that he attributes this coincidence to descent with modification. +</p> + +<p> +The question of single or multiple centres of creation differs from another +though allied question, namely, whether all the individuals of the same species +are descended from a single pair, or single hermaphrodite, or whether, as some +authors suppose, from many individuals simultaneously created. With organic +beings which never intercross, if such exist, each species, must be descended +from a succession of modified varieties, that have supplanted each other, but +have never blended with other individuals or varieties of the same species, so +that, at each successive stage of modification, all the individuals of the same +form will be descended from a single parent. But in the great majority of +cases, namely, with all organisms which habitually unite for each birth, or +which occasionally intercross, the individuals of the same species inhabiting +the same area will be kept nearly uniform by intercrossing; so that many +individuals will go on simultaneously changing, and the whole amount of +modification at each stage will +<a name="Page323"></a> +not be due to descent from a single parent. To illustrate what I mean: our +English race-horses differ 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 the selecting and training of many individuals during each +generation. +</p> + +<p> +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. +</p> + +<p class="center"> +<i>Means of Dispersal.</i> +</p> + +<p> +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 now impassable to certain +organisms from the nature of its climate, might have been a high road for +migration, when the climate was different. 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 together, 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 disputes that great mutations of level have +occurred within the period of existing organisms. Edward Forbes insisted that +all the islands in the Atlantic must have been recently connected with Europe +or Africa, and Europe likewise with America. Other authors have thus +hypothetically bridged over every ocean, and united almost every island with +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 in the level of the land or +sea; but not of such vast changes in the position and extension of our +continents, 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 +<a name="Page324"></a> +halting-places for plants and for many animals during their migration. In the +coral-producing oceans such sunken islands are now marked by rings of coral or +atolls standing over them. Whenever it is fully admitted, as 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 most of our continents which now stand 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—the degree of affinity between +the mammals inhabiting islands with those of the nearest continent, being in +part determined (as we shall hereafter see) by the depth of the intervening +ocean—these and other such facts are opposed to the admission of such +prodigious geographical revolutions within the recent period, as are necessary +on the view advanced by Forbes and admitted by his followers. The nature and +relative proportions of the inhabitants of oceanic islands are likewise opposed +to the belief of their former continuity of continents. Nor does the almost +universally volcanic composition of such islands favour the admission that they +are the wrecks of sunken continents; if they had originally existed as +continental mountain ranges, some at least of the islands would have been +formed, like other mountain summits, of granite, metamorphic schists, old +fossiliferous and other rocks, instead of consisting of mere piles of volcanic +matter. +</p> + +<p> +I must now say a few words on what are called accidental means, but which more +properly should be called occasional means of distribution. I shall here +confine myself to plants. In botanical works, this or that plant is often +stated to be ill adapted for wide dissemination; but the greater or less +facilities for transport across the sea 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 eighty-seven kinds, sixty-four +germinated after an immersion of twenty-eight days, and a few survived an +immersion of 137 days. It deserves notice that certain orders were far more +injured than others: nine Leguminosæ were tried, and, with one exception, they +resisted the salt-water badly; seven species of the allied orders, +Hydrophyllaceæ and Polemoniaceæ, were all killed by a month’s +<a name="Page325"></a> +immersion. For convenience’ sake I chiefly tried small seeds without the +capsules or fruit; and as all of these sank in a few days, they could not have +been floated across wide spaces of the sea, whether or not they were injured by +salt water. Afterwards I tried some larger fruits, capsules, &c., 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 +would often wash into the sea dried plants or branches with seed-capsules or +fruit attached to them. Hence I was led to dry the stems and branches of +ninety-four 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 ninety days, and afterwards when +planted germinated; an asparagus plant with ripe berries floated for +twenty-three days, when dried it floated for eighty-five days, and the seeds +afterwards germinated: the ripe seeds of Helosciadium sank in two days, when +dried they floated for above ninety days, and afterwards germinated. +Altogether, out of the ninety-four dried plants, eighteen floated for above +twenty-eight days; and some of the eighteen floated for a very much longer +period. So that as 64/87 kinds of seeds germinated after an immersion of +twenty-eight days; and as 18/94 distinct species with ripe fruit (but not all +the same species as in the foregoing experiment) floated, after being dried, +for above twenty-eight days, we may conclude, as far as anything can be +inferred from these scanty facts, that the seeds of 14/100 kinds of plants of +any country might be floated by sea-currents during twenty-eight days, and +would retain their power of germination. In Johnston’s Physical Atlas, +the average rate of the several Atlantic currents is thirty-three miles per +diem (some currents running at the rate of sixty 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 by an +inland gale to a favourable spot, would germinate. +</p> + +<p> +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 ninety-eight 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 both the average length of their flotation and 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 +<a name="Page326"></a> +much longer. The result was that 18/98 of his seeds of different kinds floated +for forty-two 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 +which, as Alph. de Candolle has shown, generally have restricted ranges, could +hardly be transported by any other means. +</p> + +<p> +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 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 during +the longest transport: out of one small portion of earth thus <i>completely</i> +enclosed by the roots of an oak about fifty 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 many kinds of seeds 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 sea-water for thirty +days, to my surprise nearly all germinated. +</p> + +<p> +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 +safely assume that under such circumstances their rate of flight would often be +thirty-five 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 pass uninjured through even the digestive +organs of a turkey. In the course of two months, I picked up in my garden +twelve kinds of seeds, out of the excrement of small birds, and these seemed +perfect, and some of them, which were tried, germinated. But the following fact +is more important: the crops of birds do not secrete gastric juice, and do not, +as I know by trial, injure in the least the germination of seeds; now, after a +bird +<a name="Page327"></a> +has found and devoured a large supply of food, it is positively asserted that +all the grains do not pass into the gizzard for twelve or even eighteen hours. +A bird in this interval might easily be blown to the distance of five hundred +miles, and hawks are known to look out for tired birds, and the contents of +their torn crops might thus readily get scattered. 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. +Fresh-water 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 the power +of germination. Certain seeds, however, were always killed by this process. +</p> + +<p> +Locusts are sometimes blown to great distances from the land. I myself caught +one 370 miles from the coast of Africa, and have heard of others caught at +greater distances. The Rev. R.T. Lowe informed Sir C. Lyell that in November, +1844, swarms of locusts visited the island of Madeira. They were in countless +numbers, as thick as the flakes of snow in the heaviest snowstorm, and extended +upward as far as could be seen with a telescope. During two or three days they +slowly careered round and round in an immense ellipse, at least five or six +miles in diameter, and at night alighted on the taller trees, which were +completely coated with them. They then disappeared over the sea, as suddenly as +they had appeared, and have not since visited the island. Now, in parts of +Natal it is believed by some farmers, though on insufficient evidence, that +injurious seeds are introduced into their grass-land in the dung left by the +great flights of locusts which often visit that country. In consequence of this +belief Mr. Weale sent me in a letter a small packet of the dried pellets, out +of which I extracted under the microscope several seeds, and raised from them +seven grass plants, belonging to two species, of two genera. Hence a swarm of +locusts, such as that which visited Madeira, might readily be the means of +introducing several kinds of plants into an island lying far from the mainland. +</p> + +<p> +<a name="Page328"></a> +Although the beaks and feet of birds are generally clean, earth sometimes +adheres to them: in one case I removed sixty-one grains, and in another case +twenty-two grains of dry argillaceous earth from the foot of a partridge, and +in the earth there was a pebble as large as the seed of a vetch. Here is a +better case: the leg of a woodcock was sent to me by a friend, with a little +cake of dry earth attached to the shank, weighing only nine grains; and this +contained a seed of the toad-rush (Juncus bufonius) which germinated and +flowered. Mr. Swaysland, of Brighton, who during the last forty years has paid +close attention to our migratory birds, informs me that he has often shot +wagtails (Motacillæ), wheatears, and whinchats (Saxicolæ), on their first +arrival on our shores, before they had alighted; and he has several times +noticed little cakes of earth attached to their feet. Many facts could be given +showing how generally soil is charged with seeds. For instance, Professor +Newton sent me the leg of a red-legged partridge (Caccabis rufa) which had been +wounded and could not fly, with a ball of hard earth adhering to it, and +weighing six and a half ounces. The earth had been kept for three years, but +when broken, watered and placed under a bell glass, no less than eighty-two +plants sprung from it: these consisted of twelve monocotyledons, including the +common oat, and at least one kind of grass, and of seventy dicotyledons, which +consisted, judging from the young leaves, of at least three distinct species. +With such facts before us, can we doubt that the many birds which are annually +blown by gales across great spaces of ocean, and which annually +migrate—for instance, the millions of quails across the +Mediterranean—must occasionally transport a few seeds embedded in dirt +adhering to their feet or beaks? But I shall have to recur to this subject. +</p> + +<p> +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, it can hardly be +doubted that they must occasionally, as suggested by Lyell, have transported +seeds from one part to another of the arctic and antarctic regions; and during +the Glacial period from one part of the now temperate regions to another. In +the Azores, from the large number of plants common to Europe, in comparison +with the species on the other islands of the Atlantic, which stand nearer to +the mainland, and (as remarked by Mr. H.C. Watson) from their somewhat northern +character, 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 +<a name="Page329"></a> +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 burdens 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. +</p> + +<p> +Considering that these several means of transport, and that other means, which +without doubt remain to be discovered, have been in action year after year for +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 sea water; 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; but would remain as distinct as they now are. +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 their very long 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 +rare wanderers only by one means, namely, by dirt adhering to their feet or +beaks, which is in itself a rare accident. Even in this case, how small would +be the chance 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. Out of a hundred kinds of seeds or animals +transported to an island, even if far less well-stocked than Britain, perhaps +not more than one would be so well fitted to its new home, as to become +naturalised. But this is no valid argument against what would be effected by +occasional means of transport, during the long +<a name="Page330"></a> +lapse of geological time, whilst the island was being upheaved, 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, if fitted for the climate, would germinate and survive. +</p> + +<p class="center"> +<i>Dispersal during the Glacial Period.</i> +</p> + +<p> +The identity of many plants and animals, on mountain-summits, separated from +each other by hundreds of miles of lowlands, where 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 point to the other. It is indeed a remarkable fact to see so +many plants of the same species 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 many 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 scored rocks +plainly reveal a former cold period. +</p> + +<p> +The former influence of the glacial climate on the distribution of the +inhabitants of Europe, as explained by Edward Forbes, is substantially as +follows. But we shall follow the changes more readily, by supposing a new +glacial period slowly to come on, and then pass away, as formerly occurred. As +the cold came on, and as each more southern zone became fitted for the +inhabitants of the north, these would take the places of the former inhabitants +of the temperate regions. The latter, at the same time would travel +<a name="Page331"></a> +further and further 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 an 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. +</p> + +<p> +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, as the warmth +increased and the snow still further disappeared, higher and higher, whilst +their brethren were pursuing their northern journey. Hence, when the warmth had +fully returned, the same species, which had lately lived together on the +European and North American lowlands, would again be found in the arctic +regions of the Old and New Worlds, and on many isolated mountain-summits far +distant from each other. +</p> + +<p> +Thus we can understand the identity of many plants at points so immensely +remote as the mountains of the United States and those 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 first migration when the cold came on, and the re-migration on +the returning warmth, would 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 formerly +permitted their migration across the intervening lowlands, now become too warm +for their existence. +</p> + +<p> +<a name="Page332"></a> +As the arctic forms moved first southward and afterwards backward to the north, +in unison with the changing climate, they will not have been exposed during +their long migrations to any great diversity of temperature; and as they all +migrated in a body together, their mutual relations will not have been much +disturbed. Hence, in accordance with the principles inculcated in this volume, +these forms will not have been liable to much modification. But with the 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 far 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 the coldest +period will have been temporarily driven down to the plains; they will, also, +have been subsequently exposed to somewhat different climatical influences. +Their mutual relations will thus have been in some degree disturbed; +consequently they will have been liable to modification; and they have been +modified; for if we compare the present Alpine plants and animals of the +several great European mountain ranges, one with another, though many of the +species remain identically the same, some exist as varieties, some as doubtful +forms or sub-species and some as distinct yet closely allied species +representing each other on the several ranges. +</p> + +<p> +In the foregoing illustration, I have assumed that at the commencement of our +imaginary Glacial period, the arctic productions were as uniform round the +polar regions as they are at the present day. But it is also necessary to +assume that many sub-arctic and some few temperate forms were the same round +the world, for some of the species which now exist on the lower mountain slopes +and on the plains of North America and Europe are the same; and it may be asked +how I account for this degree of uniformity of the sub-arctic and temperate +forms round the world, at the commencement of the real 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 whole Atlantic Ocean and by the +northern part of the Pacific. During the Glacial period, when the inhabitants +of the Old and New Worlds lived further southwards than they do at present, +they must have been still more completely separated from each other by wider +spaces of ocean; so that it may well be asked how the +<a name="Page333"></a> +same species could then or previously have entered the two continents. The +explanation, I believe, lies in the nature of the climate before the +commencement of the Glacial period. At this, the newer Pliocene period, the +majority of the inhabitants of the world were specifically the same as now, and +we have good reason to believe that the climate was warmer than at the present +day. Hence, we may suppose that the organisms which now live under latitude +60°, lived during the Pliocene period further north, under the Polar Circle, in +latitude 66°–67°; and that the present arctic productions then lived on the +broken land still nearer to the pole. Now, if we look at a terrestrial globe, +we see under the Polar Circle that there is almost continuous land from western +Europe through Siberia, to eastern America. And this continuity of the +circumpolar land, with the consequent freedom under a more favourable climate +for intermigration, will account for the supposed uniformity of the sub-arctic +and temperate productions of the Old and New Worlds, at a period anterior to +the Glacial epoch. +</p> + +<p> +Believing, from reasons before alluded to, that our continents have long +remained in nearly the same relative position, though subjected to great +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 highly remarkable, considering +the distance of the two areas, and their separation by the whole 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 intermigration of their inhabitants. +</p> + +<p> +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 will have been +<a name="Page334"></a> +completely cut off from each other. This separation, as far as the more +temperate productions are concerned, must have taken place long ages ago. As +the plants and animals migrated southward, they will have become mingled in the +one great region with the native American productions, and would 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 Europe and North America. 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. +</p> + +<p> +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 +marine areas completely sundered. Thus, I think, we can understand the presence +of some closely allied, still existing and extinct tertiary forms, on the +eastern and western shores of temperate North America; and the still more +striking fact of many closely allied crustaceans (as described in Dana’s +admirable work), some fish and other marine animals, inhabiting the +Mediterranean and the seas of Japan—these two areas being now completely +separated by the breadth of a whole continent and by wide spaces of ocean. +</p> + +<p> +These cases of close relationship in species either now or formerly inhabiting +the seas on the eastern and western shores of North America, the Mediterranean +and Japan, and the temperate lands of North America and Europe, are +inexplicable on the theory of creation. We cannot maintain that such species +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 parts of South Africa or Australia, we see countries closely +similar in all their physical conditions, with their inhabitants utterly +dissimilar. +</p> + +<p class="center"> +<a name="Page335"></a> +<i>Alternate Glacial Periods in the North and South.</i> +</p> + +<p> +But we must return to our more immediate subject. I am convinced that +Forbes’s view may be largely extended. In Europe we meet with the +plainest evidence of the Glacial period, from the western shores of Britain to +the Ural range, and southward to the Pyrenees. We may infer from the frozen +mammals and nature of the mountain vegetation, that Siberia was similarly +affected. In the Lebanon, according to Dr. Hooker, perpetual snow formerly +covered the central axis, and fed glaciers which rolled 4,000 feet down the +valleys. The same observer has recently found great moraines at a low level on +the Atlas range in North Africa. 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 ancient and gigantic moraines. Southward of the +Asiatic continent, on the opposite side of the equator, we know, from the +excellent researches of Dr. J. Haast and Dr. Hector, that in New Zealand +immense glaciers formerly descended to a low level; and the same plants, found +by Dr. Hooker on widely separated mountains in this island tell the same story +of a former cold period. From facts communicated to me by the Rev. W.B. Clarke, +it appears also that there are traces of former glacial action on the mountains +of the south-eastern corner of Australia. +</p> + +<p> +Looking to America: in the northern half, ice-borne fragments of rock have been +observed on the eastern side of the continent, as far south as latitude 36° and +37°, and on the shores of the Pacific, where the climate is now so different, +as far south as latitude 46°. Erratic boulders have, also, been noticed on the +Rocky Mountains. In the Cordillera of South America, nearly under the equator, +glaciers once extended far below their present level. In central Chile I +examined a vast mound of detritus with great boulders, crossing the Portillo +valley, which, there can hardly be a doubt, once formed a huge moraine; and Mr. +D. Forbes informs me that he found in various parts of the Cordillera, from +latitude 13° to 30° south, at about the height of 12,000 feet, deeply-furrowed +rocks, resembling those with which he was familiar in Norway, and likewise +great masses of detritus, including grooved pebbles. Along this whole space of +the Cordillera true glaciers do not now exist even at much more considerable +heights. Further south, on both sides of the continent, from latitude 41° to +the southernmost extremity, we have the clearest evidence of former glacial +action, in numerous immense boulders transported far from their parent source. +</p> + +<p> +From these several facts, namely, from the glacial action having +<a name="Page336"></a> +extended all round the northern and southern hemispheres—from the period +having been in a geological sense recent in both hemispheres—from its +having lasted in both during a great length of time, as may be inferred from +the amount of work effected—and lastly, from glaciers having recently +descended to a low level along the whole line of the Cordillera, it at one time +appeared to me that we could not avoid the conclusion that the temperature of +the whole world had been simultaneously lowered during the Glacial period. But +now, Mr. Croll, in a series of admirable memoirs, has attempted to show that a +glacial condition of climate is the result of various physical causes, brought +into operation by an increase in the eccentricity of the earth’s orbit. +All these causes tend towards the same end; but the most powerful appears to be +the indirect influence of the eccentricity of the orbit upon oceanic currents. +According to Mr. Croll, cold periods regularly recur every ten or fifteen +thousand years; and these at long intervals are extremely severe, owing to +certain contingencies, of which the most important, as Sir C. Lyell has shown, +is the relative position of the land and water. Mr. Croll believes that the +last great glacial period occurred about 240,000 years ago, and endured, with +slight alterations of climate, for about 160,000 years. With respect to more +ancient glacial periods, several geologists are convinced, from direct +evidence, that such occurred during the miocene and eocene formations, not to +mention still more ancient formations. But the most important result for us, +arrived at by Mr. Croll, is that whenever the northern hemisphere passes +through a cold period the temperature of the southern hemisphere is actually +raised, with the winters rendered much milder, chiefly through changes in the +direction of the ocean currents. So conversely it will be with the northern +hemisphere, while the southern passes through a glacial period. This conclusion +throws so much light on geographical distribution that I am strongly inclined +to trust in it; but I will first give the facts which demand an explanation. +</p> + +<p> +In South America, Dr. Hooker has shown that besides many closely allied +species, between forty and fifty of the flowering plants of Tierra del Fuego, +forming no inconsiderable part of its scanty flora, are common to North America +and Europe, enormously remote as these areas in opposite hemispheres are from +each other. On the lofty mountains of equatorial America a host of peculiar +species belonging to European genera occur. On the Organ Mountains of Brazil +some few temperate European, some Antarctic and some Andean genera were found +by Gardner which do not exist in the low intervening hot countries. On the +Silla of Caraccas +<a name="Page337"></a> +the illustrious Humboldt long ago found species belonging to genera +characteristic of the Cordillera. +</p> + +<p> +In Africa, several forms characteristic of Europe, and some few representatives +of the flora of the Cape of Good Hope, occur on the mountains of Abyssinia. At +the Cape of Good Hope a very few European species, believed not to have been +introduced by man, and on the mountains several representative European forms +are found which have not been discovered in the intertropical parts of Africa. +Dr. Hooker has also lately shown that several of the plants living on the upper +parts of the lofty island of Fernando Po, and on the neighbouring Cameroon +Mountains, in the Gulf of Guinea, are closely related to those on the mountains +of Abyssinia, and likewise to those of temperate Europe. It now also appears, +as I hear from Dr. Hooker, that some of these same temperate plants have been +discovered by the Rev. R.T. Lowe on the mountains of the Cape Verde Islands. +This extension of the same temperate forms, almost under the equator, across +the whole continent of Africa and to the mountains of the Cape Verde +archipelago, is one of the most astonishing facts ever recorded in the +distribution of plants. +</p> + +<p> +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 of plants collected on the loftier peaks of Java, raises a +picture of a collection made on a hillock in Europe. Still more striking is the +fact that peculiar Australian forms are represented by certain 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 hand as far +north as Japan. +</p> + +<p> +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 certain plants growing on the more lofty +mountains of the tropics in all parts of the world, and on the temperate plains +of the north and south, are either the same species or varieties of the same +species. It should, however, be observed +<a name="Page338"></a> +that these plants are not strictly arctic forms; for, as Mr. H.C. Watson has +remarked, “in receding from polar toward equatorial latitudes, the Alpine +or mountain flora really become less and less Arctic.” Besides these +identical and closely allied forms, many species inhabiting the same widely +sundered areas, belong to genera not now found in the intermediate tropical +lowlands. +</p> + +<p> +These brief remarks apply to plants alone; but some few analogous facts could +be given in regard to terrestrial animals. In marine productions, similar cases +likewise occur; as an example, I may quote a statement by the highest +authority, Prof. 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, &c., 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. +</p> + +<p> +From the foregoing facts, namely, the presence of temperate forms on the +highlands across the whole of equatorial Africa, and along the Peninsula of +India, to Ceylon and the Malay Archipelago, and in a less well-marked manner +across the wide expanse of tropical South America, it appears almost certain +that at some former period, no doubt during the most severe part of a Glacial +period, the lowlands of these great continents were everywhere tenanted under +the equator by a considerable number of temperate forms. At this period the +equatorial climate at the level of the sea was probably about the same with +that now experienced at the height of from five to six thousand feet under the +same latitude, or perhaps even rather cooler. During this, the coldest period, +the lowlands under the equator must have been clothed with a mingled tropical +and temperate vegetation, like that described by Hooker as growing luxuriantly +at the height of from four to five thousand feet on the lower slopes of the +Himalaya, but with perhaps a still greater preponderance of temperate forms. So +again in the mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann +found temperate European forms beginning to appear at the height of about five +thousand feet. On the mountains of Panama, at the height of only two thousand +feet, Dr. Seemann found the vegetation like that of Mexico, “with forms +of the torrid zone harmoniously blended with those of the temperate.” +</p> + +<p> +Now let us see whether Mr. Croll’s conclusion that when the northern +hemisphere suffered from the extreme cold of the great Glacial period, the +southern hemisphere was actually warmer, throws +<a name="Page339"></a> +any clear light on the present apparently inexplicable distribution of various +organisms in the temperate parts of both hemispheres, and on the mountains of +the tropics. 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 became more and more intense, we know +that Arctic forms invaded the temperate regions; and from the facts just given, +there can hardly be a doubt that some of the more vigorous, dominant and +widest-spreading temperate forms invaded the equatorial lowlands. The +inhabitants of these hot lowlands would at the same time have migrated to the +tropical and subtropical regions of the south, for the southern hemisphere was +at this period warmer. On the decline of the Glacial period, as both +hemispheres gradually recovered their former temperature, the northern +temperate forms living on the lowlands under the equator, would have been +driven to their former homes or have been destroyed, being replaced by the +equatorial forms returning from the south. Some, however, of the northern +temperate forms would almost certainly have ascended any adjoining high land, +where, if sufficiently lofty, they would have long survived like the Arctic +forms on the mountains of Europe. They might have survived, even if the climate +was not perfectly fitted for them, for the change of temperature must have been +very slow, and plants undoubtedly possess a certain capacity for +acclimatisation, as shown by their transmitting to their offspring different +constitutional powers of resisting heat and cold. +</p> + +<p> +In the regular course of events the southern hemisphere would in its turn be +subjected to a severe Glacial period, with the northern hemisphere rendered +warmer; and then the southern temperate forms would invade the equatorial +lowlands. The northern forms which had before been left on the mountains would +now descend and mingle with the southern forms. These latter, when the warmth +returned, would return to their former homes, leaving some few species on the +mountains, and carrying southward with them some of the northern temperate +forms which had descended from their mountain fastnesses. Thus, we should have +some few species identically the same in the northern and southern temperate +zones and on the mountains of the intermediate tropical regions. But the +species left during a long time on these mountains, or in opposite hemispheres, +would have to compete with many new forms and would be exposed to somewhat +different physical conditions; hence, they would be eminently liable to +modification, and would generally now exist as varieties or as representative +species; and this is the +<a name="Page340"></a> +case. We must, also, bear in mind the occurrence in both hemispheres of former +Glacial periods; for these will account, in accordance with the same +principles, for the many quite distinct species inhabiting the same widely +separated areas, and belonging to genera not now found in the intermediate +torrid zones. +</p> + +<p> +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 or +slightly modified species have migrated from the north to the south, than in a +reversed direction. We see, however, a few southern forms on the mountains of +Borneo and Abyssinia. I suspect that this preponderant migration from the north +to the 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 the two sets became commingled in the equatorial regions, during the +alternations of the Glacial periods, the northern forms were the more powerful +and were able to hold their places on the mountains, and afterwards migrate +southward with the southern forms; but not so the southern in regard to the +northern forms. In the same manner, at the present day, we see that very many +European productions cover the ground in La Plata, New Zealand, and to a lesser +degree in Australia, and have beaten the natives; whereas extremely few +southern forms have become naturalised in any part of the northern hemisphere, +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 forty or fifty years from Australia. The Neilgherrie Mountains +in India, however, offer a partial exception; for here, as I hear from Dr. +Hooker, Australian forms are rapidly sowing themselves and becoming +naturalised. Before the last great Glacial period, no doubt the intertropical +mountains were stocked with endemic Alpine forms; but these have almost +everywhere 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 those which have become +naturalised; and this is the first stage towards their extinction. Mountains +are islands on the land; and their inhabitants have yielded to those produced +within the larger areas of the north, just in the same way as the inhabitants +of real islands have everywhere yielded and are still yielding to continental +forms naturalised through man’s agency. +</p> + +<p> +<a name="Page341"></a> +The same principles apply to the distribution of terrestrial animals and of +marine productions, in the northern and southern temperate zones, and on the +intertropical mountains. When, during the height of the Glacial period, the +ocean-currents were widely different to what they now are, some of the +inhabitants of the temperate seas might have reached the equator; of these a +few would perhaps at once be able to migrate southwards, by keeping to the +cooler currents, while others might remain and survive in the colder depths +until the southern hemisphere was in its turn subjected to a glacial climate +and permitted their further progress; in nearly the same manner as, according +to Forbes, isolated spaces inhabited by Arctic productions exist to the present +day in the deeper parts of the northern temperate seas. +</p> + +<p> +I am far from supposing that all the difficulties in regard to the distribution +and affinities of the identical and allied species, which now live so widely +separated in the north and south, and sometimes on the intermediate mountain +ranges, are removed on the views above given. The exact lines of migration +cannot be indicated. We cannot say why certain species and not others have +migrated; why certain species have been modified and have given rise to new +forms, while 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 species ranges twice or thrice as +far, and is twice or thrice as common, as another species within their own +homes. +</p> + +<p> +Various special difficulties also remain to be solved; for instance, the +occurrence, as shown by Dr. Hooker, of the same plants at points so enormously +remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs, as suggested +by Lyell, may have been concerned in their dispersal. The existence at these +and other distant points of the southern hemisphere, of species, which, though +distinct, belong to genera exclusively confined to the south, is a more +remarkable case. Some of these species are so distinct, that we cannot suppose +that there has been time since the commencement of the last Glacial period for +their migration and subsequent modification to the necessary degree. The facts +seem to indicate that distinct species belonging to the same genera have +migrated in radiating lines from a 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 last Glacial period, when the Antarctic lands, +now covered with ice, supported a highly peculiar and isolated flora. It may be +suspected that before this flora was exterminated during the last Glacial +epoch, a few forms had +<a name="Page342"></a> +been already widely dispersed to various points of the southern hemisphere by +occasional means of transport, and by the aid, as halting-places, of now sunken +islands. Thus the southern shores of America, Australia, and New Zealand may +have become slightly tinted by the same peculiar forms of life. +</p> + +<p> +Sir C. Lyell in a striking passage has speculated, in language almost identical +with mine, on the effects of great alternations of climate throughout the world +on geographical distribution. And we have now seen that Mr. Croll’s +conclusion that successive Glacial periods in the one hemisphere coincide with +warmer periods in the opposite hemisphere, together with the admission of the +slow modification of species, explains a multitude of facts in the distribution +of the same and of the allied forms of life in all parts of the globe. The +living waters have flowed during one period from the north and during another +from the south, and in both cases have reached the equator; but the stream of +life has flowed with greater force from the north than in the opposite +direction, and has consequently more freely inundated the south. As the tide +leaves its drift in horizontal lines, 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 +latitude 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 serves as a record, full of interest to +us, of the former inhabitants of the surrounding lowlands. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page343"></a><a name="link2HCH0013" id="link2HCH0013"></a> +CHAPTER XIII.<br/> +GEOGRAPHICAL DISTRIBUTION—<i>continued</i>.</h2> + +<p class="letter"> +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. +</p> + +<p class="center"> +<i>Fresh-water Productions.</i> +</p> + +<p> +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 +formidable barrier, that they would never have extended to distant countries. +But the case is exactly the reverse. Not only have many fresh-water species, +belonging to different classes, an enormous range, but allied species prevail +in a remarkable manner throughout the world. When first collecting in the fresh +waters of Brazil, I well remember feeling much surprise at the similarity of +the fresh-water insects, shells, &c., and at the dissimilarity of the +surrounding terrestrial beings, compared with those of Britain. +</p> + +<p> +But the wide ranging power of fresh-water productions 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, within their own countries; and liability to wide dispersal would +follow from this capacity as an almost necessary consequence. We can here +consider only a few cases; of these, some of the most difficult to explain are +presented by fish. It was formerly believed that the same fresh-water species +never existed on two continents distant from each other. But Dr. Günther has +lately shown that the Galaxias attenuatus inhabits Tasmania, New Zealand, the +Falkland Islands and the mainland of South America. This is a wonderful case, +and probably indicates dispersal from an Antarctic centre during a former warm +period. This case, however, is rendered in some degree less +<a name="Page344"></a> +surprising by the species of this genus having the power of crossing by some +unknown means considerable spaces of open ocean: thus there is one species +common to New Zealand and to the Auckland Islands, though separated by a +distance of about 230 miles. On the same continent fresh-water fish often range +widely, and as if capriciously; for in two adjoining river systems some of the +species may be the same and some wholly different. +</p> + +<p> +It is probable that they are occasionally transported by what may be called +accidental means. Thus fishes still alive are not very rarely dropped at +distant points by whirlwinds; and it is known that the ova retain their +vitality for a considerable time after removal from the water. Their dispersal +may, however, be mainly attributed to changes in the level of the land within +the recent period, causing rivers to flow into each other. Instances, also, +could be given of this having occurred during floods, without any change of +level. The wide differences of the fish on the opposite sides of most +mountain-ranges, which are continuous and consequently must, from an early +period, have completely prevented the inosculation of the river systems on the +two sides, leads to the same conclusion. 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. +Moreover, Dr. Günther has recently been led by several considerations to infer +that with fishes the same forms have a long endurance. 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 which all the members are +confined to fresh water, so that a marine species belonging to a fresh-water +group might travel far along the shores of the sea, and could, it is probable, +become adapted without much difficulty to the fresh waters of a distant land. +</p> + +<p> +Some species of fresh-water shells have very wide ranges, and allied species +which, on our 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 the ova, as well as the adults, are immediately +killed by sea-water. I could not even understand how some naturalised species +have spread rapidly throughout the same country. But two facts, which I have +observed—and many others no doubt will be discovered—throw some +light on this subject. When ducks suddenly emerge from a pond covered with +duck-weed, I have twice seen these little plants adhering to their backs; and +it has happened to me, in removing a little duck-weed from one aquarium to +another, that I +<a name="Page345"></a> +have unintentionally stocked the one with fresh-water shells from the other. +But another agency is perhaps more effectual: I suspended the feet of a duck 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 if blown across the sea to an oceanic island, or to +any other distant point, would be sure to alight on a pool or rivulet. Sir +Charles Lyell 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 been blown by a favouring gale no one can tell. +</p> + +<p> +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 illustrated, according to Alph. de +Candolle, in those large groups of terrestrial plants, which have very few +aquatic members; for the latter seem immediately to acquire, as if in +consequence, a wide range. I think favourable means of dispersal explain this +fact. I have before mentioned that earth occasionally 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 wander more than those of any other; and are occasionally found +on the most remote and barren islands of the open ocean; they would not be +likely to alight on the surface of the sea, so that any dirt on their feet +would not be washed off; and when gaining the 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 tablespoonfuls of mud from three different points, beneath water, on the +edge of a little pond; this mud when dry weighed only 6 and 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 +<a name="Page346"></a> +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 unstocked ponds and streams, situated at very distant +points. The same agency may have come into play with the eggs of some of the +smaller fresh-water animals. +</p> + +<p> +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 many hours afterwards in pellets or +in the excrement. When I saw the great size of the seeds of that fine +water-lily, the Nelumbium, and remembered Alph. de Candolle’s remarks on +the distribution of this plant, I thought that the means of its dispersal must +remain 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. Now this bird must often have flown with its stomach +thus well stocked to distant ponds, and, then getting a hearty meal of fish, +analogy makes me believe that it would have rejected the seeds in the pellet in +a fit state for germination. +</p> + +<p> +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 +inhabitants of the same pond, however few in kind, yet as the number even in a +well-stocked pond is small in comparison with the number of species inhabiting +an equal area of land, the competition between them will probably be less +severe than between terrestrial species; consequently an intruder from the +waters of a foreign country would have a better chance of seizing on a new +place, than in the case of terrestrial colonists. We should also remember that +many fresh-water productions are low in the scale of nature, and we have reason +to believe that such beings become modified more slowly than the high; and this +will give time for the migration of aquatic species. We should not forget the +probability of many fresh-water forms having formerly ranged continuously over +immense areas, and then having become extinct at intermediate points. But the +wide distribution of fresh-water plants, and of the lower animals, whether +<a name="Page347"></a> +retaining the same identical form, or in some degree modified, apparently +depends in main part on the wide dispersal of their seeds and eggs by animals, +more especially by fresh-water birds, which have great powers of flight, and +naturally travel from one piece of water to another. +</p> + +<p class="center"> +<i>On the Inhabitants of Oceanic Islands.</i> +</p> + +<p> +We now come to the last of the three classes of facts, which I have selected as +presenting the greatest amount of difficulty with respect to distribution, on +the view that not only all the individuals of the same species have migrated +from some one area, but that allied species, although now inhabiting the most +distant points, have proceeded from a single area, the birthplace of their +early progenitors. I have already given my reasons for disbelieving in +continental extensions within the period of existing species on so enormous a +scale that all the many islands of the several oceans were thus stocked with +their present terrestrial inhabitants. This view removes many difficulties, but +it does not accord with all the facts in regard to the productions of islands. +In the following remarks I shall not confine myself to the mere question of +dispersal, but shall consider some other cases bearing on the truth of the two +theories of independent creation and of descent with modification. +</p> + +<p> +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. New Zealand, for instance, with its +lofty mountains and diversified stations, extending over 780 miles of latitude, +together with the outlying islands of Auckland, Campbell and Chatham, contain +altogether only 960 kinds of flowering plants; if we compare this moderate +number with the species which swarm over equal areas in Southwestern Australia +or at the Cape of Good Hope, we must admit that some cause, independently of +different physical conditions, has given rise to 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 less than half-a-dozen flowering plants; yet many species have now +become naturalised on it, as they have in 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 +<a name="Page348"></a> +of the creation of each separate species, will have to admit that a sufficient +number of the best adapted plants and animals were not created for oceanic +islands; for man has unintentionally stocked them far more fully and perfectly +than did nature. +</p> + +<p> +Although in oceanic islands the species are few in number, the proportion of +endemic kinds (<i>i.e.</i> those found nowhere else in the world) is often +extremely large. If we compare, for instance, the number of endemic land-shells +in Madeira, or of endemic birds in the Galapagos Archipelago, with the number +found on any continent, and then compare the area of the island with that of +the continent, we shall see that this is true. This fact might have been +theoretically expected, for, as already explained, species occasionally +arriving, after long intervals of time in the new and isolated district, and +having to compete with new associates, would be eminently liable to +modification, and would 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 partly on the species which +are not modified having immigrated in a body, so that their mutual relations +have not been much disturbed; and partly on the frequent arrival of unmodified +immigrants from the mother-country, with which the insular forms have +intercrossed. It should be borne in mind that the offspring of such crosses +would certainly gain in vigour; so that even an occasional cross would produce +more effect than might have been anticipated. I will give a few illustrations +of the foregoing remarks: in the Galapagos Islands there are twenty-six land +birds; of these twenty-one (or perhaps twenty-three) are peculiar; whereas of +the eleven marine birds only two are peculiar; and it is obvious that marine +birds could arrive at these islands much more easily and frequently 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 a single endemic land bird; and we +know from Mr. J.M. Jones’s admirable account of Bermuda, that very many +North American birds occasionally or even frequently visit this island. Almost +every year, as I am informed by Mr. E.V. Harcourt, many European and African +birds are blown to Madeira; this island is inhabited by ninety-nine kinds, of +which one alone is peculiar, though very closely related to a European form; +and three or four other species are confined to this island and to the +Canaries. So that the islands of Bermuda and Madeira have been stocked from the +neighbouring continents with birds, +<a name="Page349"></a> +which for long ages have there struggled together, and have become mutually +co-adapted. Hence, when settled in their new homes, each kind will have been +kept by the others to its proper place and habits, and will consequently have +been but little liable to modification. Any tendency to modification will also +have been checked by intercrossing with the unmodified immigrants, often +arriving from the mother-country. Madeira again is inhabited by a wonderful +number of peculiar land-shells, whereas not one species of sea-shell is +peculiar to its shores: now, though we do not know how sea-shells 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 across +three or four hundred miles of open sea far more easily than land-shells. The +different orders of insects inhabiting Madeira present nearly parallel cases. +</p> + +<p> +Oceanic islands are sometimes deficient in animals of certain whole classes, +and their places are occupied by other classes; thus in the Galapagos Islands +reptiles, and in New Zealand gigantic wingless birds, take, or recently took, +the place of mammals. Although New Zealand is here spoken of as an oceanic +island, it is in some degree doubtful whether it should be so ranked; it is of +large size, and is not separated from Australia by a profoundly deep sea; from +its geological character and the direction of its mountain ranges, the Rev. +W.B. Clarke has lately maintained that this island, as well as New Caledonia, +should be considered as appurtenances of Australia. Turning to plants, Dr. +Hooker has shown that in the Galapagos Islands the proportional numbers of the +different orders are very different from what they are elsewhere. All such +differences in number, and the absence of certain whole groups of animals and +plants, are generally accounted for by supposed differences in the physical +conditions of the islands; but this explanation is not a little doubtful. +Facility of immigration seems to have been fully as important as the nature of +the conditions. +</p> + +<p> +Many remarkable little facts could be given with respect to the inhabitants of +oceanic islands. For instance, in certain islands not tenanted by a single +mammal, some of the endemic plants have beautifully hooked seeds; yet few +relations are more manifest than that hooks serve for the transportal of seeds +in the wool or fur of quadrupeds. But a hooked seed might be carried to an +island by other means; and the plant then becoming modified would form an +endemic species, still retaining its hooks, which would form a useless +appendage, like the shrivelled wings under +<a name="Page350"></a> +the soldered wing-covers 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, which had no chance of +successfully competing with the many fully developed trees growing on a +continent, might, when established on an island, gain an advantage over other +herbaceous plants by growing taller and taller and overtopping them. In this +case, natural selection would tend to add to the stature of the plant, to +whatever order it belonged, and thus first convert it into a bush and then into +a tree. +</p> + +<p class="center"> +<i>Absence of Batrachians and Terrestrial mammals on Oceanic Islands.</i> +</p> + +<p> +With respect to the absence of whole orders of animals on oceanic islands, Bory +St. Vincent long ago remarked that Batrachians (frogs, toads, newts) are never +found on any of the many islands with which the great oceans are studded. I +have taken pains to verify this assertion, and have found it true, with the +exception of New Zealand, New Caledonia, the Andaman Islands, and perhaps the +Solomon Islands and the Seychelles. But I have already remarked that it is +doubtful whether New Zealand and New Caledonia ought to be classed as oceanic +islands; and this is still more doubtful with respect to the Andaman and +Solomon groups and the Seychelles. This general absence of frogs, toads and +newts on so many true oceanic islands cannot be accounted for by their physical +conditions; indeed it seems that islands are peculiarly 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 immediately killed (with the exception, as far as known, of +one Indian species) by sea-water, there would be great difficulty in their +transportal across the sea, and therefore we can see why they do not exist on +strictly oceanic islands. But why, on the theory of creation, they should not +have been created there, it would be very difficult to explain. +</p> + +<p> +Mammals offer another and similar case. I have carefully searched the oldest +voyages, and 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 +<a name="Page351"></a> +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 in +connection with the mainland at a distance of about 280 miles; moreover, +icebergs formerly brought boulders to its western shores, and they may have +formerly transported foxes, as now frequently happens in the arctic regions. +Yet it cannot be said that small islands will not support at least small +mammals, for they occur in many parts of the world on very small islands, when +lying 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 known that new species of +mammals appear and disappear at a quicker rate than other and lower animals. +Although terrestrial mammals do not occur on oceanic islands, aërial 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 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 +in their new homes in relation to their new position, and we can understand the +presence of endemic bats on oceanic islands, with the absence of all other +terrestrial mammals. +</p> + +<p> +Another interesting relation exists, namely, between the depth of the sea +separating islands from each other, or from the nearest continent, and the +degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has made +some striking observations on this head, since greatly extended by Mr. +Wallace’s admirable researches, in regard to the great Malay Archipelago, +which is +<a name="Page352"></a> +traversed near Celebes by a space of deep ocean, and this separates two widely +distinct mammalian faunas. On either side, the islands stand on a moderately +shallow submarine bank, and these islands are inhabited by the same or by +closely allied quadrupeds. I have not as yet had time to follow up this subject +in all quarters of the world; but as far as I have gone, the relation holds +good. For instance, Britain is separated by a shallow channel from Europe, and +the mammals are the same on both sides; and so it is with all the islands near +the shores of Australia. The West Indian Islands, on the other hand, stand on a +deeply submerged bank, nearly one thousand fathoms in depth, and here we find +American forms, but the species and even the genera are quite distinct. As the +amount of modification which animals of all kinds undergo partly depends on the +lapse of time, and as the islands which are separated from each other, or from +the mainland, by shallow channels, are more likely to have been continuously +united within a recent period than the islands separated by deeper channels, we +can understand how it is that a relation exists between the depth of the sea +separating two mammalian faunas, and the degree of their affinity, a relation +which is quite inexplicable on the theory of independent acts of creation. +</p> + +<p> +The foregoing statements in regard to the inhabitants of oceanic islands, +namely, the fewness of the species, with a large proportion consisting of +endemic forms—the members of certain groups, but not those of other +groups in the same class, having been modified—the absence of certain +whole orders, as of batrachians and of terrestrial mammals, notwithstanding the +presence of aërial bats, the singular proportions of certain orders of plants, +herbaceous forms having been developed into trees, &c., seem to me to +accord better with the belief in the efficiency of occasional means of +transport, carried on during a long course of time, than with the belief in the +former connection of all oceanic islands with the nearest continent; for on +this latter view it is probable that the various classes would have immigrated +more uniformly, and from the species having entered in a body, their mutual +relations would not have been much disturbed, and consequently, they would +either have not been modified, or all the species in a more equable manner. +</p> + +<p> +I do not deny that there are many and serious difficulties in understanding how +many of the inhabitants of the more remote islands, whether still retaining the +same specific form or subsequently modified, have reached their present homes. +But the probability of other islands having once existed as halting-places, of +which not a wreck now remains, must not be overlooked. I will +<a name="Page353"></a> +specify one difficult case. 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 striking instances of which have been +given by Dr. A.A. Gould in relation to the Pacific. Now it is notorious that +land-shells are easily killed by sea-water; their eggs, at least such as I have +tried, sink in it and are killed. Yet there must be some unknown, but +occasionally efficient means for their transportal. Would the just-hatched +young sometimes 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 find that several +species in this state withstand uninjured an immersion in sea-water during +seven days. One shell, the Helix pomatia, after having been thus treated, and +again hybernating, was put into sea-water for twenty days and perfectly +recovered. During this length of time the shell might have been carried by a +marine country of average swiftness to a distance of 660 geographical miles. As +this Helix has a thick calcareous operculum I removed it, and when it had +formed a new membranous one, I again immersed it for fourteen days in +sea-water, and again it recovered and crawled away. Baron Aucapitaine has since +tried similar experiments. He placed 100 land-shells, belonging to ten species, +in a box pierced with holes, and immersed it for a fortnight in the sea. Out of +the hundred shells twenty-seven recovered. The presence of an operculum seems +to have been of importance, as out of twelve specimens of Cyclostoma elegans, +which is thus furnished, eleven revived. It is remarkable, seeing how well the +Helix pomatia resisted with me the salt-water, that not one of fifty-four +specimens belonging to four other species of Helix tried by Aucapitaine +recovered. It is, however, not at all probable that land-shells have often been +thus transported; the feet of birds offer a more probable method. +</p> + +<p class="center"> +<i>On the Relations of the Inhabitants of Islands to those of the nearest +Mainland.</i> +</p> + +<p> +The most striking and important fact for us is the affinity of the species +which inhabit islands to those of the nearest mainland, without being actually +the same. Numerous instances could be given. The Galapagos Archipelago, +situated under the equator, lies at a distance of between 500 and 600 miles +from the shores of South America. Here almost every product of the land and of +the water bears the unmistakable stamp of the American continent. +<a name="Page354"></a> +There are twenty-six land-birds. Of these twenty-one, or perhaps twenty-three, +are ranked as distinct species, and would commonly be assumed to have been here +created; yet the close affinity of most of these birds to American species is +manifest in every character in their habits, gestures, and tones of voice. So +it is with the other animals, and with a large proportion of the plants, as +shown by Dr. Hooker in his admirable Flora of this archipelago. The naturalist, +looking at the inhabitants of these volcanic islands in the Pacific, distant +several hundred miles from the continent, 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 plainly +the 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 closely resembles 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 the climate, height, and size of the islands, between +the Galapagos and Cape Verde Archipelagos: but what an entire and absolute +difference in their inhabitants! The inhabitants of the Cape Verde Islands are +related to those of Africa, like those of the Galapagos to America. Facts, such +as these, admit of 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 from America, whether by +occasional means of transport or (though I do not believe in this doctrine) by +formerly continuous land, and the Cape Verde Islands from Africa; such +colonists would be liable to modification—the principle of inheritance +still betraying their original birthplace. +</p> + +<p> +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 the nearest large island. The exceptions are few, and most of +them can be explained. Thus, although Kerguelen Land stands nearer to Africa +than to America, the plants 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 +<a name="Page355"></a> +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 +partially disappears on the view that New Zealand, South America, and the other +southern lands, have been stocked in part from a nearly intermediate though +distant point, namely, from the antarctic islands, when they were clothed with +vegetation, during a warmer tertiary period, before the commencement of the +last 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; but this affinity is +confined to the plants, and will, no doubt, some day be explained. +</p> + +<p> +The same law which has determined the relationship between the inhabitants of +islands and the nearest mainland, is sometimes displayed on a small scale, but +in a most interesting manner, within the limits of the same archipelago. Thus +each separate island of the Galapagos Archipelago is tenanted, and the fact is +a marvellous one, by many distinct species; but these species are related to +each other in a very much closer manner than to the inhabitants of the American +continent, or of any other quarter of the world. This is what might have been +expected, for islands situated so near to each other would almost necessarily +receive immigrants from the same original source, and from each other. But how +is it that many of the immigrants have been differently modified, though only +in a small degree, in islands situated within sight of each other, having the +same geological nature, the same height, climate, etc? 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; whereas +it cannot be disputed that the nature of the other species 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 the species which inhabit the Galapagos +Archipelago, and are likewise found in other parts of the world, we find that +they differ considerably in the several islands. This difference might indeed +have been expected if the islands have 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, though all proceeding from +the same general source. Hence, when in former times an immigrant first settled +on one of the islands, or when it subsequently spread from one to another, it +would undoubtedly be exposed to different conditions in the different +<a name="Page356"></a> +islands, for it would have to compete with a different set of organisms; a +plant, for instance, would find the ground best-fitted for it occupied by +somewhat different species in the different islands, and 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 some species spreading widely throughout a +continent and remaining the same. +</p> + +<p> +The really surprising fact in this case of the Galapagos Archipelago, and in a +lesser degree in some analogous cases, is that each new species after being +formed in any one island, did not spread quickly 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 deep between the islands, and gales of wind are +extraordinarily rare; so that the islands are far more effectually separated +from each other than they appear on a map. Nevertheless, some of the species, +both of those found in other parts of the world and of those confined to the +archipelago, are common to the several islands; and we may infer from the +present manner of distribution that they have spread from 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 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, both will probably hold +their separate places 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 wide areas, we are apt to infer that most +species would thus spread; but we should remember that the species which become +naturalised in new countries are not generally closely allied to the aboriginal +inhabitants, but are very distinct forms, 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, differ on the different islands; 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 +<a name="Page357"></a> +may safely infer that Charles Island is well stocked with its own species, for +annually more eggs are laid and young birds hatched 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 species of 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 species which inhabit the several +islands of the Galapagos Archipelago not having all spread from island to +island. On the same continent, also, pre-occupation has probably played an +important part in checking the commingling of the species which inhabit +different districts with nearly the same physical conditions. 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; so it is, according to Mr. +Bates, with the butterflies and other animals inhabiting the great, open, and +continuous valley of the Amazons. +</p> + +<p> +The same principle which governs the general character of the inhabitants of +oceanic islands, namely, the relation to the source whence colonists could have +been most easily derived, together with their subsequent modification, is of +the widest application throughout nature. We see this on every mountain-summit, +in every lake and marsh. For Alpine species, excepting in as far as the same +species have become widely spread during the Glacial epoch, are related to +those of the surrounding lowlands; thus we have in South America, Alpine +humming-birds, Alpine rodents, Alpine plants, &c., all strictly belonging +to American forms; and it is obvious that a mountain, as it became slowly +upheaved, would 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 allowed the same forms to prevail throughout large portions of +the world. We see the same principle in the character of most of the blind +animals inhabiting the caves of America and of Europe. Other analogous facts +could be given. +<a name="Page358"></a> +It will, I believe, be found universally 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; and wherever many +closely-allied species occur, there will be found many forms which some +naturalists rank as distinct species, and others as mere varieties; these +doubtful forms showing us the steps in the process of modification. +</p> + +<p> +The relation between the power and extent of migration in certain species, +either at the present or at some former period, and the existence at remote +points of the world of closely allied species, 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 difficult of proof. Among +mammals, we see it strikingly displayed in Bats, and in a lesser degree in the +Felidæ and Canidæ. We see the same rule in the distribution of butterflies and +beetles. So it is with most of the inhabitants of fresh water, for many of the +genera in the most distinct classes range over the world, and many of the +species have enormous ranges. It is not meant that all, but that some of the +species have very wide ranges in the genera which range very widely. Nor is it +meant that the species in such genera have, on an average, a very wide range; +for this will largely depend on how far the process of modification has gone; +for instance, two varieties of the same species inhabit America and Europe, and +thus the species has an immense range; but, if variation were to be carried a +little further, the two varieties would be ranked as distinct species, and +their range would be greatly reduced. Still less is it meant, that species +which have 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 according to the view that +all the species of a genus, though distributed to the most remote points of the +world, are descended from a single progenitor, we ought to find, and I believe +as a general rule we do find, that some at least of the species range very +widely. +</p> + +<p> +We should bear in mind that many genera in all classes are of ancient origin, +and the species in this case will have had ample time for dispersal and +subsequent modification. There is also reason to believe, from geological +evidence, that within each great +<a name="Page359"></a> +class the lower organisms change at a slower rate than the higher; consequently +they will have had a better chance of ranging widely and of still retaining the +same specific character. This fact, together with that of the seeds and eggs of +most lowly organised forms being very minute and better fitted for distant +transportal, probably accounts for a law which has long been observed, and +which has lately been discussed by Alph. de Candolle in regard to plants, +namely, that the lower any group of organisms stands the more widely it ranges. +</p> + +<p> +The relations just discussed—namely, lower organisms ranging more widely +than the higher—some of the species of widely-ranging genera themselves +ranging widely—such facts, as alpine, lacustrine, and marsh productions +being generally related to those which live on the surrounding low lands and +dry lands—the striking relationship between the inhabitants of islands +and those of the nearest mainland—the still closer relationship of the +distinct inhabitants of the islands of the same archipelago—are +inexplicable on the ordinary view of the independent creation of each species, +but are explicable if we admit colonisation from the nearest or readiest +source, together with the subsequent adaptation of the colonists to their new +homes. +</p> + +<p class="center"> +<i>Summary of the last and present Chapters.</i> +</p> + +<p> +In these chapters I have endeavoured to show that if we make due allowance for +our ignorance of the full effects of changes of climate and of the level of the +land, which have certainly occurred within the recent period, and of other +changes which have probably occurred—if we remember how ignorant we are +with respect to the many curious means of occasional transport—if we bear +in mind, and this is a very important consideration, how often a species may +have ranged continuously over a wide area, and then have become extinct in the +intermediate tracts—the difficulty is not insuperable in believing that +all the individuals of the same species, wherever found, are descended from +common parents. And we are led to this conclusion, which has been arrived at by +many naturalists under the designation of single centres of creation, by +various general considerations, more especially from the importance of barriers +of all kinds, and from the analogical distribution of subgenera, genera, and +families. +</p> + +<p> +With respect to distinct species belonging to the same genus, which on our +theory have spread from one parent-source; if we make the same allowances as +before for our ignorance, and remember that some forms of life have changed +very slowly, +<a name="Page360"></a> +enormous periods of time having been thus granted for their migration, the +difficulties are far from insuperable; though in this case, as in that of the +individuals of the same species, they are often great. +</p> + +<p> +As exemplifying the effects of climatical changes on distribution, I have +attempted to show how important a part the last Glacial period has played, +which affected even the equatorial regions, and which, during the alternations +of the cold in the north and the south, allowed the productions of opposite +hemispheres to mingle, and left some of them stranded on the mountain-summits +in all parts of the world. 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. +</p> + +<p> +If the difficulties be not insuperable in admitting that in the long course of +time all the individuals of the same species, and likewise of the several +species belonging to the same genus, have proceeded from some one source; then +all the grand leading facts of geographical distribution are explicable on the +theory of migration, together with subsequent modification and the +multiplication of new forms. We can thus understand the high importance of +barriers, whether of land or water, in not only separating but in apparently +forming the several zoological and botanical provinces. We can thus understand +the concentration of related species within the same areas; 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 linked +together in so mysterious a manner, and are likewise linked to the extinct +beings which formerly inhabited the same continent. Bearing in mind that the +mutual relation of organism to organism is 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 the colonists entered one of the regions, or both; +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 into 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 the to or more +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 some groups of beings greatly, and some only +slightly modified; some developed in great +<a name="Page361"></a> +force, some existing in scanty numbers—and this we do find in the several +great geographical provinces of the world. +</p> + +<p> +On these same principles we can understand, as I have endeavoured to show, why +oceanic islands should have few inhabitants, but that of these, a large +proportion should be endemic or peculiar; and why, in relation to the means of +migration, one group of beings should have all its species peculiar, and +another group, even within the same class, should have all its species the same +with those in an adjoining quarter 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 should possess their own +peculiar species of aërial mammals or bats. We can see why, in islands, there +should be some relation between the presence of mammals, in a more or less +modified condition, and the depth of the sea between such islands 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 should likewise be related, but less closely, to those of the +nearest continent, or other source whence immigrants might have been derived. +We can see why, if there exist very closely allied or representative species in +two areas, however distant from each other, some identical species will almost +always there be found. +</p> + +<p> +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 +apparent exceptions to the rule are so few that they may fairly be attributed +to our not having as yet discovered in an intermediate deposit certain forms +which are absent in it, 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 former migrations under +different circumstances, or through occasional means of transport, or 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, living during the same period of time, or living within the +same area, are often characterised by trifling features in common, as of +sculpture or colour. In looking to the long succession of past ages, as in +<a name="Page362"></a> +looking to distant provinces throughout the world, we find that species in +certain classes differ little from each other, whilst those in another class, +or only in a different section of the same order, differ greatly from each +other. In both time and space the lowly organised members of each class +generally change less than the highly organised; but there are in both cases +marked exceptions to the rule. According to our theory, these several relations +throughout time and space are intelligible; for whether we look to the allied +forms of life which have changed during successive ages, or to those which have +changed after having migrated into distant quarters, in both cases they are +connected by the same bond of ordinary generation; in both cases the laws of +variation have been the same, and modifications have been accumulated by the +same means of natural selection. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page363"></a><a name="link2HCH0014" id="link2HCH0014"></a> +CHAPTER XIV.<br/> +MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY +ORGANS.</h2> + +<p class="letter"> +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. +</p> + +<p class="center"> +<i>Classification.</i> +</p> + +<p> +From the most remote period in the history of the world organic beings have +been found to resemble each other in descending degrees, so that they can be +classed in groups under groups. This classification is not arbitrary like the +grouping of the stars in constellations. The existence of groups would have +been of simple significance, 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, for it is +notorious how commonly members of even the same subgroup have different habits. +In the second and fourth chapters, on Variation and on Natural Selection, I +have attempted to show that within each country it is the widely ranging, the +much diffused and common, that is the dominant species, belonging to the larger +genera in each class, which vary most. The varieties, or incipient species, +thus produced, ultimately become converted 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 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, +they constantly tend to diverge in character. This latter conclusion +<a name="Page364"></a> +is supported by observing the great diversity of forms, which, in any small +area, come into the closest competition, and by certain facts in +naturalisation. +</p> + +<p> +I attempted also to show that there is a steady tendency in the forms which are +increasing in number and diverging in character, to supplant and exterminate +the preceding, less divergent and less improved 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 the whole of the genera +along this upper line form together one class, for all are descended from one +ancient 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 subfamily, distinct from that containing 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 in common, though less than when grouped in +subfamilies; and they form a family distinct from that containing the three +genera still further to the right hand, which diverged at an 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 into subfamilies, +families and orders, all under one great class. The grand fact of the natural +subordination of organic beings in groups under groups, which, from its +familiarity, does not always sufficiently strike us, is in my judgment thus +explained. No doubt organic beings, like all other objects, can be classed in +many ways, either artificially by single characters, or more naturally by a +number of characters. We know, for instance, that minerals and the elemental +substances can be thus arranged. In this case there is of course no relation to +genealogical succession, and no cause can at present be assigned for their +falling into groups. But with organic beings the case is different, and the +view above given accords with their natural arrangement in group under group; +and no other explanation has ever been attempted. +</p> + +<p> +Naturalists, as we have seen, 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 +<a name="Page365"></a> +an artificial method of 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 both, or +what else is meant by the plan of the Creator, it seems to me that nothing is +thus added to our knowledge. Expressions such as that famous one by Linnæus, +which we often meet with in a more or less concealed form, namely, that the +characters do not make the genus, but that the genus gives the characters, seem +to imply that some deeper bond is included in our classifications than mere +resemblance. I believe that this is the case, and that community of +descent—the one known cause of close similarity in organic +beings—is the bond, which, though observed by various degrees of +modification, is partially revealed to us by our classifications. +</p> + +<p> +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 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.” With plants how remarkable it is that the organs of +vegetation, on which their nutrition and life depend, are of little +signification; whereas the +<a name="Page366"></a> +organs of reproduction, with their product the seed and embryo, are of +paramount importance! So again, in formerly discussing certain morphological +characters which are not functionally important, we have seen that they are +often of the highest service in classification. This depends on their constancy +throughout many allied groups; and their constancy chiefly depends on any +slight deviations not having been preserved and accumulated by natural +selection, which acts only on serviceable characters. +</p> + +<p> +That the mere physiological importance of an organ does not determine its +classificatory value, is almost proved by the 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 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 among 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 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. +</p> + +<p> +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 much 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 position of the rudimentary florets is of the highest importance +in the classification of the Grasses. +</p> + +<p> +<a name="Page367"></a> +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 lower jaw 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 have been considered by +naturalists as an important aid in determining the degree of affinity of this +strange creature to birds. +</p> + +<p> +The importance, for classification, of trifling characters, mainly depends on +their being correlated with many 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 invariably constant. The importance of an aggregate of +characters, even when none are important, alone explains the aphorism +enunciated by Linnæus, namely, that the characters do not give the genus, but +the genus gives the character; for this seems founded on the 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.” When Aspicarpa produced +in France, during several years, only these 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 among the Malpighiaceæ. This case well +illustrates the spirit of our classifications. +</p> + +<p> +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. +<a name="Page368"></a> +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 several trifling +characters are always found in combination, 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 aerating it, or those for propagating the race, are found nearly +uniform, they are considered as highly serviceable in classification; but in +some groups all these, the most important vital organs, are found to offer +characters of quite subordinate value. Thus, as Fritz Müller has lately +remarked, in the same group of crustaceans, Cypridina is furnished with a +heart, while in two closely allied genera, namely Cypris and Cytherea, there is +no such organ; one species of Cypridina has well-developed branchiæ, while +another species is destitute of them. +</p> + +<p> +We can see why characters derived from the embryo should be of equal importance +with those derived from the adult, for a natural classification of course +includes all ages. 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 +embryological characters are the most important of all; and this doctrine has +very generally been admitted as true. Nevertheless, their importance has +sometimes been exaggerated, owing to the adaptive characters of larvæ not +having been excluded; in order to show this, Fritz Müller arranged, by the aid +of such characters alone, the great class of crustaceans, and the arrangement +did not prove a natural one. But there can be no doubt that embryonic, +excluding larval characters, are of the highest value for classification, not +only with animals but with plants. Thus the main divisions of flowering plants +are founded on differences in the embryo—on the number and position of +the cotyledons, and on the mode of development of the plumule and radicle. We +shall immediately see why these characters possess so high a value in +classification, namely, from the natural system being genealogical in its +arrangement. +</p> + +<p> +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 with crustaceans, any such definition has +<a name="Page369"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Finally, with respect to the comparative value of the various groups of +species, such as orders, suborders, families, subfamilies, 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 among plants and insects, of a group +first ranked by practised naturalists as only a genus, and then raised to the +rank of a subfamily 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. +</p> + +<p> +All the foregoing rules and aids and difficulties in classification may be +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, all true +classification being 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. +</p> + +<p> +But I must explain my meaning more fully. I believe that the <i>arrangement</i> +of the groups within each class, in due subordination and relation to each +other, must be strictly genealogical in order to be natural; but that the +<i>amount</i> 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 to refer to the diagram in the fourth chapter. We will suppose +the letters A to L to represent +<a name="Page370"></a> +allied genera existing during the Silurian epoch, and descended from some still +earlier form. In three of these genera (A, F, and I) a species has transmitted +modified descendants to the present day, represented by the fifteen genera +(<i>a</i><sup>14</sup> to <i>z</i><sup>14</sup>) on the uppermost horizontal +line. Now, all these modified descendants from a single species are related in +blood or descent in 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 parent I. But the +existing genus <small>F</small><sup>14</sup> 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 organisms belong to Silurian genera. So that the comparative +value of the differences between these organic beings, which are all related to +each other in the same degree in blood, has come to be widely different. +Nevertheless, their genealogical <i>arrangement</i> 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 stage. If, however, we +suppose any descendant of A or of I to have become so much modified as to have +lost all traces of its parentage in this case, its place in the natural system +will be lost, as seems to have occurred with some few 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 form a single genus. But this genus, +though much isolated, will still occupy its proper intermediate position. The +representation of the groups as here given in the diagram on a flat surface, is +much too simple. The branches ought to have diverged in all directions. If the +names of the groups had been simply written down in a linear series the +representation would have been still less natural; and it is notoriously not +possible to represent in a series, on a flat surface, the affinities which we +discover in nature among the beings of the same group. Thus, the natural system +is genealogical in its arrangement, like a pedigree. But the amount of +modification which the different groups have undergone has to be expressed by +ranking them under different so-called genera, subfamilies, families, sections, +orders, and classes. +</p> + +<p> +It may be worth while to illustrate this view of classification, by +<a name="Page371"></a> +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, were +to be included, such an arrangement would be the only possible one. Yet it +might be that some ancient languages had altered very little and had given rise +to few new languages, whilst others had altered much owing to the spreading, +isolation and state of civilisation of the several co-descended races, and had +thus given rise to many new dialects and languages. The various degrees of +difference between the languages of the same stock would have to be expressed +by groups subordinate to groups; but the proper or even the only possible +arrangement would still be genealogical; and this would be strictly natural, as +it would connect together all languages, extinct and recent, by the closest +affinities, and would give the filiation and origin of each tongue. +</p> + +<p> +In confirmation of this view, let us glance at the classification of varieties, +which are known or believed to be descended from a single species. These are +grouped under the species, with the subvarieties under the varieties; and in +some cases, as with the domestic pigeon, with several other grades of +difference. Nearly the same rules are followed as in classifying species. +Authors have insisted on the necessity of arranging 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 turnip 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, &c.; whereas with sheep the horns are much less +serviceable, because less constant. In classing varieties, I apprehend that if +we had a real pedigree, a genealogical classification would be universally +preferred; and it has been attempted in some cases. For we might feel sure, +whether there had been more or less modification, that the principle of +inheritance would keep the forms together which were allied in the greatest +number of points. In tumbler pigeons, though some of the subvarieties differ in +the important character of the length of the 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, +<a name="Page372"></a> +without any thought on the subject, these tumblers are kept in the same group, +because allied in blood and alike in some other respects. +</p> + +<p> +With species in a state of nature, every naturalist has in fact brought descent +into his classification; for he includes in his lowest grade, that of 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 adult males and hermaphrodites of certain +cirripedes, and yet no one dreams of separating them. As soon as the three +Orchidean forms, Monachanthus, Myanthus, and Catasetum, which had previously +been ranked as three distinct genera, were known to be sometimes produced on +the same plant, they were immediately considered as varieties; and now I have +been able to show that they are the male, female, and hermaphrodite forms of +the same species. The naturalist includes as one species the various larval +stages of the same individual, however much they may differ from each other and +from the adult; as well as the so-called alternate generations of Steenstrup, +which can only in a technical sense be considered as the same individual. He +includes monsters and varieties, not from their partial resemblance to the +parent-form, but because they are descended from it. +</p> + +<p> +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, all under the so-called +natural system? I believe it has been unconsciously used; and thus only can I +understand the several rules and guides which have been followed by our best +systematists. As we have no written pedigrees, we are forced to trace community +of descent by resemblances of any kind. Therefore, we choose those characters +which 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 +<a name="Page373"></a> +for its presence in so many forms with such different habits, only by +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, concur 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 aggregated characters +have especial value in classification. +</p> + +<p> +We can understand why a species or a group of species may depart from its +allies, in several of its most important characteristics, 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 presently see why embryological characters are of such +high classificatory importance. Geographical distribution may sometimes be +brought usefully into play in classing large genera, because all the species of +the same genus, inhabiting any distinct and isolated region, are in all +probability descended from the same parents. +</p> + +<p> +<i>Analogical Resemblances.</i>—We can understand, on the above views, +the very important distinction between real affinities and analogical or +adaptive resemblances. Lamarck first called attention to this subject, 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 dugongs and whales, and +between these two orders of mammals and fishes, are analogical. So is the +resemblance between a mouse and a shrew-mouse (Sorex), which belong to +different orders; and the still closer resemblance, insisted on by Mr. Mivart, +between the mouse and a small marsupial animal (Antechinus) of Australia. These +latter resemblances may be accounted for, as it seems to me, by adaptation for +similarly active movements through thickets and herbage, together with +concealment from enemies. +</p> + +<p> +Among insects there are innumerable instances; thus +<a name="Page374"></a> +Linnæus, misled by external appearances, actually classed an homopterous insect +as a moth. We see something of the same kind even with our domestic varieties, +as in the strikingly similar shape of the body in the improved breeds of the +Chinese and common pig, which are descended from distinct species; and in the +similarly thickened stems of the common and specifically distinct Swedish +turnip. The resemblance between the greyhound and race-horse is hardly more +fanciful than the analogies which have been drawn by some authors between +widely different animals. +</p> + +<p> +On the 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 characters, 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 have become adapted to similar conditions, +and thus have assumed a close external resemblance; but such resemblances will +not reveal—will rather tend to conceal their blood-relationship. We can +thus also understand the apparent paradox, that the very same characters are +analogical when one group is compared with another, but give true affinities +when the members of the same group are compared together: 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 +between the the several members of the whale family, the shape of the body and +the fin-like limbs offer characters exhibiting true affinity; for as these +parts are so nearly similar throughout the whole family, we cannot doubt that +they have been inherited from a common ancestor. So it is with fishes. +</p> + +<p> +Numerous cases could be given of striking resemblances in quite distinct beings +between single parts or organs, which have been adapted for the same functions. +A good instance is afforded by the close resemblance of the jaws of the dog and +Tasmanian wolf or Thylacinus—animals which are widely sundered in the +natural system. But this resemblance is confined to general appearance, as in +the prominence of the canines, and in the cutting shape of the molar teeth. For +the teeth really differ much: thus the dog has on each side of the upper jaw +four pre-molars and only two molars; while the Thylacinus has three pre-molars +and four molars. The molars also differ much in the two animals in relative +size and structure. The adult dentition is preceded by a widely different milk +dentition. Any one may, of course, deny that the teeth +<a name="Page375"></a> +in either case have been adapted for tearing flesh, through the natural +selection of successive variations; but if this be admitted in the one case, it +is unintelligible to me that it should be denied in the other. I am glad to +find that so high an authority as Professor Flower has come to this same +conclusion. +</p> + +<p> +The extraordinary cases given in a former chapter, of widely different fishes +possessing electric organs—of widely different insects possessing +luminous organs—and of orchids and asclepiads having pollen-masses with +viscid discs, come under this same head of analogical resemblances. But these +cases are so wonderful that they were introduced as difficulties or objections +to our theory. In all such cases some fundamental difference in the growth or +development of the parts, and generally in their matured structure, can be +detected. The end gained is the same, but the means, though appearing +superficially to be the same, are essentially different. The principle formerly +alluded to under the term of <i>analogical variation</i> has probably in these +cases often come into play; that is, the members of the same class, although +only distantly allied, have inherited so much in common in their constitution, +that they are apt to vary under similar exciting causes in a similar manner; +and this would obviously aid in the acquirement through natural selection of +parts or organs, strikingly like each other, independently of their direct +inheritance from a common progenitor. +</p> + +<p> +As species belonging to 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 subgroups of distinct classes. A naturalist, struck with a +parallelism of this nature, by arbitrarily raising or sinking the value of the +groups in several classes (and all our experience shows that their valuation is +as yet arbitrary), could easily extend the parallelism over a wide range; and +thus the septenary, quinary, quaternary and ternary classifications have +probably arisen. +</p> + +<p> +There is another and curious class of cases in which close external resemblance +does not depend on adaptation to similar habits of life, but has been gained +for the sake of protection. I allude to the wonderful manner in which certain +butterflies imitate, as first described by Mr. Bates, other and quite distinct +species. This excellent observer has shown that in some districts of South +America, where, for instance, an Ithomia abounds in gaudy swarms, another +butterfly, namely, a Leptalis, is often found mingled in the same flock; and +the latter so closely resembles the Ithomia in every +<a name="Page376"></a> +shade and stripe of colour, and even in the shape of its wings, that Mr. Bates, +with his eyes sharpened by collecting during eleven years, was, though always +on his guard, continually deceived. When the mockers and the mocked are caught +and compared, they are found to be very different in essential structure, and +to belong not only to distinct genera, but often to distinct families. Had this +mimicry occurred in only one or two instances, it might have been passed over +as a strange coincidence. But, if we proceed from a district where one Leptalis +imitates an Ithomia, another mocking and mocked species, belonging to the same +two genera, equally close in their resemblance, may be found. Altogether no +less than ten genera are enumerated, which include species that imitate other +butterflies. The mockers and mocked always inhabit the same region; we never +find an imitator living remote from the form which it imitates. The mockers are +almost invariably rare insects; the mocked in almost every case abounds in +swarms. In the same district in which a species of Leptalis closely imitates an +Ithomia, there are sometimes other Lepidoptera mimicking the same Ithomia: so +that in the same place, species of three genera of butterflies and even a moth +are found all closely resembling a butterfly belonging to a fourth genus. It +deserves especial notice that many of the mimicking forms of the Leptalis, as +well as of the mimicked forms, can be shown by a graduated series to be merely +varieties of the same species; while others are undoubtedly distinct species. +But why, it may be asked, are certain forms treated as the mimicked and others +as the mimickers? Mr. Bates satisfactorily answers this question by showing +that the form which is imitated keeps the usual dress of the group to which it +belongs, while the counterfeiters have changed their dress and do not resemble +their nearest allies. +</p> + +<p> +We are next led to enquire what reason can be assigned for certain butterflies +and moths so often assuming the dress of another and quite distinct form; why, +to the perplexity of naturalists, has nature condescended to the tricks of the +stage? Mr. Bates has, no doubt, hit on the true explanation. The mocked forms, +which always abound in numbers, must habitually escape destruction to a large +extent, otherwise they could not exist in such swarms; and a large amount of +evidence has now been collected, showing that they are distasteful to birds and +other insect-devouring animals. The mocking forms, on the other hand, that +inhabit the same district, are comparatively rare, and belong to rare groups; +hence, they must suffer habitually from some danger, for otherwise, from the +number of eggs laid by all butterflies, they would in three or +<a name="Page377"></a> +four generations swarm over the whole country. Now if a member of one of these +persecuted and rare groups were to assume a dress so like that of a +well-protected species that it continually deceived the practised eyes of an +entomologist, it would often deceive predaceous birds and insects, and thus +often escape destruction. Mr. Bates may almost be said to have actually +witnessed the process by which the mimickers have come so closely to resemble +the mimicked; for he found that some of the forms of Leptalis which mimic so +many other butterflies, varied in an extreme degree. In one district several +varieties occurred, and of these one alone resembled, to a certain extent, the +common Ithomia of the same district. In another district there were two or +three varieties, one of which was much commoner than the others, and this +closely mocked another form of Ithomia. From facts of this nature, Mr. Bates +concludes that the Leptalis first varies; and when a variety happens to +resemble in some degree any common butterfly inhabiting the same district, this +variety, from its resemblance to a flourishing and little persecuted kind, has +a better chance of escaping destruction from predaceous birds and insects, and +is consequently oftener preserved; “the less perfect degrees of +resemblance being generation after generation eliminated, and only the others +left to propagate their kind.” So that here we have an excellent +illustration of natural selection. +</p> + +<p> +Messrs. Wallace and Trimen have likewise described several equally striking +cases of imitation in the Lepidoptera of the Malay Archipelago and Africa, and +with some other insects. Mr. Wallace has also detected one such case with +birds, but we have none with the larger quadrupeds. The much greater frequency +of imitation with insects than with other animals, is probably the consequence +of their small size; insects cannot defend themselves, excepting indeed the +kinds furnished with a sting, and I have never heard of an instance of such +kinds mocking other insects, though they are mocked; insects cannot easily +escape by flight from the larger animals which prey on them; therefore, +speaking metaphorically, they are reduced, like most weak creatures, to +trickery and dissimulation. +</p> + +<p> +It should be observed that the process of imitation probably never commenced +between forms widely dissimilar in colour. But, starting with species already +somewhat like each other, the closest resemblance, if beneficial, could readily +be gained by the above means, and if the imitated form was subsequently and +gradually modified through any agency, the imitating form would be led along +the same track, and thus be altered to almost any extent, so that it might +ultimately assume an appearance or colouring wholly unlike +<a name="Page378"></a> +that of the other members of the family to which it belonged. There is, +however, some difficulty on this head, for it is necessary to suppose in some +cases that ancient members belonging to several distinct groups, before they +had diverged to their present extent, accidentally resembled a member of +another and protected group in a sufficient degree to afford some slight +protection, this having given the basis for the subsequent acquisition of the +most perfect resemblance. +</p> + +<p> +<i>On the Nature of the Affinities connecting Organic Beings.</i>—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 within each class 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 and under still fewer classes. As showing how few the higher groups are +in number, and how widely they are spread throughout the world, the fact is +striking that the discovery of Australia has not added an insect belonging to a +new class, and that in the vegetable kingdom, as I learn from Dr. Hooker, it +has added only two or three families of small size. +</p> + +<p> +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 degree intermediate between existing +groups. As some few of the old and intermediate forms having transmitted to the +present day descendants but little modified, these constitute our so-called +osculant or aberrant groups. The more aberrant any form is, the greater must be +the number of connecting forms which have been exterminated and utterly lost. +And we have evidence of aberrant groups having suffered severely from +extinction, for they are almost always 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 as at present by a single one, or by two or three. We can, +I think, account for this fact only by looking at aberrant groups as forms +which have been conquered by more successful competitors, with a few members +still preserved under unusually favourable conditions. +</p> + +<p> +<a name="Page379"></a> +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, that is, not to +any one Marsupial species more than to another. As these points of affinity are +believed to be real and not merely adaptive, they must be due in accordance +with our view to inheritance from a common progenitor. Therefore, we must +suppose either that all Rodents, including the bizcacha, branched off from some +ancient Marsupial, which will naturally have been more or less intermediate in +character 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 +must 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 some 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 families of plants. +</p> + +<p> +On the principle of the multiplication and gradual divergence in character of +the species descended from a common progenitor, 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 progenitor of a whole +family, now broken up by extinction into distinct groups and subgroups, will +have transmitted some of its characters, modified in various ways and degrees, +to all the species; and they 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 so without this aid, we can understand the +<a name="Page380"></a> +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. +</p> + +<p> +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 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 and at that time less differentiated vertebrate +classes. There has been much less extinction of the forms of life which once +connected fishes with Batrachians. There has been still less within some whole +classes, for instance the Crustacea, for here the most wonderfully diverse +forms are still linked together by a long and only partially broken chain of +affinities. Extinction has only defined the 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, still 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, with every link in +each branch and sub-branch still alive; and the links not greater than those +between existing varieties. In this case it would be quite impossible to give +definitions by which the several members of the several groups could be +distinguished from their more immediate parents and descendants. Yet the +arrangement in the diagram would still hold good and would be natural; for, on +the principle of inheritance, all the forms descended, for instance from A, +would have something in common. In a tree we can distinguish 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 one class which have lived throughout all time +and space. Assuredly we shall never succeed in making so perfect a collection: +nevertheless, in certain classes, we are tending toward this end; and Milne +Edwards has lately insisted, in an able paper, on +<a name="Page381"></a> +the high importance of looking to types, whether or not we can separate and +define the groups to which such types belong. +</p> + +<p> +Finally, we have seen that natural selection, which follows from the struggle +for existence, and which almost inevitably leads to extinction and divergence +of character in the descendants from any one 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 under one species, +although they may have but few characters in common; we use descent in classing +acknowledged varieties, however different they may be from their parents; and I +believe that 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 expressed by the terms genera, +families, orders, &c., 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 use rudimentary and useless organs, +or others of trifling physiological importance; why, in finding the relations +between one group and another, 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 within a few great classes; 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 the +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. +</p> + +<p> +Professor Haeckel in his “Generelle Morphologie” and in another +works, has recently brought his great knowledge and abilities to bear on what +he calls phylogeny, or the lines of descent of all organic beings. In drawing +up the several series he trusts chiefly to embryological characters, but +receives aid from homologous and rudimentary organs, as well as from the +successive periods at which the various forms of life are believed to have +first appeared in our geological formations. He has thus boldly made a great +beginning, and shows us how classification will in the future be treated. +</p> + +<p class="center"> +<a name="Page382"></a> +<i>Morphology.</i> +</p> + +<p> +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 term of +Morphology. This is one of the most interesting departments of natural history, +and may almost 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 similar bones, in the same +relative positions? How curious it is, to give a subordinate though striking +instance, that the hind feet of the kangaroo, which are so well fitted for +bounding over the open plains—those of the climbing, leaf-eating koala, +equally well fitted for grasping the branches of trees—those of the +ground-dwelling, insect or root-eating, bandicoots—and those of some +other Australian marsupials—should all be constructed on the same +extraordinary type, namely with the bones of the second and third digits +extremely slender and enveloped within the same skin, so that they appear like +a single toe furnished with two claws. Notwithstanding this similarity of +pattern, it is obvious that the hind feet of these several animals are used for +as widely different purposes as it is possible to conceive. The case is +rendered all the more striking by the American opossums, which follow nearly +the same habits of life as some of their Australian relatives, having feet +constructed on the ordinary plan. Professor Flower, from whom these statements +are taken, remarks in conclusion: “We may call this conformity to type, +without getting much nearer to an explanation of the phenomenon;” and he +then adds “but is it not powerfully suggestive of true relationship, of +inheritance from a common ancestor?” +</p> + +<p> +Geoffroy St. Hilaire has strongly insisted on the high importance of relative +position or connexion in homologous parts; they may differ to almost any extent +in form and size, and yet remain connected together in the same invariable +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, +<a name="Page383"></a> +the curious folded one of a bee or bug, and the great jaws of a beetle? Yet all +these organs, serving for such widely different purposes, are formed by +infinitely numerous modifications of an upper lip, mandibles, and two pairs of +maxillæ. The same law governs the construction of the mouths and limbs of +crustaceans. So it is with the flowers of plants. +</p> + +<p> +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 pleased the Creator to construct all the animals and plants in each +great class on a uniform plan; but this is not a scientific explanation. +</p> + +<p> +The explanation is to a large extent simple, on the theory of the selection of +successive slight modifications, each being profitable in some way to the +modified form, but often affecting by correlation other parts of the +organisation. In changes of this nature, there will be little or no tendency to +alter the original pattern, or to transpose the parts. The bones of a limb +might be shortened and flattened to any extent, becoming at the same time +enveloped in thick membrane, so as to serve as a fin; or a webbed hand might +have all its bones, or certain bones, lengthened to any extent, with the +membrane connecting them increased, so as to serve as a wing; yet all these +modifications would not tend to alter the framework of the bones or the +relative connexion of the parts. If we suppose that an early +progenitor—the archetype, as it may be called—of all mammals, birds +and reptiles, 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 class. So with the +mouths of insects, we have only to suppose that their common progenitor had an +upper lip, mandibles, and two pairs 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 reduction and ultimately by the complete +abortion of certain parts, by the fusion 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 gigantic extinct sea-lizards, and in the +mouths of certain +<a name="Page384"></a> +suctorial crustaceans, the general pattern seems thus to have become partially +obscured. +</p> + +<p> +There is another and equally curious branch of our subject; namely, serial +homologies, or the comparison of the different parts or organs in the same +individual, and not of the same parts or organs in different members of the +same class. Most physiologists believe that the bones of the skull are +homologous—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 all the higher vertebrate classes are plainly +homologous. So it is with the wonderfully complex jaws and legs of 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, during the early or embryonic stages of development in flowers, as well as +in crustaceans and many other animals, that organs, which when mature become +extremely different are at first exactly alike. +</p> + +<p> +How inexplicable are the cases of serial homologies 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 apparently representing +vertebræ? As Owen has remarked, the benefit derived from the yielding of the +separate pieces in the act of parturition by mammals, will by no means explain +the same construction in the skulls of birds and reptiles. Why should similar +bones have been created to form the wing and the leg of a bat, used as they are +for such totally different purposes, namely flying and walking? 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 each +flower, though fitted for such distinct purposes, be all constructed on the +same pattern? +</p> + +<p> +On the theory of natural selection, we can, to a certain extent, answer these +questions. We need not here consider how the bodies of some animals first +became divided into a series of segments, or how they became divided into right +and left sides, with corresponding organs, for such questions are almost beyond +investigation. It is, however, probable that some serial structures are the +result of cells multiplying by division, entailing the +<a name="Page385"></a> +multiplication of the parts developed from such cells. It must suffice for our +purpose to bear in mind that an indefinite repetition of the same part or organ +is the common characteristic, as Owen has remarked, of all low or little +specialised forms; therefore the unknown progenitor of the Vertebrata probably +possessed many vertebræ; the unknown progenitor of the Articulata, many +segments; and the unknown progenitor of flowering plants, many leaves arranged +in one or more spires. We have also formerly seen that parts many times +repeated are eminently liable to vary, not only in number, but in form. +Consequently such parts, being already present in considerable numbers, and +being highly variable, would naturally afford the materials for adaptation to +the most different purposes; yet they would generally retain, through the force +of inheritance, plain traces of their original or fundamental resemblance. They +would retain this resemblance all the more, as the variations, which afforded +the basis for their subsequent modification through natural selection, would +tend from the first to be similar; the parts being at an early stage of growth +alike, and being subjected to nearly the same conditions. Such parts, whether +more or less modified, unless their common origin became wholly obscured, would +be serially homologous. +</p> + +<p> +In the great class of molluscs, though the parts in distinct species can be +shown to be homologous, only a few serial homologies; such as the valves of +Chitons, can be indicated; that is, we are seldom enabled to say that one part +is homologous with another part 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. +</p> + +<p> +But morphology is a much more complex subject than it at first appears, as has +lately been well shown in a remarkable paper by Mr. E. Ray Lankester, who has +drawn an important distinction between certain classes of cases which have all +been equally ranked by naturalists as homologous. He proposes to call the +structures which resemble each other in distinct animals, owing to their +descent from a common progenitor with subsequent modification, +<i>homogenous;</i> and the resemblances which cannot thus be accounted for, he +proposes to call <i>homoplastic</i>. For instance, he believes that the hearts +of birds and mammals are as a whole homogenous—that is, have been derived +from a common progenitor; but that the four cavities of the heart in the two +classes are homoplastic—that is, have been independently developed. Mr. +Lankester also +<a name="Page386"></a> +adduces the close resemblance of the parts on the right and left sides of the +body, and in the successive segments of the same individual animal; and here we +have parts commonly called homologous which bear no relation to the descent of +distinct species from a common progenitor. Homoplastic structures are the same +with those which I have classed, though in a very imperfect manner, as +analogous modifications or resemblances. Their formation may be attributed in +part to distinct organisms, or to distinct parts of the same organism, having +varied in an analogous manner; and in part to similar modifications, having +been preserved for the same general purpose or function, of which many +instances have been given. +</p> + +<p> +Naturalists frequently speak of the skull as formed of metamorphosed vertebræ; +the jaws of crabs as metamorphosed legs; the stamens and pistils in flowers as +metamorphosed leaves; but it would in most cases be more correct, as Professor +Huxley has remarked, to speak of both skull and vertebræ, jaws and legs, +&c., as having been metamorphosed, not one from the other, as they now +exist, but from some common and simpler element. Most 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 converted into +skulls or jaws. Yet so strong is the appearance of this having occurred that +naturalists can hardly avoid employing language having this plain +signification. According to the views here maintained, such language may be +used literally; and the wonderful fact of the jaws, for instance, of a crab +retaining numerous characters, which they probably would have retained through +inheritance, if they had really been metamorphosed from true though extremely +simple legs, is in part explained. +</p> + +<p class="center"> +<i>Development and Embryology.</i> +</p> + +<p> +This is one of the most important subjects in the whole round of natural +history. The metamorphoses of insects, with which every one is familiar, are +generally effected abruptly by a few stages; but the transformations are in +reality numerous and gradual, though concealed. A certain ephemerous insect +(Chlöeon) during its development, moults, as shown by Sir J. Lubbock, above +twenty times, and each time undergoes a certain amount of change; and in this +case we see the act of metamorphosis performed in a primary and gradual manner. +Many insects, and +<a name="Page387"></a> +especially certain crustaceans, show us what wonderful changes of structure can +be effected during development. Such changes, however, reach their acme in the +so-called alternate generations of some of the lower animals. It is, for +instance, an astonishing fact that a delicate branching coralline, studded with +polypi, and attached to a submarine rock, should produce, first by budding and +then by transverse division, a host of huge floating jelly-fishes; and that +these should produce eggs, from which are hatched swimming animalcules, which +attach themselves to rocks and become developed into branching corallines; and +so on in an endless cycle. The belief in the essential identity of the process +of alternate generation and of ordinary metamorphosis has been greatly +strengthened by Wagner’s discovery of the larva or maggot of a fly, +namely the Cecidomyia, producing asexually other larvæ, and these others, which +finally are developed into mature males and females, propagating their kind in +the ordinary manner by eggs. +</p> + +<p> +It may be worth notice that when Wagner’s remarkable discovery was first +announced, I was asked how was it possible to account for the larvæ of this fly +having acquired the power of a sexual reproduction. As long as the case +remained unique no answer could be given. But already Grimm has shown that +another fly, a Chironomus, reproduces itself in nearly the same manner, and he +believes that this occurs frequently in the order. It is the pupa, and not the +larva, of the Chironomus which has this power; and Grimm further shows that +this case, to a certain extent, “unites that of the Cecidomyia with the +parthenogenesis of the Coccidæ;” the term parthenogenesis implying that +the mature females of the Coccidæ are capable of producing fertile eggs without +the concourse of the male. Certain animals belonging to several classes are now +known to have the power of ordinary reproduction at an unusually early age; and +we have only to accelerate parthenogenetic reproduction by gradual steps to an +earlier and earlier age—Chironomus showing us an almost exactly +intermediate stage, viz., that of the pupa—and we can perhaps account for +the marvellous case of the Cecidomyia. +</p> + +<p> +It has already been stated that various parts in the same individual, which are +exactly alike during an early embryonic period, become widely different and +serve for widely different purposes in the adult state. So again it has been +shown that generally the embryos of the most distinct species belonging to the +same class are closely similar, but become, when fully developed, widely +dissimilar. A better proof of this latter fact cannot be given than the +statement by Von Baer that “the embryos of mammalia, of +<a name="Page388"></a> +birds, lizards and snakes, probably also of chelonia, are in the earliest +states exceedingly like one another, both as a whole and in the mode of +development of their parts; so much so, in fact, that we can often distinguish +the embryos only by their size. In my possession are two little embryos in +spirit, whose names I have omitted to attach, and at present I am quite unable +to say to what class they belong. They may be lizards or small birds, or very +young mammalia, so complete is the similarity in the mode of formation of the +head and trunk in these animals. The extremities, however, are still absent in +these embryos. But even if they had existed in the earliest stage of their +development we should learn nothing, for the feet of lizards and mammals, the +wings and feet of birds, no less than the hands and feet of man, all arise from +the same fundamental form.” The larvæ of most crustaceans, at +corresponding stages of development, closely resemble each other, however +different the adults may become; and so it is with very many other animals. A +trace of the law of embryonic resemblance occasionally lasts till a rather late +age: thus birds of the same genus, and of allied genera, often resemble each +other in their immature plumage; as we see in the spotted feathers in the young +of the thrush group. In the cat tribe, most of the species when adult are +striped or spotted in lines; and stripes or spots can be plainly distinguished +in the whelp of the lion and the puma. We occasionally, though rarely, see +something of the same kind in plants; thus the first leaves of the ulex or +furze, and the first leaves of the phyllodineous acacias, are pinnate or +divided like the ordinary leaves of the leguminosæ. +</p> + +<p> +The points of structure, in which the embryos of widely different animals +within 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 courses 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 similar +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 supposes that the stripes on the whelp of +a lion, or the spots on the young blackbird, are of any use to these animals. +</p> + +<p> +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 +<a name="Page389"></a> +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. In how important a manner this +has acted, has recently been well shown by Sir J. Lubbock in his remarks on the +close similarity of the larvæ of some insects belonging to very different +orders, and on the dissimilarity of the larvæ of other insects within the same +order, according to their habits of life. Owing to such adaptations the +similarity of the larvæ of allied animals is sometimes greatly obscured; +especially when there is a division of labour during the different stages of +development, as when the same larva has during one stage to search for food, +and during another stage has to search for a place of attachment. Cases can +even be given of the larvæ of allied species, or groups of species, differing +more from each other than do the adults. 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 a crustacean: but a glance at the +larva shows this in an unmistakable manner. So again the two main divisions of +cirripedes, the pedunculated and sessile, though differing widely in external +appearance, have larvæ in all their stages barely distinguishable. +</p> + +<p> +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 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 must be 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 locomotive organs, +a 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 out 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, simple eye-spot. In this last and +complete state, cirripedes may +<a name="Page390"></a> +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 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, and every other organ of importance, excepting +those for reproduction. +</p> + +<p> +We are so much accustomed to see a difference in structure between the embryo +and the adult, that we are tempted to look at this difference as in some +necessary manner contingent on growth. But there is no reason why, for +instance, the wing of a bat, or the fin of a porpoise, should not have been +sketched out with all their parts in proper proportion, as soon as any part +became visible. In some whole groups of animals and in certain members of other +groups this is the case, and 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.” Land-shells and fresh-water +crustaceans are born having their proper forms, while the marine members of the +same two great classes pass through considerable and often great changes during +their development. Spiders, again, barely undergo any metamorphosis. The larvæ +of most insects pass through a worm-like stage, whether they are active and +adapted to diversified habits, or are inactive from being placed in the midst +of proper nutriment, or from being fed by their parents; but in some few cases, +as in that of Aphis, if we look to the admirable drawings of the development of +this insect, by Professor Huxley, we see hardly any trace of the vermiform +stage. +</p> + +<p> +Sometimes it is only the earlier developmental stages which fail. Thus, Fritz +Müller has made the remarkable discovery that certain shrimp-like crustaceans +(allied to Penoeus) first appear under the simple nauplius-form, and after +passing through two or more zoëa-stages, and then through the mysis-stage, +finally acquire their mature structure: now in the whole great malacostracan +order, to which these crustaceans belong, no other member is as yet known to be +first developed under the nauplius-form, though many appear as zoëas; +nevertheless Müller assigns reasons for his belief, that if there had been no +suppression of development, all these crustaceans would have appeared as +nauplii. +</p> + +<p> +How, then, can we explain these several facts in embryology—namely, the +very general, though not universal, difference in structure between the embryo +and the adult; the various parts in the +<a name="Page391"></a> +same individual embryo, which ultimately become very unlike, and serve for +diverse purposes, being at an early period of growth alike; the common, but not +invariable, resemblance between the embryos or larvæ of the most distinct +species in the same class; the embryo often retaining, while within the egg or +womb, structures which are of no service to it, either at that or at a later +period of life; on the other hand, larvæ which have to provide for their own +wants, being perfectly adapted to the surrounding conditions; and lastly, the +fact of certain larvæ standing higher in the scale of organisation than the +mature animal into which they are developed? I believe that all these facts can +be explained as follows. +</p> + +<p> +It is commonly assumed, perhaps from monstrosities affecting the embryo at a +very early period, that slight variations or individual differences necessarily +appear at an equally early period. We have little evidence on this head, but +what we have certainly points the other way; for it is notorious that breeders +of cattle, horses and various fancy animals, cannot positively tell, until some +time after birth, what will be the merits and demerits of their young animals. +We see this plainly in our own children; we cannot tell whether a child will be +tall or short, or what its precise features will be. The question is not, at +what period of life any variation may have been caused, but at what period the +effects are displayed. The cause may have acted, and I believe often has acted, +on one or both parents before the act of generation. It deserves notice that it +is of no importance to a very young animal, as long as it is nourished and +protected by its parent, whether most of its characters are acquired a little +earlier or later in life. It would not signify, for instance, to a bird which +obtained its food by having a much-curved beak whether or not while young it +possessed a beak of this shape, as long as it was fed by its parents. +</p> + +<p> +I have stated in the first chapter, 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 full-grown horns of cattle. But variations which, +for all that we can see might have appeared either earlier or later in life, +likewise tend to reappear at a corresponding age in the offspring and parent. I +am far from meaning that this is invariably the case, and I could give several +exceptional cases of variations (taking the word in the largest sense) which +have supervened at an earlier age in the child than in the parent. +</p> + +<p> +<a name="Page392"></a> +These two principles, namely, that slight variations generally appear at a not +very early period of life, and are inherited at a corresponding not early +period, explain, as I believe, all the above specified leading facts in +embryology. But first let us look to a few analogous cases in our domestic +varieties. Some authors who have written on Dogs maintain that the greyhound +and bull-dog, though so different, are really closely allied varieties, +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 acquired nearly their full amount of +proportional difference. So, again, I was told that the foals of cart and +race-horses—breeds which have been almost wholly formed by selection +under domestication—differed as much as the full-grown animals; but +having had careful measurements made of the dams and of three-days-old colts of +race and heavy cart-horses, I find that this is by no means the case. +</p> + +<p> +As we have conclusive evidence that the breeds of the Pigeon are descended from +a single wild species, I compared the young pigeons within twelve hours after +being hatched. I carefully measured the proportions (but will not here give the +details) of the beak, width of mouth, length of nostril and of eyelid, size of +feet and length of leg, in the wild parent species, in pouters, fantails, +runts, barbs, dragons, carriers, and tumblers. Now, some of these birds, when +mature, differ in so extraordinary a manner in the length and form of beak, and +in other characters, that they would certainly have been ranked as distinct +genera if found in a state of nature. But when the nestling birds of these +several breeds were placed in a row, though most of them could just be +distinguished, the proportional differences in the above specified 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 almost exactly the same proportions as +in the adult stage. +</p> + +<p> +These facts are explained by the above two principles. Fanciers select their +dogs, horses, pigeons, &c., for breeding, when nearly grown up. They are +indifferent whether the desired qualities are acquired earlier or later in +life, if the full-grown animal possesses them. And the cases just given, more +especially that of the +<a name="Page393"></a> +pigeons, show that the characteristic differences which have been accumulated +by man’s selection, and which give value to his breeds, do not generally +appear at a very early period of life, and are inherited at a corresponding not +early period. But the case of the short-faced tumbler, which when twelve hours +old possessed its proper characters, 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 a corresponding, but at an earlier age. +</p> + +<p> +Now, let us apply these two principles to species in a state of nature. Let us +take a group of birds, descended from some ancient form and modified through +natural selection for different habits. Then, from the many slight successive +variations having supervened in the several species at a not early age, and +having been inherited at a corresponding age, the young will have been but +little modified, and they will still resemble each other much more closely than +do the adults, just as we have seen with the breeds of the pigeon. We may +extend this view to widely distinct structures and to whole classes. The +fore-limbs, for instance, which once served as legs to a remote progenitor, may +have become, through a long course of modification, adapted in one descendant +to act as hands, in another as paddles, in another as wings; but on the above +two principles the fore-limbs will not have been much modified in the embryos +of these several forms; although in each form the fore-limb will differ greatly +in the adult state. Whatever influence long continued use or disuse may have +had in modifying the limbs or other parts of any species, this will chiefly or +solely have affected it when nearly mature, when it was compelled to use its +full powers to gain its own living; and the effects thus produced will have +been transmitted to the offspring at a corresponding nearly mature age. Thus +the young will not be modified, or will be modified only in a slight degree, +through the effects of the increased use or disuse of parts. +</p> + +<p> +With some animals the successive variations may have supervened at a very early +period of life, or the steps may have been inherited at an earlier age than +that at which they first occurred. In either of these cases the young or embryo +will closely resemble the mature parent-form, as we have seen with the +short-faced tumbler. And this is the rule of development in certain whole +groups, or in certain sub-groups alone, as with cuttle-fish, land-shells, +fresh-water crustaceans, spiders, and some members of the great class of +insects. With respect to the final cause of the young in such groups not +passing through any metamorphosis, we can see that this +<a name="Page394"></a> +would follow from the following contingencies: namely, from the young having to +provide at a very early age for their own wants, and from their following the +same habits of life with their parents; for in this case it would be +indispensable for their existence that they should be modified in the same +manner as their parents. Again, with respect to the singular fact that many +terrestrial and fresh-water animals do not undergo any metamorphosis, while +marine members of the same groups pass through various transformations, Fritz +Müller has suggested that the process of slowly modifying and adapting an +animal to live on the land or in fresh water, instead of in the sea, would be +greatly simplified by its not passing through any larval stage; for it is not +probable that places well adapted for both the larval and mature stages, under +such new and greatly changed habits of life, would commonly be found unoccupied +or ill-occupied by other organisms. In this case the gradual acquirement at an +earlier and earlier age of the adult structure would be favoured by natural +selection; and all traces of former metamorphoses would finally be lost. +</p> + +<p> +If, on the other hand, it profited the young of an animal to follow habits of +life slightly different from those of the parent-form, and consequently to be +constructed on a slightly different plan, or if it profited a larva already +different from its parent to change still further, then, on the principle of +inheritance at corresponding ages, the young or the larvæ might be rendered by +natural selection more and more different from their parents to any conceivable +extent. Differences in the larva might, also, become correlated with successive +stages of its development; so that the larva, in the first stage, might come to +differ greatly from the larva in the second stage, as is the case with many +animals. The adult might also become fitted for sites or habits, in which +organs of locomotion or of the senses, &c., would be useless; and in this +case the metamorphosis would be retrograde. +</p> + +<p> +From the remarks just made we can see how by changes of structure in the young, +in conformity with changed habits of life, together with inheritance at +corresponding ages, animals might come to pass through stages of development, +perfectly distinct from the primordial condition of their adult progenitors. +Most of our best authorities are now convinced that the various larval and +pupal stages of insects have thus been acquired through adaptation, and not +through inheritance from some ancient form. The curious case of Sitaris—a +beetle which passes through certain unusual stages of development—will +illustrate how this might occur. The first larval form is described by M. +Fabre, as an active, minute insect, furnished +<a name="Page395"></a> +with six legs, two long antennæ, and four eyes. These larvæ are hatched in the +nests of bees; and when the male bees emerge from their burrows, in the spring, +which they do before the females, the larvæ spring on them, and afterwards +crawl on to the females while paired with the males. As soon as the female bee +deposits her eggs on the surface of the honey stored in the cells, the larvæ of +the Sitaris leap on the eggs and devour them. Afterwards they undergo a +complete change; their eyes disappear; their legs and antennæ become +rudimentary, and they feed on honey; so that they now more closely resemble the +ordinary larvæ of insects; ultimately they undergo a further transformation, +and finally emerge as the perfect beetle. Now, if an insect, undergoing +transformations like those of the Sitaris, were to become the progenitor of a +whole new class of insects, the course of development of the new class would be +widely different from that of our existing insects; and the first larval stage +certainly would not represent the former condition of any adult and ancient +form. +</p> + +<p> +On the other hand it is highly probable that with many animals the embryonic or +larval stages show us, more or less completely, the condition of the progenitor +of the whole group in its adult state. In the great class of the Crustacea, +forms wonderfully distinct from each other, namely, suctorial parasites, +cirripedes, entomostraca, and even the malacostraca, appear at first as larvæ +under the nauplius-form; and as these larvæ live and feed in the open sea, and +are not adapted for any peculiar habits of life, and from other reasons +assigned by Fritz Müller, it is probable that at some very remote period an +independent adult animal, resembling the Nauplius, existed, and subsequently +produced, along several divergent lines of descent, the above-named great +Crustacean groups. So again, it is probable, from what we know of the embryos +of mammals, birds, fishes and reptiles, that these animals are the modified +descendants of some ancient progenitor, which was furnished in its adult state +with branchiæ, a swim-bladder, four fin-like limbs, and a long tail, all fitted +for an aquatic life. +</p> + +<p> +As all the organic beings, extinct and recent, which have ever lived, can be +arranged within a few great classes; and as all within each class have, +according to our theory, been connected together by fine gradations, the best, +and, if our collections were nearly perfect, the only possible arrangement, +would be genealogical; descent being 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 +<a name="Page396"></a> +than that of the adult. In two or more groups of animals, however much they may +differ from each other in structure and habits in their adult condition, if +they pass through closely similar embryonic stages, we may feel assured that +they are all descended from one parent-form, and are therefore closely related. +Thus, community in embryonic structure reveals community of descent; but +dissimilarity in embryonic development does not prove discommunity of descent, +for in one of two groups the developmental stages may have been suppressed, or +may have been so greatly modified through adaptation to new habits of life as +to be no longer recognisable. Even in groups, in which the adults have been +modified to an extreme degree, community of origin is often revealed by the +structure of the larvæ; we have seen, for instance, that cirripedes, though +externally so like shell-fish, are at once known by their larvæ to belong to +the great class of crustaceans. As the embryo often shows us more or less +plainly the structure of the less modified and ancient progenitor of the group, +we can see why ancient and extinct forms so often resemble in their adult state +the embryos of existing species of the same class. Agassiz believes this to be +a universal law of nature; and we may hope hereafter to see the law proved +true. It can, however, be proved true only in those cases in which the ancient +state of the progenitor of the group has not been wholly obliterated, either by +successive variations having supervened at a very early period of growth, or by +such variations having been inherited at an earlier age than that at which they +first appeared. It should also be borne in mind, that the law 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. The law will +not strictly hold good in those cases in which an ancient form became adapted +in its larval state to some special line of life, and transmitted the same +larval state to a whole group of descendants; for such larval state will not +resemble any still more ancient form in its adult state. +</p> + +<p> +Thus, as it seems to me, the leading facts in embryology, which are second to +none in importance, are explained on the principle of variations in the many +descendants from some one ancient progenitor, having appeared at a not very +early period of life, and having been inherited at a corresponding period. +Embryology rises greatly in interest, when we look at the embryo as a picture, +more or less obscured, of the progenitor, either in its adult or larval state, +of all the members of the same great class. +</p> + +<p class="center"> +<a name="Page397"></a> +<i>Rudimentary, Atrophied, and Aborted Organs.</i> +</p> + +<p> +Organs or parts in this strange condition, bearing the plain stamp of +inutility, are extremely common, or even general, throughout nature. It would +be impossible to name one of the higher animals in which some part or other is +not in a rudimentary condition. In the mammalia, for instance, the males +possess rudimentary mammæ; in snakes one lobe of the lungs is rudimentary; in +birds the “bastard-wing” may safely be considered as a rudimentary +digit, and in some species the whole wing is so far rudimentary that it cannot +be used for flight. What can be more curious than the presence of teeth in +foetal whales, which when grown up have not a tooth in their heads; or the +teeth, which never cut through the gums, in the upper jaws of unborn calves? +</p> + +<p> +Rudimentary organs plainly declare their origin and meaning in various ways. +There are beetles belonging to closely allied species, or even to the same +identical species, which have either full-sized and perfect wings, or mere +rudiments of membrane, which not rarely lie under wing-covers firmly soldered +together; and in these cases it is impossible to doubt, that the rudiments +represent wings. Rudimentary organs sometimes retain their potentiality: this +occasionally occurs with the mammæ of male mammals, which have been known to +become well developed and to secrete milk. So again in the udders of the genus +Bos, there are normally four developed and two rudimentary teats; but the +latter in our domestic cows sometimes become well developed and yield milk. In +regard to plants, the petals are sometimes rudimentary, and sometimes well +developed in the individuals of the same species. In certain plants having +separated sexes Kölreuter found that by crossing a species, in which the male +flowers included a rudiment of a pistil, with an hermaphrodite species, having +of course a well-developed pistil, the rudiment in the hybrid offspring was +much increased in size; and this clearly shows that the rudimentary and perfect +pistils are essentially alike in nature. An animal may possess various parts in +a perfect state, and yet they may in one sense be rudimentary, for they are +useless: thus the tadpole of the common salamander or water-newt, as Mr. G.H. +Lewes remarks, “has gills, and passes its existence in the water; but the +Salamandra atra, which lives high up among the mountains, brings forth its +young full-formed. This animal never lives in the water. Yet if we open a +gravid female, we find tadpoles inside her with exquisitely feathered gills; +and when placed in water they swim about like the tadpoles of the water-newt. +Obviously this aquatic organisation has +<a name="Page398"></a> +no reference to the future life of the animal, nor has it any adaptation to its +embryonic condition; it has solely reference to ancestral adaptations, it +repeats a phase in the development of its progenitors.” +</p> + +<p> +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 within the ovarium. 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 rudimentary pistil, for it is not +crowned with a stigma; but the style remains well developed and is clothed in +the usual manner with hairs, which serve to brush the pollen out of the +surrounding and conjoined anthers. Again, an organ may become rudimentary for +its proper purpose, and be used for a distinct one: in certain fishes 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. Many +similar instances could be given. +</p> + +<p> +Useful organs, however little they may be developed, unless we have reason to +suppose that they were formerly more highly developed, ought not to be +considered as rudimentary. They may be in a nascent condition, and in progress +towards further development. Rudimentary organs, on the other hand, are either +quite useless, such as teeth which never cut through the gums, or almost +useless, such as the wings of an ostrich, which serve merely as sails. As +organs in this condition would formerly, when still less developed, have been +of even less use than at present, they cannot formerly have been produced +through variation and natural selection, which acts solely by the preservation +of useful modifications. They have been partially retained by the power of +inheritance, and relate to a former state of things. It is, however, often +difficult to distinguish between rudimentary and nascent organs; for we can +judge only by analogy whether a part is capable of further development, in +which case alone it deserves to be called nascent. Organs in this condition +will always be somewhat rare; for beings thus provided will commonly have been +supplanted by their successors with the same organ in a more perfect state, and +consequently will have become long ago extinct. The wing of the penguin is of +high service, acting as a fin; it may, therefore, represent the nascent state +of the wing: not that I believe this to be the case; it is more probably a +reduced organ, modified for a new function: the wing of the Apteryx, on the +other hand, is +<a name="Page399"></a> +quite useless, and is truly rudimentary. Owen considers the simple filamentary +limbs of the Lepidosiren as the “beginnings of organs which attain full +functional development in higher vertebrates;” but, according to the view +lately advocated by Dr. Günther, they are probably remnants, consisting of the +persistent axis of a fin, with the lateral rays or branches aborted. The +mammary glands of the Ornithorhynchus may be considered, in comparison with the +udders of a cow, as in a nascent condition. The ovigerous frena of certain +cirripedes, which have ceased to give attachment to the ova and are feebly +developed, are nascent branchiæ. +</p> + +<p> +Rudimentary organs in the individuals of the same species are very liable to +vary in the degree of their development and in other respects. In closely +allied species, also, the extent to which the same organ has been reduced +occasionally differs much. This latter fact is well exemplified in the state of +the wings of female moths belonging to the same family. Rudimentary organs may +be utterly aborted; and this implies, that in certain animals or plants, parts +are entirely absent which analogy would lead us to expect to find in them, and +which are occasionally found in monstrous individuals. Thus in most of the +Scrophulariaceæ the fifth stamen is utterly aborted; yet we may conclude that a +fifth stamen once existed, for a rudiment of it is found in many species of the +family, and this rudiment occasionally becomes perfectly developed, as may +sometimes be seen in the common snap-dragon. In tracing the homologies of any +part in different members of the same class, nothing is more common, or, in +order fully to understand the relations of the parts, more useful than the +discovery of rudiments. This is well shown in the drawings given by Owen of the +leg bones of the horse, ox, and rhinoceros. +</p> + +<p> +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 is of greater size in the embryo relatively to the adjoining +parts, 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 +rudimentary organs in the adult are often said to have retained their embryonic +condition. +</p> + +<p> +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 that most parts and organs are exquisitely +adapted for certain purposes, tells us with equal plainness that these +rudimentary or atrophied +<a name="Page400"></a> +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 is not an explanation, merely a restatement of the fact. Nor is it +consistent with itself: thus the boa-constrictor has rudiments of hind limbs +and of a pelvis, and if it be said that these bones have been retained +“to complete the scheme of nature,” why, as Professor Weismann +asks, have they not been retained by other snakes, which do not possess even a +vestige of these same bones? What would be thought of an astronomer who +maintained that the satellites revolve in elliptic courses round their planets +“for the sake of symmetry,” because the planets thus revolve round +the sun? An eminent physiologist accounts for the presence of rudimentary +organs, by supposing that they serve to excrete matter in excess, or matter +injurious to the system; but can we suppose that the minute papilla, which +often represents the pistil in male flowers, and which is formed of mere +cellular tissue, can thus act? Can we suppose that rudimentary teeth, which are +subsequently absorbed, are beneficial to the rapidly growing embryonic calf by +removing matter so precious as phosphate of lime? When a man’s fingers +have been amputated, imperfect nails have been known to appear on the stumps, +and I could as soon believe that these vestiges of nails are developed in order +to excrete horny matter, as that the rudimentary nails on the fin of the +manatee have been developed for this same purpose. +</p> + +<p> +On the view of descent with modification, the origin of rudimentary organs is +comparatively simple; and we can understand to a large extent the laws +governing their imperfect development. 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 of sheep—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 +the balance of evidence clearly indicates that species under nature do not +undergo great and abrupt changes. But we learn from the study of our domestic +productions that the disuse of parts leads to their reduced size; and that the +result is inherited. +</p> + +<p> +It appears probable that disuse has been the main agent in +<a name="Page401"></a> +rendering organs rudimentary. It would at first lead by slow steps to the more +and more complete reduction of a part, until at last it became +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 by beasts of prey 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 will have aided in reducing +the organ, until it was rendered harmless and rudimentary. +</p> + +<p> +Any change in structure and function, which can be effected by small stages, is +within the power of natural selection; so that an organ rendered, through +changed habits of life, useless or injurious for one purpose, might be modified +and used for another purpose. An organ might, also, be retained for one alone +of its former functions. Organs, originally formed by the aid of natural +selection, when rendered useless may well be variable, for their variations can +no longer be checked by natural selection. All this agrees well with what we +see under nature. Moreover, at whatever period of life either disuse or +selection reduces an organ, and this will generally be when the being has come +to maturity and to exert its full powers of action, the principle of +inheritance at corresponding ages will tend to reproduce the organ in its +reduced state at the same mature age, but will seldom affect it in the embryo. +Thus we can understand the greater size of rudimentary organs in the embryo +relatively to the adjoining parts, and their lesser relative size in the adult. +If, for instance, the digit of an adult animal was used less and less during +many generations, owing to some change of habits, or if an organ or gland was +less and less functionally exercised, we may infer that it would become reduced +in size in the adult descendants of this animal, but would retain nearly its +original standard of development in the embryo. +</p> + +<p> +There remains, however, this difficulty. After an organ has ceased being used, +and has become in consequence much reduced, how can it be still further reduced +in size until the merest vestige is left; and how can it be finally quite +obliterated? It is scarcely possible that disuse can go on producing any +further effect after the organ has once been rendered functionless. Some +additional explanation is here requisite which I cannot give. If, for instance, +it could be proved that every part of the organisation tends to vary in a +greater degree towards diminution than toward augmentation of size, then we +should be able to understand how an organ which has become useless would be +rendered, independently of the +<a name="Page402"></a> +effects of disuse, rudimentary and would at last be wholly suppressed; for the +variations towards diminished size would no longer be checked by natural +selection. The principle of the economy of growth, explained in a former +chapter, by which the materials forming any part, if not useful to the +possessor, are saved as far as is possible, will perhaps come into play in +rendering a useless part rudimentary. But this principle will almost +necessarily be confined to the earlier stages of the process of reduction; for +we cannot suppose that a minute papilla, for instance, representing in a male +flower the pistil of the female flower, and formed merely of cellular tissue, +could be further reduced or absorbed for the sake of economising nutriment. +</p> + +<p> +Finally, as rudimentary organs, by whatever steps they may have been degraded +into their present useless condition, are the record of a former state of +things, and have been retained solely through the power of inheritance—we +can understand, on the genealogical view of classification, how it is that +systematists, in placing organisms in their proper places in the natural +system, have often 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 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 old +doctrine of creation, might even have been anticipated in accordance with the +views here explained. +</p> + +<p class="center"> +<i>Summary.</i> +</p> + +<p> +In this chapter I have attempted to show that the arrangement of all organic +beings throughout all time in groups under groups—that the nature of the +relationships by which all living and extinct organisms are united by complex, +radiating, and circuitous lines of affinities into a few grand +classes—the rules followed and the difficulties encountered by +naturalists in their classifications—the value set upon characters, if +constant and prevalent, whether of high or of the most trifling importance, or, +as with 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 if we admit the common parentage of +allied forms, together with their modification through variation and natural +selection, with the contingencies of extinction +<a name="Page403"></a> +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, dimorphic forms, and acknowledged +varieties of the same species, however much they may differ from each other in +structure. If we extend the use of this element of descent—the one +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. +</p> + +<p> +On this same view of descent with modification, most of the great facts in +Morphology become intelligible—whether we look to the same pattern +displayed by the different species of the same class in their homologous +organs, to whatever purpose applied, or to the serial and lateral homologies in +each individual animal and plant. +</p> + +<p> +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 leading facts in embryology; +namely, the close resemblance in the individual embryo of the parts which are +homologous, and which when matured become widely different in structure and +function; and the resemblance of the homologous parts or organs in allied +though distinct species, though fitted in the adult state for habits as +different as is possible. Larvæ are active embryos, which have become specially +modified in a greater or less degree in relation to their habits of life, with +their modifications inherited at a corresponding early age. On these same +principles, and bearing in mind that when organs are reduced in size, either +from disuse or through natural 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 force of inheritance—the occurrence of rudimentary +organs might even have been anticipated. The importance of embryological +characters and of rudimentary organs in classification is intelligible, on the +view that a natural arrangement must be genealogical. +</p> + +<p> +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, with which this world is peopled, are 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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page404"></a><a name="link2HCH0015" id="link2HCH0015"></a> +CHAPTER XV.<br/> +RECAPITULATION AND CONCLUSION.</h2> + +<p class="letter"> +Recapitulation of the objections to 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. +</p> + +<p> +As this whole volume is one long argument, it may be convenient to the reader +to have the leading facts and inferences briefly recapitulated. +</p> + +<p> +That many and serious objections may be advanced against the theory of descent +with modification through variation and 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 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 all parts of the organisation and +instincts offer, at least individual differences—that there is a struggle +for existence leading to the preservation of profitable deviations of structure +or instinct—and, lastly, that gradations in the state of perfection of +each organ may have existed, each good of its kind. The truth of these +propositions cannot, I think, be disputed. +</p> + +<p> +It is, no doubt, extremely difficult even to conjecture by what gradations many +structures have been perfected, more especially among broken and failing groups +of organic beings, which have suffered much extinction; but we see so many +strange gradations in nature, that we ought to be extremely cautious in saying +that any organ or instinct, or any whole structure, could not have arrived at +its present state by many graduated steps. There are, it must be admitted, +cases of special difficulty opposed to the theory of natural selection; and one +of the most curious of these +<a name="Page405"></a> +is the existence in the same community of two or three defined castes of +workers or sterile female ants; but I have attempted to show how these +difficulties can be mastered. +</p> + +<p> +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 ninth chapter, which seem to me conclusively to +show that this sterility is no more a special endowment than is the incapacity +of two distinct kinds of trees to be grafted together; but that it is +incidental on differences confined to the reproductive systems of the +intercrossed species. We see the truth of this conclusion in the vast +difference in the results of crossing the same two species +reciprocally—that is, when one species is first used as the father and +then as the mother. Analogy from the consideration of dimorphic and trimorphic +plants clearly leads to the same conclusion, for when the forms are +illegitimately united, they yield few or no seed, and their offspring are more +or less sterile; and these forms belong to the same undoubted species, and +differ from each other in no respect except in their reproductive organs and +functions. +</p> + +<p> +Although the fertility of varieties when intercrossed, and of their mongrel +offspring, has been asserted by so many authors to be universal, this cannot be +considered as quite correct after the facts given on the high authority of +Gärtner and Kölreuter. Most of the varieties which have been experimented on +have been produced under domestication; and as domestication (I do not mean +mere confinement) almost certainly tends to eliminate that sterility which, +judging from analogy, would have affected the parent-species if intercrossed, +we ought not to expect that domestication would likewise induce sterility in +their modified descendants when crossed. This elimination of sterility +apparently follows from the same cause which allows our domestic animals to +breed freely under diversified circumstances; and this again apparently follows +from their having been gradually accustomed to frequent changes in their +conditions of life. +</p> + +<p> +A double and parallel series of facts seems to throw much light on the +sterility of species, when first crossed, and of their hybrid offspring. On the +one side, there is good reason to believe that slight changes in the conditions +of life give vigour and fertility to all organic beings. We know also that a +cross between the distinct individuals of the same variety, and between +distinct varieties, increases the number of their offspring, and certainly +gives to them +<a name="Page406"></a> +increased size and vigour. This is chiefly owing to the forms which are crossed +having been exposed to somewhat different conditions of life; for I have +ascertained by a labourious series of experiments that if all the individuals +of the same variety be subjected during several generations to the same +conditions, the good derived from crossing is often much diminished or wholly +disappears. This is one side of the case. On the other side, we know that +species which have long been exposed to nearly uniform conditions, when they +are subjected under confinement to new and greatly changed conditions, either +perish, or if they survive, are rendered sterile, though retaining perfect +health. This does not occur, or only in a very slight degree, with our +domesticated productions, which have long been exposed to fluctuating +conditions. Hence when we find that hybrids produced by a cross between two +distinct species are few in number, owing to their perishing soon after +conception or at a very early age, or if surviving that they are rendered more +or less sterile, it seems highly probable that this result is due to their +having been in fact subjected to a great change in their conditions of life, +from being compounded of two distinct organisations. He who will explain in a +definite manner why, for instance, an elephant or a fox will not breed under +confinement in its native country, whilst the domestic pig or dog will breed +freely under the most diversified conditions, will at the same time be able to +give a definite answer to the question why two distinct species, when crossed, +as well as their hybrid offspring, are generally rendered more or less sterile, +while two domesticated varieties when crossed and their mongrel offspring are +perfectly fertile. +</p> + +<p> +Turning to geographical distribution, the difficulties encountered on the +theory of descent with modification are serious enough. All the individuals of +the same species, and all the species of the same genus, or even higher group, +are descended from common parents; and therefore, in however distant and +isolated parts of the world they may now be found, they must in the course of +successive generations have travelled from some one point to all 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 of time, immensely 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 there will always have been 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 +<a name="Page407"></a> +that we are as yet very ignorant as to the full extent of the various +climatical and geographical changes which have affected the earth during modern +periods; and such changes will often have facilitated migration. As an example, +I have attempted to show how potent has been the influence of the Glacial +period on the distribution of the same and of allied 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 +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 the +species of the same genus is in some degree lessened. +</p> + +<p> +As according to 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 existing 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 <i>directly</i> 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 climatic and other conditions +of life change insensibly in proceeding 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 zones. +For we have reason to believe that only a few species of a genus ever undergo +change; the other species becoming utterly extinct and leaving no modified +progeny. Of the species which do change, only a few within the same country +change at the same time; and all modifications are slowly effected. I have also +shown that the intermediate varieties which probably at first existed in the +intermediate zones, would be liable to be supplanted by the allied forms on +either hand; for the latter, from existing in greater numbers, would generally +be modified and improved at a quicker rate than the intermediate varieties, +which existed in lesser numbers; so that the intermediate varieties would, in +the long run, be supplanted and exterminated. +</p> + +<p> +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 +<a name="Page408"></a> +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? Although geological research has +undoubtedly revealed the former existence of many links, bringing numerous +forms of life much closer together, it does not yield the infinitely many fine +gradations between past and present species required on the theory, and this is +the most obvious of the many objections which may be urged against it. Why, +again, do whole groups of allied species appear, though this appearance is +often false, to have come in suddenly on the successive geological stages? +Although we now know that organic beings appeared on this globe, at a period +incalculably remote, long before the lowest bed of the Cambrian system was +deposited, why do we not find beneath this system great piles of strata stored +with the remains of the progenitors of the Cambrian fossils? For on the theory, +such strata must somewhere have been deposited at these ancient and utterly +unknown epochs of the world’s history. +</p> + +<p> +I can answer these questions and objections only on the supposition that the +geological record is far more imperfect than most geologists believe. The +number of specimens in all our museums is absolutely as nothing compared with +the countless generations of countless species which have certainly existed. +The parent form of any two or more species would not be in all its characters +directly intermediate between its modified offspring, any more than the +rock-pigeon is directly intermediate in crop and tail between its descendants, +the pouter and fantail pigeons. We should not be able to recognise a species as +the parent of another and modified species, if we were to examine the two ever +so closely, unless we possessed most of the intermediate links; and owing to +the imperfection of the geological record, we have no just right to expect to +find so many links. If two or three, or even more linking forms were +discovered, they would simply be ranked by many naturalists as so many new +species, more especially if found in different geological substages, let their +differences be ever so slight. 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 +whether or not these doubtful forms ought to be called varieties? 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. Many species when once formed never undergo any further change but +become extinct +<a name="Page409"></a> +without leaving modified descendants; and the periods during which species have +undergone modification, though long as measured by years, have probably been +short in comparison with the periods during which they retained the same form. +It is the dominant and widely ranging species which vary most frequently and +vary most, and varieties are often at first local—both causes rendering +the discovery of intermediate links in any one formation less likely. Local +varieties will not spread into other and distant regions until they are +considerably modified and improved; and when they have spread, and are +discovered in a geological formation, they 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 has probably been +shorter than the average duration of specific forms. Successive formations are +in most cases separated from each other by blank intervals of time of great +length, for fossiliferous formations thick enough to resist future degradation +can, as a general rule, 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 generally be blank. During these latter +periods there will probably be more variability in the forms of life; during +periods of subsidence, more extinction. +</p> + +<p> +With respect to the absence of strata rich in fossils beneath the Cambrian +formation, I can recur only to the hypothesis given in the tenth chapter; +namely, that though our continents and oceans have endured for an enormous +period in nearly their present relative positions, we have no reason to assume +that this has always been the case; consequently formations much older than any +now known may lie buried beneath the great oceans. With respect to the lapse of +time not having been sufficient since our planet was consolidated for the +assumed amount of organic change, and this objection, as urged by Sir William +Thompson, is probably one of the gravest as yet advanced, I can only say, +firstly, that we do not know at what rate species change, as measured by years, +and secondly, that many philosophers are not as yet willing to admit that we +know enough of the constitution of the universe and of the interior of our +globe to speculate with safety on its past duration. +</p> + +<p> +That the geological record is imperfect all will admit; but that it is +imperfect to the degree required by our theory, few will be inclined to admit. +If we look to long enough intervals of time, geology plainly declares that +species have all changed; and they have changed in the manner required by the +theory, for they have +<a name="Page410"></a> +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 widely separated formations. +</p> + +<p> +Such is the sum of the several chief objections and difficulties which may +justly be urged against the theory; and I have now briefly recapitulated the +answers and explanations which, as far as I can see, may be given. 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 is the Geological Record. Serious as these several +objections are, in my judgment they are by no means sufficient to overthrow the +theory of descent with subsequent modification. +</p> + +<p class="p2"> +Now let us turn to the other side of the argument. Under domestication we see +much variability, caused, or at least excited, by changed conditions of life; +but often in so obscure a manner, that we are tempted to consider the +variations as spontaneous. Variability is governed by many complex laws, by +correlated growth, compensation, the increased use and disuse of parts, and the +definite action of the surrounding conditions. There is much difficulty in +ascertaining how largely our domestic productions have been modified; 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 cease under domestication for a very +long period; nor do we know that it ever ceases, for new varieties are still +occasionally produced by our oldest domesticated productions. +</p> + +<p> +Variability is not actually caused by man; he only unintentionally exposes +organic beings to new conditions of life and then nature acts on the +organisation and causes it to vary. But man can and does select the variations +given to him by nature, and thus accumulates them in any desired manner. He +thus adapts animals and plants for his own benefit or pleasure. He may do this +<a name="Page411"></a> +methodically, or he may do it unconsciously by preserving the individuals most +useful or pleasing to him without any intention 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 +inappreciable except by an educated eye. This unconscious process of selection +has been the great agency in the formation of the most distinct and useful +domestic breeds. That many breeds produced by man have to a large extent the +character of natural species, is shown by the inextricable doubts whether many +of them are varieties or aboriginally distinct species. +</p> + +<p> +There is no reason why the principles which have acted so efficiently under +domestication should not have acted under nature. In the survival of favoured +individuals and races, during the constantly recurrent Struggle for Existence, +we see a powerful and ever-acting form 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 rapid increase of many animals and plants during a +succession of peculiar seasons, and when naturalised in new countries. More +individuals are born than can possibly survive. A grain in the balance may +determine which individuals 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. On the other hand the struggle will often be severe between beings +remote in the scale of nature. The slightest advantage in certain individuals, +at any age or during any season, over those with which they come into +competition, or better adaptation in however slight a degree to the surrounding +physical conditions, will, in the long run, turn the balance. +</p> + +<p> +With animals having separated sexes, there will be in most cases a struggle +between the males for the possession of the females. The most vigorous males, +or those which have most successfully struggled with their conditions of life, +will generally leave most progeny. But success will often depend on the males +having special weapons or means of defence or charms; and a slight advantage +will lead to victory. +</p> + +<p> +As geology plainly proclaims that each land has undergone great +<a name="Page412"></a> +physical changes, we might have expected to find that organic beings have +varied under nature, in the same way as they have varied under domestication. +And if there has been 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 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 species present individual differences. +But, besides such differences, all naturalists admit that natural varieties +exist, which are considered sufficiently distinct to be worthy of record in +systematic works. No one has drawn any clear distinction between individual +differences and slight varieties; or between more plainly marked varieties and +subspecies and species. On separate continents, and on different parts of the +same continent, when divided by barriers of any kind, and on outlying islands, +what a multitude of forms exist, which some experienced naturalists rank as +varieties, others as geographical races or sub species, and others as distinct, +though closely allied species! +</p> + +<p> +If, then, animals and plants do vary, let it be ever so slightly or slowly, why +should not variations or individual differences, which are in any way +beneficial, be preserved and accumulated through natural selection, or the +survival of the fittest? If man can by patience select variations useful to +him, why, under changing and complex conditions of life, should not variations +useful to nature’s living products often arise, and be preserved or +selected? 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 look no further +than this, seems to be in the highest degree 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. +</p> + +<p class="p2"> +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 +<a name="Page413"></a> +produced by secondary laws. On this same view we can understand how it is that +in a 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 a larger genera apparently have restricted ranges, and in their +affinities they are clustered in little groups round other species—in +both respects resembling varieties. These are strange relations on the view +that each species was independently created, but are intelligible if each +existed first as a variety. +</p> + +<p> +As each species tends by its geometrical rate of reproduction to increase +inordinately in number; and as the modified descendants of each species will be +enabled to increase by as much as they become more diversified in habits and +structure, so as to be able 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 the 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 within +each class 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 go on 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 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 has prevailed throughout all +time. This grand fact of the grouping of all organic beings under what is +called the Natural System, is utterly inexplicable on the theory of creation. +</p> + +<p> +As natural selection acts solely by accumulating slight, successive, favourable +variations, it can produce no great or sudden +<a name="Page414"></a> +modifications; it can act only by short and slow steps. Hence, the canon of +“Natura non facit saltum,” which every fresh addition to our +knowledge tends to confirm, is on this theory intelligible. We can see why +throughout nature the same general end is gained by an almost infinite +diversity of means, for every peculiarity when once acquired is long inherited, +and structures already modified in many different ways have to be adapted for +the same general purpose. We can, in short, 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. +</p> + +<p> +Many other facts are, as it seems to me, explicable on this theory. How strange +it is that a bird, under the form of a woodpecker, should prey on insects on +the ground; that upland geese, which rarely or never swim, would possess webbed +feet; that a thrush-like bird should dive and feed on sub-aquatic insects; and +that a petrel should have the habits and structure fitting it for the life of +an auk! and so 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 might even have been +anticipated. +</p> + +<p> +We can to a certain extent understand how it is that there is so much beauty +throughout nature; for this may be largely attributed to the agency of +selection. That beauty, according to our sense of it, is not universal, must be +admitted by every one who will look at some venomous snakes, at some fishes, +and at certain hideous bats with a distorted resemblance to the human face. +Sexual selection has given the most brilliant colours, elegant patterns, and +other ornaments to the males, and sometimes to both sexes of many birds, +butterflies and other animals. With birds it has often rendered the voice of +the male musical to the female, as well as to our ears. Flowers and fruit have +been rendered conspicuous by brilliant colours in contrast with the green +foliage, in order that the flowers may be easily seen, visited and fertilised +by insects, and the seeds disseminated by birds. How it comes that certain +colours, sounds and forms should give pleasure to man and the lower animals, +that is, how the sense of beauty in its simplest form was first acquired, we do +not know any more than how certain odours and flavours were first rendered +agreeable. +</p> + +<p> +As natural selection acts by competition, it adapts and improves the +inhabitants of each country only in relation to their co-inhabitants; so that +we need feel no surprise at the species of any +<a name="Page415"></a> +one country, although on the ordinary view supposed to have been created and +specially 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; as +in the case even of the human eye; or if some of them be abhorrent to our ideas +of fitness. We need not marvel at the sting of the bee, when used against the +enemy, causing the bee’s own death; at drones being produced in such +great 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 living 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 detected. +</p> + +<p> +The complex and little known laws governing the production of varieties are the +same, as far as we can judge, with the laws which have governed the production +of distinct species. In both cases physical conditions seem to have produced +some direct and definite effect, but how much we cannot say. Thus, when +varieties enter any new station, they occasionally assume some of the +characters proper to the species of that station. With both varieties and +species, use and disuse seem to have produced a considerable effect; for it is +impossible 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 tucu-tucu, +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. With varieties and +species, correlated variation seems to have played an important part, so that +when one part has been modified other parts have been necessarily modified. +With both varieties and species, reversions to long-lost characters +occasionally occur. How inexplicable on the theory of creation is the +occasional appearance of stripes on the shoulders and legs of the several +species of the horse-genus and of their hybrids! How simply is this fact +explained if we believe that these species are all descended from a striped +progenitor, in the same manner as the several domestic breeds of the pigeon are +descended from the blue and barred rock-pigeon! +</p> + +<p> +On the ordinary view of each species having been independently created, why +should specific characters, or those by which the +<a name="Page416"></a> +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 possess differently coloured flowers, than if all possessed 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; therefore these same characters would be more likely again to vary +than the generic characters which have been inherited without change for an +immense period. It is inexplicable on the theory of creation why a part +developed in a very unusual manner in one species alone of a genus, and +therefore, as we may naturally infer, of great importance to that species, +should be eminently liable to variation; but, on our 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 +the 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. +</p> + +<p> +Glancing at instincts, marvellous as some are, they offer no greater difficulty +than do corporeal structures 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 often 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 widely different conditions of life, yet follow +nearly the same instincts; why the thrushes of tropical and temperate South +America, for instance, line their nests with mud like our British species. On +the view of instincts having been slowly acquired through natural selection, we +need not marvel +<a name="Page417"></a> +at some instincts being not perfect and liable to mistakes, and at many +instincts causing other animals to suffer. +</p> + +<p> +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. This similarity would be a strange fact, +if species had been independently created and varieties had been produced +through secondary laws. +</p> + +<p> +If we admit that the geological record is imperfect to an extreme degree, then +the facts, which the record does give, strongly 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 from the principle of natural selection; for +old forms are supplanted by new and improved forms. Neither single species nor +groups of species reappear when the chain of ordinary generation is once +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 beings can be classed with all recent beings, naturally +follows from the living and the extinct being the offspring of common parents. +As species have generally diverged in character during their long course of +descent and modification, we can understand why it is that the more ancient +forms, or early progenitors of each group, so often occupy a position in some +degree intermediate between existing groups. Recent forms are generally looked +upon as being, on the whole, higher in the scale of organisation than ancient +forms; and they must be higher, in so far as the later and more improved forms +have conquered the older and less improved forms in the struggle for life; they +have also generally had their organs more specialised for different functions. +This fact is perfectly compatible with numerous beings still retaining simple +and but little improved structures, fitted for simple conditions of life; it is +likewise compatible with some +<a name="Page418"></a> +forms having retrograded in organisation, by having become at each stage of +descent better fitted for new and degraded habits of life. Lastly, the +wonderful 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 the same country the existing and +the extinct will be closely allied by descent. +</p> + +<p> +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 climatical 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 has 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 are the 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, and in the northern and southern +temperate zones; and likewise the close alliance of some of the inhabitants of +the sea in the northern and southern temperate latitudes, though separated by +the whole intertropical ocean. Although two countries may present physical +conditions as closely similar as the same species ever require, we need feel no +surprise at their inhabitants being widely different, if they have been for a +long period completely sundered from each other; for as the relation of +organism to organism is the most important of all relations, and as the two +countries will have received colonists at various periods and in different +proportions, from some other country or from each other, the course of +modification in the two areas will inevitably have been different. +</p> + +<p> +On this view of migration, with subsequent modification, we see why oceanic +islands are inhabited by only few species, but of these, why many are peculiar +or endemic forms. We clearly see why species belonging to those groups of +animals which cannot +<a name="Page419"></a> +cross wide spaces of the ocean, as frogs and terrestrial mammals, do not +inhabit oceanic islands; and why, on the other hand, new and peculiar species +of bats, animals which can traverse the ocean, are often found on islands far +distant from any continent. Such cases as the presence of peculiar species of +bats on oceanic islands and the absence of all other terrestrial mammals, are +facts utterly inexplicable on the theory of independent acts of creation. +</p> + +<p> +The existence of closely allied representative species in any two areas, +implies, on the theory of descent with modification, that the same parent-forms +formerly inhabited both areas; and we almost invariably find that wherever many +closely allied species inhabit two areas, some identical species are still +common to both. Wherever many closely allied yet distinct species occur, +doubtful forms and varieties belonging to the same groups 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 the striking relation of nearly all the plants and animals of the +Galapagos Archipelago, of Juan Fernandez, and of the other American islands, to +the plants and animals of the neighbouring American mainland; and of those of +the Cape de Verde Archipelago, and of the other African islands to the African +mainland. It must be admitted that these facts receive no explanation on the +theory of creation. +</p> + +<p> +The fact, as we have seen, that all past and present organic beings can be +arranged within a few great classes, in groups subordinate to groups, and with +the extinct groups often falling in between the 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 forms 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 +beings, are of hardly any importance in classification; why characters derived +from rudimentary parts, though of no service to the beings, are often of high +classificatory value; and why embryological characters are often the most +valuable of all. The real affinities of all organic beings, in +contradistinction to their adaptive resemblances, are due to inheritance or +community of descent. The Natural System is a genealogical arrangement, with +the acquired grades of difference, marked by the terms, varieties, species, +genera, families, &c.; and we have to discover the lines of descent by the +most permanent characters, whatever they may be, and of however slight vital +importance. +</p> + +<p> +<a name="Page420"></a> +The similar framework of bones 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 in the 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, to a +large extent, intelligible on the view of the gradual modification of parts or +organs, which were aboriginally alike in an early progenitor in each of these +classes. 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 +clearly see why the embryos of mammals, birds, reptiles, and fishes should be +so closely similar, and 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 of a fish which has to breathe the air +dissolved in water by the aid of well-developed branchiæ. +</p> + +<p> +Disuse, aided sometimes by natural selection, will often have reduced organs +when rendered useless under changed habits or conditions of life; and we can +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 on an organ during early life; hence the organ will not be 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 formerly reduced by disuse owing to the tongue +and palate, or lips, having become excellently fitted through natural selection +to browse without their aid; whereas in the calf, the teeth have been left +unaffected, 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 organism +with all its separate parts having been specially created, how utterly +inexplicable is it that organs bearing the plain stamp of inutility, such as +the teeth in the embryonic calf or the shrivelled wings under the soldered +wing-covers of many beetles, should so frequently occur. Nature may be said to +have taken pains to reveal her scheme of modification, by means of rudimentary +organs, of +<a name="Page421"></a> +embryological and homologous structures, but we are too blind to understand her +meaning. +</p> + +<p class="p2"> +I have now recapitulated the facts and considerations which have thoroughly +convinced me that species have been modified, during a long course of descent. +This has been effected chiefly through the natural selection of numerous +successive, slight, favourable variations; aided in an important manner by the +inherited effects of the use and disuse of parts; and in an unimportant manner, +that is, in relation to adaptive structures, whether past or present, by the +direct action of external conditions, and by variations which seem to us in our +ignorance to arise spontaneously. It appears that I formerly underrated the +frequency and value of these latter forms of variation, as leading to permanent +modifications of structure independently of natural selection. But as my +conclusions have lately been much misrepresented, and it has been stated that I +attribute the modification of species exclusively to natural selection, I may +be permitted to remark that in the first edition of this work, and +subsequently, I placed in a most conspicuous position—namely, at the +close of the Introduction—the following words: “I am convinced that +natural selection has been the main but not the exclusive means of +modification.” This has been of no avail. Great is the power of steady +misrepresentation; but the history of science shows that fortunately this power +does not long endure. +</p> + +<p> +It can hardly be supposed that a false theory would explain, in so satisfactory +a manner as does the theory of natural selection, the several large classes of +facts above specified. It has recently been objected that this is an unsafe +method of arguing; but it is a method used in judging of the common events of +life, and has often been used by the greatest natural philosophers. The +undulatory theory of light has thus been arrived at; and the belief in the +revolution of the earth on its own axis was until lately supported by hardly +any direct evidence. It is no valid objection that science as yet throws no +light on the far higher problem of the essence or origin of life. Who can +explain what is the essence of the attraction of gravity? No one now objects to +following out the results consequent on this unknown element of attraction; +notwithstanding that Leibnitz formerly accused Newton of introducing +“occult qualities and miracles into philosophy.” +</p> + +<p> +I see no good reasons why the views given in this volume should shock the +religious feelings of any one. It is satisfactory, as showing how transient +such impressions are, to remember that the greatest discovery ever made by man, +namely, the law of the +<a name="Page422"></a> +attraction of gravity, was also attacked by Leibnitz, “as subversive of +natural, and inferentially of revealed, religion.” A celebrated author +and divine has written to me that “he has gradually learned to see that +it is just as noble a conception of the Deity to believe that He created a few +original forms capable of self-development into other and needful forms, as to +believe that He required a fresh act of creation to supply the voids caused by +the action of His laws.” +</p> + +<p> +Why, it may be asked, until recently did nearly all the most eminent living +naturalists and geologists disbelieve in 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. +</p> + +<p> +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 changes of which we do not see the 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 agencies which we still see at work. The mind cannot possibly grasp the +full meaning of the term of even a million years; it cannot add up and perceive +the full effects of many slight variations, accumulated during an almost +infinite number of generations. +</p> + +<p> +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,” &c., 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 name="Page423"></a> +a certain number of facts will certainly reject the theory. A few naturalists, +endowed with much flexibility of mind, and who have already begun to doubt 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 thus only can the load of prejudice by which this subject is +overwhelmed be removed. +</p> + +<p> +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 all +the external characteristic features of true species—they admit that +these have been produced by variation, but they refuse to extend the same view +to other and 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? Undoubtedly some of these same questions cannot +be answered by those who believe in the appearance or creation of only a few +forms of life or of some one form alone. It has been maintained by several +authors that it is as easy to believe in the creation of a million beings as of +one; but Maupertuis’ philosophical axiom “of least action” +leads the mind more willingly to admit the smaller number; and certainly we +ought not to believe that innumerable beings within each great class have been +created with plain, but deceptive, marks of descent from a single parent. +</p> + +<p> +<a name="Page424"></a> +As a record of a former state of things, I have retained in the foregoing +paragraphs, and elsewhere, several sentences which imply that naturalists +believe in the separate creation of each species; and I have been much censured +for having thus expressed myself. But undoubtedly this was the general belief +when the first edition of the present work appeared. I formerly spoke to very +many naturalists on the subject of evolution, and never once met with any +sympathetic agreement. It is probable that some did then believe in evolution, +but they were either silent or expressed themselves so ambiguously that it was +not easy to understand their meaning. Now, things are wholly changed, and +almost every naturalist admits the great principle of evolution. There are, +however, some who still think that species have suddenly given birth, through +quite unexplained means, to new and totally different forms. But, as I have +attempted to show, weighty evidence can be opposed to the admission of great +and abrupt modifications. Under a scientific point of view, and as leading to +further investigation, but little advantage is gained by believing that new +forms are suddenly developed in an inexplicable manner from old and widely +different forms, over the old belief in the creation of species from the dust +of the earth. +</p> + +<p> +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 consider, by so much the arguments in favour of community of descent +become fewer in number and less in force. But some arguments of the greatest +weight extend very far. All the members of whole classes are connected together +by a chain of affinities, and all can be classed on the same principle, in +groups subordinate to groups. Fossil remains sometimes tend to fill up very +wide intervals between existing orders. +</p> + +<p> +Organs in a rudimentary condition plainly show that an early progenitor had the +organ in a fully developed condition, and this in some cases implies an +enormous amount of modification in the descendants. Throughout whole classes +various structures are formed on the same pattern, and at a very early age the +embryos closely resemble each other. Therefore I cannot doubt that the theory +of descent with modification embraces all the members of the same great class +or kingdom. I believe that animals are descended from at most only four or five +progenitors, and plants from an equal or lesser number. +</p> + +<p> +Analogy would lead me one step further, namely, to the belief that all animals +and plants are descended from some one prototype. But analogy may be a +deceitful guide. Nevertheless all living +<a name="Page425"></a> +things have much in common, in their chemical composition, their cellular +structure, their laws of growth, and their liability to injurious influences. +We see this even in so trifling a fact 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. With all organic +beings, excepting perhaps some of the very lowest, sexual reproduction seems to +be essentially similar. With all, as far as is at present known, the germinal +vesicle is the same; so that all organisms start from a common origin. If we +look even to the two main divisions—namely, to the animal and vegetable +kingdoms—certain low forms are so far intermediate in character that +naturalists have disputed to which kingdom they should be referred. As +Professor Asa Gray has remarked, “the spores and other reproductive +bodies of many of the lower algæ may claim to have first a characteristically +animal, and then an unequivocally vegetable existence.” Therefore, on the +principle of natural selection with divergence of character, it does not seem +incredible that, from some such low and intermediate form, both animals and +plants may have been developed; and, if we admit this, we must likewise admit +that all the organic beings which have ever lived on this earth may be +descended from some one primordial form. But this inference is chiefly grounded +on analogy, and it is immaterial whether or not it be accepted. No doubt it is +possible, as Mr. G.H. Lewes has urged, that at the first commencement of life +many different forms were evolved; but if so, we may conclude that only a very +few have left modified descendants. For, as I have recently remarked in regard +to the members of each great kingdom, such as the Vertebrata, Articulata, +&c., we have distinct evidence in their embryological, homologous, and +rudimentary structures, that within each kingdom all the members are descended +from a single progenitor. +</p> + +<p> +When the views advanced by me in this volume, and by Mr. Wallace or when +analogous views on the origin of species 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 a +true 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 good 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, +<a name="Page426"></a> +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. +</p> + +<p> +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 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; 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. +</p> + +<p> +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, &c., 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 something wholly beyond his comprehension; when we regard every +production of nature as one which has had a long history; when we contemplate +every complex structure and instinct as the summing up of many contrivances, +each useful to the possessor, in the same way as any great mechanical invention +is 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—does the study of natural +history become! +</p> + +<p> +A grand and almost untrodden field of inquiry will be opened, on the causes and +laws of variation, on correlation, 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 +<a name="Page427"></a> +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 pedigree 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 often reveal to us the +structure, in some degree obscured, of the prototypes of each great class. +</p> + +<p> +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 birth-place; +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 between 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. +</p> + +<p> +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 occurrence of favourable +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 do not include many identical species, by +the general succession of the forms of life. As species are produced and +exterminated by slowly acting and still existing causes, and not by miraculous +acts of creation; 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 +<a name="Page428"></a> +mutual relation of organism to organism—the improvement of one organism +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 relative, though not actual lapse of time. A +number of species, however, keeping in a body might remain for a long period +unchanged, whilst within the 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. +</p> + +<p> +In the future I see open fields for far more important researches. Psychology +will be securely based on the foundation already well laid by Mr. Herbert +Spencer, that of the necessary acquirement of each mental power and capacity by +gradation. Much light will be thrown on the origin of man and his history. +</p> + +<p> +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 Cambrian 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 distinct 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 in each genus, and all the species in many genera, have left +no descendants, but have become utterly extinct. We can so far take a prophetic +glance into futurity as to foretell that it will be the common and widely +spread species, belonging to the larger and dominant groups within each class, +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 Cambrian 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 +great 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. +</p> + +<p> +<a name="Page429"></a> +It is interesting to contemplate a tangled 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 upon 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 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 by the Creator into a few forms or into one; and that, +whilst this planet has gone circling 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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page430"></a><a name="link2H_GLOS" id="link2H_GLOS"></a> +GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.*</h2> + +<p class="footnote"> +* I am indebted to the kindness of Mr. W.S. Dallas for this Glossary, which has +been given because several readers have complained to me that some of the terms +used were unintelligible to them. Mr. Dallas has endeavoured to give the +explanations of the terms in as popular a form as possible. +</p> + +<p> +ABERRANT.—Forms or groups of animals or plants which deviate in important +characters from their nearest allies, so as not to be easily included in the +same group with them, are said to be aberrant. +</p> + +<p> +ABERRATION (in Optics).—In the refraction of light by a convex lens the +rays passing through different parts of the lens are brought to a focus at +slightly different distances—this is called <i>spherical aberration;</i> +at the same time the coloured rays are separated by the prismatic action of the +lens and likewise brought to a focus at different distances—this is +<i>chromatic aberration</i>. +</p> + +<p> +ABNORMAL.—Contrary to the general rule. +</p> + +<p> +ABORTED.—An organ is said to be aborted, when its development has been +arrested at a very early stage. +</p> + +<p> +ALBINISM.—Albinos are animals in which the usual colouring matters +characteristic of the species have not been produced in the skin and its +appendages. Albinism is the state of being an albino. +</p> + +<p> +ALGÆ.—A class of plants including the ordinary sea-weeds and the +filamentous fresh-water weeds. +</p> + +<p> +ALTERNATION OF GENERATIONS.—This term is applied to a peculiar mode of +reproduction which prevails among many of the lower animals, in which the egg +produces a living form quite different from its parent, but from which the +parent-form is reproduced by a process of budding, or by the division of the +substance of the first product of the egg. +</p> + +<p> +AMMONITES.—A group of fossil, spiral, chambered shells, allied to the +existing pearly Nautilus, but having the partitions between the chambers waved +in complicated patterns at their junction with the outer wall of the shell. +</p> + +<p> +ANALOGY.—That resemblance of structures which depends upon similarity of +function, as in the wings of insects and birds. Such structures are said to be +<i>analogous</i>, and to be <i>analogues</i> of each other. +</p> + +<p> +<a name="Page431"></a> +ANIMALCULE.—A minute animal: generally applied to those visible only by +the microscope. +</p> + +<p> +ANNELIDS.—A class of worms in which the surface of the body exhibits a +more or less distinct division into rings or segments, generally provided with +appendages for locomotion and with gills. It includes the ordinary marine +worms, the earth-worms, and the leeches. +</p> + +<p> +ANTENNÆ.—Jointed organs appended to the head in Insects, Crustacea and +Centipedes, and not belonging to the mouth. +</p> + +<p> +ANTHERS.—The summits of the stamens of flowers, in which the pollen or +fertilising dust is produced. +</p> + +<p> +APLACENTALIA, APLACENTATA or Aplacental Mammals.—See <i>mammalia</i>. +</p> + +<p> +ARCHETYPAL.—Of or belonging to the Archetype, or ideal primitive form +upon which all the beings of a group seem to be organised. +</p> + +<p> +ARTICULATA.—A great division of the Animal Kingdom characterised +generally by having the surface of the body divided into rings called segments, +a greater or less number of which are furnished with jointed legs (such as +Insects, Crustaceans and Centipedes). +</p> + +<p> +ASYMMETRICAL.—Having the two sides unlike. +</p> + +<p> +ATROPHIED.—Arrested in development at a very early stage. +</p> + +<p> +BALANUS.—The genus including the common Acorn-shells which live in +abundance on the rocks of the sea-coast. +</p> + +<p> +BATRACHIANS.—A class of animals allied to the Reptiles, but undergoing a +peculiar metamorphosis, in which the young animal is generally aquatic and +breathes by gills. (<i>Examples</i>, Frogs, Toads, and Newts.) +</p> + +<p> +BOULDERS.—Large transported blocks of stone generally embedded in clays +or gravels. +</p> + +<p> +BRACHIOPODA.—A class of marine Mollusca, or soft-bodied animals, +furnished with a bivalve shell, attached to submarine objects by a stalk which +passes through an aperture in one of the valves, and furnished with fringed +arms, by the action of which food is carried to the mouth. +</p> + +<p> +BRANCHIÆ.—Gills or organs for respiration in water. +</p> + +<p> +BRANCHIAL.—Pertaining to gills or branchiæ. +</p> + +<p> +CAMBRIAN SYSTEM.—A series of very ancient Palæozoic rocks, between the +Laurentian and the Silurian. Until recently these were regarded as the oldest +fossiliferous rocks. +</p> + +<p> +CANIDÆ.—The Dog-family, including the Dog, Wolf, Fox, Jackal, &c. +</p> + +<p> +CARAPACE.—The shell enveloping the anterior part of the body in +Crustaceans generally; applied also to the hard shelly pieces of the +Cirripedes. +</p> + +<p> +CARBONIFEROUS.—This term is applied to the great formation which +includes, among other rocks, the coal-measures. It belongs to the oldest, or +Palæozoic, system of formations. +</p> + +<p> +CAUDAL.—Of or belonging to the tail. +</p> + +<p> +CEPHALOPODS.—The highest class of the Mollusca, or soft-bodied animals, +characterised by having the mouth surrounded by a greater or less number of +fleshy arms or tentacles, which, in most living species, are furnished with +sucking-cups. (<i>Examples</i>, Cuttle-fish, Nautilus.) +</p> + +<p> +CETACEA.—An order of Mammalia, including the Whales, Dolphins, &c., +<a name="Page432"></a> +having the form of the body fish-like, the skin naked, and only the fore limbs +developed. +</p> + +<p> +CHELONIA.—An order of Reptiles including the Turtles, Tortoises, &c. +</p> + +<p> +CIRRIPEDES.—An order of Crustaceans including the Barnacles and +Acorn-shells. Their young resemble those of many other Crustaceans in form; but +when mature they are always attached to other objects, either directly or by +means of a stalk, and their bodies are enclosed by a calcareous shell composed +of several pieces, two of which can open to give issue to a bunch of curled, +jointed tentacles, which represent the limbs. +</p> + +<p> +COCCUS.—The genus of Insects including the Cochineal. In these the male +is a minute, winged fly, and the female generally a motionless, berry-like +mass. +</p> + +<p> +COCOON.—A case usually of silky material, in which insects are frequently +enveloped during the second or resting-stage (pupa) of their existence. The +term “cocoon-stage” is here used as equivalent to +“pupa-stage.” +</p> + +<p> +CŒLOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which +have the seed hollowed on the inner face. +</p> + +<p> +COLEOPTERA.—Beetles, an order of Insects, having a biting mouth and the +first pair of wings more or less horny, forming sheaths for the second pair, +and usually meeting in a straight line down the middle of the back. +</p> + +<p> +COLUMN.—A peculiar organ in the flowers of Orchids, in which the stamens, +style and stigma (or the reproductive parts) are united. +</p> + +<p> +COMPOSITÆ or COMPOSITOUS PLANTS.—Plants in which the inflorescence +consists of numerous small flowers (florets) brought together into a dense +head, the base of which is enclosed by a common envelope. (<i>Examples</i>, the +Daisy, Dandelion, &c.) +</p> + +<p> +CONFERVÆ.—The filamentous weeds of fresh water. +</p> + +<p> +CONGLOMERATE.—A rock made up of fragments of rock or pebbles, cemented +together by some other material. +</p> + +<p> +COROLLA.—The second envelope of a flower usually composed of coloured, +leaf-like organs (petals), which may be united by their edges either in the +basal part or throughout. +</p> + +<p> +CORRELATION.—The normal coincidence of one phenomenon, character, +&c., with another. +</p> + +<p> +CORYMB.—A bunch of flowers in which those springing from the lower part +of the flower stalks are supported on long stalks so as to be nearly on a level +with the upper ones. +</p> + +<p> +COTYLEDONS.—The first or seed-leaves of plants. +</p> + +<p> +CRUSTACEANS.—A class of articulated animals, having the skin of the body +generally more or less hardened by the deposition of calcareous matter, +breathing by means of gills. (<i>Examples</i>, Crab, Lobster, Shrimp, &c.) +</p> + +<p> +CURCULIO.—The old generic term for the Beetles known as Weevils, +characterised by their four-jointed feet, and by the head being produced into a +sort of beak, upon the sides of which the antennæ are inserted. +</p> + +<p> +CUTANEOUS.—Of or belonging to the skin. +</p> + +<p> +DEGRADATION.—The wearing down of land by the action of the sea or of +meteoric agencies. +</p> + +<p> +<a name="Page433"></a> +DENUDATION.—The wearing away of the surface of the land by water. +</p> + +<p> +DEVONIAN SYSTEM or FORMATION.—A series of Palæozoic rocks, including the +Old Red Sandstone. +</p> + +<p> +DICOTYLEDONS, or DICOTYLEDONOUS PLANTS.—A class of plants characterised +by having two seed-leaves, by the formation of new wood between the bark and +the old wood (exogenous growth) and by the reticulation of the veins of the +leaves. The parts of the flowers are generally in multiples of five. +</p> + +<p> +DIFFERENTATION.—The separation or discrimination of parts or organs which +in simpler forms of life are more or less united. +</p> + +<p> +DIMORPHIC.—Having two distinct forms.—DIMORPHISM is the condition +of the appearance of the same species under two dissimilar forms. +</p> + +<p> +DIOECIOUS.—Having the organs of the sexes upon distinct individuals. +</p> + +<p> +DIORITE.—A peculiar form of Greenstone. +</p> + +<p> +DORSAL.—Of or belonging to the back. +</p> + +<p> +EDENTATA.—A peculiar order of Quadrupeds, characterised by the absence of +at least the middle incisor (front) teeth in both jaws. (<i>Examples</i>, the +Sloths and Armadillos.) +</p> + +<p> +ELYTRA.—The hardened fore-wings of Beetles, serving as sheaths for the +membranous hind-wings, which constitute the true organs of flight. +</p> + +<p> +EMBRYO.—The young animal undergoing development within the egg or womb. +</p> + +<p> +EMBRYOLOGY.—The study of the development of the embryo. +</p> + +<p> +ENDEMIC.—Peculiar to a given locality. +</p> + +<p> +ENTOMOSTRACA.—A division of the class Crustacea, having all the segments +of the body usually distinct, gills attached to the feet or organs of the +mouth, and the feet fringed with fine hairs. They are generally of small size. +</p> + +<p> +EOCENE.—The earliest of the three divisions of the Tertiary epoch of +geologists. Rocks of this age contain a small proportion of shells identical +with species now living. +</p> + +<p> +EPHEMEROUS INSECTS.—Insects allied to the May-fly. +</p> + +<p> +FAUNA.—The totality of the animals naturally inhabiting a certain country +or region, or which have lived during a given geological period. +</p> + +<p> +FELIDÆ.—The Cat-family. +</p> + +<p> +FERAL.—Having become wild from a state of cultivation or domestication. +</p> + +<p> +FLORA.—The totality of the plants growing naturally in a country, or +during a given geological period. +</p> + +<p> +FLORETS.—Flowers imperfectly developed in some respects, and collected +into a dense spike or head, as in the Grasses, the Dandelion, &c. +</p> + +<p> +FOETAL.—Of or belonging to the foetus, or embryo in course of +development. +</p> + +<p> +FORAMINIFERA.—A class of animals of very low organisation and generally +of small size, having a jelly-like body, from the surface of which delicate +filaments can be given off and retracted for the prehension of external +objects, and having a calcareous or sandy shell, usually divided into chambers +and perforated with small apertures. +</p> + +<p> +<a name="Page434"></a> +FOSSILIFEROUS.—Containing fossils. +</p> + +<p> +FOSSORIAL.—Having a faculty of digging. The Fossorial Hymenoptera are a +group of Wasp-like Insects, which burrow in sandy soil to make nests for their +young. +</p> + +<p> +FRENUM (pl. FRENA).—A small band or fold of skin. +</p> + +<p> +FUNGI (sing. FUNGUS).—A class of cellular plants, of which Mushrooms, +Toadstools, and Moulds, are familiar examples. +</p> + +<p> +FURCULA.—The forked bone formed by the union of the collar-bones in many +birds, such as the common Fowl. +</p> + +<p> +GALLINACEOUS BIRDS.—An order of birds of which the common Fowl, Turkey, +and Pheasant, are well-known examples. +</p> + +<p> +GALLUS.—The genus of birds which includes the common Fowl. +</p> + +<p> +GANGLION.—A swelling or knot from which nerves are given off as from a +centre. +</p> + +<p> +GANOID FISHES.—Fishes covered with peculiar enamelled bony scales. Most +of them are extinct. +</p> + +<p> +GERMINAL VESICLE.—A minute vesicle in the eggs of animals, from which the +development of the embryo proceeds. +</p> + +<p> +GLACIAL PERIOD.—A period of great cold and of enormous extension of ice +upon the surface of the earth. It is believed that glacial periods have +occurred repeatedly during the geological history of the earth, but the term is +generally applied to the close of the Tertiary epoch, when nearly the whole of +Europe was subjected to an arctic climate. +</p> + +<p> +GLAND.—An organ which secretes or separates some peculiar product from +the blood or sap of animals or plants. +</p> + +<p> +GLOTTIS.—The opening of the windpipe into the œsophagus or gullet. +</p> + +<p> +GNEISS.—A rock approaching granite in composition, but more or less +laminated, and really produced by the alteration of a sedimentary deposit after +its consolidation. +</p> + +<p> +GRALLATORES.—The so-called wading-birds (storks, cranes, snipes, +&c.), which are generally furnished with long legs, bare of feathers above +the heel, and have no membranes between the toes. +</p> + +<p> +GRANITE.—A rock consisting essentially of crystals of felspar and mica in +a mass of quartz. +</p> + +<p> +HABITAT.—The locality in which a plant or animal naturally lives. +</p> + +<p> +HEMIPTERA.—An order or sub-order of insects, characterised by the +possession of a jointed beak or rostrum, and by having the fore-wings horny in +the basal portion and membranous at the extremity, where they cross each other. +This group includes the various species of bugs. +</p> + +<p> +HERMAPHRODITE.—Possessing the organs of both sexes. +</p> + +<p> +HOMOLOGY.—That relation between parts which results from their +development from corresponding embryonic parts, either in different animals, as +in the case of the arm of man, the fore-leg of a quadruped, and the wing of a +bird; or in the same individual, as in the case of the fore and hind legs in +quadrupeds, and the segments or rings and their appendages of which the body of +a worm, a centipede, &c., is composed. The latter is called <i>serial +homology</i>. The parts which stand in such a relation to each other are said +to be <i>homologous</i>, and one such part or organ is +<a name="Page435"></a> +called the <i>homologue</i> of the other. In different plants the parts +of the flower are homologous, and in general these parts are regarded +as homologous with leaves. +</p> + +<p> +HOMOPTERA.—An order or sub-order of insects having (like the Hemiptera) a +jointed beak, but in which the fore-wings are either wholly membranous or +wholly leathery, The <i>Cicadæ</i>, frog-hoppers, and <i>Aphides</i>, are +well-known examples. +</p> + +<p> +HYBRID.—The offspring of the union of two distinct species. +</p> + +<p> +HYMENOPTERA.—An order of insects possessing biting jaws and usually four +membranous wings in which there are a few veins. Bees and wasps are familiar +examples of this group. +</p> + +<p> +HYPERTROPHIED.—Excessively developed. +</p> + +<p> +ICHNEUMONIDÆ.—A family of hymenopterous insects, the members of which lay +their eggs in the bodies or eggs of other insects. +</p> + +<p> +IMAGO.—The perfect (generally winged) reproductive state of an insect. +</p> + +<p> +INDIGENES.—The aboriginal animal or vegetable inhabitants of a country or +region. +</p> + +<p> +INFLORESCENCE.—The mode of arrangement of the flowers of plants. +</p> + +<p> +INFUSORIA.—A class of microscopic animalcules, so called from their +having originally been observed in infusions of vegetable matters. They consist +of a gelatinous material enclosed in a delicate membrane, the whole or part of +which is furnished with short vibrating hairs (called cilia), by means of which +the animalcules swim through the water or convey the minute particles of their +food to the orifice of the mouth. +</p> + +<p> +INSECTIVOROUS.—Feeding on insects. +</p> + +<p> +INVERTEBRATA, or INVERTEBRATE ANIMALS.—Those animals which do not possess +a backbone or spinal column. +</p> + +<p> +LACUNÆ.—Spaces left among the tissues in some of the lower animals and +serving in place of vessels for the circulation of the fluids of the body. +</p> + +<p> +LAMELLATED.—Furnished with lamellæ or little plates. +</p> + +<p> +LARVA (pl. LARVÆ).—The first condition of an insect at its issuing from +the egg, when it is usually in the form of a grub, caterpillar, or maggot. +</p> + +<p> +LARYNX.—The upper part of the windpipe opening into the gullet. +</p> + +<p> +LAURENTIAN.—A group of greatly altered and very ancient rocks, which is +greatly developed along the course of the St. Laurence, whence the name. It is +in these that the earliest known traces of organic bodies have been found. +</p> + +<p> +LEGUMINOSÆ.—An order of plants represented by the common peas and beans, +having an irregular flower in which one petal stands up like a wing, and the +stamens and pistil are enclosed in a sheath formed by two other petals. The +fruit is a pod (or legume). +</p> + +<p> +LEMURIDÆ.—A group of four-handed animals, distinct from the monkeys and +approaching the insectivorous quadrupeds in some of their characters and +habits. Its members have the nostrils curved or twisted, and a claw instead of +a nail upon the first finger of the hind hands. +</p> + +<p> +LEPIDOPTERA.—An order of insects, characterised by the possession of a +spiral proboscis, and of four large more or less scaly wings. It includes the +well-known butterflies and moths. +</p> + +<p> +<a name="Page436"></a> +LITTORAL.—Inhabiting the seashore. +</p> + +<p> +LOESS.—A marly deposit of recent (Post-Tertiary) date, which occupies a +great part of the valley of the Rhine. +</p> + +<p> +MALACOSTRACA.—The higher division of the Crustacea, including the +ordinary crabs, lobsters, shrimps, &c., together with the woodlice and +sand-hoppers. +</p> + +<p> +MAMMALIA.—The highest class of animals, including the ordinary hairy +quadrupeds, the whales and man, and characterised by the production of living +young which are nourished after birth by milk from the teats (<i>Mammæ</i>, +<i>Mammary glands</i>) of the mother. A striking difference in embryonic +development has led to the division of this class into two great groups; in one +of these, when the embryo has attained a certain stage, a vascular connection, +called the <i>placenta</i>, is formed between the embryo and the mother; in the +other this is wanting, and the young are produced in a very incomplete state. +The former, including the greater part of the class, are called <i>Placental +Mammals;</i> the latter, or <i>Aplacental Mammals</i>, include the Marsupials +and Monotremes (<i>Ornithorhynchus</i>). +</p> + +<p> +MAMMIFEROUS.—Having mammæ or teats (see M<small>AMMALIA</small>). +</p> + +<p> +MANDIBLES.—in insects, the first or uppermost pair of jaws, which are +generally solid, horny, biting organs. In birds the term is applied to both +jaws with their horny coverings. In quadrupeds the mandible is properly the +lower jaw. +</p> + +<p> +MARSUPIALS.—An order of Mammalia in which the young are born in a very +incomplete state of development, and carried by the mother, while sucking, in a +ventral pouch (marsupium), such as the kangaroos, opossums, &c. (see +MAMMALIA). +</p> + +<p> +MAXILLÆ.—in insects, the second or lower pair of jaws, which are composed +of several joints and furnished with peculiar jointed appendages called palpi, +or feelers. +</p> + +<p> +MELANISM.—The opposite of albinism; an undue development of colouring +material in the skin and its appendages. +</p> + +<p> +METAMORPHIC ROCKS.—Sedimentary rocks which have undergone alteration, +generally by the action of heat, subsequently to their deposition and +consolidation. +</p> + +<p> +MOLLUSCA.—One of the great divisions of the animal kingdom, including +those animals which have a soft body, usually furnished with a shell, and in +which the nervous ganglia, or centres, present no definite general arrangement. +They are generally known under the denomination of “shellfish”; the +cuttle-fish, and the common snails, whelks, oysters, mussels, and cockles, may +serve as examples of them. +</p> + +<p> +MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS.—Plants in which the seed +sends up only a single seed-leaf (or cotyledon); characterised by the absence +of consecutive layers of wood in the stem (endogenous growth), by the veins of +the leaves being generally straight, and by the parts of the flowers being +generally in multiples of three. (<i>Examples</i>, Grasses, Lilies, Orchids, +Palms, &c.) +</p> + +<p> +MORAINES.—The accumulations of fragments of rock brought down by +glaciers. +</p> + +<p> +MORPHOLOGY.—The law of form or structure independent of function. +</p> + +<p> +<a name="Page437"></a> +MYSIS-STAGE.—A stage in the development of certain crustaceans (prawns), +in which they closely resemble the adults of a genus (<i>Mysis</i>) belonging +to a slightly lower group. +</p> + +<p> +NASCENT.—Commencing development. +</p> + +<p> +NATATORY.—Adapted for the purpose of swimming. +</p> + +<p> +NAUPLIUS-FORM.—The earliest stage in the development of many Crustacea, +especially belonging to the lower groups. In this stage the animal has a short +body, with indistinct indications of a division into segments, and three pairs +of fringed limbs. This form of the common fresh-water <i>Cyclops</i> was +described as a distinct genus under the name of <i>Nauplius</i>. +</p> + +<p> +NEURATION.—The arrangement of the veins or nervures in the wings of +insects. +</p> + +<p> +NEUTERS.—Imperfectly developed females of certain social insects (such as +ants and bees), which perform all the labours of the community. Hence, they are +also called <i>workers</i>. +</p> + +<p> +NICTITATING MEMBRANE.—A semi-transparent membrane, which can be drawn +across the eye in birds and reptiles, either to moderate the effects of a +strong light or to sweep particles of dust, &c., from the surface of the +eye. +</p> + +<p> +OCELLI.—The simple eyes or stemmata of insects, usually situated on the +crown of the head between the great compound eyes. +</p> + +<p> +ŒSOPHAGUS.—The gullet. +</p> + +<p> +OOLITIC.—A great series of secondary rocks, so called from the texture of +some of its members, which appear to be made up of a mass of small EGG-LIKE +calcareous bodies. +</p> + +<p> +OPERCULUM.—A calcareous plate employed by many Molluscæ to close the +aperture of their shell. The OPERCULAR VALVES of Cirripedes are those which +close the aperture of the shell. +</p> + +<p> +ORBIT.—The bony cavity for the reception of the eye. +</p> + +<p> +ORGANISM.—An organised being, whether plant or animal. +</p> + +<p> +ORTHOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which +have the seed straight. +</p> + +<p> +OSCULANT.—Forms or groups apparently intermediate between and connecting +other groups are said to be osculant. +</p> + +<p> +OVA.—Eggs. +</p> + +<p> +OVARIUM or OVARY (in plants).—The lower part of the pistil or female +organ of the flower, containing the ovules or incipient seeds; by growth after +the other organs of the flower have fallen, it usually becomes converted into +the fruit. +</p> + +<p> +OVIGEROUS.—Egg-bearing. +</p> + +<p> +OVULES (of plants).—The seeds in the earliest condition. +</p> + +<p> +PACHYDERMS.—A group of Mammalia, so called from their thick skins, and +including the elephant, rhinoceros, hippopotamus, &c. +</p> + +<p> +PALÆOZOIC.—The oldest system of fossiliferous rocks. +</p> + +<p> +PALPI.—Jointed appendages to some of the organs of the mouth in insects +and Crustacea. +</p> + +<p> +<a name="Page438"></a> +PAPILIONACEÆ.—An order of plants (see LEGUMINOSÆ), The flowers of these +plants are called <i>papilionaceous</i>, or butterfly-like, from the fancied +resemblance of the expanded superior petals to the wings of a butterfly. +</p> + +<p> +PARASITE.—An animal or plant living upon or in, and at the expense of, +another organism. +</p> + +<p> +PARTHENOGENESIS.—The production of living organisms from unimpregnated +eggs or seeds. +</p> + +<p> +PEDUNCULATED.—Supported upon a stem or stalk. The pedunculated oak has +its acorns borne upon a footstool. +</p> + +<p> +PELORIA or PELORISM.—The appearance of regularity of structure in the +flowers of plants which normally bear irregular flowers. +</p> + +<p> +PELVIS.—The bony arch to which the hind limbs of vertebrate animals are +articulated. +</p> + +<p> +PETALS.—The leaves of the corolla, or second circle of organs in a +flower. They are usually of delicate texture and brightly coloured. +</p> + +<p> +PHYLLODINEOUS.—Having flattened, leaf-like twigs or leafstalks instead of +true leaves. +</p> + +<p> +PIGMENT.—The colouring material produced generally in the superficial +parts of animals. The cells secreting it are called <i>pigment-cells</i>. +</p> + +<p> +PINNATE.—Bearing leaflets on each side of a central stalk. +</p> + +<p> +PISTILS.—The female organs of a flower, which occupy a position in the +centre of the other floral organs. The pistil is generally divisible into the +ovary or germen, the style and the stigma. +</p> + +<p> +PLACENTALIA, PLACENTATA.—or PLACENTAL MAMMALS, See MAMMALIA. +</p> + +<p> +PLANTIGRADES.—Quadrupeds which walk upon the whole sole of the foot, like +the bears. +</p> + +<p> +PLASTIC.—Readily capable of change. +</p> + +<p> +PLEISTOCENE PERIOD.—The latest portion of the Tertiary epoch. +</p> + +<p> +PLUMULE (in plants).—The minute bud between the seed-leaves of +newly-germinated plants. +</p> + +<p> +PLUTONIC ROCKS.—Rocks supposed to have been produced by igneous action in +the depths of the earth. +</p> + +<p> +POLLEN.—The male element in flowering plants; usually a fine dust +produced by the anthers, which, by contact with the stigma effects the +fecundation of the seeds. This impregnation is brought about by means of tubes +(<i>pollen-tubes</i>) which issue from the pollen-grains adhering to the +stigma, and penetrate through the tissues until they reach the ovary. +</p> + +<p> +POLYANDROUS (flowers).—Flowers having many stamens. +</p> + +<p> +POLYGAMOUS PLANTS.—Plants in which some flowers are unisexual and others +hermaphrodite. The unisexual (male and female) flowers, may be on the same or +on different plants. +</p> + +<p> +POLYMORPHIC.—Presenting many forms. +</p> + +<p> +POLYZOARY.—The common structure formed by the cells of the Polyzoa, such +as the well-known seamats. +</p> + +<p> +PREHENSILE.—Capable of grasping. +</p> + +<p> +PREPOTENT.—Having a superiority of power. +</p> + +<p> +<a name="Page439"></a> +PRIMARIES.—The feathers forming the tip of the wing of a bird, and +inserted upon that part which represents the hand of man. +</p> + +<p> +PROCESSES.—Projecting portions of bones, usually for the attachment of +muscles, ligaments, &c. +</p> + +<p> +PROPOLIS.—A resinous material collected by the hivebees from the opening +buds of various trees. +</p> + +<p> +PROTEAN.—Exceedingly variable. +</p> + +<p> +PROTOZOA.—The lowest great division of the animal kingdom. These animals +are composed of a gelatinous material, and show scarcely any trace of distinct +organs. The Infusoria, Foraminifera, and sponges, with some other forms, belong +to this division. +</p> + +<p> +PUPA (pl. PUPÆ).—The second stage in the development of an insect, from +which it emerges in the perfect (winged) reproductive form. In most insects the +<i>pupal stage</i> is passed in perfect repose. The <i>chrysalis</i> is the +pupal state of butterflies. +</p> + +<p> +RADICLE.—The minute root of an embryo plant. +</p> + +<p> +RAMUS.—One half of the lower jaw in the Mammalia. The portion which rises +to articulate with the skull is called the <i>ascending ramus</i>. +</p> + +<p> +RANGE.—The extent of country over which a plant or animal is naturally +spread. <i>Range in time</i> expresses the distribution of a species or group +through the fossiliferous beds of the earth’s crust. +</p> + +<p> +RETINA.—The delicate inner coat of the eye, formed by nervous filaments +spreading from the optic nerve, and serving for the perception of the +impressions produced by light. +</p> + +<p> +RETROGRESSION.—Backward development. When an animal, as it approaches +maturity, becomes less perfectly organised than might be expected from its +early stages and known relationships, it is said to undergo a <i>retrogade +development</i> or <i>metamorphosis</i>. +</p> + +<p> +RHIZOPODS.—A class of lowly organised animals (Protozoa), having a +gelatinous body, the surface of which can be protruded in the form of root-like +processes or filaments, which serve for locomotion and the prehension of food. +The most important order is that of the Foraminifera. +</p> + +<p> +RODENTS.—The gnawing Mammalia, such as the rats, rabbits, and squirrels. +They are especially characterised by the possession of a single pair of +chisel-like cutting teeth in each jaw, between which and the grinding teeth +there is a great gap. +</p> + +<p> +RUBUS.—The bramble genus. +</p> + +<p> +RUDIMENTARY.—Very imperfectly developed. +</p> + +<p> +RUMINANTS.—The group of quadrupeds which ruminate or chew the cud, such +as oxen, sheep, and deer. They have divided hoofs, and are destitute of front +teeth in the upper jaw. +</p> + +<p> +SACRAL.—Belonging to the sacrum, or the bone composed usually of two or +more united vertebræ to which the sides of the pelvis in vertebrate animals are +attached. +</p> + +<p> +SARCODE.—The gelatinous material of which the bodies of the lowest +animals (Protozoa) are composed. +</p> + +<p> +SCUTELLÆ.—The horny plates with which the feet of birds are generally +more or less covered, especially in front. +</p> + +<p> +SEDIMENTARY FORMATIONS.—Rocks deposited as sediments from water. +</p> + +<p> +<a name="Page440"></a> +SEGMENTS.—The transverse rings of which the body of an articulate animal +or annelid is composed. +</p> + +<p> +SEPALS.—The leaves or segments of the calyx, or outermost envelope of an +ordinary flower. They are usually green, but sometimes brightly coloured. +</p> + +<p> +SERRATURES.—Teeth like those of a saw. +</p> + +<p> +SESSILE.—Not supported on a stem or footstalk. +</p> + +<p> +SILURIAN SYSTEM.—A very ancient system of fossiliferous rocks belonging +to the earlier part of the Palæozoic series. +</p> + +<p> +SPECIALISATION.—The setting apart of a particular organ for the +performance of a particular function. +</p> + +<p> +SPINAL CORD.—The central portion of the nervous system in the Vertebrata, +which descends from the brain through the arches of the vertebræ, and gives off +nearly all the nerves to the various organs of the body. +</p> + +<p> +STAMENS.—The male organs of flowering plants, standing in a circle within +the petals. They usually consist of a filament and an anther, the anther being +the essential part in which the pollen, or fecundating dust, is formed. +</p> + +<p> +STERNUM.—The breast-bone. +</p> + +<p> +STIGMA.—The apical portion of the pistil in flowering plants. +</p> + +<p> +STIPULES.—Small leafy organs placed at the base of the footstalks of the +leaves in many plants. +</p> + +<p> +STYLE.—The middle portion of the perfect pistil, which rises like a +column from the ovary and supports the stigma at its summit. +</p> + +<p> +SUBCUTANEOUS.—Situated beneath the skin. +</p> + +<p> +SUCTORIAL.—Adapted for sucking. +</p> + +<p> +SUTURES (in the skull).—The lines of junction of the bones of which the +skull is composed. +</p> + +<p> +TARSUS (pl. TARSI).—The jointed feet of articulate animals, such as +insects. +</p> + +<p> +TELEOSTEAN FISHES.—Fishes of the kind familiar to us in the present day, +having the skeleton usually completely ossified and the scales horny. +</p> + +<p> +TENTACULA or TENTACLES.—Delicate fleshy organs of prehension or touch +possessed by many of the lower animals. +</p> + +<p> +TERTIARY.—The latest geological epoch, immediately preceding the +establishment of the present order of things. +</p> + +<p> +TRACHEA.—The windpipe or passage for the admission of air to the lungs. +</p> + +<p> +TRIDACTYLE.—Three-fingered, or composed of three movable parts attached +to a common base. +</p> + +<p> +TRILOBITES.—A peculiar group of extinct crustaceans, somewhat resembling +the woodlice in external form, and, like some of them, capable of rolling +themselves up into a ball. Their remains are found only in the Palæozoic rocks, +and most abundantly in those of Silurian age. +</p> + +<p> +TRIMORPHIC.—Presenting three distinct forms. +</p> + +<p> +UMBELLIFERÆ.—An order of plants in which the flowers, which contain five +stamens and a pistil with two styles, are supported upon footstalks which +spring from the top of the flower stem and spread out like the wires of an +umbrella, so as to bring all the flowers in the same head (<i>umbel</i>) nearly +to the same level. (<i>Examples</i>, Parsley and Carrot.) +</p> + +<p> +<a name="Page441"></a> +UNGULATA.—Hoofed quadrupeds. +</p> + +<p> +UNICELLULAR.—Consisting of a single cell. +</p> + +<p> +VASCULAR.—Containing blood-vessels. +</p> + +<p> +VERMIFORM.—Like a worm. +</p> + +<p> +VERTEBRATA or VERTEBRATE ANIMALS.—The highest division of the animal +kingdom, so called from the presence in most cases of a backbone composed of +numerous joints or <i>vertebræ</i>, which constitutes the centre of the +skeleton and at the same time supports and protects the central parts of the +nervous system. +</p> + +<p> +WHORLS.—The circles or spiral lines in which the parts of plants are +arranged upon the axis of growth. +</p> + +<p> +WORKERS.—See neuters. +</p> + +<p> +ZOËA-STAGE.—The earliest stage in the development of many of the higher +Crustacea, so called from the name of <i>Zoëa</i> applied to these young +animals when they were supposed to constitute a peculiar genus. +</p> + +<p> +ZOOIDS.—In many of the lower animals (such as the Corals, Medusæ, +&c.) reproduction takes place in two ways, namely, by means of eggs and by +a process of budding with or without separation from the parent of the product +of the latter, which is often very different from that of the egg. The +individuality of the species is represented by the whole of the form produced +between two sexual reproductions; and these forms, which are apparently +individual animals, have been called <i>zooids</i>.<a name="Page442"></a> +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page443"></a><a name="link2H_4_0029" id="link2H_4_0029"></a>INDEX.</h2> + +<p class="noindent"><br/> +Aberrant groups, <a href="#Page379">379</a>.<br/> +<br/> +Abyssinia, plants of, <a href="#Page340">340</a>.<br/> +<br/> +Acclimatisation, <a href="#Page112">112</a>.<br/> +<br/> +Adoxa, <a href="#Page173">173</a>.<br/> +<br/> +Affinities of extinct species, <a href="#Page301">301</a>.<br/> +—, of organic beings, <a href="#Page378">378</a>.<br/> +<br/> +Agassiz on Amblyopsis, <a href="#Page112">112</a>.<br/> +—, on groups of species suddenly appearing, <a href="#Page289">289</a>.<br/> +—, on prophetic forms, <a href="#Page301">301</a>.<br/> +—, on embryological succession, <a href="#Page310">310</a>.<br/> +—, on the Glacial period, <a href="#Page330">330</a>.<br/> +—, on embryological characters, <a href="#Page368">368</a>.<br/> +—, on the latest tertiary forms, <a href="#Page278">278</a>.<br/> +—, on parallelism of embryological development and geological +succession, <a href="#Page396">396</a>.<br/> +—, Alex., on pedicellariæ, <a href="#Page191">191</a>.<br/> +<br/> +Algæ of New Zealand, <a href="#Page338">338</a>.<br/> +<br/> +Alligators, males, fighting, <a href="#Page69">69</a>.<br/> +<br/> +Alternate generations, <a href="#Page387">387</a>.<br/> +<br/> +Amblyopsis, blind fish, <a href="#Page112">112</a>.<br/> +<br/> +America, North, productions allied to those of Europe, <a href="#Page333">333</a>.<br/> +—, boulders and glaciers of, <a href="#Page335">335</a>.<br/> +—, South, no modern formations on west coast, <a href="#Page272">272</a>.<br/> +<br/> +Ammonites, sudden extinction of, <a href="#Page297">297</a>.<br/> +<br/> +Anagallis, sterility of, <a href="#Page236">236</a>.<br/> +<br/> +Analogy of variations, <a href="#Page127">127</a>.<br/> +<br/> +Andaman Islands inhabited by a toad, <a href="#Page350">350</a>.<br/> +<br/> +Ancylus, <a href="#Page345">345</a>.<br/> +<br/> +Animals, not domesticated from being variable, <a href="#Page13">13</a>.<br/> +—, domestic; descended from several stocks, <a href="#Page14">14</a>.<br/> +—, acclimatisation of, <a href="#Page112">112</a>.<br/> +<br/> +Animals of Australia, <a href="#Page90">90</a>.<br/> +—, with thicker fur in cold climates, <a href="#Page107">107</a>.<br/> +—, blind, in caves, <a href="#Page110">110</a>.<br/> +—, extinct, of Australia, <a href="#Page310">310</a>.<br/> +<br/> +Anomma, <a href="#Page232">232</a>.<br/> +<br/> +Antarctic islands, ancient flora of, <a href="#Page355">355</a>.<br/> +<br/> +Antechinus, <a href="#Page373">373</a>.<br/> +<br/> +Ants attending aphides, <a href="#Page207">207</a>.<br/> +—, slave-making instinct, <a href="#Page217">217</a>.<br/> +—, neuters, structure of, <a href="#Page230">230</a>.<br/> +<br/> +Apes, not having acquired intellectual powers, <a href="#Page181">181</a>.<br/> +<br/> +Aphides attended by ants, <a href="#Page207">207</a>.<br/> +<br/> +Aphis, development of, <a href="#Page390">390</a>.<br/> +<br/> +Apteryx, <a href="#Page140">140</a>.<br/> +<br/> +Arab horses, <a href="#Page26">26</a>.<br/> +<br/> +Aralo-Caspian Sea, <a href="#Page311">311</a>.<br/> +<br/> +Archeopteryx, <a href="#Page284">284</a>.<br/> +<br/> +Archiac, M. de, on the succession of species, <a href="#Page299">299</a>.<br/> +<br/> +Artichoke, Jerusalem, <a href="#Page114">114</a>.<br/> +<br/> +Ascension, plants of, <a href="#Page347">347</a>.<br/> +<br/> +Asclepias, pollen of, <a href="#Page151">151</a>.<br/> +<br/> +Asparagus, <a href="#Page325">325</a>.<br/> +<br/> +Aspicarpa, <a href="#Page367">367</a>.<br/> +<br/> +Asses, striped, <a href="#Page127">127</a>.<br/> +—, improved by selection, <a href="#Page30">30</a>.<br/> +<br/> +Ateuchus, <a href="#Page109">109</a>.<br/> +<br/> +Aucapitaine, on land-shells, <a href="#Page353">353</a>.<br/> +<br/> +Audubon, on habits of frigate-bird, <a href="#Page142">142</a>.<br/> +—, on variation in birds’ nests, <a href="#Page208">208</a>.<br/> +—, on heron eating seeds, <a href="#Page346">346</a>.<br/> +<br/> +Australia, animals of, <a href="#Page90">90</a>.<br/> +—, dogs of, <a href="#Page211">211</a>.<br/> +—, extinct animals of, <a href="#Page310">310</a>.<br/> +—, European plants in, <a href="#Page337">337</a>.<br/> +—, glaciers of, <a href="#Page335">335</a>.<br/> +<br/> +Azara, on flies destroying cattle, <a href="#Page56">56</a>.<br/> +<br/> +Azores, flora of, <a href="#Page328">328</a>.<br/> +<br/> +Babington, Mr., on British plants, <a href="#Page37">37</a>.<br/> +<br/> +Baer, Von, standard of Highness, <a href="#Page97">97</a>.<br/> +—, comparison of bee and fish, <a href="#Page308">308</a>.<br/> +—, embryonic similarity of the Vertebrata, <a href="#Page387">387</a>.<br/> +<br/> +Baker, Sir S., on the giraffe, <a href="#Page178">178</a>.<br/> +<br/> +Balancement of growth, <a href="#Page117">117</a>.<br/> +<br/> +Baleen, <a href="#Page182">182</a>.<br/> +<br/> +Barberry, flowers of, <a href="#Page77">77</a>.<br/> +<br/> +Barrande, M., on Silurian colonies, <a href="#Page291">291</a>.<br/> +—, on the succession of species, <a href="#Page299">299</a>.<br/> +—, on parallelism of palæozoic formations, <a href="#Page301">301</a>.<br/> +—, on affinities of ancient species, <a href="#Page302">302</a>.<br/> +<br/> +Barriers, importance of, <a href="#Page317">317</a>.<br/> +<br/> +Bates, Mr., on mimetic butterflies, <a href="#Page375">375</a>, <a href="#Page376">376</a>.<br/> +<br/> +Batrachians on islands, <a href="#Page350">350</a>.<br/> +<br/> +Bats, how structure acquired, <a href="#Page140">140</a>.<br/> +—, distribution of, <a href="#Page351">351</a>.<br/> +<br/> +Bear, catching water-insects, <a href="#Page141">141</a>.<br/> +<br/> +Beauty, how acquired, <a href="#Page159">159</a>, <a href="#Page414">414</a>.<br/> +<br/> +Bee, sting of, <a href="#Page163">163</a>.<br/> +—, queen, killing rivals, <a href="#Page164">164</a>.<br/> +—, Australian, extermination of, <a href="#Page59">59</a>.<br/> +<br/> +Bees, fertilizing flowers, <a href="#Page57">57</a>.<br/> +—, hive, not sucking the red clover, <a href="#Page75">75</a>.<br/> +—, Ligurian, <a href="#Page75">75</a>.<br/> +—, hive, cell-making instinct, <a href="#Page220">220</a>.<br/> +—, variation in habits, <a href="#Page208">208</a>.<br/> +—, parasitic, <a href="#Page216">216</a>.<br/> +—, humble, cells of, <a href="#Page220">220</a>.<br/> +<br/> +Beetles, wingless, in Madeira, <a href="#Page109">109</a>.<br/> +—, with deficient tarsi, <a href="#Page109">109</a>.<br/> +<br/> +Bentham, Mr., on British plants, <a href="#Page37">37</a>.<br/> +—, on classification, <a href="#Page369">369</a>.<br/> +<br/> +Berkeley, Mr., on seeds in salt-water, <a href="#Page324">324</a>.<br/> +<br/> +Bermuda, birds of, <a href="#Page348">348</a>.<br/> +<br/> +Birds acquiring fear, <a href="#Page208">208</a>.<br/> +—, beauty of, <a href="#Page161">161</a>.<br/> +—, annually cross the Atlantic, <a href="#Page329">329</a>.<br/> +—, colour of, on continents, <a href="#Page107">107</a>.<br/> +—, footsteps, and remains of, in secondary rocks, <a href="#Page284">284</a>.<br/> +—, fossil, in caves of Brazil, <a href="#Page310">310</a>.<br/> +—, of Madeira, Bermuda, and Galapagos, <a href="#Page348">349</a>, <a href="#Page349">349</a>.<br/> +—, song of males, <a href="#Page70">70</a>.<br/> +—, transporting seeds, <a href="#Page328">328</a>.<br/> +—, waders, <a href="#Page345">345</a>.<br/> +—, wingless, <a href="#Page108">108</a>, <a href="#Page140">140</a>.<br/> +<br/> +Bizcacha, <a href="#Page318">318</a>.<br/> +—, , affinities of, <a href="#Page379">379</a>.<br/> +<br/> +Bladder for swimming, in fish, <a href="#Page147">147</a>.<br/> +<br/> +Blindness of cave animals, <a href="#Page110">110</a>.<br/> +<br/> +Blyth, Mr., on distinctness of Indian cattle, <a href="#Page14">14</a>.<br/> +—, on striped Hemionus, <a href="#Page128">128</a>.<br/> +—, on crossed geese, <a href="#Page240">240</a>.<br/> +<br/> +Borrow, Mr., on the Spanish pointer, <a href="#Page26">26</a>.<br/> +<br/> +Bory St. Vincent, on Batrachians, <a href="#Page350">350</a>.<br/> +<br/> +Bosquet, M., on fossil Chthamalus, <a href="#Page284">284</a>.<br/> +<br/> +Boulders, erratic, on the Azores, <a href="#Page328">328</a>.<br/> +<br/> +Branchiæ, <a href="#Page148">148</a>, <a href="#Page149">149</a>.<br/> +—, of crustaceans, <a href="#Page152">152</a>.<br/> +<br/> +Braun, Prof., on the seeds of Fumariaceæ, <a href="#Page174">174</a>.<br/> +<br/> +Brent, Mr., on house-tumblers, <a href="#Page210">210</a>.<br/> +<br/> +Britain, mammals of, <a href="#Page352">352</a>.<br/> +<br/> +Broca, Prof., on Natural Selection, <a href="#Page170">170</a>.<br/> +<br/> +Bronn, Prof., on duration of specific forms, <a href="#Page275">275</a>.<br/> +—, various objections by, <a href="#Page170">170</a>.<br/> +<br/> +Brown, Robert, on classification, <a href="#Page366">366</a>.<br/> +<br/> +Brown-Sequard, on inherited mutilations, <a href="#Page108">108</a>.<br/> +<br/> +Busk, Mr., on the Polyzoa, <a href="#Page193">193</a>.<br/> +<br/> +Butterflies, mimetic, <a href="#Page375">375</a>, <a href="#Page376">376</a>.<br/> +<br/> +Buzareingues, on sterility of varieties, <a href="#Page258">258</a>.<br/> +<br/> +Cabbage, varieties of, crossed, <a href="#Page78">78</a>.<br/> +<br/> +Calceolaria, <a href="#Page239">239</a>.<br/> +<br/> +Canary-birds, sterility of hybrids, <a href="#Page240">240</a>.<br/> +<br/> +Cape de Verde Islands, productions of, <a href="#Page354">354</a>.<br/> +—, plants of, on mountains, <a href="#Page337">337</a>.<br/> +<br/> +Cape of Good Hope, plants of, <a href="#Page101">101</a>, <a href="#Page347">347</a>.<br/> +<br/> +Carpenter, Dr., on foraminifera, <a href="#Page308">308</a>.<br/> +<br/> +Carthemus, <a href="#Page173">173</a>.<br/> +<br/> +Catasetum, <a href="#Page155">155</a>, <a href="#Page372">372</a>.<br/> +<br/> +Cats, with blue eyes, deaf, <a href="#Page09">9</a>.<br/> +—, variation in habits of, <a href="#Page209">209</a>.<br/> +—, curling tail when going to spring, <a href="#Page162">162</a>.<br/> +<br/> +Cattle destroying fir-trees, <a href="#Page56">56</a>.<br/> +—, destroyed by flies in Paraguay, <a href="#Page56">56</a>.<br/> +—, breeds of, locally extinct, <a href="#Page86">86</a>.<br/> +—, fertility of Indian and European breeds, <a href="#Page241">241</a>.<br/> +—, Indian, <a href="#Page14">14</a>, <a href="#Page241">241</a>.<br/> +<br/> +Cave, inhabitants of, blind, <a href="#Page110">110</a>.<br/> +<br/> +Cecidomyia, <a href="#Page387">387</a>.<br/> +<br/> +Celts, proving antiquity of man, <a href="#Page13">13</a>.<br/> +<br/> +Centres of creation, <a href="#Page320">320</a>.<br/> +<br/> +Cephalopodæ, structures of eyes, <a href="#Page151">151</a>.<br/> +—, development of, <a href="#Page390">390</a>.<br/> +<br/> +Cercopithecus, tail of, <a href="#Page189">189</a>.<br/> +<br/> +Ceroxylus laceratus, <a href="#Page182">182</a>.<br/> +<br/> +Cervulus, <a href="#Page240">240</a>.<br/> +<br/> +Cetacea, teeth and hair, <a href="#Page115">115</a>.<br/> +—, development of the whalebone, <a href="#Page182">182</a>.<br/> +<br/> +Cetaceans, <a href="#Page182">182</a>.<br/> +<br/> +Ceylon, plants of, <a href="#Page338">338</a>.<br/> +<br/> +Chalk formation, <a href="#Page297">297</a>.<br/> +<br/> +Characters, divergence of, <a href="#Page86">86</a>.<br/> +—, sexual, variable, <a href="#Page119">119</a>, <a href="#Page123">123</a>.<br/> +—, adaptive or analogical, <a href="#Page373">373</a>.<br/> +<br/> +Charlock, <a href="#Page59">59</a>.<br/> +<br/> +Checks to increase, <a href="#Page53">53</a>.<br/> +—, mutual, <a href="#Page55">55</a>.<br/> +<br/> +Chelæ of Crustaceans, <a href="#Page193">193</a>.<br/> +<br/> +Chickens, instinctive tameness of, <a href="#Page211">211</a>.<br/> +<br/> +Chironomus, its asexual reproduction, <a href="#Page387">387</a>.<br/> +<br/> +Chthamalinæ, <a href="#Page271">271</a>.<br/> +<br/> +Chthamalus, cretacean species of, <a href="#Page284">384</a>.<br/> +<br/> +Circumstances favourable to selection of domestic products, <a href="#Page29">29</a>.<br/> +—, to natural selection, <a href="#Page80">80</a>.<br/> +<br/> +Cirripedes capable of crossing, <a href="#Page79">79</a>.<br/> +—, carapace aborted, <a href="#Page118">118</a>.<br/> +—, their ovigerous frena, <a href="#Page148">148</a>.<br/> +—, fossil, <a href="#Page284">284</a>.<br/> +—, larvæ of, <a href="#Page389">389</a>.<br/> +<br/> +Claparède, Prof., on the hair-claspers of the Acaridæ, <a href="#Page153">153</a>.<br/> +<br/> +Clarke, Rev. W.B., on old glaciers in Australia, <a href="#Page335">335</a>.<br/> +<br/> +Classification, <a href="#Page363">363</a>.<br/> +<br/> +Clift, Mr., on the succession of types, <a href="#Page310">310</a>.<br/> +<br/> +Climate, effects of, in checking increase of beings, <a href="#Page54">54</a>.<br/> +—, adaptation of, to organisms, <a href="#Page112">112</a>.<br/> +<br/> +Climbing plants, <a href="#Page147">147</a>.<br/> +—, development of, <a href="#Page96">96</a>.<br/> +<br/> +Clover visited by bees, <a href="#Page75">75</a>.<br/> +<br/> +Cobites, intestine of, <a href="#Page147">147</a>.<br/> +<br/> +Cockroach, <a href="#Page59">59</a>.<br/> +<br/> +Collections, palæontological, poor, <a href="#Page270">270</a>.<br/> +<br/> +Colour, influenced by climate, <a href="#Page107">107</a>.<br/> +—, in relation to attacks by flies, <a href="#Page159">159</a>.<br/> +<br/> +Columba livia, parent of domestic pigeons, <a href="#Page17">17</a>.<br/> +<br/> +Colymbetes, <a href="#Page345">345</a>.<br/> +<br/> +Compensation of growth, <a href="#Page117">117</a>.<br/> +<br/> +Compositæ, flowers and seeds of, <a href="#Page116">116</a>.<br/> +—, outer and inner florets of, <a href="#Page173">173</a>.<br/> +—, male flowers of, <a href="#Page398">398</a>.<br/> +<br/> +Conclusion, general, <a href="#Page421">421</a>.<br/> +<br/> +Conditions, slight changes in, favourable to fertility, <a href="#Page251">251</a>.<br/> +<br/> +Convergence of genera, <a href="#Page100">100</a>.<br/> +<br/> +Coot, <a href="#Page142">142</a>.<br/> +<br/> +Cope, Prof., on the acceleration or retardation of the period of<br/> +reproduction, <a href="#Page149">149</a>.<br/> +<br/> +Coral-islands, seeds drifted to, <a href="#Page326">326</a>.<br/> +—, reefs, indicating movements of earth, <a href="#Page326">326</a>.<br/> +<br/> +Corn-crake, <a href="#Page143">143</a>.<br/> +<br/> +Correlated variation in domestic productions, <a href="#Page09">9</a>.<br/> +<br/> +Coryanthes, <a href="#Page154">154</a>.<br/> +<br/> +Creation, single centres of, <a href="#Page320">320</a>.<br/> +<br/> +Crinum, <a href="#Page238">238</a>.<br/> +<br/> +Croll, Mr., on subaërial denudation, <a href="#Page267">267</a>, <a href="#Page269">269</a>.<br/> +—, on the age of our oldest formations, <a href="#Page286">286</a>.<br/> +—, on alternate Glacial periods in the North and South, <a href="#Page336">336</a>.<br/> +<br/> +Crosses, reciprocal, <a href="#Page244">244</a>.<br/> +<br/> +Crossing of domestic animals, importance in altering breeds, <a href="#Page15">15</a>.<br/> +—, advantages of, <a href="#Page76">76</a>, <a href="#Page77">77</a>.<br/> +—, unfavourable to selection, <a href="#Page80">80</a>.<br/> +<br/> +Crüger, Dr., on Coryanthes, <a href="#Page154">154</a>.<br/> +<br/> +Crustacea of New Zealand, <a href="#Page338">338</a>.<br/> +<br/> +Crustacean, blind, <a href="#Page110">110</a>.<br/> +air-breathers, <a href="#Page152">152</a>.<br/> +<br/> +Crustaceans, their chelæ, <a href="#Page193">193</a>.<br/> +<br/> +Cryptocerus, <a href="#Page231">231</a>.<br/> +<br/> +Ctenomys, blind, <a href="#Page110">110</a>.<br/> +<br/> +Cuckoo, instinct of, <a href="#Page205">205</a>, <a href="#Page212">212</a>.<br/> +<br/> +Cunningham, Mr., on the flight of the logger-headed duck, <a href="#Page108">108</a>.<br/> +<br/> +Currants, grafts of, <a href="#Page246">246</a>.<br/> +<br/> +Currents of sea, rate of, <a href="#Page325">325</a>.<br/> +<br/> +Cuvier on conditions of existence, <a href="#Page205">205</a>.<br/> +—, on fossil monkeys, <a href="#Page283">283</a>, <a href="#Page284">284</a>.<br/> +<br/> +Cuvier, Fred., on instinct, <a href="#Page205">205</a>.<br/> +<br/> +Cyclostoma, resisting salt water, <a href="#Page353">353</a>.<br/> +<br/> +Dana, Prof., on blind cave-animals, <a href="#Page111">111</a>.<br/> +—, on relations of crustaceans of Japan, <a href="#Page334">334</a>.<br/> +—, on crustaceans of New Zealand, <a href="#Page338">338</a>.<br/> +<br/> +Dawson, Dr., on eozoon, <a href="#Page287">287</a>.<br/> +<br/> +De Candolle, Aug. Pyr., on struggle for existence, <a href="#Page49">49</a>.<br/> +—, on umbelliferæ, <a href="#Page116">116</a>.<br/> +—, on general affinities, <a href="#Page379">379</a>.<br/> +<br/> +De Candolle, Alph., on the variability of oaks, <a href="#Page40">40</a>.<br/> +—, on low plants, widely dispersed, <a href="#Page359">359</a>.<br/> +—, on widely-ranging plants being variable, <a href="#Page43">43</a>.<br/> +—, on naturalisation, <a href="#Page89">89</a>.<br/> +—, on winged seeds, <a href="#Page117">117</a>.<br/> +—, on Alpine species suddenly becoming rare, <a href="#Page135">135</a>.<br/> +—, on distribution of plants with large seeds, <a href="#Page326">326</a>.<br/> +—, on vegetation of Australia, <a href="#Page340">340</a>.<br/> +—, on fresh-water plants, <a href="#Page345">345</a>.<br/> +—, on insular plants, <a href="#Page347">347</a>.<br/> +<br/> +Degradation of rocks, <a href="#Page266">266</a>.<br/> +<br/> +Denudation, rate of, <a href="#Page268">268</a>.<br/> +—, of oldest rocks, <a href="#Page287">287</a>.<br/> +—, of granitic areas, <a href="#Page274">274</a>.<br/> +<br/> +Development of ancient forms, <a href="#Page307">307</a>.<br/> +<br/> +Devonian system, <a href="#Page305">305</a>.<br/> +<br/> +Dianthus, fertility of crosses, <a href="#Page243">243</a>.<br/> +<br/> +Dimorphism in plants, <a href="#Page35">35</a>, <a href="#Page252">252</a>.<br/> +<br/> +Dirt on feet of birds, <a href="#Page328">328</a>.<br/> +<br/> +Dispersal, means of, <a href="#Page323">323</a>.<br/> +—, during Glacial period, <a href="#Page330">330</a>.<br/> +<br/> +Distribution, geographical, <a href="#Page316">316</a>.<br/> +—, means of, <a href="#Page323">323</a>.<br/> +<br/> +Disuse, effect of, under nature, <a href="#Page108">108</a>.<br/> +<br/> +Diversification of means for same general purpose, <a href="#Page153">153</a>.<br/> +<br/> +Division, physiological, of labour, <a href="#Page89">89</a>.<br/> +<br/> +Divergence of character, <a href="#Page86">86</a>.<br/> +<br/> +Dog, resemblance of jaw to that of the Thylacinus, <a href="#Page374">374</a>.<br/> +<br/> +Dogs, hairless, with imperfect teeth, <a href="#Page09">9</a>.<br/> +—, descended from several wild stocks, <a href="#Page15">15</a>.<br/> +—, domestic instincts of, <a href="#Page210">210</a>.<br/> +—, inherited civilisation of, <a href="#Page210">210</a>.<br/> +—, fertility of breeds together, <a href="#Page241">241</a>.<br/> +—, of crosses, <a href="#Page256">256</a>.<br/> +—, proportions of body in different breeds, when young, <a href="#Page392">392</a>.<br/> +<br/> +Domestication, variation under, <a href="#Page05">5</a>.<br/> +<br/> +Double flowers, <a href="#Page230">230</a>.<br/> +<br/> +Downing, Mr., on fruit-trees in America, <a href="#Page66">66</a>.<br/> +<br/> +Dragon-flies, intestines of, <a href="#Page147">147</a>.<br/> +<br/> +Drift-timber, <a href="#Page326">326</a>.<br/> +<br/> +Driver-ant, <a href="#Page232">232</a>.<br/> +<br/> +Drones killed by other bees, <a href="#Page164">164</a>.<br/> +<br/> +Duck, domestic, wings of, reduced, <a href="#Page08">8</a>.<br/> +—, beak of, <a href="#Page183">183</a>.<br/> +—, logger-headed, <a href="#Page140">140</a>.<br/> +<br/> +Duckweed, <a href="#Page344">344</a>.<br/> +<br/> +Dugong, affinities of, <a href="#Page365">365</a>.<br/> +<br/> +Dung-beetles with deficient tarsi, <a href="#Page108">108</a>.<br/> +<br/> +Dyticus, <a href="#Page345">345</a>.<br/> +<br/> +Earl, Mr., W., on the Malay Archipelago, <a href="#Page351">351</a>.<br/> +<br/> +Ears, drooping, in domestic animals, <a href="#Page08">8</a>.<br/> +—, rudimentary, <a href="#Page400">400</a>.<br/> +<br/> +Earth, seeds in roots of trees, <a href="#Page326">326</a>.<br/> +—, charged with seeds, <a href="#Page328">328</a>.<br/> +<br/> +Echinodermata, their pedicellariæ, <a href="#Page191">191</a>.<br/> +<br/> +Eciton, <a href="#Page230">230</a>.<br/> +<br/> +Economy of organisation, <a href="#Page117">117</a>.<br/> +<br/> +Edentata, teeth and hair, <a href="#Page115">115</a>.<br/> +—, fossil species of, <a href="#Page417">417</a>.<br/> +<br/> +Edwards, Milne, on physiological division of labour, <a href="#Page89">89</a>.<br/> +—, on gradations of structure, <a href="#Page156">156</a>.<br/> +<br/> +Edwards, on embryological characters, <a href="#Page368">368</a>.<br/> +<br/> +Eggs, young birds escaping from, <a href="#Page68">68</a>.<br/> +<br/> +Egypt, productions of, not modified, <a href="#Page169">169</a>.<br/> +<br/> +Electric organs, <a href="#Page150">150</a>.<br/> +<br/> +Elephant, rate of increase, <a href="#Page51">51</a>.<br/> +—, of Glacial period, <a href="#Page113">113</a>.<br/> +<br/> +Embryology, <a href="#Page386">386</a>.<br/> +<br/> +Eozoon Canadense, <a href="#Page287">287</a>.<br/> +<br/> +Epilipsy inherited, <a href="#Page108">108</a>.<br/> +<br/> +Existence, struggle for, <a href="#Page48">48</a>.<br/> +—, condition of, <a href="#Page167">167</a>.<br/> +<br/> +Extinction, as bearing on natural selection, <a href="#Page96">96</a>.<br/> +—, of domestic varieties, <a href="#Page93">93</a>.<br/> +—, , <a href="#Page293">293</a>.<br/> +<br/> +Eye, structure of, <a href="#Page144">144</a>.<br/> +—, correction for aberration, <a href="#Page163">163</a>.<br/> +<br/> +Eyes, reduced, in moles, <a href="#Page110">110</a>.<br/> +<br/> +Fabre, M., on hymenoptera fighting, <a href="#Page69">69</a>.<br/> +—, on parasitic sphex, <a href="#Page216">216</a>.<br/> +—, on Sitaris, <a href="#Page394">394</a>.<br/> +<br/> +Falconer, Dr., on naturalisation of plants in India, <a href="#Page51">51</a>.<br/> +—, on elephants and mastodons, <a href="#Page306">306</a>.<br/> +—, and Cautley on mammals of sub-Himalayan beds, <a href="#Page311">311</a>.<br/> +<br/> +Falkland Islands, wolf of, <a href="#Page351">351</a>.<br/> +<br/> +Faults, <a href="#Page268">268</a>.<br/> +<br/> +Faunas, marine, <a href="#Page317">317</a>.<br/> +<br/> +Fear, instinctive, in birds, <a href="#Page211">211</a>.<br/> +<br/> +Feet of birds, young molluscs adhering to, <a href="#Page345">345</a>.<br/> +<br/> +Fertilisation variously effected, <a href="#Page154">154</a>, <a href="#Page161">161</a>.<br/> +<br/> +Fertility of hybrids, <a href="#Page238">238</a>.<br/> +—, from slight changes in conditions, <a href="#Page252">252</a>.<br/> +—, of crossed varieties, <a href="#Page255">255</a>.<br/> +<br/> +Fir-trees destroyed by cattle, <a href="#Page56">56</a>.<br/> +—, pollen of, <a href="#Page164">164</a>.<br/> +<br/> +Fish, flying, <a href="#Page140">140</a>.<br/> +—, teleostean, sudden appearance of, <a href="#Page285">285</a>.<br/> +—, eating seeds, <a href="#Page327">327</a>, <a href="#Page346">346</a>.<br/> +—, fresh-water, distribution of, <a href="#Page343">343</a>.<br/> +<br/> +Fishes, ganoid, now confined to fresh water, <a href="#Page83">83</a>.<br/> +—, electric organs of, <a href="#Page150">150</a>.<br/> +—, ganoid, living in fresh water, <a href="#Page296">296</a>.<br/> +—, of southern hemisphere, <a href="#Page338">338</a>.<br/> +<br/> +Flat-fish, their structure, <a href="#Page186">186</a>.<br/> +<br/> +Flight, powers of, how acquired, <a href="#Page140">140</a>.<br/> +<br/> +Flint-tools, proving antiquity of man, <a href="#Page13">13</a>.<br/> +<br/> +Flower, Prof., on the larynx, <a href="#Page190">190</a>.<br/> +—, on Halitherium, <a href="#Page302">302</a>.<br/> +—, on the resemblance between the jaws of the dog and Thylacinus, <a href="#Page375">375</a>.<br/> +—, on the homology of the feet of certain marsupials, <a href="#Page382">382</a>.<br/> +<br/> +Flowers, structure of in relation to crossing, <a href="#Page73">73</a>.<br/> +—, of compositæ and umbelliferæ, <a href="#Page116">116</a>, <a href="#Page173">173</a>.<br/> +—, beauty of, <a href="#Page161">161</a>.<br/> +—, double, <a href="#Page230">230</a>.<br/> +<br/> +Flysch formation, destitute of organic remains, <a href="#Page271">271</a>.<br/> +<br/> +Forbes, Mr. D., on glacial action in the Andes, <a href="#Page335">335</a>.<br/> +<br/> +Forbes, E., on colours of shells, <a href="#Page107">107</a>.<br/> +—, on abrupt range of shells in depth, <a href="#Page135">135</a>.<br/> +—, on poorness of palæontological collections, <a href="#Page270">270</a>.<br/> +—, on continuous succession of genera, <a href="#Page293">293</a>.<br/> +—, on continental extensions, <a href="#Page323">323</a>.<br/> +—, on distribution during Glacial period, <a href="#Page330">330</a>.<br/> +—, on parallelism in time and space, <a href="#Page361">361</a>.<br/> +<br/> +Forests, changes in, in America, <a href="#Page58">58</a>.<br/> +<br/> +Formation, Devonian, <a href="#Page305">305</a>.<br/> +—, Cambrian, <a href="#Page287">287</a>.<br/> +Formations, thickness of, in Britain, <a href="#Page268">268</a>.<br/> +—, intermittent, <a href="#Page277">277</a>.<br/> +<br/> +Formica rufescens, <a href="#Page216">216</a>.<br/> +—, sanguinea, <a href="#Page217">217</a>.<br/> +—, flava, neuter of, <a href="#Page231">231</a>.<br/> +<br/> +Forms, lowly organised, long enduring, <a href="#Page99">99</a>.<br/> +<br/> +Frena, ovigerous, of cirripedes, <a href="#Page148">148</a>.<br/> +<br/> +Fresh-water productions, dispersal of, <a href="#Page343">343</a>.<br/> +<br/> +Fries on species in large genera being closely allied to other species, <a href="#Page45">45</a>.<br/> +<br/> +Frigate-bird, <a href="#Page142">142</a>.<br/> +<br/> +Frogs on islands, <a href="#Page350">350</a>.<br/> +<br/> +Fruit-trees, gradual improvement of, <a href="#Page27">27</a>.<br/> +—, in United States, <a href="#Page66">66</a>.<br/> +—, varieties of, acclimatised in United States, <a href="#Page114">114</a>.<br/> +<br/> +Fuci, crossed, <a href="#Page249">249</a>, <a href="#Page343">343</a>.<br/> +<br/> +Fur, thicker in cold climates, <a href="#Page107">107</a>.<br/> +<br/> +Furze, <a href="#Page388">388</a>.<br/> +<br/> +Galapagos Archipelago, birds of, <a href="#Page348">348</a>.<br/> +—, productions of, <a href="#Page353">353</a>, <a href="#Page355">355</a>.<br/> +<br/> +Galaxias, its wide range, <a href="#Page343">343</a>.<br/> +<br/> +Galeopithecus, <a href="#Page139">139</a>.<br/> +<br/> +Game, increase of, checked by vermin, <a href="#Page55">55</a>.<br/> +<br/> +Gärtner on sterility of hybrids, <a href="#Page237">237</a>, <a href="#Page241">241</a>.<br/> +—, on reciprocal crosses, <a href="#Page243">243</a>.<br/> +—, on crossed maize and verbascum, <a href="#Page257">257</a>, <a href="#Page258">258</a>.<br/> +—, on comparison of hybrids and mongrels, <a href="#Page259">259</a>, <a href="#Page260">260</a>.<br/> +<br/> +Gaudry, Prof., on intermediate genera of fossil mammals in Attica, <a href="#Page301">301</a>.<br/> +<br/> +Geese, fertility when crossed, <a href="#Page307">307</a>.<br/> +—, upland, <a href="#Page142">142</a>.<br/> +<br/> +Geikie, Mr., on subaërial denudation, <a href="#Page267">267</a>.<br/> +<br/> +Genealogy, important in classification, <a href="#Page369">369</a>.<br/> +<br/> +Generations, alternate, <a href="#Page387">387</a>.<br/> +<br/> +Geoffroy St. Hilaire, on balancement, <a href="#Page117">117</a>.<br/> +—, on homologous organs, <a href="#Page382">382</a>.<br/> +—, , Isidore, on variability of repeated parts, <a href="#Page118">118</a>.<br/> +—, on correlation, in monstrosities, <a href="#Page09">9</a>.<br/> +—, on correlation, <a href="#Page115">115</a>.<br/> +—, on variable parts being often monstrous, <a href="#Page122">122</a>.<br/> +<br/> +Geographical distribution, <a href="#Page316">316</a>.<br/> +<br/> +Geography, ancient, <a href="#Page427">427</a>.<br/> +<br/> +Geology, future progress of, <a href="#Page427">427</a>.<br/> +—, imperfection of the record, <a href="#Page427">427</a>.<br/> +<br/> +Gervais, Prof., on Typotherium, <a href="#Page302">302</a>.<br/> +<br/> +Giraffe, tail of, <a href="#Page157">157</a>.<br/> +—, structure of, <a href="#Page177">177</a>.<br/> +<br/> +Glacial period, <a href="#Page330">330</a>.<br/> +—, affecting the North and South, <a href="#Page335">335</a>.<br/> +<br/> +Glands, mammary, <a href="#Page189">189</a>.<br/> +<br/> +Gmelin, on distribution, <a href="#Page330">330</a>.<br/> +<br/> +Godwin-Austin, Mr., on the Malay Archipelago, <a href="#Page280">280</a>.<br/> +<br/> +Goethe, on compensation of growth, <a href="#Page117">117</a>.<br/> +<br/> +Gomphia, <a href="#Page174">174</a>.<br/> +<br/> +Gooseberry, grafts of, <a href="#Page246">246</a>.<br/> +<br/> +Gould, Dr. Aug. A., on land-shells, <a href="#Page353">353</a>.<br/> +<br/> +Gould, Mr., on colours of birds, <a href="#Page107">107</a>.<br/> +—, on instincts of cuckoo, <a href="#Page214">214</a>.<br/> +—, on distribution of genera of birds, <a href="#Page358">358</a>.<br/> +<br/> +Gourds, crossed, <a href="#Page258">258</a>.<br/> +<br/> +Graba, on the Uria lacrymans, <a href="#Page72">72</a>.<br/> +<br/> +Grafting, capacity of, <a href="#Page245">245</a>, <a href="#Page246">246</a>.<br/> +<br/> +Granite, areas of denuded, <a href="#Page274">274</a>.<br/> +<br/> +Grasses, varieties of, <a href="#Page88">88</a>.<br/> +<br/> +Gray, Dr. Asa, on the variability of oaks, <a href="#Page40">40</a>.<br/> +—, on man not causing variability, <a href="#Page62">62</a>.<br/> +—, on sexes of the holly, <a href="#Page74">74</a>.<br/> +—, on trees of the United States, <a href="#Page79">79</a>.<br/> +—, on naturalised plants in the United States, <a href="#Page89">89</a>.<br/> +—, on æstivation, <a href="#Page174">174</a>.<br/> +—, on Alpine plants, <a href="#Page330">330</a>.<br/> +—, on rarity of intermediate varieties, <a href="#Page136">136</a>.<br/> +<br/> +Gray, Dr. J.E., on striped mule, <a href="#Page128">128</a>.<br/> +<br/> +Grebe, <a href="#Page142">142</a>.<br/> +<br/> +Grimm, on asexual reproduction, <a href="#Page387">387</a>.<br/> +<br/> +Groups, aberrant, <a href="#Page378">378</a>.<br/> +<br/> +Grouse, colours of, <a href="#Page66">66</a>.<br/> +—, red, a doubtful species, <a href="#Page38">38</a>.<br/> +<br/> +Growth, compensation of, <a href="#Page117">117</a>.<br/> +<br/> +Günther, Dr., on flat-fish, <a href="#Page187">187</a>.<br/> +—, on prehensile tails, <a href="#Page189">189</a>.<br/> +—, on the fishes of Panama, <a href="#Page317">317</a>.<br/> +—, on the range of fresh-water fishes, <a href="#Page343">343</a>.<br/> +—, on the limbs of Lepidosiren, <a href="#Page399">399</a>.<br/> +<br/> +Haast, Dr., on glaciers of New Zealand, <a href="#Page335">335</a>.<br/> +<br/> +Habit, effect of, under domestication, <a href="#Page08">8</a>.<br/> +—, effect of, under nature, <a href="#Page108">108</a>.<br/> +—, diversified, of same species, <a href="#Page141">141</a>.<br/> +<br/> +Häckel, Prof., on classification and the lines of descent, <a href="#Page381">381</a>.<br/> +<br/> +Hair and teeth, correlated, <a href="#Page115">115</a>.<br/> +<br/> +Halitherium, <a href="#Page302">302</a>.<br/> +<br/> +Harcourt, Mr. E.V., on the birds of Madeira, <a href="#Page348">348</a>.<br/> +<br/> +Hartung, M., on boulders in the Azores, <a href="#Page328">328</a>.<br/> +<br/> +Hazel-nuts, <a href="#Page325">325</a>.<br/> +<br/> +Hearne, on habits of bears, <a href="#Page141">141</a>.<br/> +<br/> +Heath, changes in vegetation, <a href="#Page55">55</a>.<br/> +<br/> +Hector, Dr., on glaciers of New Zealand, <a href="#Page335">335</a>.<br/> +<br/> +Heer, Oswald, on ancient cultivated plants, <a href="#Page13">13</a>.<br/> +—, on plants of Madeira, <a href="#Page83">83</a>.<br/> +<br/> +Helianthemum, <a href="#Page174">174</a>.<br/> +<br/> +Helix, resisting salt water, <a href="#Page353">353</a>.<br/> +<br/> +Helix pomatia, <a href="#Page353">353</a>.<br/> +<br/> +Helmholtz, M., on the imperfection of the human eye, <a href="#Page163">163</a>.<br/> +<br/> +Helosciadium, <a href="#Page325">325</a>.<br/> +<br/> +Hemionus, striped, <a href="#Page128">128</a>.<br/> +<br/> +Hensen, Dr., on the eyes of Cephalopods, <a href="#Page152">152</a>.<br/> +<br/> +Herbert, W., on struggle for existence, <a href="#Page49">49</a>.<br/> +—, on sterility of hybrids, <a href="#Page238">238</a>.<br/> +<br/> +Hermaphrodites crossing, <a href="#Page76">76</a>.<br/> +<br/> +Heron eating seed, <a href="#Page346">346</a>.<br/> +<br/> +Heron, Sir R., on peacocks, <a href="#Page70">70</a>.<br/> +<br/> +Heusinger, on white animals poisoned by certain plants, <a href="#Page09">9</a>.<br/> +<br/> +Hewitt, Mr., on sterility of first crosses, <a href="#Page249">249</a>.<br/> +<br/> +Hildebrand, Prof., on the self-sterility of Corydalis, <a href="#Page238">238</a>.<br/> +<br/> +Hilgendorf, on intermediate varieties, <a href="#Page275">275</a>.<br/> +<br/> +Himalaya, glaciers of, <a href="#Page335">335</a>.<br/> +—, plants of, <a href="#Page337">337</a>.<br/> +<br/> +Hippeastrum, <a href="#Page238">238</a>.<br/> +<br/> +Hippocampus, <a href="#Page189">189</a>.<br/> +<br/> +Hofmeister, Prof., on the movements of plants, <a href="#Page197">197</a>.<br/> +<br/> +Holly-trees, sexes of, <a href="#Page73">73</a>.<br/> +<br/> +Hooker, Dr., on trees of New Zealand, <a href="#Page78">78</a>.<br/> +—, on acclimatisation of Himalayan trees, <a href="#Page112">112</a>.<br/> +—, on flowers of umbelliferæ, <a href="#Page116">116</a>.<br/> +—, on the position of ovules, <a href="#Page172">172</a>.<br/> +—, on glaciers of Himalaya, <a href="#Page335">335</a>.<br/> +—, on algæ of New Zealand, <a href="#Page338">338</a>.<br/> +—, on vegetation at the base of the Himalaya, <a href="#Page338">338</a>.<br/> +—, on plants of Tierra del Fuego, <a href="#Page336">336</a>.<br/> +—, on Australian plants, <a href="#Page337">337</a>, <a href="#Page355">355</a>.<br/> +—, on relations of flora of America, <a href="#Page340">340</a>.<br/> +—, on flora of the Antarctic lands, <a href="#Page341">341</a>, <a href="#Page354">354</a>.<br/> +—, on the plants of the Galapagos, <a href="#Page349">349</a>, <a href="#Page354">354</a>.<br/> +—, on glaciers of the Lebanon, <a href="#Page335">335</a>.<br/> +—, on man not causing variability, <a href="#Page62">62</a>.<br/> +—, on plants of mountains of Fernando Po, <a href="#Page337">337</a>.<br/> +<br/> +Hooks on palms, <a href="#Page158">158</a>.<br/> +—, on seeds, on islands, <a href="#Page349">349</a>.<br/> +<br/> +Hopkins, Mr., on denudation, <a href="#Page274">274</a>.<br/> +<br/> +Hornbill, remarkable instinct of, <a href="#Page234">234</a>.<br/> +<br/> +Horns, rudimentary, <a href="#Page400">400</a>.<br/> +<br/> +Horse, fossil in La Plata, <a href="#Page294">294</a>.<br/> +—, proportions of, when young, <a href="#Page392">392</a>.<br/> +<br/> +Horses destroyed by flies in Paraguay, <a href="#Page56">56</a>.<br/> +—, striped, <a href="#Page128">128</a>.<br/> +<br/> +Horticulturists, selection applied by, <a href="#Page23">23</a>.<br/> +<br/> +Huber on cells of bees, <a href="#Page224">224</a>.<br/> +<br/> +Huber, P., on reason blended with instinct, <a href="#Page205">205</a>.<br/> +—, on habitual nature of instincts, <a href="#Page206">206</a>.<br/> +—, on slave-making ants, <a href="#Page216">216</a>.<br/> +—, on Melipona domestica, <a href="#Page220">220</a>.<br/> +<br/> +Hudson, Mr., on the Ground-woodpecker of La Plata, <a href="#Page142">142</a>.<br/> +—, on the Molothrus, <a href="#Page215">215</a>.<br/> +<br/> +Humble-bees, cells of, <a href="#Page221">221</a>.<br/> +<br/> +Hunter, J., on secondary sexual characters, <a href="#Page119">119</a>.<br/> +<br/> +Hutton, Captain, on crossed geese, <a href="#Page240">240</a>.<br/> +<br/> +Huxley, Prof., on structure of hermaphrodites, <a href="#Page79">79</a>.<br/> +—, on the affinities of the Sirenia, <a href="#Page302">302</a>.<br/> +—, on forms connecting birds and reptiles, <a href="#Page302">302</a>.<br/> +—, on homologous organs, <a href="#Page386">386</a>.<br/> +—, on the development of aphis, <a href="#Page390">390</a>.<br/> +<br/> +Hybrids and mongrels compared, <a href="#Page259">259</a>.<br/> +<br/> +Hybridism, <a href="#Page235">235</a>.<br/> +<br/> +Hydra, structure of, <a href="#Page147">147</a>.<br/> +<br/> +Hymenoptera, fighting, <a href="#Page69">69</a>.<br/> +<br/> +Hymenopterous insect, diving, <a href="#Page142">142</a>.<br/> +<br/> +Hyoseris, <a href="#Page173">173</a>.<br/> +<br/> +Ibla, <a href="#Page118">118</a>.<br/> +<br/> +Icebergs transporting seeds, <a href="#Page329">329</a>.<br/> +<br/> +Increase, rate of, <a href="#Page50">50</a>.<br/> +<br/> +Individuals, numbers favourable to selection, <a href="#Page80">80</a>.<br/> +—, many, whether simultaneously created, <a href="#Page322">322</a>.<br/> +<br/> +Inheritance, laws of, <a href="#Page10">10</a>.<br/> +—, at corresponding ages, <a href="#Page10">10</a>, <a href="#Page67">67</a>.<br/> +<br/> +Insects, colour of, fitted for their stations, <a href="#Page66">66</a>.<br/> +—, sea-side, colours of, <a href="#Page107">107</a>.<br/> +—, blind, in caves, <a href="#Page110">110</a>.<br/> +—, luminous, <a href="#Page151">151</a>.<br/> +—, their resemblance to various objects, <a href="#Page181">181</a>.<br/> +—, neuter, <a href="#Page230">2320</a>.<br/> +<br/> +Instinct, <a href="#Page205">205</a>.<br/> +—, , not varying simultaneously with structure, <a href="#Page229">229</a>.<br/> +<br/> +Instincts, domestic, <a href="#Page209">209</a>.<br/> +<br/> +Intercrossing, advantages of, <a href="#Page76">76</a>, <a href="#Page251">251</a>.<br/> +<br/> +Islands, oceanic, <a href="#Page347">347</a>.<br/> +<br/> +Isolation favourable to selection, <a href="#Page81">81</a>.<br/> +<br/> +Japan, productions of, <a href="#Page334">334</a>.<br/> +<br/> +Java, plants of, <a href="#Page337">337</a>.<br/> +<br/> +Jones, Mr. J.M., on the birds of Bermuda, <a href="#Page348">348</a>.<br/> +<br/> +Jordain, M., on the eye-spots of star fishes, <a href="#Page144">144</a>.<br/> +<br/> +Jukes, Prof., on subaërial denudation, <a href="#Page267">267</a>.<br/> +<br/> +Jussieu on classification, <a href="#Page367">367</a>.<br/> +<br/> +Kentucky, caves of, <a href="#Page111">111</a>.<br/> +<br/> +Kerguelen-land, flora of, <a href="#Page341">341</a>, <a href="#Page354">354</a>.<br/> +<br/> +Kidney-bean, acclimatisation of, <a href="#Page114">114</a>.<br/> +<br/> +Kidneys of birds, <a href="#Page115">115</a>.<br/> +<br/> +Kirby, on tarsi deficient in beetles, <a href="#Page108">108</a>.<br/> +<br/> +Knight, Andrew, on cause of variation, <a href="#Page05">5</a>.<br/> +<br/> +Kölreuter, on intercrossing, <a href="#Page76">76</a>.<br/> +—, on the barberry, <a href="#Page77">77</a>.<br/> +—, on sterility of hybrids, <a href="#Page237">237</a>.<br/> +—, on reciprocal crosses, <a href="#Page243">243</a>.<br/> +—, on crossed varieties of nicotiana, <a href="#Page258">258</a>.<br/> +—, on crossing male and hermaphrodite flowers, <a href="#Page397">397</a>.<br/> +<br/> +Lamarck, on adaptive characters, <a href="#Page373">373</a>.<br/> +<br/> +Lancelet, <a href="#Page99">99</a>.<br/> +—, , eyes of, <a href="#Page145">145</a>.<br/> +<br/> +Landois, on the development of the wings of insects, <a href="#Page148">148</a>.<br/> +<br/> +Land-shells, distribution of, <a href="#Page353">353</a>.<br/> +—, of Madeira, naturalised, <a href="#Page357">357</a>.<br/> +—, resisting salt water, <a href="#Page353">353</a>.<br/> +<br/> +Languages, classification of, <a href="#Page371">371</a>.<br/> +<br/> +Lankester, Mr. E. Ray, on longevity, <a href="#Page169">169</a>.<br/> +—, on homologies, <a href="#Page385">385</a>.<br/> +<br/> +Lapse, great, of time, <a href="#Page266">266</a>.<br/> +<br/> +Larvæ, <a href="#Page388">388</a>, <a href="#Page389">389</a>.<br/> +<br/> +Laurel, nectar secreted by the leaves, <a href="#Page73">73</a>.<br/> +<br/> +Laurentian formation, <a href="#Page287">287</a>.<br/> +<br/> +Laws of variation, <a href="#Page106">106</a>.<br/> +<br/> +Leech, varieties of, <a href="#Page59">59</a>.<br/> +<br/> +Leguminosæ, nectar secreted by glands, <a href="#Page73">73</a>.<br/> +<br/> +Leibnitz’ attack on Newton, <a href="#Page421">421</a>.<br/> +<br/> +Lepidosiren, <a href="#Page83">83</a>, <a href="#Page303">303</a>.<br/> +—, , limbs in a nascent condition, <a href="#Page398">398</a>, <a href="#Page399">399</a>.<br/> +<br/> +Lewes, Mr. G.H., on species not having changed in Egypt, <a href="#Page169">169</a>.<br/> +—, on the Salamandra atra, <a href="#Page397">397</a>.<br/> +—, on many forms of life having been at first evolved, <a href="#Page425">425</a>.<br/> +<br/> +Life, struggle for, <a href="#Page49">49</a>.<br/> +<br/> +Lingula, Silurian, <a href="#Page286">286</a>.<br/> +<br/> +Linnæus, aphorism of, <a href="#Page365">365</a>.<br/> +<br/> +Lion, mane of, <a href="#Page69">69</a>.<br/> +—, young of, striped, <a href="#Page388">388</a>.<br/> +<br/> +Lobelia fulgens, <a href="#Page57">57</a>, <a href="#Page77">77</a>.<br/> +<br/> +Lobelia, sterility of crosses, <a href="#Page238">238</a>.<br/> +<br/> +Lockwood, Mr., on the ova of the Hippocampus, <a href="#Page189">189</a>.<br/> +<br/> +Locusts transporting seeds, <a href="#Page327">327</a>.<br/> +<br/> +Logan, Sir W., on Laurentian formation, <a href="#Page287">287</a>.<br/> +<br/> +Lowe, Rev. R.T., on locusts visiting Madeira, <a href="#Page327">327</a>.<br/> +<br/> +Lowness, of structure connected with variability, <a href="#Page118">118</a>.<br/> +—, related to wide distribution, <a href="#Page359">359</a>.<br/> +<br/> +Lubbock, Sir J., on the nerves of coccus, <a href="#Page35">35</a>.<br/> +—, on secondary sexual characters, <a href="#Page124">124</a>.<br/> +—, on a diving hymenopterous insect, <a href="#Page142">142</a>.<br/> +—, on affinities, <a href="#Page280">280</a>.<br/> +—, on metamorphoses, <a href="#Page386">386</a>, <a href="#Page389">389</a>.<br/> +<br/> +Lucas, Dr. P., on inheritance, <a href="#Page09">9</a>.<br/> +—, on resemblance of child to parent, <a href="#Page261">261</a>.<br/> +<br/> +Lund and Clausen, on fossils of Brazil, <a href="#Page310">310</a>.<br/> +<br/> +Lyell, Sir C., on the struggle for existence, <a href="#Page49">49</a>.<br/> +—, on modern changes of the earth, <a href="#Page75">75</a>.<br/> +—, on terrestrial animals not having been developed on islands, <a href="#Page180">180</a>.<br/> +—, on a carboniferous land-shell, <a href="#Page271">271</a>.<br/> +—, on strata beneath Silurian system, <a href="#Page287">287</a>.<br/> +—, on the imperfection of the geological record, <a href="#Page289">289</a>.<br/> +—, on the appearance of species, <a href="#Page289">289</a>.<br/> +—, on Barrande’s colonies, <a href="#Page291">291</a>.<br/> +—, on tertiary formations of Europe and North America, <a href="#Page298">298</a>.<br/> +—, on parallelism of tertiary formations, <a href="#Page301">301</a>.<br/> +—, on transport of seeds by icebergs, <a href="#Page328">328</a>.<br/> +—, on great alternations of climate, <a href="#Page342">342</a>.<br/> +—, on the distribution of fresh-water shells, <a href="#Page345">345</a>.<br/> +—, on land-shells of Madeira, <a href="#Page357">357</a>.<br/> +<br/> +Lyell and Dawson, on fossilized trees in Nova Scotia, <a href="#Page278">278</a>.<br/> +<br/> +Lythrum salicaria, trimorphic, <a href="#Page254">254</a>.<br/> +<br/> +Macleay, on analogical characters, <a href="#Page373">373</a>.<br/> +<br/> +Macrauchenia, <a href="#Page302">302</a>.<br/> +<br/> +McDonnell, Dr., on electric organs, <a href="#Page150">150</a>.<br/> +<br/> +Madeira, plants of, <a href="#Page83">83</a>.<br/> +—, beetles of, wingless, <a href="#Page109">109</a>.<br/> +—, fossil land-shells of, <a href="#Page311">311</a>.<br/> +—, birds of, <a href="#Page348">348</a>.<br/> +<br/> +Magpie tame in Norway, <a href="#Page209">209</a>.<br/> +<br/> +Males, fighting, <a href="#Page69">69</a>.<br/> +<br/> +Maize, crossed, <a href="#Page257">257</a>.<br/> +<br/> +Malay Archipelago, compared with Europe, <a href="#Page280">280</a>.<br/> +—, mammals of, <a href="#Page352">352</a>.<br/> +<br/> +Malm, on flat-fish, <a href="#Page186">186</a>.<br/> +<br/> +Malpighiaceæ, small imperfect flowers of, <a href="#Page173">173</a>.<br/> +<br/> +Malpighiaceæ, <a href="#Page367">367</a>.<br/> +<br/> +Mammæ, their development, <a href="#Page189">189</a>.<br/> +—, rudimentary, <a href="#Page397">397</a>.<br/> +<br/> +Mammals, fossil, in secondary formation, <a href="#Page283">283</a>.<br/> +—, insular, <a href="#Page351">351</a>.<br/> +<br/> +Man, origin of, <a href="#Page428">428</a>.<br/> +<br/> +Manatee, rudimentary nails of, <a href="#Page400">400</a>.<br/> +<br/> +Marsupials of Australia, <a href="#Page90">90</a>.<br/> +—, , fossil species of, <a href="#Page382">382</a>.<br/> +—, , structure of their feet, <a href="#Page310">310</a>.<br/> +<br/> +Martens, M., experiment on seeds, <a href="#Page325">325</a>.<br/> +<br/> +Martin, Mr. W.C., on striped mules, <a href="#Page129">129</a>.<br/> +<br/> +Masters, Dr., on Saponaria, <a href="#Page174">174</a>.<br/> +<br/> +Matteucci, on the electric organs of rays, <a href="#Page150">150</a>.<br/> +<br/> +Matthiola, reciprocal crosses of, <a href="#Page244">244</a>.<br/> +<br/> +Maurandia, <a href="#Page197">197</a>.<br/> +<br/> +Means of dispersal, <a href="#Page323">323</a>.<br/> +<br/> +Melipona domestica, <a href="#Page220">220</a>.<br/> +<br/> +Merrill, Dr., on the American cuckoo, <a href="#Page212">212</a>.<br/> +<br/> +Metamorphism of oldest rocks, <a href="#Page287">287</a>.<br/> +<br/> +Mice destroying bees, <a href="#Page56">56</a>.<br/> +—, acclimatisation of, <a href="#Page113">113</a>.<br/> +—, tails of, <a href="#Page189">189</a>.<br/> +<br/> +Miller, Prof., on the cells of bees, <a href="#Page221">221</a>, <a href="#Page224">224</a>.<br/> +<br/> +Mirabilis, crosses of, <a href="#Page243">243</a>.<br/> +<br/> +Missel-thrush, <a href="#Page59">59</a>.<br/> +<br/> +Mistletoe, complex relations of, <a href="#Page02">2</a>.<br/> +<br/> +Mivart, Mr., on the relation of hair and teeth, <a href="#Page115">115</a>.<br/> +—, on the eyes of cephalopods, <a href="#Page151">151</a>.<br/> +—, various objections to Natural Selection, <a href="#Page174">174</a>.<br/> +—, on abrupt modifications, <a href="#Page201">201</a>.<br/> +—, on the resemblance of the mouse and antechinus, <a href="#Page373">373</a>.<br/> +<br/> +Mocking-thrush of the Galapagos, <a href="#Page357">357</a>.<br/> +<br/> +Modification of species, not abrupt, <a href="#Page424">424</a>.<br/> +<br/> +Moles, blind, <a href="#Page110">110</a>.<br/> +<br/> +Molothrus, habits of, <a href="#Page215">215</a>.<br/> +<br/> +Mongrels, fertility and sterility of, <a href="#Page255">255</a>.<br/> +—, and hybrids compared, <a href="#Page259">259</a>.<br/> +<br/> +Monkeys, fossil, <a href="#Page284">284</a>, <a href="#Page285">285</a>.<br/> +<br/> +Monachanthus, <a href="#Page372">372</a>.<br/> +<br/> +Mons, Van, on the origin of fruit-trees, <a href="#Page21">21</a>.<br/> +<br/> +Monstrosities, <a href="#Page33">33</a>.<br/> +<br/> +Moquin-Tandon, on sea-side plants, <a href="#Page107">107</a>.<br/> +<br/> +Morphology, <a href="#Page382">382</a>.<br/> +<br/> +Morren, on the leaves of Oxalis, <a href="#Page197">197</a>.<br/> +<br/> +Moths, hybrid, <a href="#Page240">240</a>.<br/> +<br/> +Mozart, musical powers of, <a href="#Page206">206</a>.<br/> +<br/> +Mud, seeds in, <a href="#Page345">345</a>.<br/> +<br/> +Mules, striped, <a href="#Page129">129</a>.<br/> +<br/> +Müller, Adolph, on the instincts of the cuckoo, <a href="#Page213">213</a>.<br/> +<br/> +Müller, Dr. Ferdinand, on Alpine Australian plants, <a href="#Page337">337</a>.<br/> +<br/> +Müller, Fritz, on dimorphic crustaceans, <a href="#Page35">35</a>, <a href="#Page233">233</a>.<br/> +—, on the lancelet, <a href="#Page99">99</a>.<br/> +—, on air-breathing crustaceans, <a href="#Page152">152</a>.<br/> +—, on climbing plants, <a href="#Page197">197</a>.<br/> +—, on the self-sterility of orchids, <a href="#Page238">238</a>.<br/> +—, on embryology in relation to classification, <a href="#Page368">368</a>.<br/> +—, on the metamorphoses of crustaceans, <a href="#Page390">390</a>, <a href="#Page395">395</a>.<br/> +—, on terrestrial and fresh-water organisms not undergoing any +metamorphosis, <a href="#Page394">394</a>.<br/> +<br/> +Multiplication of species not indefinite, <a href="#Page101">101</a>.<br/> +<br/> +Murchison, Sir, R., on the formations of Russia, <a href="#Page272">272</a>.<br/> +—, on azoic formations, <a href="#Page286">286</a>.<br/> +—, on extinction, <a href="#Page293">293</a>.<br/> +<br/> +Murie, Dr., on the modification of the skull in old age, <a href="#Page149">149</a>.<br/> +<br/> +Murray, Mr. A., on cave-insects, <a href="#Page111">111</a>.<br/> +<br/> +Mustela vison, <a href="#Page138">138</a>.<br/> +<br/> +Myanthus, <a href="#Page372">372</a>.<br/> +<br/> +Myrmecocystus, <a href="#Page231">231</a>.<br/> +<br/> +Myrmica, eyes of, <a href="#Page232">232</a>.<br/> +<br/> +Nägeli, on morphological characters, <a href="#Page170">170</a>.<br/> +<br/> +Nails, rudimentary, <a href="#Page400">400</a>.<br/> +<br/> +Nathusius, Von, on pigs, <a href="#Page159">159</a>.<br/> +<br/> +Natural history, future progress of, <a href="#Page426">426</a>.<br/> +—, selection, <a href="#Page62">62</a>.<br/> +—, system, <a href="#Page364">364</a>.<br/> +<br/> +Naturalisation of forms distinct from the indigenous species, <a href="#Page89">89</a>.<br/> +—, in New Zealand, <a href="#Page163">163</a>.<br/> +<br/> +Naudin, on analagous variations in gourds, <a href="#Page125">125</a>.<br/> +—, on hybrid gourds, <a href="#Page258">258</a>.<br/> +—, on reversion, <a href="#Page260">260</a>.<br/> +<br/> +Nautilus, Silurian, <a href="#Page286">286</a>.<br/> +<br/> +Nectar of plants, <a href="#Page73">73</a>.<br/> +<br/> +Nectaries, how formed, <a href="#Page73">73</a>.<br/> +<br/> +Nelumbium luteum, <a href="#Page346">346</a>.<br/> +<br/> +Nests, variation in, <a href="#Page208">208</a>, <a href="#Page228">228</a>, <a href="#Page234">234</a>.<br/> +<br/> +Neuter insects, <a href="#Page230">230</a>, <a href="#Page231">231</a>.<br/> +<br/> +New Zealand, productions of, not perfect, <a href="#Page163">163</a>.<br/> +—, naturalised products of, <a href="#Page309">309</a>.<br/> +—, fossil birds of, <a href="#Page310">310</a>.<br/> +—, glaciers of, <a href="#Page335">335</a>.<br/> +—, crustaceans of, <a href="#Page338">338</a>.<br/> +—, algæ of, <a href="#Page338">338</a>.<br/> +—, flora of, <a href="#Page354">354</a>.<br/> +—, number of plants of, <a href="#Page374">374</a>.<br/> +<br/> +Newman, Col., on humble-bees, <a href="#Page57">57</a>.<br/> +<br/> +Newton, Prof., on earth attached to a partridge’s foot, <a href="#Page328">328</a>.<br/> +<br/> +Newton, Sir I., attacked for irreligion, <a href="#Page421">421</a>.<br/> +<br/> +Nicotiana, crossed varieties of, <a href="#Page258">258</a>.<br/> +—, certain species very sterile, <a href="#Page243">243</a>.<br/> +<br/> +Nitsche, Dr., on the Polyzoa, <a href="#Page193">193</a>.<br/> +<br/> +Noble, Mr., on fertility of Rhododendron, <a href="#Page239">239</a>.<br/> +<br/> +Nodules, phosphatic, in azoic rocks, <a href="#Page287">287</a>.<br/> +<br/> +Oaks, variability of, <a href="#Page40">40</a>.<br/> +<br/> +Œnonis, small imperfect flowers of, <a href="#Page173">173</a>.<br/> +<br/> +Onites apelles, <a href="#Page108">108</a>.<br/> +<br/> +Orchids, fertilisation of, <a href="#Page154">154</a>.<br/> +—, the development of their flowers, <a href="#Page195">195</a>.<br/> +—, forms of, <a href="#Page372">372</a>.<br/> +<br/> +Orchis, pollen of, <a href="#Page151">151</a>.<br/> +<br/> +Organisation, tendency to advance, <a href="#Page97">97</a>.<br/> +<br/> +Organs of extreme perfection, <a href="#Page143">143</a>.<br/> +—, electric, of fishes, <a href="#Page150">150</a>.<br/> +—, of little importance, <a href="#Page156">156</a>.<br/> +—, homologous, <a href="#Page382">382</a>.<br/> +—, rudiments of, and nascent, <a href="#Page397">397</a>.<br/> +<br/> +Ornithorhynchus, <a href="#Page83">83</a>, <a href="#Page367">367</a>.<br/> +—, mammæ of, <a href="#Page190">190</a>.<br/> +<br/> +Ostrich not capable of flight, <a href="#Page180">180</a>.<br/> +—, habit of laying eggs together, <a href="#Page215">215</a>.<br/> +—, American, two species of, <a href="#Page318">318</a>.<br/> +<br/> +Otter, habits of, how acquired, <a href="#Page138">138</a>.<br/> +<br/> +Ouzel, water, <a href="#Page142">142</a>.<br/> +<br/> +Owen, Prof., on birds not flying, <a href="#Page108">108</a>.<br/> +—, on vegetative repetition, <a href="#Page118">118</a>.<br/> +—, on variability of unusually developed parts, <a href="#Page119">119</a>.<br/> +—, on the eyes of fishes, <a href="#Page145">145</a>.<br/> +—, on the swim-bladder of fishes, <a href="#Page148">148</a>.<br/> +—, on fossil horse of La Plata, <a href="#Page294">294</a>.<br/> +—, on generalised form, <a href="#Page301">301</a>.<br/> +—, on relation of ruminants and pachyderms, <a href="#Page303">303</a>.<br/> +—, on fossil birds of New Zealand, <a href="#Page310">310</a>.<br/> +—, on succession of types, <a href="#Page310">310</a>.<br/> +—, on affinities of the dugong, <a href="#Page365">365</a>.<br/> +—, on homologous organs, <a href="#Page383">383</a>.<br/> +—, on the metamorphosis of cephalopods, <a href="#Page390">390</a>.<br/> +<br/> +Pacific Ocean, faunas of, <a href="#Page317">317</a>.<br/> +<br/> +Pacini, on electric organs, <a href="#Page151">151</a>.<br/> +<br/> +Paley, on no organ formed to give pain, <a href="#Page163">163</a>.<br/> +<br/> +Pallas, on the fertility of the domesticated descendants of wild stocks, <a href="#Page241">241</a>.<br/> +<br/> +Palm with hooks, <a href="#Page158">158</a>.<br/> +<br/> +Papaver bracteatum, <a href="#Page174">174</a>.<br/> +<br/> +Paraguay, cattle destroyed by flies, <a href="#Page56">56</a>.<br/> +<br/> +Parasites, <a href="#Page215">215</a>.<br/> +<br/> +Partridge, with ball of dirt attached to foot, <a href="#Page328">328</a>.<br/> +<br/> +Parts greatly developed, variable, <a href="#Page119">119</a>.<br/> +<br/> +Parus major, <a href="#Page141">141</a>.<br/> +<br/> +Passiflora, <a href="#Page238">238</a>.<br/> +<br/> +Peaches in United States, <a href="#Page66">66</a>.<br/> +<br/> +Pear, grafts of, <a href="#Page246">246</a>.<br/> +<br/> +Pedicellariæ, <a href="#Page191">191</a>.<br/> +<br/> +Pelargonium, flowers of, <a href="#Page166">166</a>.<br/> +—, sterility of, <a href="#Page239">239</a>.<br/> +<br/> +Peloria, <a href="#Page116">116</a>.<br/> +<br/> +Pelvis of women, <a href="#Page115">115</a>.<br/> +<br/> +Period, glacial, <a href="#Page330">330</a>.<br/> +<br/> +Petrels, habits of, <a href="#Page142">142</a>.<br/> +<br/> +Phasianus, fertility of hybrids, <a href="#Page240">240</a>.<br/> +<br/> +Pheasant, young, wild, <a href="#Page211">211</a>.<br/> +<br/> +Pictet, Prof., on groups of species suddenly appearing, <a href="#Page282">282</a>.<br/> +—, on rate of organic change, <a href="#Page291">291</a>.<br/> +—, on continuous succession of genera, <a href="#Page293">293</a>.<br/> +—, on change in latest tertiary forms, <a href="#Page278">278</a>.<br/> +—, on close alliance of fossils in consecutive formations, <a href="#Page306">306</a>.<br/> +—, on early transitional links, <a href="#Page283">283</a>.<br/> +<br/> +Pierce, Mr., on varieties of wolves, <a href="#Page71">71</a>.<br/> +<br/> +Pigeons with feathered feet and skin between toes, <a href="#Page09">9</a>.<br/> +—, breeds described, and origin of, <a href="#Page15">15</a>.<br/> +—, breeds of, how produced, <a href="#Page28">28</a>, <a href="#Page30">30</a>.<br/> +—, tumbler, not being able to get out of egg, <a href="#Page68">68</a>.<br/> +—, reverting to blue colour, <a href="#Page127">127</a>.<br/> +—, instinct of tumbling, <a href="#Page210">210</a>.<br/> +—, young of, <a href="#Page392">392</a>.<br/> +<br/> +Pigs, black, not affected by the paint-root, <a href="#Page09">9</a>.<br/> +—, modified by want of exercise, <a href="#Page159">159</a>.<br/> +<br/> +Pistil, rudimentary, <a href="#Page397">397</a>.<br/> +<br/> +Plants, poisonous, not affecting certain coloured animals, <a href="#Page09">9</a>.<br/> +—, selection, applied to, <a href="#Page27">27</a>.<br/> +—, gradual improvement of, <a href="#Page27">27</a>.<br/> +—, not improved in barbarous countries, <a href="#Page27">27</a>.<br/> +—, dimorphic, <a href="#Page35">35</a>, <a href="#Page253">253</a>.<br/> +—, destroyed by insects, <a href="#Page53">53</a>.<br/> +—, in midst of range, have to struggle with other plants, <a href="#Page60">60</a>.<br/> +—, nectar of, <a href="#Page73">73</a>.<br/> +—, fleshy, on sea-shores, <a href="#Page107">107</a>.<br/> +—, climbing, <a href="#Page147">147</a>, <a href="#Page196">196</a>.<br/> +—, fresh-water, distribution of, <a href="#Page345">345</a>.<br/> +—, low in scale, widely distributed, <a href="#Page359">359</a>.<br/> +<br/> +Pleuronectidæ, their structure, <a href="#Page186">186</a>.<br/> +<br/> +Plumage, laws of change in sexes of birds, <a href="#Page70">70</a>.<br/> +<br/> +Plums in the United States, <a href="#Page66">66</a>.<br/> +<br/> +Pointer dog, origin of, <a href="#Page25">25</a>.<br/> +—, habits of, <a href="#Page210">210</a>.<br/> +<br/> +Poison not affecting certain coloured animals, <a href="#Page09">9</a>.<br/> +<br/> +Poison, similar effect of, on animals and plants, <a href="#Page425">425</a>.<br/> +<br/> +Pollen of fir-trees, <a href="#Page164">164</a>.<br/> +—, transported by various means, <a href="#Page154">154</a>, <a href="#Page161">161</a>.<br/> +<br/> +Pollinia, their development, <a href="#Page195">195</a>.<br/> +<br/> +Polyzoa, their avicularia, <a href="#Page193">193</a>.<br/> +<br/> +Poole, Col., on striped hemionus, <a href="#Page128">128</a>.<br/> +<br/> +Potemogeton, <a href="#Page346">346</a>.<br/> +<br/> +Pouchet, on the colours of flat-fish, <a href="#Page188">188</a>.<br/> +<br/> +Prestwich, Mr., on English and French eocene formations, <a href="#Page300">300</a>.<br/> +<br/> +Proctotrupes, <a href="#Page142">142</a>.<br/> +<br/> +Proteolepas, <a href="#Page118">118</a>.<br/> +<br/> +Proteus, <a href="#Page112">112</a>.<br/> +<br/> +Psychology, future progress of, <a href="#Page428">428</a>.<br/> +<br/> +Pyrgoma, found in the chalk, <a href="#Page284">284</a>.<br/> +<br/> +Quagga, striped, <a href="#Page129">129</a>.<br/> +<br/> +Quatrefages, M., on hybrid moths, <a href="#Page240">240</a>.<br/> +<br/> +Quercus, variability of, <a href="#Page40">40</a>.<br/> +<br/> +Quince, grafts of, <a href="#Page246">246</a>.<br/> +<br/> +Rabbit, disposition of young, <a href="#Page211">211</a>.<br/> +<br/> +Races, domestic, characters of, <a href="#Page12">12</a>.<br/> +<br/> +Race-horses, Arab, <a href="#Page26">26</a>.<br/> +—, English, <a href="#Page323">323</a>.<br/> +<br/> +Radcliffe, Dr., the electrical organs of the torpedo, <a href="#Page150">150</a>.<br/> +<br/> +Ramond, on plants of Pyrenees, <a href="#Page331">331</a>.<br/> +<br/> +Ramsay, Prof., on subaërial denudation, <a href="#Page267">267</a>.<br/> +—, on thickness of the British formations, <a href="#Page268">268</a>, <a href="#Page269">269</a>.<br/> +—, on faults, <a href="#Page268"></a>.<br/> +<br/> +Ramsay, Mr., on instincts of cuckoo, <a href="#Page213">213</a>.<br/> +<br/> +Ratio of increase, <a href="#Page50">50</a>.<br/> +<br/> +Rats, supplanting each other, <a href="#Page59">59</a>.<br/> +—, acclimatisation of, <a href="#Page113">113</a>.<br/> +—, blind, in cave, <a href="#Page110">110</a>.<br/> +<br/> +Rattle-snake, <a href="#Page162">162</a>.<br/> +<br/> +Reason and instinct, <a href="#Page205">205</a>.<br/> +<br/> +Recapitulation, general, <a href="#Page404">404</a>.<br/> +<br/> +Reciprocity of crosses, <a href="#Page243">243</a>.<br/> +<br/> +Record, geological, imperfect, <a href="#Page264">264</a>.<br/> +<br/> +Rengger, on flies destroying cattle, <a href="#Page56">56</a>.<br/> +<br/> +Reproduction, rate of, <a href="#Page50">50</a>.<br/> +<br/> +Resemblance, protective, of insects, <a href="#Page181">181</a>.<br/> +—, to parents in mongrels and hybrids, <a href="#Page260">260</a>.<br/> +<br/> +Reversion, law of inheritance, <a href="#Page11">11</a>.<br/> +—, in pigeons, to blue colour, <a href="#Page127">127</a>.<br/> +<br/> +Rhododendron, sterility of, <a href="#Page239">239</a>.<br/> +<br/> +Richard, Prof., on Aspicarpa, <a href="#Page367">367</a>.<br/> +<br/> +Richardson, Sir J., on structure of squirrels, <a href="#Page139">139</a>.<br/> +—, on fishes of the southern hemisphere, <a href="#Page338">338</a>.<br/> +<br/> +Robinia, grafts of, <a href="#Page246">246</a>.<br/> +<br/> +Rodents, blind, <a href="#Page110">110</a>.<br/> +<br/> +Rogers, Prof., Map of N. America, <a href="#Page274">274</a>.<br/> +<br/> +Rudimentary organs, <a href="#Page397">397</a>.<br/> +<br/> +Rudiments important for classification, <a href="#Page367">367</a>.<br/> +<br/> +Rütimeyer, on Indian cattle, <a href="#Page14">14</a>, <a href="#Page241">241</a>.<br/> +<br/> +Sageret, on grafts, <a href="#Page246">246</a>.<br/> +<br/> +Salamandra atra, <a href="#Page397">397</a>.<br/> +<br/> +Saliva used in nests, <a href="#Page228">228</a>.<br/> +<br/> +Salmons, males fighting, and hooked jaws of, <a href="#Page69">69</a>.<br/> +<br/> +Salt-water, how far injurious to seeds, <a href="#Page325">325</a>.<br/> +—, not destructive to land-shells, <a href="#Page353">353</a>.<br/> +<br/> +Salter, Mr., on early death of hybrid embryos, <a href="#Page249">249</a>.<br/> +<br/> +Salvin, Mr., on the beaks of ducks, <a href="#Page184">184</a>.<br/> +<br/> +Saurophagus sulphuratus, <a href="#Page141">141</a>.<br/> +<br/> +Schacht, Prof., on Phyllotaxy, <a href="#Page173">173</a>.<br/> +<br/> +Schiödte, on blind insects, <a href="#Page110">110</a>.<br/> +—, on flat-fish, <a href="#Page186">186</a>.<br/> +<br/> +Schlegel, on snakes, <a href="#Page115">115</a>.<br/> +<br/> +Schöbl, Dr., on the ears of mice, <a href="#Page172">172</a>.<br/> +<br/> +Scott, Mr. J., on the self-sterility of orchids, <a href="#Page238">238</a>.<br/> +—, on the crossing of varieties of verbascum, <a href="#Page258">258</a>.<br/> +<br/> +Sea-water, how far injurious to seeds, <a href="#Page325">325</a>.<br/> +—, not destructive to land-shells, <a href="#Page325">325</a>.<br/> +<br/> +Sebright, Sir J., on crossed animals, <a href="#Page15">15</a>.<br/> +<br/> +Sedgwick, Prof., on groups of species suddenly appearing, <a href="#Page282">282</a>.<br/> +<br/> +Seedlings destroyed by insects, <a href="#Page53">53</a>.<br/> +<br/> +Seeds, nutriment in, <a href="#Page60">60</a>.<br/> +—, winged, <a href="#Page117">117</a>.<br/> +—, means of dissemination, <a href="#Page154">154</a>, <a href="#Page161">161</a>, <a href="#Page327">327</a>, <a href="#Page328">328</a>.<br/> +—, power of resisting salt-water, <a href="#Page325">325</a>.<br/> +—, in crops and intestines of birds, <a href="#Page326">326</a>, <a href="#Page327">327</a>.<br/> +—, eaten by fish, <a href="#Page327">327</a>, <a href="#Page346">346</a>.<br/> +—, in mud, <a href="#Page345">345</a>.<br/> +—, hooked, on islands, <a href="#Page349">349</a>.<br/> +<br/> +Selection of domestic products, <a href="#Page22">22</a>.<br/> +—, principle not of recent origin, <a href="#Page27">27</a>.<br/> +—, unconscious, <a href="#Page27">27</a>.<br/> +—, natural, <a href="#Page62">62</a>.<br/> +—, sexual, <a href="#Page69">69</a>.<br/> +—, objections to term, <a href="#Page63">63</a>.<br/> +—, natural, has not induced sterility, <a href="#Page247">247</a>.<br/> +<br/> +Sexes, relations of, <a href="#Page69">69</a>.<br/> +<br/> +Sexual characters variable, <a href="#Page123">123</a>.<br/> +—, selection, <a href="#Page69">69</a>.<br/> +<br/> +Sheep, Merino, their selection, <a href="#Page23">23</a>.<br/> +—, two sub-breeds, unintentionally produced, <a href="#Page26">26</a>.<br/> +—, mountain, varieties of, <a href="#Page59">59</a>.<br/> +<br/> +Shells, colours of, <a href="#Page107">107</a>.<br/> +—, hinges of, <a href="#Page154">154</a>.<br/> +—, littoral, seldom embedded, <a href="#Page270">270</a>.<br/> +—, fresh-water, long retain the same forms, <a href="#Page308">308</a>.<br/> +—, fresh-water, dispersal of, <a href="#Page344">344</a>.<br/> +—, of Madeira, <a href="#Page349">349</a>.<br/> +—, land, distribution of, <a href="#Page349">349</a>.<br/> +—, land, resisting salt water, <a href="#Page325">325</a>.<br/> +<br/> +Shrew-mouse, <a href="#Page373">373</a>.<br/> +<br/> +Silene, infertility of crosses, <a href="#Page243">243</a>.<br/> +<br/> +Silliman, Prof., on blind rat, <a href="#Page110">110</a>.<br/> +<br/> +Sirenia, their affinities, <a href="#Page302">302</a>.<br/> +<br/> +Sitaris, metamorphosis of, <a href="#Page394">394</a>.<br/> +<br/> +Skulls of young mammals, <a href="#Page159">159</a>, <a href="#Page384">384</a>.<br/> +<br/> +Slave-making instinct, <a href="#Page216">216</a>.<br/> +<br/> +Smith, Col. Hamilton, on striped horses, <a href="#Page129">129</a>.<br/> +<br/> +Smith, Dr., on the Polyzoa, <a href="#Page193">193</a>.<br/> +<br/> +Smith, Mr. Fred., on slave-making ants, <a href="#Page217">217</a>.<br/> +—, on neuter ants, <a href="#Page231">231</a>.<br/> +<br/> +Snake with tooth for cutting through egg-shell, <a href="#Page214">214</a>.<br/> +<br/> +Somerville, Lord, on selection of sheep, <a href="#Page23">23</a>.<br/> +<br/> +Sorbus, grafts of, <a href="#Page246">246</a>.<br/> +<br/> +Sorex, <a href="#Page373">373</a>.<br/> +<br/> +Spaniel, King Charles’ breed, <a href="#Page25">25</a>.<br/> +<br/> +Specialisation of organs, <a href="#Page98">98</a>.<br/> +<br/> +Species, polymorphic, <a href="#Page35">35</a>.<br/> +—, dominant, <a href="#Page43">43</a>.<br/> +—, common, variable, <a href="#Page42">42</a>.<br/> +—, in large genera variable, <a href="#Page44">44</a>.<br/> +—, groups of, suddenly appearing, <a href="#Page282">282</a>, <a href="#Page285">285</a>.<br/> +—, beneath Silurian formations, <a href="#Page287">287</a>.<br/> +—, successively appearing, <a href="#Page290">290</a>.<br/> +—, changing simultaneously throughout the world, <a href="#Page297">297</a>.<br/> +<br/> +Spencer, Lord, on increase in size of cattle, <a href="#Page26">26</a>.<br/> +<br/> +Spencer, Mr. Herbert, on the first steps in differentiation, <a href="#Page100">100</a>.<br/> +—, on the tendency to an equilibrium in all forces, <a href="#Page252">252</a>.<br/> +<br/> +Sphex, parasitic, <a href="#Page216">216</a>.<br/> +<br/> +Spiders, development of, <a href="#Page390">390</a>.<br/> +<br/> +Sports in plants, <a href="#Page08">8</a>.<br/> +<br/> +Sprengel, C.C., on crossing, <a href="#Page76">76</a>.<br/> +—, on ray-florets, <a href="#Page116">116</a>.<br/> +<br/> +Squalodon, <a href="#Page302">302</a>.<br/> +<br/> +Squirrels, gradations in structure, <a href="#Page139">139</a>.<br/> +<br/> +Staffordshire, heath, changes in, <a href="#Page55">55</a>.<br/> +<br/> +Stag-beetles, fighting, <a href="#Page69">69</a>.<br/> +<br/> +Star fishes, eyes of, <a href="#Page144">144</a>.<br/> +—, their pedicellariæ, <a href="#Page192">192</a>.<br/> +<br/> +Sterility from changed conditions of life, <a href="#Page07">7</a>.<br/> +—, of hybrids, <a href="#Page236">236</a>.<br/> +—, laws of, <a href="#Page241">241</a>.<br/> +—, causes of, <a href="#Page247">247</a>.<br/> +—, from unfavourable conditions, <a href="#Page250">250</a>.<br/> +—, not induced through natural selection, <a href="#Page247">247</a>.<br/> +<br/> +St. Helena, productions of, <a href="#Page347">347</a>.<br/> +<br/> +St. Hilaire, Aug., on variability of certain plants, <a href="#Page174">174</a>.<br/> +—, on classification, <a href="#Page368">368</a>.<br/> +<br/> +St. John, Mr., on habits of cats, <a href="#Page209">209</a>.<br/> +<br/> +Sting of bee, <a href="#Page163">163</a>.<br/> +<br/> +Stocks, aboriginal, of domestic animals, <a href="#Page14">14</a>.<br/> +<br/> +Strata, thickness of, in Britain, <a href="#Page268">268</a>, <a href="#Page269">269</a>.<br/> +<br/> +Stripes on horses, <a href="#Page128">128</a>.<br/> +<br/> +Structure, degrees of utility of, <a href="#Page159">159</a>.<br/> +<br/> +Struggle for existence, <a href="#Page48">48</a>.<br/> +<br/> +Succession, geological, <a href="#Page290">290</a>.<br/> +—, of types in same areas, <a href="#Page310">310</a>.<br/> +<br/> +Swallow, one species supplanting another, <a href="#Page59">59</a>.<br/> +<br/> +Swaysland, Mr., on earth adhering to the feet of migratory birds, <a href="#Page328">328</a>.<br/> +<br/> +Swifts, nests of, <a href="#Page228">228</a>.<br/> +<br/> +Swim-bladder, <a href="#Page148">148</a>.<br/> +<br/> +Switzerland, lake habitations of, <a href="#Page13">13</a>.<br/> +<br/> +System, natural, <a href="#Page364">364</a>.<br/> +<br/> +Tail of giraffe, <a href="#Page157">157</a>.<br/> +—, of aquatic animals, <a href="#Page157">157</a>.<br/> +—, prehensile, <a href="#Page188">188</a>.<br/> +—, rudimentary, <a href="#Page400">400</a>.<br/> +<br/> +Tanais, dimorphic, <a href="#Page36">36</a>.<br/> +<br/> +Tarsi deficient, <a href="#Page108">108</a>.<br/> +<br/> +Tausch, Dr., on umbelliferæ, <a href="#Page173">173</a>.<br/> +<br/> +Teeth and hair correlated, <a href="#Page115">115</a>.<br/> +—, rudimentary, in embryonic calf, <a href="#Page397">397</a>, <a href="#Page420">420</a>.<br/> +<br/> +Tegetmeier, Mr., on cells of bees, <a href="#Page222">222</a>, <a href="#Page226">226</a>.<br/> +<br/> +Temminck, on distribution aiding classification, <a href="#Page369">369</a>.<br/> +<br/> +Tendrils, their development, <a href="#Page196">196</a>.<br/> +<br/> +Thompson, Sir W., on the age of the habitable world, <a href="#Page286">286</a>.<br/> +—, on the consolidation of the crust of the earth, <a href="#Page409">409</a>.<br/> +<br/> +Thouin, on grafts, <a href="#Page246">246</a>.<br/> +<br/> +Thrush, aquatic species of, <a href="#Page142">142</a>.<br/> +—, mocking, of the Galapagos, <a href="#Page356">356</a>.<br/> +—, young of, spotted, <a href="#Page388">388</a>.<br/> +—, nest of, <a href="#Page234">234</a>.<br/> +<br/> +Thuret, M., on crossed fuci, <a href="#Page243">243</a>.<br/> +<br/> +Thwaites, Mr., on acclimatisation, <a href="#Page112">112</a>.<br/> +<br/> +Thylacinus, <a href="#Page374">374</a>.<br/> +<br/> +Tierra del Fuego, dogs of, <a href="#Page211">211</a>.<br/> +—, plants of, <a href="#Page341">341</a>.<br/> +<br/> +Timber-drift, <a href="#Page326">326</a>.<br/> +<br/> +Time, lapse of, <a href="#Page266">266</a>.<br/> +—, by itself not causing modification, <a href="#Page81">81</a>.<br/> +<br/> +Titmouse, <a href="#Page141">141</a>.<br/> +<br/> +Toads on islands, <a href="#Page350">350</a>.<br/> +<br/> +Tobacco, crossed varieties of, <a href="#Page258">258</a>.<br/> +<br/> +Tomes, Mr., on the distribution of bats, <a href="#Page351">351</a>.<br/> +<br/> +Transitions in varieties rare, <a href="#Page134">134</a>.<br/> +<br/> +Traquair, Dr., on flat-fish, <a href="#Page188">188</a>.<br/> +<br/> +Trautschold, on intermediate varieties, <a href="#Page275">275</a>.<br/> +<br/> +Trees on islands belong to peculiar orders, <a href="#Page350">350</a>.<br/> +—, with separated sexes, <a href="#Page78">78</a>.<br/> +<br/> +Trifolium pratense, <a href="#Page57">57</a>, <a href="#Page75">75</a>.<br/> +—, incarnatum, <a href="#Page75">75</a>.<br/> +<br/> +Trigonia, <a href="#Page296">296</a>.<br/> +<br/> +Trilobites, <a href="#Page286">286</a>.<br/> +—, sudden extinction of, <a href="#Page297">297</a>.<br/> +<br/> +Trimen, Mr., on imitating-insects, <a href="#Page377">377</a>.<br/> +<br/> +Trimorphism in plants, <a href="#Page35">35</a>, <a href="#Page252">252</a>.<br/> +<br/> +Troglodytes, <a href="#Page234">234</a>.<br/> +<br/> +Tuco-tuco, blind, <a href="#Page110">110</a>.<br/> +<br/> +Tumbler pigeons, habits of, hereditary, <a href="#Page210">210</a>.<br/> +—, young of, <a href="#Page392">392</a>.<br/> +<br/> +Turkey-cock, tuft of hair on breast, <a href="#Page70">70</a>.<br/> +<br/> +—, naked skin on head, <a href="#Page158">158</a>.<br/> +—, young of, instinctively wild, <a href="#Page265">265</a>.<br/> +<br/> +Turnip and cabbage, analogous variations of, <a href="#Page125">125</a>.<br/> +<br/> +Type, unity of, <a href="#Page166">166</a>, <a href="#Page167">167</a>.<br/> +<br/> +Types, succession of, in same areas, <a href="#Page310">310</a>.<br/> +<br/> +Typotherium, <a href="#Page302">302</a>.<br/> +<br/> +Udders enlarged by use, <a href="#Page08">8</a>.<br/> +—, rudimentary, <a href="#Page397">397</a>.<br/> +<br/> +Ulex, young leaves of, <a href="#Page388">388</a>.<br/> +<br/> +Umbelliferæ, flowers and seeds of, <a href="#Page116">116</a>.<br/> +—, outer and inner florets of, <a href="#Page173">173</a>.<br/> +<br/> +Unity of type, <a href="#Page166">166</a>, <a href="#Page167">167</a>.<br/> +<br/> +Uria lacrymans, <a href="#Page72">72</a>.<br/> +<br/> +Use, effects of, under domestication, <a href="#Page08">8</a>.<br/> +—, effects of, in a state of nature, <a href="#Page108">108</a>.<br/> +<br/> +Utility, how far important in the construction of each part, <a href="#Page159">159</a>.<br/> +<br/> +Valenciennes, on fresh-water fish, <a href="#Page344">344</a>.<br/> +<br/> +Variability of mongrels and hybrids, <a href="#Page259">259</a>.<br/> +<br/> +Variation, under domestication, <a href="#Page05">5</a>.<br/> +—, caused by reproductive system being affected by conditions of life, <a href="#Page07">7</a>.<br/> +—, under nature, <a href="#Page33">33</a>.<br/> +—, laws of, <a href="#Page106">106</a>.<br/> +—, correlated, <a href="#Page09">9</a>, <a href="#Page114">114</a>, <a href="#Page159">159</a>.<br/> +<br/> +Variations appear at corresponding ages, <a href="#Page10">10</a>, <a href="#Page67">67</a>.<br/> +—, analogous in distinct species, <a href="#Page124">124</a>.<br/> +<br/> +Varieties, natural, <a href="#Page32">32</a>.<br/> +—, struggle between, <a href="#Page59">59</a>.<br/> +—, domestic, extinction of, <a href="#Page86">86</a>.<br/> +—, transitional, rarity of, <a href="#Page134">134</a>.<br/> +—, when crossed, fertile, <a href="#Page257">257</a>.<br/> +—, when crossed, sterile, <a href="#Page256">256</a>.<br/> +—, classification of, <a href="#Page371">371</a>.<br/> +<br/> +Verbascum, sterility of, <a href="#Page238">238</a>.<br/> +—, varieties of, crossed, <a href="#Page258">258</a>.<br/> +<br/> +Verlot, M., on double stocks, <a href="#Page230">230</a>.<br/> +<br/> +Verneuil, M. de, on the succession of species, <a href="#Page299">299</a>.<br/> +<br/> +Vibracula of the Polyzoa, <a href="#Page193">193</a>.<br/> +<br/> +Viola, small imperfect flowers of, <a href="#Page173">173</a>.<br/> +—, tricolor, <a href="#Page57">57</a>.<br/> +<br/> +Virchow, on the structure of the crystalline lens, <a href="#Page145">145</a>.<br/> +<br/> +Virginia, pigs of, <a href="#Page66">66</a>.<br/> +<br/> +Volcanic islands, denudation of, <a href="#Page268">268</a>.<br/> +<br/> +Vulture, naked skin on head, <a href="#Page158">158</a>.<br/> +<br/> +Wading-birds, <a href="#Page375">375</a>.<br/> +<br/> +Wagner, Dr., on Cecidomyia, <a href="#Page387">387</a>.<br/> +<br/> +Wagner, Moritz, on the importance of isolation, <a href="#Page81">81</a>.<br/> +<br/> +Wallace, Mr., on origin of species, <a href="#Page01">1</a>.<br/> +—, on the limit of variation under domestication, <a href="#Page31">31</a>.<br/> +—, on dimorphic lepidoptera, <a href="#Page36">36</a>, <a href="#Page232">232</a>.<br/> +—, on races in the Malay Archipelago, <a href="#Page37">37</a>.<br/> +—, on the improvement of the eye, <a href="#Page145">145</a>.<br/> +—, on the walking-stick insect, <a href="#Page182">182</a>.<br/> +—, on laws of geographical distribution, <a href="#Page322">322</a>.<br/> +—, on the Malay Archipelago, <a href="#Page351">351</a>.<br/> +—, on mimetic animals, <a href="#Page377">377</a>.<br/> +<br/> +Walsh, Mr. B.D., on phytophagic forms, <a href="#Page38">38</a>.<br/> +—, on equal variability, <a href="#Page125">125</a>.<br/> +<br/> +Water, fresh, productions of, <a href="#Page343">343</a>.<br/> +<br/> +Water-hen, <a href="#Page143">143</a>.<br/> +<br/> +Waterhouse, Mr., on Australian marsupials, <a href="#Page90">90</a>.<br/> +—, on greatly developed parts being variable, <a href="#Page119">119</a>.<br/> +—, on the cells of bees, <a href="#Page220">220</a>.<br/> +—, on general affinities, <a href="#Page379">379</a>.<br/> +<br/> +Water-ouzel, <a href="#Page142">142</a>.<br/> +<br/> +Watson, Mr. H.C., on range of varieties of British plants, <a href="#Page37">37</a>, <a href="#Page46">46</a>.<br/> +—, on acclimatisation, <a href="#Page112">112</a>.<br/> +—, on flora of Azores, <a href="#Page328">328</a>.<br/> +—, on rarity of intermediate varieties, <a href="#Page136">136</a>.<br/> +—, on Alpine plants, <a href="#Page331">331</a>.<br/> +—, on convergence, <a href="#Page100">100</a>.<br/> +—, on the indefinite multiplication of species, <a href="#Page101">101</a>.<br/> +<br/> +Weale, Mr., on locusts transporting seeds, <a href="#Page327">327</a>.<br/> +<br/> +Web of feet in water-birds, <a href="#Page142">142</a>.<br/> +<br/> +Weismann, Prof., on the causes of variability, <a href="#Page06">6</a>.<br/> +—, on rudimentary organs, <a href="#Page400">400</a>.<br/> +<br/> +West Indian islands, mammals of, <a href="#Page352">352</a>.<br/> +<br/> +Westwood, on species in large genera being closely allied to others, <a href="#Page45">45</a>.<br/> +—, on the tarsi of Engidæ, <a href="#Page124">124</a>.<br/> +—, on the antennæ of hymenopterous insects, <a href="#Page366">366</a>.<br/> +<br/> +Whales, <a href="#Page182">182</a>.<br/> +<br/> +Wheat, varieties of, <a href="#Page88">88</a>.<br/> +<br/> +White Mountains, flora of, <a href="#Page330">330</a>.<br/> +<br/> +Whittaker, Mr., on lines of escarpment, <a href="#Page267">267</a>.<br/> +<br/> +Wichura, Max, on hybrids, <a href="#Page249">249</a>, <a href="#Page251">251</a>, <a href="#Page260">260</a>.<br/> +<br/> +Wings, reduction of size, <a href="#Page109">109</a>.<br/> +—, of insects homologous with branchiæ, <a href="#Page148">148</a>.<br/> +—, rudimentary, in insects, <a href="#Page397">397</a>.<br/> +<br/> +Wolf crossed with dog, <a href="#Page210">210</a>.<br/> +—, of Falkland Isles, <a href="#Page351">351</a>.<br/> +<br/> +Wollaston, Mr., on varieties of insects, <a href="#Page38">38</a>.<br/> +—, on fossil varieties of shells in Madeira, <a href="#Page42">42</a>.<br/> +—, on colours of insects on sea-shore, <a href="#Page107">107</a>.<br/> +—, on wingless beetles, <a href="#Page109">109</a>.<br/> +—, on rarity of intermediate varieties, <a href="#Page136">136</a>.<br/> +—, on insular insects, <a href="#Page347">347</a>.<br/> +—, on land-shells of Madeira naturalised, <a href="#Page357">357</a>.<br/> +<br/> +Wolves, varieties of, <a href="#Page71">71</a>.<br/> +<br/> +Woodcock with earth attached to leg, <a href="#Page328">328</a>.<br/> +<br/> +Woodpecker, habits of, <a href="#Page141">141</a>.<br/> +—, green colour of, <a href="#Page158">158</a>.<br/> +<br/> +Woodward, Mr., on the duration of specific forms, <a href="#Page276">276</a>.<br/> +—, on Pyrgoma, <a href="#Page284">284</a>.<br/> +—, on the continuous succession of genera, <a href="#Page293">293</a>.<br/> +—, on the succession of types, <a href="#Page311">311</a>.<br/> +<br/> +World, species changing simultaneously throughout, <a href="#Page297">297</a>.<br/> +<br/> +Wrens, nest of, <a href="#Page234">234</a>.<br/> +<br/> +Wright, Mr. Chauncey, on the giraffe, <a href="#Page178">178</a>.<br/> +—, on abrupt modifications, <a href="#Page203">203</a>.<br/> +<br/> +Wyman, Prof., on correlation of colour and effects of poison, <a href="#Page09">9</a>.<br/> +—, on the cells of the bee, <a href="#Page222">22</a>.<br/> +<br/> +Youatt, Mr., on selection, <a href="#Page23">23</a>.<br/> +—, on sub-breeds of sheep, <a href="#Page26">26</a>.<br/> +—, on rudimentary horns in young cattle, <a href="#Page400">400</a>.<br/> +<br/> +Zanthoxylon, <a href="#Page174">174</a>.<br/> +<br/> +Zebra, stripes on, <a href="#Page128">128</a>.<br/> +<br/> +Zeuglodon, <a href="#Page302">302</a>.<br/> +</p> + +</div><!--end chapter--> + +<hr /> + +<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK ON THE ORIGIN OF SPECIES ***</div> +<div style='text-align:left'> + +<div style='display:block; margin:1em 0'> +Updated editions will replace the previous one—the old editions will +be renamed. +</div> + +<div style='display:block; margin:1em 0'> +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. 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