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diff --git a/old/2009.txt b/old/2009.txt new file mode 100644 index 0000000..52f2291 --- /dev/null +++ b/old/2009.txt @@ -0,0 +1,21649 @@ +The Project Gutenberg EBook of On the Origin of Species, by Charles Darwin + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: On the Origin of Species + 6th Edition + +Author: Charles Darwin + +Release Date: December, 1999 [Etext #2009] +Posting Date: November 23, 2009 + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK ON THE ORIGIN OF SPECIES *** + + + + +Produced by Sue Asscher + + + + + +THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION; + +or, the PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE. + +By Charles Darwin, M.A., F.R.S., + +Author of "The Descent of Man," etc., etc. + + +Sixth London Edition, with all Additions and Corrections. + +The 6th Edition is often considered the definitive edition. + +Also see Project Gutenberg Etext #1228 for the First Edition. + + + +"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."--Whewell: "Bridgewater Treatise". + +"The only distinct meaning of the word 'natural' is STATED, FIXED or +SETTLED; since what is natural as much requires and presupposes an +intelligent agent to render it so, i.e., to effect it continually or at +stated times, as what is supernatural or miraculous does to effect it +for once."--Butler: "Analogy of Revealed Religion". + +"To conclude, therefore, let no man out of a weak conceit of sobriety, +or an ill-applied moderation, think or maintain, that a man can search +too far or be too well studied in the book of God's word, or in the book +of God's works; divinity or philosophy; but rather let men endeavour +an endless progress or proficience in both."--Bacon: "Advancement of +Learning". + + + + +AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF +SPECIES, + +PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK. + +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 (Aristotle, in his "Physicae +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.), 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. + +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 Vertebres". In these 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. (I have taken the date of the first publication of Lamarck +from Isidore Geoffroy Saint-Hilaire's ("Hist. Nat. Generale", 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.) + +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 "monde ambiant" 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 probleme a reserver entierement a l'avenir, suppose meme que +l'avenir doive avoir prise sur lui." + +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. + +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 "Amaryllidaceae" (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. + +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 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. + +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. + +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. + +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." + +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. + +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, +FIRST, 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 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; SECOND, 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. + +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. + +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 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." + +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 palaeontologists 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," etc.(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. + +M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850 (of +which a Resume appeared in the "Revue et Mag. de Zoolog.", Jan., 1851), +briefly gives his reason for believing that specific characters "sont +fixes, pour chaque espece, tant qu'elle se perpetue au milieu des memes +circonstances: ils se modifient, si les circonstances ambiantes viennent +a changer. En resume, L'OBSERVATION des animaux sauvages demontre deja +la variabilite LIMITEE des especes. Les EXPERIENCES sur les animaux +sauvages devenus domestiques, et sur les animaux domestiques redevenus +sauvages, la demontrent plus clairment encore. Ces memes experiences +prouvent, de plus, que les differences produites peuvent etre de VALEUR +GENERIQUE." In his "Hist. Nat. Generale" (tom. ii, page 430, 1859) he +amplifies analogous conclusions. + +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. + +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. + +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 Museum", tom. i, +page 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 mysterieuse, indeterminee; fatalite +pour les uns; pour les autres volonte providentielle, dont l'action +incessante sur les etres vivantes determine, a toutes les epoques de +l'existence du monde, la forme, le volume, et la duree de chacun d'eux, +en raison de sa destinee dans l'ordre de choses dont il fait partie. +C'est cette puissance qui harmonise chaque membre a l'ensemble, en +l'appropriant a la fonction qu'il doit remplir dans l'organisme general +de la nature, fonction qui est pour lui sa raison d'etre." (From +references in Bronn's "Untersuchungen uber die Entwickelungs-Gesetze", +it appears that the celebrated botanist and palaeontologist 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'Espece", it seems that +Bory St. Vincent, Burdach, Poiret and Fries, have all admitted that +new species are continually being produced. 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.) + +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. + +In this same year, 1853, Dr. Schaaffhausen published an excellent +pamphlet ("Verhand. des Naturhist. Vereins der Preuss. Rheinlands", +etc.), 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." + +A well-known French botanist, M. Lecoq, writes in 1854 ("Etudes sur +Geograph." Bot. tom. i, page 250), "On voit que nos recherches sur la +fixite ou la variation de l'espece, nous conduisent directement aux +idees emises par deux hommes justement celebres, 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. + +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, not a casual phenomenon," or, +as Sir John Herschel expresses it, "a natural in contradistinction to a +miraculous process." + +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. + +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. + +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." + +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. + +The first edition of this work was published on November 24, 1859, and +the second edition on January 7, 1860. + + + +CONTENTS. + + + INTRODUCTION + + + CHAPTER I. + + VARIATION UNDER DOMESTICATION. + + 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. + + + CHAPTER II. + + VARIATION UNDER NATURE. + + Variability--Individual Differences--Doubtful species--Wide ranging, + much diffused, and common species, vary most--Species of the larger + genera in 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. + + + CHAPTER III. + + STRUGGLE FOR EXISTENCE. + + 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. + + + CHAPTER IV. + + NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST. + + 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. + + + CHAPTER V. + + LAWS OF VARIATION. + + 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. + + + CHAPTER VI. + + DIFFICULTIES OF THE THEORY. + + 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. + + + CHAPTER VII. + + MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION. + + 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. + + + CHAPTER VIII. + + INSTINCT. + + Instincts comparable with habits, but different in their + origin--Instincts graduated--Aphides and ants--Instincts + variable--Domestic instincts, their origin--Natural instincts of + the cuckoo, 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. + + + CHAPTER IX. + + HYBRIDISM. + + Distinction between the sterility of first crosses and of + hybrids--Sterility various in degree, not universal, affected by close + interbreeding, removed by domestication--Laws governing the sterility + of hybrids--Sterility not a special endowment, but incidental on + other differences, 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. + + + CHAPTER X. + + ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + + On the absence of intermediate varieties at the present day--On the + nature of extinct intermediate varieties; on their number--On the 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 + palaeontological 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. + + + CHAPTER XI. + + ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + + On the slow and successive appearance of new species--On their different + rates of change--Species once lost do not reappear--Groups of species + follow the same general rules in their appearance and disappearance as + do single species--On extinction--On simultaneous changes in the forms + of life throughout the world--On the affinities of extinct species to + each other and to living species--On the state of development of + ancient forms--On the succession of the same types within the same + areas--Summary of preceding and present chapter. + + + CHAPTER XII. + + GEOGRAPHICAL DISTRIBUTION. + + Present distribution cannot be accounted for by differences in physical + conditions--Importance of barriers--Affinity of the productions of the + same continent--Centres of creation--Means of dispersal by changes of + climate and of the level of the land, and by occasional means--Dispersal + during the Glacial period--Alternate Glacial periods in the north and + south. + + + CHAPTER XIII. + + GEOGRAPHICAL DISTRIBUTION--CONTINUED. + + Distribution of fresh-water productions--On the inhabitants of oceanic + islands--Absence of Batrachians and of terrestrial Mammals--On + the relation of the inhabitants of islands to those of the nearest + mainland--On colonisation from the nearest source with subsequent + modification--Summary of the last and present chapter. + + + CHAPTER XIV. + + MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY--EMBRYOLOGY--RUDIMENTARY + ORGANS. + + Classification, groups subordinate to groups--Natural system--Rules and + difficulties in classification, explained on the theory of descent + with modification--Classification of varieties--Descent always used in + classification--Analogical or adaptive characters--Affinities, + general, complex and radiating--Extinction separates and defines + groups--Morphology, between members of the same class, between parts of + the same individual--Embryology, laws of, explained by variations not + supervening at an early age, and being inherited at a corresponding + age--Rudimentary Organs; their origin explained--Summary. + + + CHAPTER XV. + + RECAPITULATION AND CONCLUSION. + + 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. + + + GLOSSARY OF SCIENTIFIC TERMS. + + + INDEX. + + + + +ORIGIN OF SPECIES. + + + + +INTRODUCTION. + +When on board H.M.S. Beagle, as naturalist, I was much struck with +certain facts in the distribution of the 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. + +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. + +This abstract, which I now publish, must necessarily be imperfect. I +cannot here give references and authorities for my several statements; +and I must trust to the reader reposing some confidence in my accuracy. +No doubt errors 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. + +I much regret that want of space prevents my having the satisfaction of +acknowledging the generous assistance which I have received from very +many naturalists, some of them personally unknown to me. I cannot, +however, let this opportunity pass without expressing my deep +obligations to Dr. Hooker, who, for the last fifteen years, has aided me +in every possible way by his large stores of knowledge and his excellent +judgment. + +In considering the origin of species, it is quite conceivable that a +naturalist, reflecting on the mutual affinities of organic beings, +on their embryological relations, their geographical distribution, +geological succession, and other such facts, might come to the +conclusion that 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, etc., +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 of external conditions, +or of habit, or of the volition of the plant itself. + +It is, therefore, of the highest importance to gain a clear insight into +the means of modification and coadaptation. At the commencement of +my observations it seemed to me probable that a careful study of +domesticated animals and of cultivated plants would offer the best +chance of making out this obscure problem. Nor have I been disappointed; +in this and in all other perplexing cases I have invariably found that +our knowledge, imperfect though it be, of variation under domestication, +afforded the best and safest clue. I may venture to express my +conviction of the high value of such studies, although they have been +very commonly neglected by naturalists. + +From these considerations, I shall devote the first chapter of this +abstract to variation under domestication. We shall thus see that a +large amount of hereditary modification is at least possible; and, what +is equally or more important, we shall see how great is the power of man +in accumulating by his selection successive slight variations. I will +then pass on to the variability of species in a state of nature; but +I shall, unfortunately, be compelled to treat this subject far too +briefly, as it can be treated properly only by giving long catalogues of +facts. We shall, however, be enabled to discuss what circumstances +are most favourable to variation. In the next chapter the struggle +for existence 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 NATURALLY SELECTED. From +the strong principle of inheritance, any selected variety will tend to +propagate its new and modified form. + +This fundamental subject of natural selection will be treated at +some length in the fourth chapter; and we shall then see how natural +selection almost inevitably causes much extinction of the less improved +forms of life, and 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 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. + +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. + + + + +CHAPTER I. VARIATION UNDER DOMESTICATION. + + 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. + + +CAUSES OF VARIABILITY. + +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. + +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 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, etc. 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. + +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 of body or constitution, causing coughs or colds, rheumatism, or +inflammation of various organs. + +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 Kolreuter 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 cultivation, and vary very +slightly--perhaps hardly more than in a state of nature. + +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, etc., 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. + +EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED +VARIATION; INHERITANCE. + +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. + +Many laws regulate variation, some few of which can be dimly 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.e. 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. + +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, etc.; 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. + +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 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, etc., appearing in several members +of the same family. If strange and rare deviations of structure are +truly inherited, less strange and commoner deviations may be freely +admitted to be inheritable. Perhaps the correct way of viewing the whole +subject would be, to look at the inheritance of every character whatever +as the rule, and non-inheritance as the anomaly. + +The laws governing inheritance are 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 APPEARANCE of the peculiarity, and not +to the primary cause 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. + +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 DEFINITE 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. + +CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN +VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE +SPECIES. + +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. + +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 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. + +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. + +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 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. + +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 Canidae 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 Rutimeyer. 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. + +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, +etc., but that each of these kingdoms possesses several peculiar breeds +of cattle, sheep, etc., we must admit that many domestic breeds must +have originated in Europe; for whence otherwise could they have been +derived? So it is 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, etc.--so unlike all wild Canidae--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, +etc., 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. + +BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN. + +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, 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 oesophagus. The Jacobin has the feathers so much +reversed along the back of the neck that they form a hood, and it +has, proportionally to its size, 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. + +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 vertebrae +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 oesophagus; 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 scutellae on +the toes, the development of skin between the toes, are all points of +structure which are variable. The period at which the perfect plumage is +acquired varies, as does the state of the down with which the nestling +birds are clothed when hatched. The shape and size of the eggs vary. The +manner of flight, 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. + +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. + +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 +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. + +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 Columbidae 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. + +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 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. + +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, +inter se, seems to me rash in the extreme. + +From these several reasons, namely, the improbability of man having +formerly made seven or eight supposed species of pigeons to 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 +Columbidae, 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. + +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 Aegyptian +dynasty, about 3000 B.C., as was pointed out to me by Professor Lepsius; +but Mr. Birch informs me that pigeons are given in a bill of fare in +the previous dynasty. In the time of the Romans, as we hear from Pliny, +immense prices were given for pigeons; "nay, they are come to this pass, +that they can reckon up their pedigree and race." Pigeons were much +valued by Akber Khan in India, about the year 1600; never less than +20,000 pigeons were taken with the court. "The monarchs of Iran and +Turan sent him some very rare birds;" and, continues the courtly +historian, "His Majesty, by crossing the breeds, which method was never +practised before, has improved them astonishingly." About this same +period the Dutch were as eager about pigeons as were the old Romans. The +paramount importance of these considerations in explaining the immense +amount of variation which pigeons have undergone, will likewise be +obvious when 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. + +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? + +PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS. + +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. + +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 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. + +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. + +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 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. + +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. + +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 encyclopaedia. 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. + +UNCONSCIOUS SELECTION. + +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, etc., 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; but what concerns us is, that the change has been +effected unconsciously and gradually, and yet so effectually that, +though the old Spanish pointer certainly came from Spain, Mr. Borrow +has not seen, as I am informed by him, any native dog in Spain like our +pointer. + +By a similar process of selection, and by careful training, 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. + +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." + +If there exist savages so barbarous as never to think of the inherited +character of the offspring of their domestic animals, yet any one animal +particularly useful to them, for any special purpose, would be carefully +preserved during famines and other accidents, to which savages are +so liable, and such choice animals would thus generally leave more +offspring than the inferior ones; so that in this case there would be a +kind of unconscious selection going on. We see the value set on animals +even by the barbarians of Tierra del Fuego, by their killing and +devouring their old women, in times of dearth, as of less value than +their dogs. + +In plants the same gradual process of improvement through the occasional +preservation of the best individuals, whether or not sufficiently +distinct to be ranked at their first appearance as distinct varieties, +and whether or not two or more species or races have become blended +together by crossing, may plainly be recognised in the increased size +and beauty which we now see in the varieties of the heartsease, rose, +pelargonium, dahlia, and other plants, when compared with the older +varieties or with their parent-stocks. No one would ever expect to get +a first-rate heartsease or dahlia from the seed of a wild plant. No one +would expect to raise a first-rate melting pear from the seed of a wild +pear, though he might succeed from a poor seedling growing wild, if it +had come from a garden-stock. The pear, though cultivated in classical +times, appears, from Pliny's description, to have been a fruit of very +inferior quality. I have seen great surprise expressed in horticultural +works at the wonderful skill of gardeners in having produced such +splendid results from such poor materials; but the art 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. + +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. + +In regard to the domestic animals kept by uncivilised man, it should +not be overlooked that they almost always have to struggle for their +own food, at least during certain seasons. And in two countries very +differently circumstanced, individuals of the same species, having +slightly different constitutions or structure, would often succeed +better in the one country than in the other, and thus by a process of +"natural selection," as will hereafter be more fully explained, two +sub-breeds might be formed. This, perhaps, partly explains 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. + +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 oesophagus--a habit which is disregarded by +all fanciers, as it is not one of the points of the breed. + +Nor let it be thought that some great deviation of structure would +be necessary to catch the fancier's eye: he perceives extremely small +differences, and it is in human nature to value any novelty, however +slight, in one's own possession. Nor must the value which would formerly +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 fleeting of characters, have lately been exhibited as distinct +at our poultry-shows. + +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. + +CIRCUMSTANCES FAVOURABLE TO MAN'S POWER OF SELECTION. + +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 IN SMALL LOTS, 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 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. + +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, etc., may be attributed in main part +to selection not having been brought into play: in cats, from the +difficulty in pairing them; in donkeys, from only a few being kept by +poor people, and little attention paid to their breeding; 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 +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. + +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. + +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 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, etc., 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. + + + + +CHAPTER II. VARIATION UNDER NATURE. + + Variability--Individual differences--Doubtful species--Wide ranging, + much diffused, and common species, vary most--Species of the larger + genera in 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. + + +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. + +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 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. + +INDIVIDUAL DIFFERENCES. + +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 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 larvae +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. + +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. + +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 Muller has described +analogous but more extraordinary cases 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 antennae 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. + +DOUBTFUL SPECIES. + +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 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. + +Hence, in determining whether a form should be ranked as a species or +a variety, the opinion of naturalists having sound judgment and wide +experience seems the only guide to follow. We must, however, in many +cases, decide by a majority of naturalists, for few well-marked and +well-known varieties can be named which have not been ranked as species +by at least some competent judges. + +That varieties of this doubtful nature are far from uncommon cannot be +disputed. Compare the several floras of Great Britain, of France, or +of the United States, drawn up by different botanists, and see what +a surprising number of forms have been ranked by one botanist as good +species, and by another as mere varieties. Mr. H.C. Watson, to whom I +lie under deep obligation for assistance of all kinds, has marked for +me 182 British plants, which are generally considered as varieties, but +which have all been ranked by botanists as species; and in making this +list he has omitted many trifling varieties, but which nevertheless have +been ranked by some botanists as species, and he has entirely omitted +several highly polymorphic genera. Under genera, including the most +polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham +gives only 112--a difference of 139 doubtful forms! 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 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. + +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? + +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 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. + +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. + +Many of the cases of strongly marked varieties or doubtful species well +deserve consideration; for several interesting lines of argument, from +geographical distribution, analogical variation, hybridism, etc., have +been brought to bear 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 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. + +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 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 palaeontology, geographical botany and zoology, of +anatomical structure and classification." + +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. + +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. + +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 toward 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. + +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. + +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. + +WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST. + +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 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. + +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. + +SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN +THE SPECIES OF THE SMALLER GENERA. + +If the plants inhabiting a country as described in any Flora, be divided +into two equal masses, all those in the larger genera (i.e., 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. + +From looking at species as only strongly marked and well-defined +varieties, I was led to anticipate that the species of the larger genera +in each country would oftener present varieties, than the species of the +smaller genera; for wherever many closely related species (i.e., species +of the same genus) have been formed, many varieties or incipient species +ought, as a general rule, to be now forming. Where many large trees +grow, we expect to find saplings. Where many species of a genus have +been formed through variation, circumstances have been favourable +for variation; and hence we might expect that the circumstances would +generally 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. + +To test the truth of this anticipation I have arranged the plants of +twelve countries, and the coleopterous insects of two districts, into +two nearly equal masses, the species of the larger genera on one side, +and those of the smaller genera on the other side, and it has invariably +proved to be the case that a larger proportion of the species on the +side of the larger genera 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. + +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. + +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 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. + +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. + +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; 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. + +SUMMARY. + +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. + +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. + + + + +CHAPTER III. STRUGGLE FOR EXISTENCE. + + 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. + +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. + +Again, it may be asked, how is it that varieties, which I have called +incipient species, become ultimately converted into good and distinct +species, which in most cases obviously differ from each other far +more than do the varieties of the same species? How do those groups +of species, which constitute what are called distinct genera and which +differ from each other more than do the species of the same genus, +arise? All these results, as we shall more fully see in the next +chapter, follow 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. + +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. + +THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE. + +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. + +GEOMETRICAL RATIO OF INCREASE. + +A struggle for existence inevitably follows from the high rate at which +all organic beings tend to increase. Every being, which during its +natural lifetime produces several eggs or seeds, must suffer destruction +during some period of its life, and during some season or occasional +year, otherwise, on the principle of geometrical increase, its numbers +would quickly become so inordinately great that no country could support +the product. Hence, as more individuals are produced than can possibly +survive, there must in every case be a struggle for existence, either +one individual with another of the same species, or with the individuals +of distinct species, or with the physical conditions of life. It is the +doctrine of Malthus applied with manifold force to the whole animal and +vegetable kingdoms; for in this case there can be no artificial increase +of food, and no prudential restraint from marriage. Although some +species may be now increasing, more or less rapidly, in numbers, all +cannot do so, for the world would not hold them. + +There is no exception to the rule that every organic being naturally +increases at so high a rate, that, if not destroyed, the earth would +soon be covered by the progeny of a single pair. Even slow-breeding +man has doubled in twenty-five years, and at this rate, in less than +a thousand years, there would literally not be standing room for his +progeny. Linnaeus 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. + +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. + +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. + +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. + +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 check, mitigate the +destruction ever so little, and the number of the species will almost +instantaneously increase to any amount. + +NATURE OF THE CHECKS TO INCREASE. + +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. + +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 destroyed by beasts of prey; for even the tiger in India most +rarely dares to attack a young elephant protected by its dam. + +Climate plays an important part in determining the average numbers of +a species, and periodical seasons of extreme cold or drought 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 DIRECTLY 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. + +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. + +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. + +On the other hand, in many cases, a large stock of individuals of the +same species, relatively to the numbers of its enemies, is absolutely +necessary for its preservation. Thus we can easily raise plenty of +corn and rape-seed, etc., in our fields, because the seeds are in great +excess compared with the number of birds which feed on them; nor can +the birds, though having a superabundance of food at this one season, +increase in number proportionally to the supply of seed, as their +numbers are checked during 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. + +COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE +STRUGGLE FOR EXISTENCE. + +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 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. + +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 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! + +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, 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! + +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! + +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 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. + +STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE +SAME SPECIES. + +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. + +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. + +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. + +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. + +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 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. + +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. + + + + +CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST. + + 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. + + +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 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. + +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. + +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 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. + +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 +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. + +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? + +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 good; silently +and insensibly working, WHENEVER AND WHEREVER OPPORTUNITY OFFERS, at +the improvement of each organic being in relation to its organic and +inorganic conditions of life. We see nothing of these slow changes in +progress, until the hand of time has marked the long lapse of ages, and +then so imperfect is our view into long-past geological ages that we see +only that the forms of life are now different from what they formerly +were. + +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. + +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 +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. + +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, etc., 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. + +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. + +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 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. + +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 undergo +modification and improvement at the same time in the same area. + +SEXUAL SELECTION. + +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. + +Among birds, the contest is often of a more peaceful character. All +those who have attended to the subject, believe that there is the +severest rivalry between the males of many species to attract, by +singing, the females. The rock-thrush of Guiana, birds of paradise, +and some others, congregate, and successive males display 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. + +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. + +ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE +FITTEST. + +In order to make it clear how, as I believe, natural selection acts, I +must beg permission to give one or two imaginary illustrations. Let us +take the case of a wolf, which preys on various animals, securing some +by craft, some by strength, and some by fleetness; and let us suppose +that the fleetest prey, a deer for instance, had from any change in +the country increased in numbers, or that other prey had decreased +in numbers, during that season of the year when the wolf 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. + +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. + +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 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. + +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. + +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 district, competing with and conquering the unchanged +individuals on the margins of an ever-increasing circle. + +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 Leguminosae, 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. + +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. 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. + +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, +etc., 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 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. + +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 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. + +ON THE INTERCROSSING OF INDIVIDUALS. + +I must here introduce a short digression. In the case of animals +and plants with separated sexes, it is of course obvious that two +individuals must always (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 +Kolreuter. We shall presently see its importance; but I must here treat +the subject with extreme brevity, though I have the materials prepared +for an ample discussion. All vertebrate animals, all insects and some +other large groups of animals, pair for each birth. Modern research +has much diminished the number of supposed hermaphrodites and of real +hermaphrodites a large number pair; that is, two individuals regularly +unite for reproduction, which is all that concerns us. But still there +are many hermaphrodite animals which certainly do not habitually pair, +and a vast majority of plants are hermaphrodites. What reason, it may be +asked, is there for supposing in these cases that two individuals ever +concur in reproduction? As it is impossible here to enter on details, I +must trust to some general considerations alone. + +In the first place, I have collected so large a body of facts, 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 CLOSE 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. + +On the belief that this is a law of nature, we can, I think, understand +several large classes of facts, such as the following, which on any +other view are inexplicable. Every hybridizer knows how unfavourable +exposure to wet is to the fertilisation of a flower, yet what a +multitude of flowers have their anthers and stigmas fully exposed to the +weather! 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 Gartner, the influence of the +foreign pollen. + +When the stamens of a flower suddenly spring towards the pistil, or +slowly move one after the other towards it, the contrivance seems +adapted solely to ensure self-fertilisation; and no doubt it is useful +for this end: but the agency of insects is often required to cause the +stamens to spring forward, as Kolreuter 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 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! + +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 +VARIETY 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 +SPECIES 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. + +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 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. + +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. + +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. + +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. + +CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL +SELECTION. + +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? + +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 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. + +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. + +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. + +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 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, etc., 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. + +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. + +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. + +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 +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. + +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 +Europaeo-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 area, and from having been exposed to less varied, and +therefore less severe, competition. + +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. + +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 +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. + +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. + +EXTINCTION CAUSED BY NATURAL SELECTION. + +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. + +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. + +From these several considerations I think it inevitably follows, +that as new species in the course of time are formed through natural +selection, others will become rarer and rarer, and finally extinct. +The forms which stand in closest competition with those undergoing +modification and improvement, will naturally suffer most. And we have +seen in the chapter on the Struggle for Existence that it is the most +closely-allied forms,--varieties of the same species, and species of +the same genus or 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." + +DIVERGENCE OF CHARACTER. + +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. + +As has always been my practice, I have sought light on this 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, etc., 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. + +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. + +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 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. + +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 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. + +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. + +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. + +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 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. + +THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH +DIVERGENCE OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON +ANCESTOR. + +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. + +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 (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. + +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 a1 and m1. 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. + +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 a2, which will, owing to the principle +of divergence, differ more from (A) than did variety a1. Variety m1 is +supposed to have produced two varieties, namely m2 and s2, 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. + +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 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. + +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 a1 to a10. 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. + +After ten thousand generations, species (A) is supposed to have produced +three forms, a10, f10, and m10, 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 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 +a14 and m14, all descended from (A). Thus, as I believe, species are +multiplied and genera are formed. + +In a large genus it is probable that more than one species would vary. +In the diagram I have assumed that a second species (I) has produced, by +analogous steps, after ten thousand generations, either two well-marked +varieties (w10 and z10) 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 +n14 to z14, 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. + +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 quite new station, in which +offspring and progenitor do not come into competition, both may continue +to exist. + +If, then, our diagram be assumed to represent a considerable amount of +modification, species (A) and all the earlier varieties will have become +extinct, being replaced by eight new species (a14 to m14); and species +(I) will be replaced by six (n14 to z14) new species. + +But we may go further than this. The original species of our genus were +supposed to resemble each other in unequal degrees, as is so generally +the case in nature; species (A) being more nearly related to B, C, and +D than to the other species; and species (I) more to G, H, K, L, than to +the others. These two species (A and I), were also supposed to be very +common and widely diffused species, so that they must originally have +had some advantage over most of the other species of the genus. Their +modified descendants, fourteen in number at the fourteen-thousandth +generation, will probably have inherited some of the same advantages: +they have also been modified and improved in a diversified manner at +each stage of descent, so as to have become adapted to many related +places in the natural economy of their country. It seems, therefore, +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. + +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 a14 and z14 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 a14, q14, p14, will be nearly +related from having recently branched off from a10; b14 and f14, from +having diverged at an earlier period from a5, will be in some degree +distinct from the three first-named species; and lastly, o14, e14, and +m14, 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. + +The six descendants from (I) will form two sub-genera or genera. But as +the original species (I) differed largely from (A), standing 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. + +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. + +It is worth while to reflect for a moment on the character of the new +species F14, 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 (F14) 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. + +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. + +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 a14 to p14, those marked +b14 and f14, and those marked o14 to m14, 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. + +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 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. + +ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE. + +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, probably with more +truth, look at the plants which have their several organs much modified +and reduced in number as the highest. + +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. + +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, 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. + +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 +aerial 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 +Muller, 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. + +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 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. + +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. + +CONVERGENCE OF CHARACTER. + +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 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. + +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, etc.; 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 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, etc. 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 VERY 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. + +SUMMARY OF CHAPTER. + +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 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. + +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. + +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. + +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 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. + +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 occasionally see an animal like the Ornithorhynchus or +Lepidosiren, which in some small degree connects by its affinities two +large branches of life, and which has apparently been saved from fatal +competition by having inhabited a protected station. As buds give rise +by growth to fresh buds, and these, if vigorous, branch out and overtop +on all sides many a feebler branch, so by generation I believe it has +been with the great Tree of Life, which fills with its dead and broken +branches the crust of the earth, and covers the surface with its +ever-branching and beautiful ramifications. + + + + +CHAPTER V. LAWS OF VARIATION. + + Effects of 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. + + +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. + +It is very difficult to decide how far changed conditions, such as of +climate, food, etc., have acted in a definite manner. There is 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. + +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. + +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. + +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 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. + +EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED BY +NATURAL SELECTION. + +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. + +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 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. + +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. + +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 swimmers if they had not been able to swim +at all and had stuck to the wreck. + +The eyes of moles and of some burrowing rodents are rudimentary in size, +and in some cases are quite covered 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. + +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. + +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, Schiodte 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 +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 Schiodte 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 Schiodte'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 antennae 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 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. + +ACCLIMATISATION. + +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, +etc., 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, 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. + +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. + +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 Encyclopaedias +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 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. + +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. + +CORRELATED VARIATION. + +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 larvae 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 +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. + +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. + +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, etc.), 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. + +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 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 Compositae countenances this idea; +but with the Umbelliferae 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 +Compositae 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. + +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 +Umbelliferae these differences are of such apparent importance--the +seeds being sometimes orthospermous in the exterior flowers and +coelospermous 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. + +We may often falsely attribute to correlated variation structures which +are common to whole groups of species, and which in truth are simply +due to inheritance; for an ancient progenitor may have acquired through +natural selection some one modification in structure, and, after +thousands of generations, some other and independent modification; and +these two modifications, having been transmitted to a whole group of +descendants with diverse habits, would naturally be thought to be 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. + +COMPENSATION AND ECONOMY OF GROWTH. + +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. + +I suspect, also, that some of the cases of compensation which have been +advanced, and likewise some other facts, may be merged under a more +general principle, namely, that natural selection is continually +trying to economise in every part of the organisation. If under changed +conditions of life a structure, before useful, becomes less useful, 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 antennae. 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. + +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. + +MULTIPLE, RUDIMENTARY, AND LOWLY-ORGANISED STRUCTURES ARE VARIABLE. + +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 vertebrae 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, 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. + +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. + +A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, +IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY +VARIABLE. + +Several years ago I was much struck 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 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. + +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. + +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 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, etc., 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. + +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 GENERATIVE VARIABILITY, as it may be called, +still present in a high degree. For in this case the variability +will seldom as yet have been fixed by the continued selection of the +individuals varying in the required manner and degree, and by the +continued rejection of those tending to revert to a former and less +modified condition. + +SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS. + +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, +DIFFERS considerably in closely-allied species, it is often VARIABLE +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. + +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 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. + +SECONDARY SEXUAL CHARACTERS VARIABLE. + +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. + +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 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 Engidae, 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 +antennae 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. + +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 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. + +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. + +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. + +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 rock-pigeon, I presume that no +one will doubt that this is a case of reversion, and not of a new yet +analogous variation appearing in the several breeds. We may, I think, +confidently come to this conclusion, because, as we have seen, these +coloured marks are eminently liable to appear in the crossed offspring +of two distinct and differently coloured breeds; and in this case there +is nothing in the external conditions of life to cause the reappearance +of the slaty-blue, with the several marks, beyond the influence of the +mere act of crossing on the laws of inheritance. + +No doubt it is a very surprising fact that characters should reappear +after having been lost for many, 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 BOTH 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. + +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 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. + +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. + +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 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 NOT an albino, has been described without +either spinal or shoulder stripe; and these stripes are sometimes very +obscure, or actually quite lost, in dark-coloured asses. The koulan +of Pallas is said to have been seen with a double shoulder-stripe. +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. + +With respect to the horse, I have collected cases in England of the +spinal stripe in horses of the most distinct breeds, and of ALL colours; +transverse bars on the legs are not rare in duns, mouse-duns, and in one +instance in a chestnut; a faint shoulder-stripe may sometimes be seen +in duns, and I have seen a trace in a bay horse. My son made a careful +examination and sketch for me of a dun Belgian cart-horse with a double +stripe on each shoulder and with leg-stripes. I have myself seen a dun +Devonshire pony, and a small dun Welsh pony has been carefully described +to me, both with THREE parallel stripes on each shoulder. + +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 Eastern +China; and from Norway in the north to the Malay Archipelago in the +south. In all parts of the world these stripes occur far oftenest +in duns and mouse-duns; by the term dun a large range of colour is +included, from one between brown and black to a close approach to cream +colour. + +I am aware that Colonel Hamilton Smith, who has written on this subject, +believes that the several breeds of the horse 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, etc., inhabiting the most distant parts of the +world, should have all have been crossed with one supposed aboriginal +stock. + +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. + +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 TENDENCY in the young of each successive +generation to produce the long-lost character, and that this tendency, +from unknown causes, sometimes prevails. And we have just seen that in +several species of the horse genus the stripes are either plainer or +appear more commonly in the young than in the old. Call the breeds of +pigeons, some of which have bred true for centuries, species; and how +exactly parallel is the case with that of the species of the horse +genus! For myself, I venture confidently to look back thousands on +thousands of generations, and I see an animal striped like a zebra, but +perhaps otherwise very differently constructed, the common parent of +our domestic horse (whether or not it be descended from one or more wild +stocks) of the ass, the hemionus, quagga, and zebra. + +He who believes that each equine species was independently created, +will, I presume, assert that each species has been created with a +tendency to vary, both under nature and under domestication, in this +particular manner, so as often to become striped like 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, but had been created in stone so as to mock the shells now +living on the sea-shore. + +SUMMARY. + +Our ignorance of the laws of variation is profound. Not in one case out +of a hundred can we pretend to assign any reason why this or that part +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, 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. + +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. + + + + +CHAPTER VI. DIFFICULTIES OF THE THEORY. + + 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. + + +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. + +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? + +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? + +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? + +Fourthly, how can we account for species, when crossed, being sterile +and producing sterile offspring, whereas, when varieties are crossed, +their fertility is unimpaired? + +The two first heads will be here discussed; some miscellaneous +objections in the following chapter; Instinct and Hybridism in the two +succeeding chapters. + +ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES. + +As natural selection acts solely by the preservation of profitable +modifications, each new form will tend in a fully-stocked country to +take the place of, and finally to exterminate, its own less improved +parent-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. + +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. + +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 quite confounded +me. But I think it can be in large part explained. + +In the first place we should be extremely cautious in inferring, because +an area is now continuous, that it has been continuous during a long +period. Geology would lead us to believe that 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. + +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 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. + +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. + +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 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. + +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. + +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. + +Thirdly, when two or more varieties have been formed in different +portions of a strictly continuous area, intermediate varieties will, it +is probable, at first have been formed in the intermediate zones, but +they will generally have had a short duration. For these intermediate +varieties will, from reasons already assigned (namely from what we know +of the actual distribution of closely allied or representative species, +and likewise of acknowledged varieties), exist in the intermediate zones +in lesser numbers than the varieties which they tend to connect. From +this cause alone the intermediate varieties will be liable to accidental +extermination; and during the process of further modification through +natural selection, they will almost certainly be beaten and supplanted +by the forms which they connect; for these, from existing in greater +numbers will, in the aggregate, present more varieties, and thus be +further improved through natural selection and gain further advantages. + +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. + +ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS AND +STRUCTURE. + +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. + +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 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. + +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. + +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 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. + +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. + +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? + +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 had come to a high stage of perfection, +so as to have given them a decided advantage over other animals in +the battle for life. Hence the chance of discovering species with +transitional grades of structure in a fossil condition will always be +less, from their having existed in lesser numbers, than in the case of +species with fully developed structures. + +I will now give two or three instances, 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. + +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 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. + +Petrels are the most aerial 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. + +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, 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. + +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. + +ORGANS OF EXTREME PERFECTION AND COMPLICATION. + +To suppose that the eye with all its inimitable contrivances for +adjusting the focus to different distances, for admitting different +amounts of light, and for the correction of spherical and chromatic +aberration, could have been formed by natural selection, seems, I freely +confess, absurd in the highest 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 Vox populi, vox Dei, +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 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. + +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. + +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. + +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 organs in the Articulata are so +much diversified that Muller formerly made three main classes with seven +subdivisions, besides a fourth main class of aggregated simple eyes. + +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. + +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 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. + +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? + +MODES Of TRANSITION. + +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 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. + +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. + +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 branchiae 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 aerial 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. + +The illustration of the swim-bladder in fishes is a good one, because +it shows us clearly the highly important fact that an organ originally +constructed for one purpose, namely flotation, may be converted into +one for a 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. + +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 branchiae 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 branchiae 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. + +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 branchiae, the whole surface of +the body and of the sack, together with the small frena, serving for +respiration. The Balanidae 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 lacunae of the sack and body, and which +have been considered by all naturalists to act as branchiae. Now I +think no one will dispute that the ovigerous frena in the one family are +strictly homologous with the branchiae of the other family; indeed, 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 branchiae, 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 branchiae in this latter family had +originally existed as organs for preventing the ova from being washed +out of the sack? + +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 +Muller, 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. + +SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION. + +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. + +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. + +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 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. + +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 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. + +Fritz Muller, 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 +Muller, 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 branchiae, 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, +while all the other important organs were closely similar, or rather, +identical. + +Fritz Muller 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. Muller 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 Muller +to accept the views maintained by me in this volume. + +Another distinguished zoologist, the late Professor Claparede, has +argued in the same manner, and has arrived at the same result. He +shows that there are parasitic mites (Acaridae), 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 maxillae or lips, and of appendages on the under side of the hind +part of the body. + +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 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. + +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. Cruger 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 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. Cruger 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. Cruger 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. Cruger 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. + +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. + +How, it may be asked, in the foregoing and in innumerable other +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. + +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. + +ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL SELECTION. + +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 great, though of +a very different kind, as in the case of the most perfect and complex +organs. + +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. + +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. + +In the second place, we may easily err in attributing importance to +characters, and in believing that they have been developed 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, etc., 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. + +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. + +We are profoundly ignorant of the cause of each slight variation 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. + +UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW ACQUIRED. + +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 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. + +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 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 diatomaceae: +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. + +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 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. + +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. + +Natural selection will never produce in a being any structure more +injurious than beneficial to that being, for natural selection acts +solely by and for the good of each. No organ will be formed, as Paley +has remarked, for the purpose of causing pain or for doing an injury +to its possessor. If a fair balance be struck between the good and evil +caused by each part, each will be found on the whole advantageous. After +the lapse of time, under changing conditions of life, if any part comes +to be injurious, it will be modified; or if it be not so, the being will +become extinct, as myriads have become extinct. + +Natural selection tends only to make each organic being as perfect as, +or slightly more perfect than the other inhabitants of the same country +with which it 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 Muller 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? + +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, 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? + +SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CONDITIONS OF EXISTENCE +EMBRACED BY THE THEORY OF NATURAL SELECTION. + +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. + +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. + +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. + +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. + +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. + +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, 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. + +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. + +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. + +It is generally acknowledged that all organic beings have been formed on +two great laws--Unity of Type, and the Conditions of Existence. By unity +of type is meant that fundamental agreement in structure which we see +in organic beings of the same class, and which is quite independent of +their habits of life. On my theory, unity of type is explained by unity +of descent. The expression of conditions of existence, so often insisted +on by the illustrious Cuvier, is fully embraced by the principle of +natural selection. For natural selection acts by either now adapting the +varying parts of each being to its organic and inorganic conditions +of life; or by having adapted them during 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. + + + + +CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION. + + 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. + + +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. + +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 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. + +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. + +The celebrated palaeontologist, 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, +etc., 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. + +Bronn also insists that distinct species never differ from each other 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. + +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 Nageli 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, etc. + +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. + +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. + +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 be of no special use, I may +mention that, according to Dr. Schobl, 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. + +With respect to plants, to which on account of Nageli'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. + +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. + +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 columnae five of the +alternate stamens are 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 Malpighiaceae 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. + +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 Compositae and Umbelliferae (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 Compositae the central achenes +alone are furnished with a pappus; and in Hyoseris the same head yields +achenes of three different forms. In certain Umbelliferae the exterior +seeds, according to Tausch, are orthospermous, and the central one +coelospermous, and this is a character which was considered by De +Candolle to be in other species of the highest systematic 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. + +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, etc., 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 aestivation is almost as +frequently that of the Rhinanthideae as of the Antirrhinideae, to which +latter tribe the genus belongs. Aug. St. Hilaire gives the following +cases: the genus Zanthoxylon belongs to a division of the Rutaceae 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 PLUS OU MOINS LARGE, +s'etend 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 oleaeformis two forms which he did +not at first doubt were distinct species, but he subsequently saw them +growing on the same bush; and he then adds, "Voila donc dans un meme +individu des loges et un style qui se rattachent tantot a un axe +verticale et tantot a un gynobase." + +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 Nageli'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. + +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 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, etc., 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. + +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. + +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 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. + +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. + +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, etc., 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 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. + +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. + +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. + +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 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. + +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. + +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 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, etc.--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. + +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, 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. + +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. + +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 ALL DIRECTIONS, 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." + +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. + +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. + +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 jaw, of about 300 +plates or laminae, 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 Balaenoptera +rostrata only about nine inches in length. The quality of the whalebone +also differs in the different species. + +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 Criblatores, 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 lamellae by +finely graduated steps, each of service to its possessor. + +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 lamellae, 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 lamellae. 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 length of the head of a moderately large Balaenoptera 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 Balaenoptera, +the lamellae 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 lamellae 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 lamellae 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 lamellae, 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. + +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 lamellae 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 lamellae are considerably less developed than in the common +duck; but I do not know whether they use their beaks for sifting the +water. + +Turning to another group of the same family. In the Egyptian goose +(Chenalopex) the beak closely resembles that of the common duck; but the +lamellae 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 lamellae 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 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 lamellae are less developed +than in the common goose. + +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 lamellae, 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. + +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 lamellae 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 lamellae 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 lamellae would be +two-thirds of the length of the plates of baleen in the Balaenoptera +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 struggle for existence, and +that the structure of every part of its frame must be well adapted to +its conditions of life. + +The Pleuronectidae, 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 Pleuronectidae are admirably adapted by +their flattened and asymmetrical structure for their habits of life, +is manifest from several species, such as soles, flounders, etc., 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 Schiodte +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." + +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 Pleuronectidae, 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 swim-bladder. 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. + +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 Pleuronectidae, 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. Schiodte believes, in opposition to some other +naturalists, that the Pleuronectidae 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 Pleuronectidae, 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. Gunther, concludes his abstract of Malm's paper, by +remarking that "the author gives a very simple explanation of the +abnormal condition of the Pleuronectoids." + +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 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. + +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 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. +Gunther, 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. + +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 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. + +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 IS 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. + +We will now turn for a short space to the lower divisions of the +animal kingdom. The Echinodermata (star-fishes, sea-urchins, etc.) are +furnished with remarkable organs, called pedicellariae, 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. + +With respect to these organs, Mr. Mivart, as on so many previous +occasions, asks: "What would be the utility of the FIRST RUDIMENTARY +BEGINNINGS of such structures, and how could such insipient buddings +have ever preserved the life of a single Echinus?" He adds, "not even +the SUDDEN 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 +pedicellariae, 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. + +With respect to the steps by which these curious organs have been +evolved, Mr. Agassiz infers from his own researches and those of Mr. +Muller, that both in star-fishes and sea-urchins the pedicellariae 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 pedicellariae. +The gradation extends even to the manner in which ordinary spines +and the pedicellariae, with their supporting calcareous rods, are +articulated to the shell. In certain genera of star-fishes, "the very +combinations needed to show that the pedicellariae 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 pedicellariae, 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 pedicellariae 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 +pedicellariae, would be of service. + +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 pedicellariae. All possible gradations, as he adds, +may likewise be found between the pedicellariae of the star-fishes and +the hooks of the Ophiurians, another group of the Echinodermata; and +again between the pedicellariae of sea-urchins and the anchors of the +Holothuriae, also belonging to the same great class. + +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 tremble. When the jaws are touched with a needle they seize +it so firmly that the branch can thus be shaken. + +Mr. Mivart adduces this case, chiefly on account of the supposed +difficulty of organs, namely the avicularia of the Polyzoa and the +pedicellariae 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 pedicellariae and +avicularia. The latter resembles somewhat more closely the chelae 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. + +As the chelae 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 chelae of a lobster. And all these +gradations can be actually traced. + +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 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. + +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. + +In the vegetable kingdom Mr. Mivart only alludes to two cases, 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 +ORIGIN 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. + +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, 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. + +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. + +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 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 Muller 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. + +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. + +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. + +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. + +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. + +With the giraffe, the continued preservation of the individuals of some +extinct high-reaching ruminant, which had the longest necks, legs, etc., +and could browse a little above the average height, and 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 lamellae, like those of +the domestic ducks--and then into lamellae, 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 +lamellae are first used as teeth, then partly as teeth and partly as +a sifting apparatus, and at last almost exclusively for this latter +purpose. + +With such structures as the above lamellae 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 mammae 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 +pedicellariae, 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 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. + +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." + +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. + +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. + +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, etc.; 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. + +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 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. + +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, etc., 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. + +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 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. + +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. + +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 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. + +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. + + + + +CHAPTER VIII. INSTINCT. + + Instincts comparable with habits, but different in their + origin--Instincts graduated--Aphides and ants--Instincts + variable--Domestic instincts, their origin--Natural instincts of + the cuckoo, 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. + + +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. + +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. + +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. 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. + +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. + +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 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. + +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, etc.; 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. + +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 antennae; 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 antennae on the abdomen +first of one aphis and then of another; and each, as soon as it felt the +antennae, immediately lifted up its abdomen and excreted a limpid drop +of sweet juice, which was eagerly devoured by the ant. Even the quite +young aphides behaved in this manner, showing that the action was +instinctive, and not the result of experience. 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. + +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 are not more fearful than small; and the +magpie, so wary in England, is tame in Norway, as is the hooded crow in +Egypt. + +That the 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. + +INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS. + +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 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. + +How strongly these domestic instincts, habits, and dispositions are +inherited, and how curiously they become mingled, is well shown when +different breeds of dogs are crossed. Thus it is known that a cross with +a bull-dog has affected for many generations the courage and obstinacy +of greyhounds; and a cross with a greyhound has given to a whole family +of shepherd-dogs a tendency to hunt hares. These domestic instincts, +when thus tested by crossing, resemble natural instincts, which in a +like manner become curiously blended together, and for a long period +exhibit traces of the instincts of either parent: for example, Le Roy +describes a dog, whose great-grandfather was a wolf, and this dog +showed a trace of its wild parentage only in one way, by not coming in a +straight line to his master, when called. + +Domestic instincts are sometimes spoken of as actions which have become +inherited solely from long-continued and compulsory habit, but this 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 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. + +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. + +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 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. + +SPECIAL INSTINCTS. + +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. + +INSTINCTS OF THE CUCKOO. + +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 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 Muller, 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. + +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! + +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 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. + +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. + +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?" + +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 +Gallinaceae, 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, 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. + +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 Sphegidae (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 larvae, 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. + +SLAVE-MAKING INSTINCT. + +This remarkable instinct was first discovered in the Formica (Polyerges) +rufescens by Pierre Huber, a better observer even than his celebrated +father. This ant is absolutely dependent on its slaves; without their +aid, the species would certainly become extinct in a single year. The +males and fertile females do no work 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 larvae. 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 larvae and pupae 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 larvae, 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. + +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 larvae and pupae 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 +on the slaves being captured in greater numbers in Switzerland than in +England. + +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 +pupae to rear as slaves. I then dug up a small parcel of the pupae of F. +fusca from another nest, and put them down on a bare spot near the place +of combat; they were eagerly seized and carried off by the tyrants, who +perhaps fancied that, after all, they had been victorious in their late +combat. + +At the same time I laid on the same place a small parcel of the pupae 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 pupae 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 pupae of F. +fusca, whereas they were much terrified when they came across the pupae, +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 pupae. + +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 pupae. I traced a long file of ants burdened 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. + +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 larvae, 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 +larvae. So that the masters in this country receive much less service +from their slaves than they do in Switzerland. + +By what steps the instinct of F. sanguinea originated I will not pretend +to conjecture. But as ants which are not slave-makers, will, as I have +seen, carry off pupae of other species, if scattered near their nests, +it is possible that such pupae 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 pupae, 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 instinct--always supposing each modification to be of use to +the species--until an ant was formed as abjectly dependent on its slaves +as is the Formica rufescens. + +CELL-MAKING INSTINCT OF THE HIVE-BEE. + +I will not here enter on minute details on this subject, but will merely +give an outline of the conclusions at which I have arrived. He must be +a dull man who can examine the exquisite structure of a comb, so +beautifully adapted to its end, without enthusiastic admiration. We +hear from mathematicians that bees have practically solved a recondite +problem, and have made their cells of the proper shape to hold the +greatest possible amount of honey, with the least possible consumption +of precious wax in their construction. It has been remarked that a +skilful workman, with fitting tools and measures, would find it very +difficult to make cells of wax of the true form, though this is 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. + +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 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. + +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:-- + +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 +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. + +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. + +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.e. about the +width of an ordinary cell), and were in depth about one sixth of the +diameter of the sphere of which they formed a part, the rims of the +basins intersected or broke into each other. As soon as this occurred, +the bees ceased to excavate, and began to build up flat walls of wax +on the lines of intersection between the basins, so that each hexagonal +prism was built upon the scalloped edge of a smooth basin, instead of on +the straight edges of a three-sided pyramid as in the case of ordinary +cells. + +I then put into the hive, instead of a thick, rectangular piece of wax, +a thin and narrow, knife-edged ridge, coloured with vermilion. The bees +instantly began on both sides to excavate little basins near to each +other, in the same way as before; but the ridge of wax was so thin, that +the bottoms of the basins, if they had been excavated to the same depth +as in the former experiment, would have broken into each other from the +opposite sides. The bees, however, did not suffer this to happen, and +they stopped their excavations in due time; so that the basins, as soon +as they had been a little deepened, came to have flat 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. + +Considering how flexible thin wax is, I do not see that there is any +difficulty in the bees, whilst at work on the two sides of a strip +of wax, perceiving when they have gnawed the wax away to the proper +thinness, and then stopping their work. In ordinary combs it has +appeared to me that the bees do not always succeed in working at exactly +the same rate from the opposite sides; for I have noticed half-completed +rhombs at the base of a just-commenced cell, which were slightly concave +on one side, where I suppose that the bees had excavated too quickly, +and convex on the opposed side where the bees had worked less quickly. +In one well-marked instance, I put the comb back into the hive, and +allowed the bees to go on working for a short time, and again examined +the cell, and I found that the rhombic plate had been completed, and had +become PERFECTLY FLAT: it was absolutely impossible, from the extreme +thinness of the little plate, that they could have effected this by +gnawing away the convex side; and I suspect that the bees in such cases +stand in the opposed cells and push and bend the ductile and warm wax +(which as I have tried is easily done) into its proper intermediate +plane, and thus flatten it. + +From the experiment of the ridge of vermilion wax we can see that, if +the bees were to build for themselves a thin wall of wax, they could +make their cells of the proper shape, by standing at the proper distance +from each other, by excavating at the same rate, and by endeavouring to +make equal spherical hollows, but never allowing the spheres to break +into each other. Now bees, as may be clearly seen by examining the edge +of a growing comb, do make a rough, circumferential wall or rim all +round the comb; and they gnaw 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. + +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, 1/352 of an 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. + +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. + +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 +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. + +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 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. + +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 larvae, 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. + +OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS: +NEUTER AND STERILE INSECTS. + +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; 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. + +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 from both the males +and fertile females, and yet, from being sterile, they cannot propagate +their kind. + +The subject well deserves to be discussed at great length, but I will +here take only a single case, that of working or sterile ants. How the +workers have been rendered sterile is a difficulty; but not much greater +than that of any other striking modification of structure; for it can +be shown that some insects and other articulate animals in a state of +nature occasionally become sterile; and if such insects had been social, +and it had been profitable to the community that a number should have +been annually born capable of work, but incapable of procreation, I can +see no 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? + +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 members of insect communities; the difficulty lies +in understanding how such correlated modifications of structure could +have been slowly accumulated by natural selection. + +This difficulty, though appearing insuperable, is lessened, or, as I +believe, disappears, when it is remembered that selection may be applied +to the family, as well as to the individual, and may thus gain the +desired end. 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. + +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 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. + +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 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. + +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. + +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. + +An analogous explanation has been given by Mr. Wallace, of the equally +complex case, of certain Malayan butterflies regularly appearing under +two or even three distinct female forms; and by Fritz Muller, of certain +Brazilian crustaceans likewise appearing under two widely distinct male +forms. But this subject need not here be discussed. + +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. + +SUMMARY. + +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 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. + +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 +larvae of ichneumonidae feeding within the live bodies of caterpillars, +not as specially endowed or created instincts, but as small consequences +of one general law leading to the advancement of all organic +beings--namely, multiply, vary, let the strongest live and the weakest +die. + + + + +CHAPTER IX. HYBRIDISM. + + Distinction between the sterility of first crosses and of + hybrids--Sterility various in degree, not universal, affected by close + interbreeding, removed by domestication--Laws governing the sterility + of hybrids--Sterility not a special endowment, but incidental on + other differences, 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. + + +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. + +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. + +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 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. + +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. + +DEGREES OF STERILITY. + +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, Kolreuter and Gartner, +who almost devoted their lives to this subject, without being deeply +impressed with the high generality of some degree of sterility. +Kolreuter 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. Gartner, also, makes the rule equally universal; and he +disputes the entire fertility of Kolreuter's ten cases. But in these and +in many other cases, Gartner 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 Gartner 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 Gartner +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 Leguminosae, in which there is an acknowledged +difficulty in the manipulation) half of these twenty plants had their +fertility in some degree impaired. Moreover, as Gartner 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. + +It is certain, on the one hand, that the sterility of various species +when crossed is so different in degree and graduates away so insensibly, +and, on the other hand, that the fertility of pure species is so easily +affected by various circumstances, that for all practical purposes it is +most difficult to say where perfect fertility ends and sterility begins. +I think no better evidence of this can be required than that the two +most experienced observers who have ever lived, namely Kolreuter and +Gartner, 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. + +In regard to the sterility of hybrids in successive generations; though +Gartner 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 Gartner, namely, that if even +the less fertile hybrids be artificially fertilised with hybrid pollen +of the same kind, their fertility, notwithstanding the frequent ill +effects 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 Gartner 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 ARTIFICIALLY FERTILISED hybrids, in contrast with those +spontaneously self-fertilised, may, as I believe, be accounted for by +too close interbreeding having been avoided. + +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 Kolreuter and Gartner 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 Gartner. 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. + +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 Muller, 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 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. + +The practical experiments of horticulturists, though not made with +scientific precision, deserve some notice. It is notorious in how +complicated a manner the species of Pelargonium, Fuchsia, Calceolaria, +Petunia, Rhododendron, etc., have been crossed, yet many of these +hybrids seed freely. For instance, Herbert asserts that a hybrid from +Calceolaria integrifolia and plantaginea, species most widely dissimilar +in general habit, "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 Gartner 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. + +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 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. + +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 inter se 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 inter se. This was effected by Mr. Eyton, who +raised two hybrids from the same parents, but from different hatches; +and from these two birds he raised no less than eight hybrids +(grandchildren of the pure geese) from one nest. In India, however, +these cross-bred geese must be far more fertile; for I am assured by +two eminently capable judges, namely Mr. Blyth and 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. + +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 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 Rutimeyer on their +important osteological differences, as well as from those by Mr. Blyth +on their differences in habits, voice, constitution, etc., 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. + +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. + +LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS. + +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 Gartner'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. + +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 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. + +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. + +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. + +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 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 Gartner 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. + +No one has been able to point out what kind or what amount of +difference, in any recognisable character, is sufficient to prevent two +species crossing. It can be shown that plants most widely different in +habit and general appearance, and having strongly marked differences in +every part of the flower, even in the pollen, in the fruit, and in the +cotyledons, can be crossed. Annual and perennial plants, deciduous and +evergreen trees, plants inhabiting different stations and fitted for +extremely different climates, can often be crossed with ease. + +By a reciprocal cross between two species, I mean the case, for +instance, of a 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 Kolreuter. 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 Kolreuter 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. Gartner, moreover, found that this difference of 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. + +Several other singular rules could be given from Gartner: 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. + +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, in the facility of effecting an union. The hybrids, +moreover, produced from reciprocal crosses often differ in fertility. + +Now do these complex and singular rules indicate that species have been +endowed with sterility simply to prevent their becoming confounded +in nature? I think not. For why should the sterility be so extremely +different in degree, when various species are crossed, all of which +we must suppose it would be equally important to keep from blending +together? Why should the degree of sterility be innately variable in +the individuals of the same species? Why should some species cross with +facility and yet produce very sterile hybrids; and other species cross +with extreme difficulty, and yet produce fairly fertile hybrids? +Why should there often be so great a difference in the result of a +reciprocal cross between the same two species? Why, it may even be +asked, has the production of hybrids been permitted? To grant to species +the special power of producing hybrids, and then to stop their further +propagation by different degrees of sterility, not strictly related to +the facility of the first union between their parents, seems a strange +arrangement. + +The foregoing rules and facts, on the other hand, appear to me clearly +to indicate that the sterility, both of first crosses and of hybrids, +is simply incidental or dependent on unknown differences 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 SPECIALLY endowed +quality, but will admit that it is incidental on differences in the laws +of growth of the two plants. We can sometimes see the reason why one +tree will not take on another from differences in their rate of growth, +in the hardness of their wood, in the period of the flow or nature of +their sap, etc.; but in a multitude of cases we can assign no reason +whatever. Great diversity in the size of two plants, one being woody and +the other herbaceous, one being evergreen and the other deciduous, and +adaptation to widely different climates, does not always prevent the two +grafting together. As in hybridisation, so with grafting, the capacity +is limited by systematic affinity, for no one has been able to graft +together trees belonging to quite distinct families; and, on the other +hand, closely allied species and varieties of the same species, can +usually, but not invariably, be grafted with ease. But this capacity, +as in hybridisation, is by no means absolutely governed by systematic +affinity. Although many distinct genera within the same family have been +grafted together, in other cases species of the same genus will not take +on each other. The pear can be grafted far more readily on the quince, +which is ranked as a distinct genus, than on the apple, which is a +member of the same genus. Even different varieties of the pear take with +different degrees of facility on the quince; so do different varieties +of the apricot and peach on certain varieties of the plum. + +As Gartner found that there was sometimes an innate difference in +different INDIVIDUALS of the same two species in crossing; so Sagaret +believes this to be the case with different individuals of the same +two species in being grafted together. As in reciprocal crosses, the +facility of effecting an union is often very far from equal, so it +sometimes is in grafting. The common gooseberry, for instance, cannot +be grafted on the currant, whereas the currant will take, though with +difficulty, on the gooseberry. + +We have seen that the sterility of hybrids which have their reproductive +organs in an imperfect condition, is a 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, etc., which +seed much more freely when fertilised with the pollen of a distinct +species than when fertilised with pollen from the same plant. + +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, 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. + +ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS. + +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. + +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 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. + +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 Gartner and Kolreuter 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. + +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 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 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. + +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. + +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 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 inter se, they transmit to their offspring +from generation to generation the same compounded organisation, and +hence we need not be surprised that their sterility, though in some +degree variable, 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. + +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. + +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, etc., 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. + +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. + +RECIPROCAL DIMORPHISM AND TRIMORPHISM. + +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 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. + +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. + +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 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 inter se. 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. + +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 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. + +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. + +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, Gartner, likewise concluded that +species when crossed are sterile owing to differences confined to their +reproductive systems. + +FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT +UNIVERSAL. + +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 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 Gartner 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. + +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, 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. + +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. + +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. Gartner 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 other; but only a single +head produced any seed, and this one head produced only five grains. +Manipulation in this case could not have been injurious, as the plants +have separated sexes. No one, I believe, has suspected that these +varieties of maize are distinct species; and it is important to notice +that the hybrid plants thus raised were themselves PERFECTLY fertile; +so that even Gartner did not venture to consider the two varieties as +specifically distinct. + +Girou de Buzareingues crossed three varieties of gourd, which like +the maize has separated sexes, and he asserts that their mutual +fertilisation is by so much the less easy as their differences are +greater. How far these experiments may be trusted, I know not; but +the forms 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. + +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 Gartner: 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 DISTINCT 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 Gartner'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. + +Kolreuter, 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 of this one variety must have been in some +manner and in some degree modified. + +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. + +HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY. + +Independently of the question of fertility, the offspring of species +and of varieties when crossed may be compared in several other respects. +Gartner, 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. + +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 Gartner 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. +Gartner 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. + +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 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. + +But to return to our comparison of mongrels and hybrids: Gartner 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, Gartner 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. +Gartner 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 Kolreuter. + +Such alone are the unimportant differences which Gartner 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 Gartner 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 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. + +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. + +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. + +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 having +been produced by secondary laws, this similarity would be an astonishing +fact. But it harmonises perfectly with the view that there is no +essential distinction between species and varieties. + +SUMMARY OF CHAPTER. + +First crosses between forms, sufficiently distinct to be ranked as +species, and their hybrids, are very generally, but not universally, +sterile. The sterility is of all degrees, and is often so slight that +the 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. + +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. + +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 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. + +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. + +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. + + + + +CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + + On the absence of intermediate varieties at the present day--On the + nature of extinct intermediate varieties; on their number--On the 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 palaeontological 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. + + +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 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. + +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 DIRECTLY intermediate between them. But this is a wholly +false view; we should always look for forms intermediate between each +species and a common but unknown progenitor; and the progenitor +will generally have differed in some respects from all its modified +descendants. To give a simple illustration: the fantail and pouter +pigeons 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. + +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. + +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 DIRECT intermediate links will have existed between them. +But such a case would imply that one form had remained for a very long +period unaltered, whilst its descendants had undergone a vast amount of +change; and the principle of competition between organism and organism, +between child and parent, will render this a very rare event; for in all +cases the new and improved forms of life tend to supplant the old and +unimproved forms. + +By the theory of natural selection all living species have been +connected with the parent-species of each genus, by differences not +greater than we see between the 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. + +ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF DEPOSITION AND EXTENT +OF DENUDATION. + +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. + +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. + +We have, however, recently learned from the observations of Ramsay, in +the van of many excellent observers--of Jukes, Geikie, Croll and +others, that subaerial 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 subaerial 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 subaerial denudation better than the surrounding surface; +this surface consequently has been gradually lowered, with the lines of +harder rock 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 subaerial agencies, which apparently have +so little power, and which seem to work so slowly, have produced great +results. + +When thus impressed with the slow rate at which the land is worn +away through subaerial 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. + +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 DIFFERENT parts of Great Britain; and this is the +result:-- + + Feet + + Palaeozoic strata (not including igneous beds)..57,154 + Secondary strata................................13,190 + Tertiary strata..................................2,240 + + --making altogether 72,584 feet; + + +that is, very nearly thirteen and three-quarters British miles. +Some of these formations, which are represented in England by thin beds, +are thousands of feet in thickness on the Continent. Moreover, between +each successive formation we have, in the opinion of most geologists, +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. + +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 subaerial 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 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. + +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. + +ON THE POORNESS OF PALAEONTOLOGICAL COLLECTIONS. + +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 palaeontologist, 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 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 +Chthamalinae (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. + +With respect to the terrestrial productions which lived during the +Secondary and Palaeozoic 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 palaeozoic formations. + +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 palaeontologists, 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 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. + +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. + +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 subaerial 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 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. + +I am convinced that nearly all our ancient formations, which are +throughout the greater part of their thickness RICH IN FOSSILS, have +thus been formed during subsidence. Since publishing my views on this +subject in 1845, I have watched the progress of geology, and have been +surprised to note how author after author, in treating of this or that +great formation, has come to the conclusion that it was accumulated +during subsidence. I may add, that the only ancient tertiary formation +on the west coast of South America, which has been bulky enough to +resist such degradation as it has as yet suffered, but which will +hardly last to a distant geological age, was deposited during a downward +oscillation of level, and thus gained considerable thickness. + +All geological facts tell us plainly that each area has undergone +numerous slow oscillations of level, and apparently these oscillations +have affected wide spaces. Consequently, formations rich in fossils and +sufficiently thick and extensive to resist subsequent degradation, 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, THICK +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. + +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 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. + +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, etc., 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") and granite rocks exceed, in the proportion of 19 to +12.5, the whole of the newer Palaeozoic 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. + + +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. + +ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE +FORMATION. + +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. + +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 palaeontologists, whose +opinions are worthy of much deference, namely Bronn and Woodward, have +concluded that the average duration of each formation is twice or thrice +as long as the average duration of specific forms. But insuperable +difficulties, as it seems to me, prevent us 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. + +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 palaeozoic +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, INCLUDING FOSSIL REMAINS, have gone on +accumulating within the same area during the whole of this period. It is +not, for instance, probable that sediment was deposited during the whole +of the glacial period near the mouth of the Mississippi, within that +limit of depth at which marine animals can 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 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. + +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 palaeontologist that very thick deposits are usually +barren of organic remains, except near their upper or lower limits. + +It would seem that each separate formation, like the whole pile of +formations in any country, has generally been intermittent in its +accumulation. When we see, as is so often the case, a formation composed +of beds of 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 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. + +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. + +It is notorious on what excessively slight differences many +palaeontologists 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 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. + +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. + +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. + +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 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. + +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. + +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! + +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 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. + +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. + +It may be doubted whether the duration of any one great period of +subsidence over the whole or part of the archipelago, together with +a contemporaneous accumulation of sediment, would EXCEED the average +duration of the same specific forms; and these contingencies are +indispensable for the preservation of all the transitional gradations +between any two or more species. If such gradations were not 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. + +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 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 palaeontologists, be ranked as new and +distinct species. + +If then there be some degree of truth in these remarks, we have no right +to expect to find, in our geological formations, an infinite number of +those fine transitional forms, which, on 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 palaeontologists, +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. + +ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES. + +The abrupt manner in which whole groups of species suddenly appear in +certain formations, has been urged by several palaeontologists--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 palaeontological 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, 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. + +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? + +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 +Palaeontology, 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 +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, palaeontologists +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. + +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 palaeontologist, 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 have abundant evidence of the existence +of this group of animals during the secondary period. + +The case most frequently insisted on by palaeontologists 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 palaeozoic 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 Palaeontology 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. + +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 palaeontological 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. + +ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN +FOSSILIFEROUS STRATA. + +There is another and allied difficulty, which is much more serious. I +allude to the manner in which species belonging to several of the 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, etc., 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. + +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. + +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 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 +palaeozoic 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. + +The case at present must remain inexplicable; and may be truly urged as +a valid argument against the views here entertained. To show that it +may hereafter receive some explanation, I will give the following +hypothesis. From the nature of the organic remains which do not appear +to have inhabited profound depths, in the several formations of Europe +and of the United States; and from the amount of sediment, miles in +thickness, of which the formations are composed, we may infer that from +first to last large islands or tracts of land, whence the sediment was +derived, occurred in the neighbourhood of the 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. + +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 +palaeozoic or secondary formation. Hence, we may perhaps infer, that +during the palaeozoic and secondary periods, neither continents nor +continental islands existed where our oceans now extend; for had they +existed, palaeozoic 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 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. + +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 palaeontologists, namely, Cuvier, Agassiz, Barrande, +Pictet, Falconer, E. Forbes, etc., and all our greatest geologists, as +Lyell, Murchison, Sedgwick, etc., 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 palaeontologists 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. + + + + +CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + + On the slow and successive appearance of new species--On their different + rates of change--Species once lost do not reappear--Groups of species + follow the same general rules in their appearance and disappearance as + do single species--On extinction--On simultaneous changes in the forms + of life throughout the world--On the affinities of extinct species to + each other and to living species--On the state of development of + ancient forms--On the succession of the same types within the same + areas--Summary of preceding and present chapters. + + +Let us now see whether the several facts and 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. + +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. + +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 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. + +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 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. + +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. + +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. + +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 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. + +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. + +ON EXTINCTION. + +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, etc., whose general views would naturally +lead them to this conclusion. On the contrary, we have every reason to +believe, from the study of the tertiary formations, that species and +groups of species gradually disappear, one after another, first from one +spot, then from another, and finally from the world. 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 palaeozoic 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. + +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 certain, from +the analogy of all other mammals, even of the slow-breeding elephant, +and from the history of the naturalisation of the domestic horse in +South America, that under more favourable conditions it would in a very +few years have stocked the whole continent. But we could not have +told what the unfavourable conditions were which checked its increase, +whether some one or several contingencies, and at what period of the +horse's life, and in what degree they severally acted. If the conditions +had gone on, however slowly, becoming less and less favourable, we +assuredly should not have perceived the fact, yet the fossil horse would +certainly have become rarer and rarer, and finally extinct--its place +being seized on by some more successful competitor. + +It is most difficult always to remember that the increase of every +living 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. + +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. + +The theory of natural selection is grounded on the belief that each new +variety and ultimately each new species, is produced and maintained by +having some advantage over those with which it comes into competition; +and the consequent extinction of less-favoured forms almost inevitably +follows. It is the same with our domestic productions: when a new and +slightly improved variety has been raised, it at first supplants the +less improved varieties in the same neighbourhood; when much improved it +is transported far and near, like our short-horn cattle, and takes the +place of other breeds in other countries. Thus the appearance of new +forms and the disappearance of old forms, both 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. + +The competition will generally be most severe, as formerly explained +and illustrated by examples, between the forms which are most like each +other in all respects. Hence the improved and modified descendants of +a species will generally cause the extermination of the parent-species; +and if many new forms have been developed from any one species, the +nearest allies of that species, i.e. the species of the same genus, will +be the most liable to extermination. Thus, as I believe, a number of +new species descended from one species, that is a new genus, comes to +supplant an old genus, belonging to the same family. But it must often +have happened that a new species belonging to some one group 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. + +With respect to the apparently sudden extermination of whole families or +orders, as of Trilobites at the close of the palaeozoic 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. + +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. + +ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE +WORLD. + +Scarcely any palaeontological 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, 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 palaeozoic 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 palaeozoic +and tertiary stages, would still be manifest, and the several formations +could be easily correlated. + +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. + +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 MARINE formations, namely, the upper +pliocene, the pleistocene and strictly modern beds of Europe, North and +South America, and Australia, from containing fossil remains in some +degree allied, and from not including those forms which are found +only in the older underlying deposits, would be correctly ranked as +simultaneous in a geological sense. + +The fact of the forms of life changing simultaneously in the above large +sense, at distant parts of the world, has greatly struck those admirable +observers, MM. de Verneuil and d'Archiac. After referring to the +parallelism of the palaeozoic forms of life in various parts of Europe, +they add, "If struck by this strange sequence, we turn our attention to +North America, and there discover a series of analogous phenomena, +it will appear certain that all these modifications of species, their +extinction, and the introduction of new ones, cannot be owing to mere +changes in marine currents or other causes more or less local and +temporary, but depend on general laws which govern the whole animal +kingdom." M. Barrande has made forcible remarks to precisely the same +effect. It is, indeed, quite futile to look to changes of currents, +climate, or other physical conditions, as the cause of these great +mutations in the forms of life throughout the world, under the most +different climates. We must, as Barrande has remarked, look to some +special law. We shall see this more clearly when we treat of the present +distribution of organic beings, and find how slight is the relation +between the physical conditions of various countries and the nature of +their inhabitants. + +This great fact of the parallel succession of the forms of life +throughout the world, is explicable on the theory of natural selection. +New species are formed by 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 of parallelism in the +succession of the productions of the land than with those of the sea. + +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. + +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. + +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 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. + +ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS. + +Let us now look to the mutual affinities of extinct and living species. +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 +palaeontologist, M. Gaudry, has shown in the most striking manner that +many of the fossil mammals discovered by him in Attica serve to break +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." + +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 palaeozoic 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. + +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. 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. + +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. + +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, a14, q14, p14, will form a small family; b14 and f14, a closely +allied family or subfamily; and o14, i14, m14, 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 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 F14. + +All the many forms, extinct and recent, descended from (A), make, as +before remarked, one order; and this order, from the continued effects +of extinction and divergence of character, has become divided into +several sub-families and families, some of which are supposed to have +perished at different periods, and some to have endured to the present +day. + +By looking at the diagram we can see that if many of the extinct forms +supposed to be embedded in the successive formations, were discovered +at several points low down in the series, the three existing families on +the uppermost line would be rendered less distinct from each other. If, +for instance, the genera a1, a5, a10, f8, m3, m6, m9, 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 a14, etc., and b14, etc.) 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 (a14 to m14), 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. + +Under nature the process will be far more complicated than is +represented in the diagram; for the groups will have been more numerous; +they will have endured for extremely unequal lengths of time, and will +have been modified in various degrees. As we possess only the last +volume of the geological record, and that in a very broken condition, +we have no right to expect, except in 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 palaeontologists is +frequently the case. + +Thus, on the theory of descent with modification, the main facts with +respect to the mutual affinities of the extinct forms of life to each +other and to living forms, are explained in a satisfactory manner. And +they are wholly inexplicable on any other view. + +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 +palaeontologists 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. + +It is no real objection to the truth of the statement that the fauna of +each period as a whole is nearly intermediate in character between the +preceding and succeeding faunas, that certain genera offer exceptions +to the rule. For instance, 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. + +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 palaeontologists, 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. + +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. + +ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS. + +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 palaeozoic 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 +palaeontologists would answer in the affirmative; and it seems that this +answer must be admitted as true, though difficult of proof. + +It is no valid objection to this conclusion, that certain Brachiopods +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. + +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 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. + +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 +most skilful naturalist, from an examination of the species of the two +countries, could not have foreseen this result. + +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. + +ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE +LATER TERTIARY PERIODS. + +Mr. Clift many years ago showed that the fossil mammals from the +Australian caves were closely allied to the living marsupials of that +continent. In South America, a similar relationship is manifest, even to +an uneducated eye, in the gigantic pieces of armour, like those of the +armadillo, found in several parts of La Plata; and Professor Owen has +shown in the most striking manner that most of the fossil mammals, +buried there in such numbers, are related to South American types. This +relationship is even more clearly seen in the wonderful collection of +fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was +so much impressed with these facts that I strongly insisted, in 1839 +and 1845, on this "law of the succession of types,"--on "this wonderful +relationship in the same continent between the dead and the living." +Professor Owen has subsequently extended the same generalisation to +the mammals of the Old World. We see the same law in this author's +restorations of the extinct and gigantic birds of New Zealand. We see +it also in the birds of the caves of Brazil. Mr. Woodward has shown that +the same law holds good with sea-shells, but, from the wide distribution +of most molluscs, it is not well displayed by them. Other cases could +be added, as the relation between the extinct and living land-shells of +Madeira; and between the extinct and living brackish water-shells of the +Aralo-Caspian Sea. + +Now, what does this remarkable law of the succession of the same types +within the same areas mean? He would be a bold man who, after comparing +the present climate of Australia and of parts of South America, under +the same latitude, would attempt to account, on the one hand 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. + +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. + +It may be asked in ridicule whether I suppose that the megatherium and +other allied huge monsters, which formerly lived in 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. + +SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS. + +I have attempted to show that the geological record is extremely +imperfect; that only a small portion of the globe has been geologically +explored with care; that only certain classes of organic beings have +been largely preserved in a fossil state; that the number both of +specimens and of species, preserved in our museums, is absolutely as +nothing compared with the 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; 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. + +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. + +Passing from these difficulties, the other great leading facts +in palaeontology 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 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. + +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. + +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. + +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 +palaeontologists, 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. + +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 palaeontology 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. + + + + +CHAPTER XII. GEOGRAPHICAL DISTRIBUTION. + + Present distribution cannot be accounted for by differences in physical + conditions--Importance of barriers--Affinity of the productions of the + same continent--Centres of creation--Means of dispersal by changes of + climate and of the level of the land, and by occasional means--Dispersal + during the Glacial period--Alternate Glacial periods in the North and + South. + + +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! + +In the southern hemisphere, if we compare large tracts of land in +Australia, South Africa, and western South America, between latitudes +25 and 35 degrees, we shall find parts extremely similar in all their +conditions, yet it would not be possible to point out three faunas +and floras more utterly dissimilar. Or, again, we may compare the +productions of South America south of latitude 35 degrees with those +north of 25 degrees, 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. + +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, etc., are not as impassable, or likely to have endured so long, +as the oceans separating continents, the differences are very inferior +in degree to those characteristic of distinct continents. + +Turning to the sea, we find the same law. 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. +Gunther 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 further westward from the eastern islands of the +tropical parts of the Pacific, we encounter no impassable barriers, and +we have innumerable islands as halting-places, 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. + +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. + +The bond is simply inheritance, that cause which alone, as far as 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. + +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. + +According to these views, it is obvious that the several species of the +same genus, though inhabiting the most distant quarters of the world, +must originally have proceeded from the same source, as they 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. + +SINGLE CENTRES OF SUPPOSED CREATION. + +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 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! + +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 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. + +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. + +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 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. + +Before discussing the three classes of facts, which I have selected as +presenting the greatest amount of difficulty on the theory of "single +centres of creation," I must say a few words on the means of dispersal. + +MEANS OF DISPERSAL. + +Sir C. Lyell and other authors have ably treated this subject. I can +give here only the briefest abstract of the more important facts. Change +of climate must have had a powerful influence on migration. A region 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 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. + +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 Leguminosae were tried, and, with one exception, they resisted the +salt-water badly; seven species of the allied orders, Hydrophyllaceae +and Polemoniaceae, were all killed by a month's 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, etc., +and some of these floated for a long time. It is well known what a +difference there is in the buoyancy of green and seasoned timber; and it +occurred to me that floods 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. + +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 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. + +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 COMPLETELY 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. + +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 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. + +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. + +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 (Motacillae), wheatears, and whinchats (Saxicolae), 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. + +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 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. + +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 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. + +DISPERSAL DURING THE GLACIAL PERIOD. + +The identity of many plants and animals, on mountain-summits, separated +from each other by hundreds of miles of lowlands, where 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. + +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 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. + +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. + +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. + +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. + +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 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 degrees, lived +during the Pliocene period further north, under the Polar Circle, in +latitude 66-67 degrees; 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. + +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. + +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 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. + +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. + +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. + +ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH. + +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. + +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 degrees, and on the shores of the Pacific, where +the climate is now so different, as far south as latitude 46 degrees. +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 degrees 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 degrees to the +southernmost extremity, we have the clearest evidence of former glacial +action, in numerous immense boulders transported far from their parent +source. + +From these several facts, namely, from the glacial action having +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. + +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 +the illustrious Humboldt long ago found species belonging to genera +characteristic of the Cordillera. + +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. + +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. + +On the southern mountains of Australia, Dr. F. Muller 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 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. + +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, etc., of northern forms of fish. Dr. Hooker informs +me that twenty-five species of Algae are common to New Zealand and to +Europe, but have not been found in the intermediate tropical seas. + +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." + +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 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. + +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 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. + +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. + +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. + +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. + +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 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. + +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. + + + + +CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION--continued. + + Distribution of fresh-water productions--On the inhabitants of oceanic + islands--Absence of Batrachians and of terrestrial Mammals--On + the relation of the inhabitants of islands to those of the nearest + mainland--On colonisation from the nearest source with subsequent + modification--Summary of the last and present chapters. + + +FRESH-WATER PRODUCTIONS. + +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, etc., and at the dissimilarity of the surrounding +terrestrial beings, compared with those of Britain. + +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. Gunther 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 +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. + +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. Gunther 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. + +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 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. + +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 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. + +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. + +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 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. + +ON THE INHABITANTS OF OCEANIC ISLANDS. + +We now come to the last of the three classes of facts, which I have +selected as presenting the greatest amount of difficulty 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. + +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 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. + +Although in oceanic islands the species are few in number, the +proportion of endemic kinds (i.e. 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, 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 larvae, +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. + +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. + +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 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. + +ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS. + +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. + +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 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, aerial +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. + +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 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. + +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 aerial bats, the singular +proportions of certain orders of plants, herbaceous forms having been +developed into trees, etc., seem to me to accord better with the 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. + +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 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. + +ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEAREST +MAINLAND. + +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. 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. + +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 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. + +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 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. + +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 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. + +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, etc., 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. +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. + +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 Felidae and +Canidae. 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. + +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 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. + +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. + +SUMMARY OF THE LAST AND PRESENT CHAPTERS. + +In these chapters I have endeavoured to show that if we make due +allowance for our ignorance of the full effects of 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. + +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, 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. + +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. + +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 force, some +existing in scanty numbers--and this we do find in the several great +geographical provinces of the world. + +On these same principles we can understand, as I have endeavoured to +show, why oceanic islands should have few inhabitants, but 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 +aerial 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. + +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 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. + + + + +CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS: + +MORPHOLOGY--EMBRYOLOGY--RUDIMENTARY ORGANS. + + Classification, groups subordinate to groups--Natural system--Rules and + difficulties in classification, explained on the theory of descent + with modification--Classification of varieties--Descent always used in + classification--Analogical or adaptive characters--Affinities, + general, complex and radiating--Extinction separates and defines + groups--Morphology, between members of the same class, between parts of + the same individual--Embryology, laws of, explained by variations not + supervening at an early age, and being inherited at a corresponding + age--Rudimentary organs; their origin explained--Summary. + + +CLASSIFICATION. + +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 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. + +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. + +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 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 Linnaeus, 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. + +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 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. + +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 +Proteaceae, 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 Connaraceae "differ in having +one or more ovaria, in the existence or absence of albumen, in the +imbricate or valvular aestivation. 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 antennae, 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 antennae in these two divisions of the same order are +of unequal physiological importance. Any number of instances could be +given of the varying importance for classification of the same important +organ within the same group of beings. + +Again, no one will say that rudimentary or atrophied organs are of high +physiological or vital importance; yet, undoubtedly, organs in this +condition are often of 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. + +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 Algae--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. + +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 Linnaeus, 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 Malpighiaceae, 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 Malpighiaceae. This case well +illustrates the spirit of our classifications. + +Practically, when naturalists are at work, they do not trouble +themselves about the physiological value of the characters which they +use in defining a group or in allocating any particular species. If they +find a character nearly uniform, and common to a great number of forms, +and not common to others, they use it as one of high value; if common to +some lesser number, they use it as of subordinate value. This principle +has been broadly confessed by some naturalists to be the true one; and +by none more clearly than by that excellent botanist, Aug. St. Hilaire. +If 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 Muller 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 branchiae, while +another species is destitute of them. + +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 larvae not having been excluded; in order to show this, +Fritz Muller 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. + +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 +hitherto been found impossible. There are crustaceans at the opposite +ends of the series, which have hardly a character in common; yet the +species at both ends, from being plainly allied to others, and these to +others, and so onwards, can be recognised as unequivocally belonging to +this, and to no other class of the Articulata. + +Geographical distribution has often been used, though perhaps not quite +logically, in classification, more especially in very large groups of +closely allied forms. Temminck insists on the utility or even necessity +of this practice in certain groups of birds; and it has been followed by +several entomologists and botanists. + +Finally, with respect to the comparative value of the various groups of +species, such as orders, 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. + +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. + +But I must explain my meaning more fully. I believe that the ARRANGEMENT +of the groups within each class, in due subordination and relation to +each other, must be strictly genealogical in order to be natural; but +that the AMOUNT of difference in the several branches or groups, though +allied in the same degree in blood to their common progenitor, may +differ greatly, being due to the different degrees of modification which +they have undergone; and this is expressed by the forms being ranked +under different genera, families, sections or orders. The reader will +best understand what is meant, if he will take the trouble to refer to +the diagram in the fourth chapter. We will suppose the letters A to L +to represent 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 (a14 to z14) 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 F14 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 ARRANGEMENT remains strictly +true, not only at the present time, but at each successive period +of descent. All the modified descendants from A will have inherited +something in common from their common parent, as will all the +descendants from I; so will it be with each subordinate branch of +descendants at each successive 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. + +It may be worth while to illustrate this view of classification, by +taking the case of languages. If we possessed a perfect pedigree of +mankind, a genealogical arrangement of the races of man would afford the +best classification of the various languages now spoken throughout the +world; and if all extinct languages, and all intermediate and slowly +changing dialects, 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. + +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, etc.; 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, without any thought on +the subject, these tumblers are kept in the same group, because allied +in blood and alike in some other respects. + +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. + +As descent has universally been used in classing together the +individuals of the same species, though the males and females and larvae +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 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. + +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. + +ANALOGICAL RESEMBLANCES. + +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. + +Among insects there are innumerable instances; thus Linnaeus, 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. + +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. + +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 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. + +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 ANALOGICAL VARIATION 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. + +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. + +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 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. + +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 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. + +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. + +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 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. + +ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC BEINGS. + +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. + +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. + +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. + +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 extraordinary difficulty which naturalists have experienced in +describing, without the aid of a diagram, the various affinities which +they perceive between the many living and extinct members of the same +great natural class. + +Extinction, as we have seen in the fourth chapter, has played an +important part in defining and widening the intervals between the +several groups in each class. We may thus account 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 the high +importance of looking to types, whether or not we can separate and +define the groups to which such types belong. + +Finally, we have seen that natural selection, which 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, etc., we +can understand the rules which we are compelled to follow in our +classification. We can understand why we value certain resemblances far +more than others; why we 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. + +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. + +MORPHOLOGY. + +We have seen that the members of the same class, independently of +their habits of life, resemble each other in the general plan of their +organisation. This resemblance is often expressed by the term "unity of +type;" or by saying that the several parts and organs in the different +species of the class are homologous. The whole subject is included under +the general 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?" + +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, 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 maxillae. The same law governs the +construction of the mouths and limbs of crustaceans. So it is with the +flowers of plants. + +Nothing can be more hopeless than to attempt to explain this similarity +of pattern in members of the same class, by utility or by the doctrine +of final causes. The hopelessness of the attempt has been expressly +admitted by Owen in his most interesting work on the "Nature of Limbs." +On the ordinary view of the independent creation of each being, we can +only say that so it is; that it has 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. + +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 maxillae, 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 suctorial crustaceans, the +general pattern seems thus to have become partially obscured. + +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 vertebrae. +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. + +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 vertebrae? 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? + +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 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 vertebrae; 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. + +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. + +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, "homogenous"; and the resemblances which +cannot thus be accounted for, he proposes to call "homoplastic". 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 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. + +Naturalists frequently speak of the skull as formed of metamorphosed +vertebrae; 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 vertebrae, jaws and legs, etc., 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--vertebrae 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. + +DEVELOPMENT AND EMBRYOLOGY. + +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 (Chloeon) 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 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 larvae, and these others, which +finally are developed into mature males and females, propagating their +kind in the ordinary manner by eggs. + +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 larvae 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 +Coccidae;" the term parthenogenesis implying that the mature females of +the Coccidae 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. + +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 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 larvae 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 leguminosae. + +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. + +The case, however, is different when an animal, during any part of its +embryonic career, is active, and has to provide for itself. The period +of activity may come on earlier or later in life; but whenever it comes +on, the adaptation of the larva to its conditions of life is just as +perfect and as beautiful as in the adult animal. 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 larvae of some insects +belonging to very different orders, and on the dissimilarity of the +larvae of other insects within the same order, according to their habits +of life. Owing to such adaptations the similarity of the larvae 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 larvae of allied species, or groups of species, differing +more from each other than do the adults. In most cases, however, the +larvae, 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 larvae +in all their stages barely distinguishable. + +The embryo in the course of development generally rises in organisation. +I use this expression, though I am aware that it is hardly possible to +define clearly what is meant by 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 larvae 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 antennae; 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 antennae, and their +two eyes are now reconverted into a minute, single, simple eye-spot. +In this last and complete state, cirripedes may be considered as +either more highly or more lowly organised than they were in the +larval condition. But in some genera the larvae 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. + +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 +larvae 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. + +Sometimes it is only the earlier developmental stages which fail. Thus, +Fritz Muller 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 zoea-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 zoeas; nevertheless Muller assigns +reasons for his belief, that if there had been no suppression of +development, all these crustaceans would have appeared as nauplii. + +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 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 larvae 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, larvae which +have to provide for their own wants, being perfectly adapted to the +surrounding conditions; and lastly, the fact of certain larvae 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. + +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. + +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. + +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. + +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. + +These facts are explained by the above two principles. Fanciers select +their dogs, horses, pigeons, etc., 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 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. + +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. + +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 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 Muller 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. + +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 larvae 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, etc., would be useless; and +in this case the metamorphosis would be retrograde. + +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 with six legs, two long antennae, and four +eyes. These larvae 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 larvae 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 larvae of +the Sitaris leap on the eggs and devour them. Afterwards they undergo +a complete change; their eyes disappear; their legs and antennae become +rudimentary, and they feed on honey; so that they now more closely +resemble the ordinary larvae 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. + +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 larvae under the nauplius-form; and +as these larvae live and feed in the open sea, and are not adapted for +any peculiar habits of life, and from other reasons assigned by Fritz +Muller, 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 branchiae, a swim-bladder, four +fin-like limbs, and a long tail, all fitted for an aquatic life. + +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 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 larvae; we have seen, +for instance, that cirripedes, though externally so like shell-fish, +are at once known by their larvae 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. + +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. + +RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS. + +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 mammae; 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? + +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 mammae 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 Kolreuter 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 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." + +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 +Compositae, 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. + +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 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. Gunther, 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 +branchiae. + +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 Scrophulariaceae 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. + +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. + +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 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. + +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. + +It appears probable that disuse has been the main agent in 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. + +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. + +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 +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. + +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. + +SUMMARY. + +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 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. + +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. + +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. Larvae +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. + +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. + + + + +CHAPTER XV. RECAPITULATION AND CONCLUSION. + + 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. + + +As this whole volume is one long argument, it may be convenient to the +reader to have the leading facts and inferences briefly recapitulated. + +That many and 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. + +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 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. + +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. + +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 Gartner and Kolreuter. 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. + +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 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. + +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 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. + +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 DIRECTLY +connecting links between them, but only between each and some extinct +and supplanted form. Even on a wide area, which has during a long period +remained continuous, and of which the 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. + +On this doctrine of the extermination of an infinitude of connecting +links, between the living and extinct inhabitants of the world, and at +each successive period between the extinct and still older species, why +is not every geological formation charged with such links? Why does +not every collection of fossil remains afford plain evidence of the +gradation and mutation of the forms of life? 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. + +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 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. + +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. + +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 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. + +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. + +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. + +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 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. + +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. + +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. + +As geology plainly proclaims that each land has undergone great 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! + +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. + +On the view that species are only strongly marked and permanent +varieties, and that each species first existed as a variety, we can +see why it is that no line of demarcation can be drawn between species, +commonly supposed to have been produced by special acts of creation, +and varieties which are acknowledged to have been produced by secondary +laws. On this same view we can understand how it is that in 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. + +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. + +As natural selection acts solely by accumulating slight, successive, +favourable variations, it can produce no great or sudden 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. + +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. + +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. + +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 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 ichneumonidae 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. + +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! + +On the ordinary view of each species having been independently created, +why should specific characters, or those by which the species of the +same genus differ from each other, be more variable than the generic +characters in which they all agree? Why, for instance, should the colour +of a flower be more likely to vary in any one species of a genus, if +the other species 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. + +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 at some instincts +being not perfect and liable to mistakes, and at many instincts causing +other animals to suffer. + +If species be only well-marked and permanent varieties, we can at once +see why their crossed offspring should follow the same complex laws +in their degrees and kinds of resemblance to their parents--in being +absorbed into each other by successive crosses, and in other such +points--as do the crossed offspring of acknowledged varieties. This +similarity would be a strange fact, if species had been independently +created and varieties had been produced through secondary laws. + +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 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. + +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. + +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 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. + +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. + +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, etc.; and we have to +discover the lines of descent by the most permanent characters, whatever +they may be, and of however slight vital importance. + +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 vertebrae +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 branchiae. + +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 embryological and homologous structures, but we are too blind to +understand her meaning. + +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. + +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." + +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 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." + +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. + +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. + +Although I am fully convinced of the truth of the views given in this +volume under the form of an abstract, I by no means expect to convince +experienced naturalists whose minds are stocked with a multitude of +facts all viewed, during a long course of years, from a point of view +directly opposite to mine. It is so easy to hide our ignorance under +such expressions as the "plan of creation," "unity of design," etc., and +to think that we give an explanation when we only restate a fact. Any +one whose disposition leads him to attach more weight to unexplained +difficulties than to the explanation of a certain number of facts +will certainly reject 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. + +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. + +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. + +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. + +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. + +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 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 algae 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, etc., we have +distinct evidence in their embryological, homologous, and rudimentary +structures, that within each kingdom all the members are descended from +a single progenitor. + +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, to be capable of definition; and if definable, whether the +differences be sufficiently important to deserve a specific name. This +latter point will become a far more essential consideration than it is +at present; for differences, however slight, between any two forms, +if not blended by intermediate gradations, are looked at by most +naturalists as sufficient to raise both forms to the rank of species. + +Hereafter we shall be compelled to acknowledge that the only distinction +between species and well-marked varieties is, that the latter are +known, or believed to be connected at the present day by intermediate +gradations, whereas species were formerly thus connected. Hence, without +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. + +The other and more general departments of natural history will rise +greatly in interest. The terms used by naturalists, of affinity, +relationship, community of type, paternity, morphology, adaptive +characters, rudimentary and aborted organs, etc., will cease to be +metaphorical and will have a plain signification. When we no longer look +at an organic being as a savage looks at a ship, as 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! + +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 +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. + +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. + +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 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. + +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. + +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. + +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. + + +GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME. + +(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.) + +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. + +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 SPHERICAL ABERRATION; 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 +CHROMATIC ABERRATION. + +ABNORMAL.--Contrary to the general rule. + +ABORTED.--An organ is said to be aborted, when its development has been +arrested at a very early stage. + +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. + +ALGAE.--A class of plants including the ordinary sea-weeds and the +filamentous fresh-water weeds. + +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. + +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. + +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 ANALOGOUS, and to be ANALOGUES of each other. + +ANIMALCULE.--A minute animal: generally applied to those visible only by +the microscope. + +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. + +ANTENNAE.--Jointed organs appended to the head in Insects, Crustacea and +Centipedes, and not belonging to the mouth. + +ANTHERS.--The summits of the stamens of flowers, in which the pollen or +fertilising dust is produced. + +APLACENTALIA, APLACENTATA or APLACENTAL MAMMALS.--See MAMMALIA. + +ARCHETYPAL.--Of or belonging to the Archetype, or ideal primitive form +upon which all the beings of a group seem to be organised. + +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). + +ASYMMETRICAL.--Having the two sides unlike. + +ATROPHIED.--Arrested in development at a very early stage. + +BALANUS.--The genus including the common Acorn-shells which live in +abundance on the rocks of the sea-coast. + +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. (Examples, Frogs, Toads, and Newts.) + +BOULDERS.--Large transported blocks of stone generally embedded in clays +or gravels. + +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. + +BRANCHIAE.--Gills or organs for respiration in water. + +BRANCHIAL.--Pertaining to gills or branchiae. + +CAMBRIAN SYSTEM.--A series of very ancient Palaeozoic rocks, between the +Laurentian and the Silurian. Until recently these were regarded as the +oldest fossiliferous rocks. + +CANIDAE.--The Dog-family, including the Dog, Wolf, Fox, Jackal, etc. + +CARAPACE.--The shell enveloping the anterior part of the body in +Crustaceans generally; applied also to the hard shelly pieces of the +Cirripedes. + +CARBONIFEROUS.--This term is applied to the great formation which +includes, among other rocks, the coal-measures. It belongs to the +oldest, or Palaeozoic, system of formations. + +CAUDAL.--Of or belonging to the tail. + +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. (Examples, Cuttle-fish, Nautilus.) + +CETACEA.--An order of Mammalia, including the Whales, Dolphins, etc., +having the form of the body fish-like, the skin naked, and only the fore +limbs developed. + +CHELONIA.--An order of Reptiles including the Turtles, Tortoises, etc. + +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. + +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. + +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." + +COELOSPERMOUS.--A term applied to those fruits of the Umbelliferae which +have the seed hollowed on the inner face. + +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. + +COLUMN.--A peculiar organ in the flowers of Orchids, in which the +stamens, style and stigma (or the reproductive parts) are united. + +COMPOSITAE 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. +(Examples, the Daisy, Dandelion, etc.) + +CONFERVAE.--The filamentous weeds of fresh water. + +CONGLOMERATE.--A rock made up of fragments of rock or pebbles, cemented +together by some other material. + +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. + +CORRELATION.--The normal coincidence of one phenomenon, character, etc., +with another. + +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. + +COTYLEDONS.--The first or seed-leaves of plants. + +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. (Examples, Crab, Lobster, Shrimp, +etc.) + +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 antennae are inserted. + +CUTANEOUS.--Of or belonging to the skin. + +DEGRADATION.--The wearing down of land by the action of the sea or of +meteoric agencies. + +DENUDATION.--The wearing away of the surface of the land by water. + +DEVONIAN SYSTEM or FORMATION.--A series of Palaeozoic rocks, including +the Old Red Sandstone. + +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. + +DIFFERENTATION.--The separation or discrimination of parts or organs +which in simpler forms of life are more or less united. + +DIMORPHIC.--Having two distinct forms.--DIMORPHISM is the condition of +the appearance of the same species under two dissimilar forms. + +DIOECIOUS.--Having the organs of the sexes upon distinct individuals. + +DIORITE.--A peculiar form of Greenstone. + +DORSAL.--Of or belonging to the back. + +EDENTATA.--A peculiar order of Quadrupeds, characterised by the absence +of at least the middle incisor (front) teeth in both jaws. (Examples, +the Sloths and Armadillos.) + +ELYTRA.--The hardened fore-wings of Beetles, serving as sheaths for the +membranous hind-wings, which constitute the true organs of flight. + +EMBRYO.--The young animal undergoing development within the egg or womb. + +EMBRYOLOGY.--The study of the development of the embryo. + +ENDEMIC.--Peculiar to a given locality. + +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. + +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. + +EPHEMEROUS INSECTS.--Insects allied to the May-fly. + +FAUNA.--The totality of the animals naturally inhabiting a certain +country or region, or which have lived during a given geological period. + +FELIDAE.--The Cat-family. + +FERAL.--Having become wild from a state of cultivation or domestication. + +FLORA.--The totality of the plants growing naturally in a country, or +during a given geological period. + +FLORETS.--Flowers imperfectly developed in some respects, and collected +into a dense spike or head, as in the Grasses, the Dandelion, etc. + +FOETAL.--Of or belonging to the foetus, or embryo in course of +development. + +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. + +FOSSILIFEROUS.--Containing fossils. + +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. + +FRENUM (pl. FRENA).--A small band or fold of skin. + +FUNGI (sing. FUNGUS).--A class of cellular plants, of which Mushrooms, +Toadstools, and Moulds, are familiar examples. + +FURCULA.--The forked bone formed by the union of the collar-bones in +many birds, such as the common Fowl. + +GALLINACEOUS BIRDS.--An order of birds of which the common Fowl, Turkey, +and Pheasant, are well-known examples. + +GALLUS.--The genus of birds which includes the common Fowl. + +GANGLION.--A swelling or knot from which nerves are given off as from a +centre. + +GANOID FISHES.--Fishes covered with peculiar enamelled bony scales. Most +of them are extinct. + +GERMINAL VESICLE.--A minute vesicle in the eggs of animals, from which +the development of the embryo proceeds. + +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. + +GLAND.--An organ which secretes or separates some peculiar product from +the blood or sap of animals or plants. + +GLOTTIS.--The opening of the windpipe into the oesophagus or gullet. + +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. + +GRALLATORES.--The so-called wading-birds (storks, cranes, snipes, etc.), +which are generally furnished with long legs, bare of feathers above the +heel, and have no membranes between the toes. + +GRANITE.--A rock consisting essentially of crystals of felspar and mica +in a mass of quartz. + +HABITAT.--The locality in which a plant or animal naturally lives. + +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. + +HERMAPHRODITE.--Possessing the organs of both sexes. + +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, etc., is composed. +The latter is called serial homology. The parts which stand in such a +relation to each other are said to be homologous, and one such part +or organ is called the homologue of the other. In different plants +the parts of the flower are homologous, and in general these parts are +regarded as homologous with leaves. + +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 Cicadae, frog-hoppers, and Aphides, are +well-known examples. + +HYBRID.--The offspring of the union of two distinct species. + +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. + +HYPERTROPHIED.--Excessively developed. + +ICHNEUMONIDAE.--A family of hymenopterous insects, the members of which +lay their eggs in the bodies or eggs of other insects. + +IMAGO.--The perfect (generally winged) reproductive state of an insect. + +INDIGENES.--The aboriginal animal or vegetable inhabitants of a country +or region. + +INFLORESCENCE.--The mode of arrangement of the flowers of plants. + +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. + +INSECTIVOROUS.--Feeding on insects. + +INVERTEBRATA, or INVERTEBRATE ANIMALS.--Those animals which do not +possess a backbone or spinal column. + +LACUNAE.--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. + +LAMELLATED.--Furnished with lamellae or little plates. + +LARVA (pl. LARVAE).--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. + +LARYNX.--The upper part of the windpipe opening into the gullet. + +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. + +LEGUMINOSAE.--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). + +LEMURIDAE.--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. + +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. + +LITTORAL.--Inhabiting the seashore. + +LOESS.--A marly deposit of recent (Post-Tertiary) date, which occupies a +great part of the valley of the Rhine. + +MALACOSTRACA.--The higher division of the Crustacea, including the +ordinary crabs, lobsters, shrimps, etc., together with the woodlice and +sand-hoppers. + +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 (MAMMAE, MAMMARY GLANDS) 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 PLACENTA, 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 PLACENTAL MAMMALS; the latter, +or APLACENTAL MAMMALS, include the Marsupials and Monotremes +(ORNITHORHYNCHUS). + +MAMMIFEROUS.--Having mammae or teats (see MAMMALIA). + +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. + +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, etc. (see MAMMALIA). + +MAXILLAE.--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. + +MELANISM.--The opposite of albinism; an undue development of colouring +material in the skin and its appendages. + +METAMORPHIC ROCKS.--Sedimentary rocks which have undergone alteration, +generally by the action of heat, subsequently to their deposition and +consolidation. + +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. + +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. (Examples, grasses, +lilies, orchids, palms, etc.) + +MORAINES.--The accumulations of fragments of rock brought down by +glaciers. + +MORPHOLOGY.--The law of form or structure independent of function. + +MYSIS-STAGE.--A stage in the development of certain crustaceans +(prawns), in which they closely resemble the adults of a genus (Mysis) +belonging to a slightly lower group. + +NASCENT.--Commencing development. + +NATATORY.--Adapted for the purpose of swimming. + +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 +CYCLOPS was described as a distinct genus under the name of NAUPLIUS. + +NEURATION.--The arrangement of the veins or nervures in the wings of +insects. + +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 WORKERS. + +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, etc., from the surface of +the eye. + +OCELLI.--The simple eyes or stemmata of insects, usually situated on the +crown of the head between the great compound eyes. + +OESOPHAGUS.--The gullet. + +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. + +OPERCULUM.--A calcareous plate employed by many Molluscae to close the +aperture of their shell. The OPERCULAR VALVES of Cirripedes are those +which close the aperture of the shell. + +ORBIT.--The bony cavity for the reception of the eye. + +ORGANISM.--An organised being, whether plant or animal. + +ORTHOSPERMOUS.--A term applied to those fruits of the Umbelliferae which +have the seed straight. + +OSCULANT.--Forms or groups apparently intermediate between and +connecting other groups are said to be osculant. + +OVA.--Eggs. + +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. + +OVIGEROUS.--Egg-bearing. + +OVULES (of plants).--The seeds in the earliest condition. + +PACHYDERMS.--A group of Mammalia, so called from their thick skins, and +including the elephant, rhinoceros, hippopotamus, etc. + +PALAEOZOIC.--The oldest system of fossiliferous rocks. + +PALPI.--Jointed appendages to some of the organs of the mouth in insects +and Crustacea. + +PAPILIONACEAE.--An order of plants (see LEGUMINOSAE), The flowers of +these plants are called PAPILIONACEOUS, or butterfly-like, from the +fancied resemblance of the expanded superior petals to the wings of a +butterfly. + +PARASITE.--An animal or plant living upon or in, and at the expense of, +another organism. + +PARTHENOGENESIS.--The production of living organisms from unimpregnated +eggs or seeds. + +PEDUNCULATED.--Supported upon a stem or stalk. The pedunculated oak has +its acorns borne upon a footstool. + +PELORIA or PELORISM.--The appearance of regularity of structure in the +flowers of plants which normally bear irregular flowers. + +PELVIS.--The bony arch to which the hind limbs of vertebrate animals are +articulated. + +PETALS.--The leaves of the corolla, or second circle of organs in a +flower. They are usually of delicate texture and brightly coloured. + +PHYLLODINEOUS.--Having flattened, leaf-like twigs or leafstalks instead +of true leaves. + +PIGMENT.--The colouring material produced generally in the superficial +parts of animals. The cells secreting it are called PIGMENT-CELLS. + +PINNATE.--Bearing leaflets on each side of a central stalk. + +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. + +PLACENTALIA, PLACENTATA.--or PLACENTAL MAMMALS, See MAMMALIA. + +PLANTIGRADES.--Quadrupeds which walk upon the whole sole of the foot, +like the bears. + +PLASTIC.--Readily capable of change. + +PLEISTOCENE PERIOD.--The latest portion of the Tertiary epoch. + +PLUMULE (in plants).--The minute bud between the seed-leaves of +newly-germinated plants. + +PLUTONIC ROCKS.--Rocks supposed to have been produced by igneous action +in the depths of the earth. + +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 (POLLEN-TUBES) which issue from the pollen-grains adhering to the +stigma, and penetrate through the tissues until they reach the ovary. + +POLYANDROUS (flowers).--Flowers having many stamens. + +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. + +POLYMORPHIC.--Presenting many forms. + +POLYZOARY.--The common structure formed by the cells of the Polyzoa, +such as the well-known seamats. + +PREHENSILE.--Capable of grasping. + +PREPOTENT.--Having a superiority of power. + +PRIMARIES.--The feathers forming the tip of the wing of a bird, and +inserted upon that part which represents the hand of man. + +PROCESSES.--Projecting portions of bones, usually for the attachment of +muscles, ligaments, etc. + +PROPOLIS.--A resinous material collected by the hivebees from the +opening buds of various trees. + +PROTEAN.--Exceedingly variable. + +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. + +PUPA (pl. PUPAE).--The second stage in the development of an insect, +from which it emerges in the perfect (winged) reproductive form. In most +insects the PUPAL STAGE is passed in perfect repose. The CHRYSALIS is +the pupal state of butterflies. + +RADICLE.--The minute root of an embryo plant. + +RAMUS.--One half of the lower jaw in the Mammalia. The portion which +rises to articulate with the skull is called the ASCENDING RAMUS. + +RANGE.--The extent of country over which a plant or animal is naturally +spread. RANGE IN TIME expresses the distribution of a species or group +through the fossiliferous beds of the earth's crust. + +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. + +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 +RETROGRADE DEVELOPMENT or METAMORPHOSIS. + +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. + +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. + +RUBUS.--The bramble genus. + +RUDIMENTARY.--Very imperfectly developed. + +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. + +SACRAL.--Belonging to the sacrum, or the bone composed usually of two +or more united vertebrae to which the sides of the pelvis in vertebrate +animals are attached. + +SARCODE.--The gelatinous material of which the bodies of the lowest +animals (Protozoa) are composed. + +SCUTELLAE.--The horny plates with which the feet of birds are generally +more or less covered, especially in front. + +SEDIMENTARY FORMATIONS.--Rocks deposited as sediments from water. + +SEGMENTS.--The transverse rings of which the body of an articulate +animal or annelid is composed. + +SEPALS.--The leaves or segments of the calyx, or outermost envelope +of an ordinary flower. They are usually green, but sometimes brightly +coloured. + +SERRATURES.--Teeth like those of a saw. + +SESSILE.--Not supported on a stem or footstalk. + +SILURIAN SYSTEM.--A very ancient system of fossiliferous rocks belonging +to the earlier part of the Palaeozoic series. + +SPECIALISATION.--The setting apart of a particular organ for the +performance of a particular function. + +SPINAL CORD.--The central portion of the nervous system in the +Vertebrata, which descends from the brain through the arches of the +vertebrae, and gives off nearly all the nerves to the various organs of +the body. + +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. + +STERNUM.--The breast-bone. + +STIGMA.--The apical portion of the pistil in flowering plants. + +STIPULES.--Small leafy organs placed at the base of the footstalks of +the leaves in many plants. + +STYLE.--The middle portion of the perfect pistil, which rises like a +column from the ovary and supports the stigma at its summit. + +SUBCUTANEOUS.--Situated beneath the skin. + +SUCTORIAL.--Adapted for sucking. + +SUTURES (in the skull).--The lines of junction of the bones of which the +skull is composed. + +TARSUS (pl. TARSI).--The jointed feet of articulate animals, such as +insects. + +TELEOSTEAN FISHES.--Fishes of the kind familiar to us in the present +day, having the skeleton usually completely ossified and the scales +horny. + +TENTACULA or TENTACLES.--Delicate fleshy organs of prehension or touch +possessed by many of the lower animals. + +TERTIARY.--The latest geological epoch, immediately preceding the +establishment of the present order of things. + +TRACHEA.--The windpipe or passage for the admission of air to the lungs. + +TRIDACTYLE.--Three-fingered, or composed of three movable parts attached +to a common base. + +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 Palaeozoic rocks, and most abundantly in those of Silurian +age. + +TRIMORPHIC.--Presenting three distinct forms. + +UMBELLIFERAE.--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 +(UMBEL) nearly to the same level. (Examples, parsley and carrot.) + +UNGULATA.--Hoofed quadrupeds. + +UNICELLULAR.--Consisting of a single cell. + +VASCULAR.--Containing blood-vessels. + +VERMIFORM.--Like a worm. + +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 VERTEBRAE, which constitutes the centre +of the skeleton and at the same time supports and protects the central +parts of the nervous system. + +WHORLS.--The circles or spiral lines in which the parts of plants are +arranged upon the axis of growth. + +WORKERS.--See neuters. + +ZOEA-STAGE.--The earliest stage in the development of many of the +higher Crustacea, so called from the name of ZOEA applied to these young +animals when they were supposed to constitute a peculiar genus. + +ZOOIDS.--In many of the lower animals (such as the Corals, Medusae, +etc.) 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 ZOOIDE. + + + + +INDEX. + + + Aberrant groups + + Abyssinia, plants of + + Acclimatisation + + Adoxa + + Affinities of extinct species + --of organic beings + + Agassiz on Amblyopsis + --on groups of species suddenly appearing + --on prophetic forms + --on embryological succession + --on the Glacial period + --on embryological characters + --on the latest tertiary forms + --on parallelism of embryological development and geological succession + --Alex., on pedicellariae + + Algae of New Zealand + + Alligators, males, fighting + + Alternate generations + + Amblyopsis, blind fish + + America, North, productions allied to those of Europe + --boulders and glaciers of + --South, no modern formations on west coast + + Ammonites, sudden extinction of + + Anagallis, sterility of + + Analogy of variations + + Andaman Islands inhabited by a toad + + Ancylus + + Animals, not domesticated from being variable + --domestic; descended from several stocks + --acclimatisation of + + Animals of Australia + --with thicker fur in cold climates + --blind, in caves + --extinct, of Australia + + Anomma + + Antarctic islands, ancient flora of + + Antechinus + + Ants attending aphides + --slave-making instinct + --neuters, structure of + + Apes, not having acquired intellectual powers + + Aphides attended by ants + + Aphis, development of + + Apteryx + + Arab horses + + Aralo-Caspian Sea + + Archeopteryx + + Archiac, M. de, on the succession of species + + Artichoke, Jerusalem + + Ascension, plants of + + Asclepias, pollen of + + Asparagus + + Aspicarpa + + Asses, striped + --improved by selection + + Ateuchus + + Aucapitaine, on land-shells + + Audubon, on habits of frigate-bird + --on variation in birds' nests + --on heron eating seeds + + Australia, animals of + --dogs of + --extinct animals of + --European plants in + --glaciers of + + Azara, on flies destroying cattle + + Azores, flora of + + Babington, Mr., on British plants + + Baer, Von, standard of Highness + --comparison of bee and fish + --embryonic similarity of the Vertebrata + + Baker, Sir S., on the giraffe + + Balancement of growth + + Baleen + + Barberry, flowers of + + Barrande, M., on Silurian colonies + --on the succession of species + --on parallelism of palaeozoic formations + --on affinities of ancient species + + Barriers, importance of + + Bates, Mr., on mimetic butterflies + + Batrachians on islands + + Bats, how structure acquired + --distribution of + + Bear, catching water-insects + + Beauty, how acquired + + Bee, sting of + --queen, killing rivals + --Australian, extermination of + + Bees, fertilizing flowers + --hive, not sucking the red clover + --hive, cell-making instinct + --Ligurian + --variation in habits + + Bees, parasitic + --humble, cells of + + Beetles, wingless, in Madeira + --with deficient tarsi + + Bentham, Mr., on British plants + --on classification + + Berkeley, Mr., on seeds in salt-water + + Bermuda, birds of + + Birds acquiring fear + --beauty of + --annually cross the Atlantic + --colour of, on continents + --footsteps, and remains of, in secondary rocks + --fossil, in caves of Brazil + --of Madeira, Bermuda, and Galapagos + --song of males + --transporting seeds + --waders + --wingless + + Bizcacha, affinities of + + Bladder for swimming, in fish + + Blindness of cave animals + + Blyth, Mr., on distinctness of Indian cattle + --on striped Hemionus + --on crossed geese + + Borrow, Mr., on the Spanish pointer + + Bory St. Vincent, on Batrachians + + Bosquet, M., on fossil Chthamalus + + Boulders, erratic, on the Azores + + Branchiae + --of crustaceans + + Braun, Prof., on the seeds of Fumariaceae + + Brent, Mr., on house-tumblers + + Britain, mammals of + + Broca, Prof., on Natural Selection + + Bronn, Prof., on duration of specific forms + --various objections by + + Brown, Robert, on classification + + Brown-Sequard, on inherited mutilations + + Busk, Mr., on the Polyzoa + + Butterflies, mimetic + + Buzareingues, on sterility of varieties + + Cabbage, varieties of, crossed + + Calceolaria + + Canary-birds, sterility of hybrids + + Cape de Verde Islands, productions of + --plants of, on mountains + + Cape of Good Hope, plants of + + Carpenter, Dr., on foraminifera + + Carthemus + + Catasetum + + Cats, with blue eyes, deaf + --variation in habits of + --curling tail when going to spring + + Cattle destroying fir-trees + --destroyed by flies in Paraguay + --breeds of, locally extinct + --fertility of Indian and European breeds + --Indian + + Cave, inhabitants of, blind + + Cecidomyia + + Celts, proving antiquity of man + + Centres of creation + + Cephalopodae, structures of eyes + --development of + + Cercopithecus, tail of + + Ceroxylus laceratus + + Cervulus + + Cetacea, teeth and hair + --development of the whalebone + + Cetaceans + + Ceylon, plants of + + Chalk formation + + Characters, divergence of + --sexual, variable + --adaptive or analogical + + Charlock + + Checks to increase + --mutual + + Chelae of Crustaceans + + Chickens, instinctive tameness of + + Chironomus, its asexual reproduction + + Chthamalinae + + Chthamalus, cretacean species of + + Circumstances favourable to selection of domestic products + --to natural selection + + Cirripedes capable of crossing + --carapace aborted + --their ovigerous frena + --fossil + --larvae of + + Claparede, Prof., on the hair-claspers of the Acaridae + + Clarke, Rev. W.B., on old glaciers in Australia + + Classification + + Clift, Mr., on the succession of types + + Climate, effects of, in checking increase of beings + --adaptation of, to organisms + + Climbing plants + --development of + + Clover visited by bees + + Cobites, intestine of + + Cockroach + + Collections, palaeontological, poor + + Colour, influenced by climate + --in relation to attacks by flies + + Columba livia, parent of domestic pigeons + + Colymbetes + + Compensation of growth + + Compositae, flowers and seeds of + --outer and inner florets of + --male flowers of + + Conclusion, general + + Conditions, slight changes in, favourable to fertility + + Convergence of genera + + Coot + + Cope, Prof., on the acceleration or retardation of the period of + reproduction + + Coral-islands, seeds drifted to + --reefs, indicating movements of earth + + Corn-crake + + Correlated variation in domestic productions + + Coryanthes + + Creation, single centres of + + Crinum + + Croll, Mr., on subaerial denudation + --on the age of our oldest formations + --on alternate Glacial periods in the North and South + + Crosses, reciprocal + + Crossing of domestic animals, importance in altering breeds + --advantages of + --unfavourable to selection + + Cruger, Dr., on Coryanthes + + Crustacea of New Zealand + + Crustacean, blind + air-breathers + + Crustaceans, their chelae + + Cryptocerus + + Ctenomys, blind + + Cuckoo, instinct of + + Cunningham, Mr., on the flight of the logger-headed duck + + Currants, grafts of + + Currents of sea, rate of + + Cuvier on conditions of existence + --on fossil monkeys + + Cuvier, Fred., on instinct + + Cyclostoma, resisting salt water + + Dana, Prof., on blind cave-animals + --on relations of crustaceans of Japan + --on crustaceans of New Zealand + + Dawson, Dr., on eozoon + + De Candolle, Aug. Pyr., on struggle for existence + --on umbelliferae + --on general affinities + + De Candolle, Alph., on the variability of oaks + --on low plants, widely dispersed + --on widely-ranging plants being variable + --on naturalisation + --on winged seeds + --on Alpine species suddenly becoming rare + --on distribution of plants with large seeds + --on vegetation of Australia + --on fresh-water plants + --on insular plants + + Degradation of rocks + + Denudation, rate of + --of oldest rocks + --of granite areas + + Development of ancient forms + + Devonian system + + Dianthus, fertility of crosses + + Dimorphism in plants + + Dirt on feet of birds + + Dispersal, means of + --during Glacial period + + Distribution, geographical + --means of + + Disuse, effect of, under nature + + Diversification of means for same general purpose + + Division, physiological, of labour + + Divergence of character + + Dog, resemblance of jaw to that of the Thylacinus + + Dogs, hairless, with imperfect teeth + --descended from several wild stocks + --domestic instincts of + --inherited civilisation of + --fertility of breeds together + --of crosses + --proportions of body in different breeds, when young + + Domestication, variation under + + Double flowers + + Downing, Mr., on fruit-trees in America + + Dragon-flies, intestines of + + Drift-timber + + Driver-ant + + Drones killed by other bees + + Duck, domestic, wings of, reduced + --beak of + --logger-headed + + Duckweed + + Dugong, affinities of + + Dung-beetles with deficient tarsi + + Dyticus + + Earl, Mr., W., on the Malay Archipelago + + Ears, drooping, in domestic animals + --rudimentary + + Earth, seeds in roots of trees + --charged with seeds + + Echinodermata, their pedicellariae + + Eciton + + Economy of organisation + + Edentata, teeth and hair + --fossil species of + + Edwards, Milne, on physiological division of labour + --on gradations of structure + + Edwards, on embryological characters + + Eggs, young birds escaping from + + Egypt, productions of, not modified + + Electric organs + + Elephant, rate of increase + --of Glacial period + + Embryology + + Eozoon Canadense + + Epilipsy inherited + + Existence, struggle for + --condition of + + Extinction, as bearing on natural selection + --of domestic varieties + + Eye, structure of + --correction for aberration + + Eyes, reduced, in moles + + Fabre, M., on hymenoptera fighting + --on parasitic sphex + --on Sitaris + + Falconer, Dr., on naturalisation of plants in India + --on elephants and mastodons + --and Cautley on mammals of sub-Himalayan beds + + Falkland Islands, wolf of + + Faults + + Faunas, marine + + Fear, instinctive, in birds + + Feet of birds, young molluscs adhering to + + Fertilisation variously effected + + Fertility of hybrids + --from slight changes in conditions + --of crossed varieties + + Fir-trees destroyed by cattle + --pollen of + + Fish, flying + --teleostean, sudden appearance of + --eating seeds + --fresh-water, distribution of + + Fishes, ganoid, now confined to fresh water + --ganoid, living in fresh water + --electric organs of + --of southern hemisphere + + Flight, powers of, how acquired + + Flint-tools, proving antiquity of man + + Flower, Prof., on the larynx + --on Halitherium + --on the resemblance between the jaws of the dog and Thylacinus + --on the homology of the feet of certain marsupials + + Flowers, structure of + --in relation to crossing + --of composite and umbelliferae + --beauty of + --double + + Flysch formation, destitute of organic remains + + Forbes, Mr. D., on glacial action in the Andes + + Forbes, E., on colours of shells + --on abrupt range of shells in depth + --on poorness of palaeontological collections + --on continuous succession of genera + --on continental extensions + --on distribution during Glacial period + --on parallelism in time and space + + Forests, changes in, in America + + Formation, Devonian + --Cambrian + --intermittent + --thickness of, in Britain + + Formica + --rufescens + --sanguinea + --flava, neuter of + + Forms, lowly organised, long enduring + + Frena, ovigerous, of cirripedes + + Fresh-water productions, dispersal of + + Fries on species in large genera being closely allied to other species + + Frigate-bird + + Frogs on islands + + Fruit-trees, gradual improvement of + --in United States + --varieties of, acclimatised in United States + + Fuci, crossed + + Fur, thicker in cold climates + + Furze + + Galapagos Archipelago, birds of + --productions of + + Galaxias, its wide range + + Galeopithecus + + Game, increase of, checked by vermin + + Gartner on sterility of hybrids + --on reciprocal crosses + --on crossed maize and verbascum + --on comparison of hybrids and mongrels + + Gaudry, Prof., on intermediate genera of fossil mammals in Attica + + Geese, fertility when crossed + --upland + + Geikie, Mr., on subaerial denudation + + Genealogy, important in classification + + Generations, alternate + + Geoffroy St. Hilaire, on balancement + --on homologous organs + + Geoffroy St. Hilaire, Isidore, on variability of repeated parts + --on correlation, in monstrosities + --on correlation + --on variable parts being often monstrous + + Geographical distribution + + Geography, ancient + + Geology, future progress of + --imperfection of the record + + Gervais, Prof., on Typotherium + + Giraffe, tail of + --structure of + + Glacial period + --affecting the North and South + + Glands, mammary + + Gmelin, on distribution + + Godwin-Austin, Mr., on the Malay Archipelago + + Goethe, on compensation of growth + + Gomphia + + Gooseberry, grafts of + + Gould, Dr. Aug. A., on land-shells + + Gould, Mr., on colours of birds + --on instincts of cuckoo + --on distribution of genera of birds + + Gourds, crossed + + Graba, on the Uria lacrymans + + Grafting, capacity of + + Granite, areas of denuded + + Grasses, varieties of + + Gray, Dr. Asa, on the variability of oaks + --on man not causing variability + --on sexes of the holly + --on trees of the United States + --on naturalised plants in the United States + --on aestivation + --on rarity of intermediate varieties + --on Alpine plants + + Gray, Dr. J.E., on striped mule + + Grebe + + Grimm, on asexual reproduction + + Groups, aberrant + + Grouse, colours of + --red, a doubtful species + + Growth, compensation of + + Gunther, Dr., on flat-fish + --on prehensile tails + --on the fishes of Panama + --on the range of fresh-water fishes + --on the limbs of Lepidosiren + + Haast, Dr., on glaciers of New Zealand + + Habit, effect of, under domestication + --effect of, under nature + --diversified, of same species + + Hackel, Prof., on classification and the lines of descent + + Hair and teeth, correlated + + Halitherium + + Harcourt, Mr. E.V., on the birds of Madeira + + Hartung, M., on boulders in the Azores + + Hazel-nuts + + Hearne, on habits of bears + + Heath, changes in vegetation + + Hector, Dr., on glaciers of New Zealand + + Heer, Oswald, on ancient cultivated plants + --on plants of Madeira + + Helianthemum + + Helix pomatia, resisting salt water + + Helmholtz, M., on the imperfection of the human eye + + Helosciadium + + Hemionus, striped + + Hensen, Dr., on the eyes of Cephalopods + + Herbert, W., on struggle for existence + --on sterility of hybrids + + Hermaphrodites crossing + + Heron eating seed + + Heron, Sir R., on peacocks + + Heusinger, on white animals poisoned by certain plants + + Hewitt, Mr., on sterility of first crosses + + Hildebrand, Prof., on the self-sterility of Corydalis + + Hilgendorf, on intermediate varieties + + Himalaya, glaciers of + --plants of + + Hippeastrum + + Hippocampus + + Hofmeister, Prof., on the movements of plants + + Holly-trees, sexes of + + Hooker, Dr., on trees of New Zealand + --on acclimatisation of Himalayan trees + --on flowers of umbelliferae + --on the position of ovules + --on glaciers of Himalaya + --on algae of New Zealand + --on vegetation at the base of the Himalaya + --on plants of Tierra del Fuego + --on Australian plants + --on relations of flora of America + --on flora of the Antarctic lands + --on the plants of the Galapagos + --on glaciers of the Lebanon + --on man not causing variability + --on plants of mountains of Fernando Po + + Hooks on palms + --on seeds, on islands + + Hopkins, Mr., on denudation + + Hornbill, remarkable instinct of + + Horns, rudimentary + + Horse, fossil in La Plata + --proportions of, when young + + Horses destroyed by flies in Paraguay + --striped + + Horticulturists, selection applied by + + Huber on cells of bees + + Huber, P., on reason blended with instinct + --on habitual nature of instincts + --on slave-making ants + --on Melipona domestica + + Hudson, Mr., on the Ground-woodpecker of La Plata + --on the Molothrus + + Humble-bees, cells of + + Hunter, J., on secondary sexual characters + + Hutton, Captain, on crossed geese + + Huxley, Prof., on structure of hermaphrodites + --on the affinities of the Sirenia + --on forms connecting birds and reptiles + --on homologous organs + --on the development of aphis + + Hybrids and mongrels compared + + Hybridism + + Hydra, structure of + + Hymenoptera, fighting + + Hymenopterous insect, diving + + Hyoseris + + Ibla + + Icebergs transporting seeds + + Increase, rate of + + Individuals, numbers favourable to selection + --many, whether simultaneously created + + Inheritance, laws of + --at corresponding ages + + Insects, colour of, fitted for their stations + --sea-side, colours of + --blind, in caves + --luminous + --their resemblance to certain objects + --neuter + + Instinct, not varying simultaneously with structure + + Instincts, domestic + + Intercrossing, advantages of + + Islands, oceanic + + Isolation favourable to selection + + Japan, productions of + + Java, plants of + + Jones, Mr. J.M., on the birds of Bermuda + + Jordain, M., on the eye-spots of star fishes + + Jukes, Prof., on subaerial denudation + + Jussieu on classification + + Kentucky, caves of + + Kerguelen-land, flora of + + Kidney-bean, acclimatisation of + + Kidneys of birds + + Kirby, on tarsi deficient in beetles + + Knight, Andrew, on cause of variation + + Kolreuter, on intercrossing + --on the barberry + --on sterility of hybrids + --on reciprocal crosses + --on crossed varieties of nicotiana + --on crossing male and hermaphrodite flowers + + Lamarck, on adaptive characters + + Lancelet, eyes of + + Landois, on the development of the wings of insects + + Land-shells, distribution of + --of Madeira, naturalised + --resisting salt water + + Languages, classification of + + Lankester, Mr. E. Ray, on longevity + --on homologies + + Lapse, great, of time + + Larvae + + Laurel, nectar secreted by the leaves + + Laurentian formation + + Laws of variation + + Leech, varieties of + + Leguminosae, nectar secreted by glands + + Leibnitz, attack on Newton + + Lepidosiren, limbs in a nascent condition + + Lewes, Mr. G.H., on species not having changed in Egypt + --on the Salamandra atra + --on many forms of life having been at first evolved + + Life, struggle for + + Lingula, Silurian + + Linnaeus, aphorism of + + Lion, mane of + --young of, striped + + Lobelia fulgens + + Lobelia, sterility of crosses + + Lockwood, Mr., on the ova of the Hippocampus + + Locusts transporting seeds + + Logan, Sir W., on Laurentian formation + + Lowe, Rev. R.T., on locusts visiting Madeira + + Lowness, of structure connected with variability + --related to wide distribution + + Lubbock, Sir J., on the nerves of coccus + --on secondary sexual characters + --on a diving hymenopterous insect + --on affinities + --on metamorphoses + + Lucas, Dr. P., on inheritance + --on resemblance of child to parent + + Lund and Clausen, on fossils of Brazil + + Lyell, Sir C., on the struggle for existence + --on modern changes of the earth + --on terrestrial animals not having been developed on islands + --on a carboniferous land-shell + --on strata beneath Silurian system + --on the imperfection of the geological record + --on the appearance of species + --on Barrande's colonies + --on tertiary formations of Europe and North America + --on parallelism of tertiary formations + --on transport of seeds by icebergs + --on great alternations of climate + --on the distribution of fresh-water shells + --on land-shells of Madeira + + Lyell and Dawson, on fossilized trees in Nova Scotia + + Lythrum salicaria, trimorphic + + Macleay, on analogical characters + + Macrauchenia + + McDonnell, Dr., on electric organs + + Madeira, plants of + --beetles of, wingless + --fossil land-shells of + --birds of + + Magpie tame in Norway + + Males, fighting + + Maize, crossed + + Malay Archipelago, compared with Europe + --mammals of + + Malm, on flat-fish + + Malpighiaceae, small imperfect flowers of + + Mammae, their development + --rudimentary + + Mammals, fossil, in secondary formation + --insular + + Man, origin of + + Manatee, rudimentary nails of + + Marsupials, fossil species of + + Marsupials of Australia, structure of their feet + + Martens, M., experiment on seeds + + Martin, Mr. W.C., on striped mules + + Masters, Dr., on Saponaria + + Matteucci, on the electric organs of rays + + Matthiola, reciprocal crosses of + + Maurandia + + Means of dispersal + + Melipona domestica + + Merrill, Dr., on the American cuckoo + + Metamorphism of oldest rocks + + Mice destroying bees + --acclimatisation of + --tails of + + Miller, Prof., on the cells of bees + + Mirabilis, crosses of + + Missel-thrush + + Mistletoe, complex relations of + + Mivart, Mr., on the relation of hair and teeth + --on the eyes of cephalopods + --various objections to Natural Selection + --on abrupt modifications + --on the resemblance of the mouse and antechinus + + Mocking-thrush of the Galapagos + + Modification of species, not abrupt + + Moles, blind + + Molothrus, habits of + + Mongrels, fertility and sterility of + --and hybrids compared + + Monkeys, fossil + + Monachanthus + + Mons, Van, on the origin of fruit-trees + + Monstrosities + + Moquin-Tandon, on sea-side plants + + Morphology + + Morren, on the leaves of Oxalis + + Moths, hybrid + + Mozart, musical powers of + + Mud, seeds in + + Mules, striped + + Muller, Adolph, on the instincts of the cuckoo + + Muller, Dr. Ferdinand, on Alpine Australian plants + + Muller, Fritz, on dimorphic crustaceans + --on the lancelet + --on air-breathing crustaceans + --on the self-sterility of orchids + --on embryology in relation to classification + --on the metamorphoses of crustaceans + --on terrestrial and fresh-water organisms not undergoing any metamorphosis + --on climbing plants + + Multiplication of species not indefinite + + Murchison, Sir, R., on the formations of Russia + --on azoic formations + --on extinction + + Murie, Dr., on the modification of the skull in old age + + Murray, Mr. A., on cave-insects + + Mustela vison + + Myanthus + + Myrmecocystus + + Myrmica, eyes of + + Nageli, on morphological characters + + Nails, rudimentary + + Nathusius, Von, on pigs + + Natural history, future progress of + --selection + --system + + Naturalisation of forms distinct from the indigenous species + --in New Zealand + + Naudin, on analagous variations in gourds + --on hybrid gourds + --on reversion + + Nautilus, Silurian + + Nectar of plants + + Nectaries, how formed + + Nelumbium luteum + + Nests, variation in + + Neuter insects + + New Zealand, productions of, not perfect + --naturalised products of + --fossil birds of + --glaciers of + --crustaceans of + --algae of + --number of plants of + --flora of + + Newman, Col., on humble-bees + + Newton, Prof., on earth attached to a partridge's foot + + Newton, Sir I., attacked for irreligion + + Nicotiana, crossed varieties of + --certain species very sterile + + Nitsche, Dr., on the Polyzoa + + Noble, Mr., on fertility of Rhododendron + + Nodules, phosphatic, in azoic rocks + + Oak, varieties of + + Onites apelles + + Orchids, fertilisation of + --the development of their flowers + --forms of + + Orchis, pollen of + + Organisation, tendency to advance + + Organs of extreme perfection + --electric, of fishes + --of little importance + --homologous + --rudiments of, and nascent + + Ornithorhynchus, mammae of + + Ostrich not capable of flight + --habit of laying eggs together + --American, two species of + + Otter, habits of, how acquired + + Ouzel, water + + Owen, Prof., on birds not flying + --on vegetative repetition + --on variability of unusually developed parts + --on the eyes of fishes + --on the swim-bladder of fishes + --on fossil horse of La Plata + --on generalised form + --on relation of ruminants and pachyderms + --on fossil birds of New Zealand + --on succession of types + --on affinities of the dugong + --on homologous organs + --on the metamorphosis of cephalopods + + Pacific Ocean, faunas of + + Pacini, on electric organs + + Paley, on no organ formed to give pain + + Pallas, on the fertility of the domesticated descendants of wild stocks + + Palm with hooks + + Papaver bracteatum + + Paraguay, cattle destroyed by flies + + Parasites + + Partridge, with ball of dirt attached to foot + + Parts greatly developed, variable + + Parus major + + Passiflora + + Peaches in United States + + Pear, grafts of + + Pedicellariae + + Pelargonium, flowers of + --sterility of + + Peloria + + Pelvis of women + + Period, glacial + + Petrels, habits of + + Phasianus, fertility of hybrids + + Pheasant, young, wild + + Pictet, Prof., on groups of species suddenly appearing + --on rate of organic change + --on continuous succession of genera + --on close alliance of fossils in consecutive formations + --on change in latest tertiary forms + --on early transitional links + + Pierce, Mr., on varieties of wolves + + Pigeons with feathered feet and skin between toes + --breeds described, and origin of + --breeds of, how produced + --tumbler, not being able to get out of egg + --reverting to blue colour + --instinct of tumbling + --young of + + Pigs, black, not affected by the paint-root + --modified by want of exercise + + Pistil, rudimentary + + Plants, poisonous, not affecting certain coloured animals + --selection, applied to + --gradual improvement of + --not improved in barbarous countries + --dimorphic + --destroyed by insects + --in midst of range, have to struggle with other plants + --nectar of + --fleshy, on sea-shores + --climbing + --fresh-water, distribution of + --low in scale, widely distributed + + Pleuronectidae, their structure + + Plumage, laws of change in sexes of birds + + Plums in the United States + + Pointer dog, origin of + --habits of + + Poison not affecting certain coloured animals + + Poison, similar effect of, on animals and plants + + Pollen of fir-trees + --transported by various means + + Pollinia, their development + + Polyzoa, their avicularia + + Poole, Col., on striped hemionus + + Potemogeton + + Pouchet, on the colours of flat-fish + + Prestwich, Mr., on English and French eocene formations + + Proctotrupes + + Proteolepas + + Proteus + + Psychology, future progress of + + Pyrgoma, found in the chalk + + Quagga, striped + + Quatrefages, M., on hybrid moths + + Quercus, variability of + + Quince, grafts of + + Rabbits, disposition of young + + Races, domestic, characters of + + Race-horses, Arab + --English + + Radcliffe, Dr., the electrical organs of the torpedo + + Ramond, on plants of Pyrenees + + Ramsay, Prof., on subaerial denudation + --on thickness of the British formations + --on faults + + Ramsay, Mr., on instincts of cuckoo + + Ratio of increase + + Rats, supplanting each other + --acclimatisation of + --blind, in cave + + Rattle-snake + + Reason and instinct + + Recapitulation, general + + Reciprocity of crosses + + Record, geological, imperfect + + Rengger, on flies destroying cattle + + Reproduction, rate of + + Resemblance, protective, of insects + --to parents in mongrels and hybrids + + Reversion, law of inheritance + --in pigeons, to blue colour + + Rhododendron, sterility of + + Richard, Prof., on Aspicarpa + + Richardson, Sir J., on structure of squirrels + --on fishes of the southern hemisphere + + Robinia, grafts of + + Rodents, blind + + Rogers, Prof., Map of N. America + + Rudimentary organs + + Rudiments important for classification + + Rutimeyer, on Indian cattle + + Sageret, on grafts + + Salamandra atra + + Saliva used in nests + + Salmons, males fighting, and hooked jaws of + + Salt-water, how far injurious to seeds + --not destructive to land-shells + + Salter, Mr., on early death of hybrid embryos + + Salvin, Mr., on the beaks of ducks + + Saurophagus sulphuratus + + Schacht, Prof., on Phyllotaxy + + Schiodte, on blind insects + --on flat-fish + + Schlegel, on snakes + + Schobl, Dr., on the ears of mice + + Scott, Mr. J., on the self-sterility of orchids + --on the crossing of varieties of verbascum + + Sea-water, how far injurious to seeds + --not destructive to land-shells + + Sebright, Sir J., on crossed animals + + Sedgwick, Prof., on groups of species suddenly appearing + + Seedlings destroyed by insects + + Seeds, nutriment in + --winged + --means of dissemination + --power of resisting salt-water + --in crops and intestines of birds + --eaten by fish + --in mud + --hooked, on islands + + Selection of domestic products + --principle not of recent origin + --unconscious + --natural + --sexual + --objections to term + --natural, has not induced sterility + + Sexes, relations of + + Sexual characters variable + --selection + + Sheep, Merino, their selection + --two sub-breeds, unintentionally produced + --mountain, varieties of + + Shells, colours of, littoral + --hinges of + --seldom embedded + + Shells, fresh-water, long retain the same forms + --fresh-water, dispersal of + --of Madeira + --land, distribution of + --land, resisting salt water + + Shrew-mouse + + Silene, infertility of crosses + + Silliman, Prof., on blind rat + + Sirenia, their affinities + + Sitaris, metamorphosis of + + Skulls of young mammals + + Slave-making instinct + + Smith, Col. Hamilton, on striped horses + + Smith, Dr., on the Polyzoa + + Smith, Mr. Fred., on slave-making ants + --on neuter ants + + Snake with tooth for cutting through egg-shell + + Somerville, Lord, on selection of sheep + + Sorbus, grafts of + + Sorex + + Spaniel, King Charles' breed + + Specialisation of organs + + Species, polymorphic + --dominant + --common, variable + --in large genera variable + --groups of, suddenly appearing + --beneath Silurian formations + --successively appearing + --changing simultaneously throughout the world + + Spencer, Lord, on increase in size of cattle + + Spencer, Mr. Herbert, on the first steps in differentiation + --on the tendency to an equilibrium in all forces + + Sphex, parasitic + + Spiders, development of + + Sports in plants + + Sprengel, C.C., on crossing + --on ray-florets + + Squalodon + + Squirrels, gradations in structure + + Staffordshire, heath, changes in + + Stag-beetles, fighting + + Star fishes, eyes of + --their pedicellariae + + Sterility from changed conditions of life + --of hybrids + --laws of + --causes of + --from unfavourable conditions + --not induced through natural selection + + St. Helena, productions of + + St. Hilaire, Aug., on variability of certain plants + --on classification + + St. John, Mr., on habits of cats + + Sting of bee + + Stocks, aboriginal, of domestic animals + + Strata, thickness of, in Britain + + Stripes on horses + + Structure, degrees of utility of + + Struggle for existence + + Succession, geological + --of types in same areas + + Swallow, one species supplanting another + + Swaysland, Mr., on earth adhering to the feet of migratory birds + + Swifts, nests of + + Swim-bladder + + Switzerland, lake inhabitants of + + System, natural + + Tail of giraffe + --of aquatic animals + --prehensile + --rudimentary + + Tanais, dimorphic + + Tarsi deficient + + Tausch, Dr., on umbelliferae + + Teeth and hair correlated + --rudimentary, in embryonic calf + + Tegetmeier, Mr., on cells of bees + + Temminck, on distribution aiding classification + + Tendrils, their development + + Thompson, Sir W., on the age of the habitable world + --on the consolidation of the crust of the earth + + Thouin, on grafts + + Thrush, aquatic species of + --mocking, of the Galapagos + --young of, spotted + --nest of + + Thuret, M., on crossed fuci + + Thwaites, Mr., on acclimatisation + + Thylacinus + + Tierra del Fuego, dogs of + --plants of + + Timber-drift + + Time, lapse of + --by itself not causing modification + + Titmouse + + Toads on islands + + Tobacco, crossed varieties of + + Tomes, Mr., on the distribution of bats + + Transitions in varieties rare + + Traquair, Dr., on flat-fish + + Trautschold, on intermediate varieties + + Trees on islands belong to peculiar orders + --with separated sexes + + Trifolium pratense + --incarnatum + + Trigonia + + Trilobites + --sudden extinction of + + Trimen, Mr., on imitating-insects + + Trimorphism in plants + + Troglodytes + + Tuco-tuco, blind + + Tumbler pigeons, habits of, hereditary + --young of + + Turkey-cock, tuft of hair on breast + + Turkey, naked skin on head + --young of, instinctively wild + + Turnip and cabbage, analogous variations of + + Type, unity of + + Types, succession of, in same areas + + Typotherium + + Udders enlarged by use + --rudimentary + + Ulex, young leaves of + + Umbelliferae, flowers and seeds of + --outer and inner florets of + + Unity of type + + Uria lacrymans + + Use, effects of + --under domestication + --in a state of nature + + Utility, how far important in the construction of each part + + Valenciennes, on fresh-water fish + + Variability of mongrels and hybrids + + Variation, under domestication + --caused by reproductive system being affected by conditions of life + --under nature + --laws of + --correlated + + Variations appear at corresponding ages + --analogous in distinct species + + Varieties, natural + --struggle between + --domestic, extinction of + --transitional, rarity of + + Varieties, when crossed + --fertile + --sterile + --classification of + + Verbascum, sterility of + --varieties of, crossed + + Verlot, M., on double stocks + + Verneuil, M. de, on the succession of species + + Vibracula of the Polyzoa + + Viola, small imperfect flowers of + --tricolor + + Virchow, on the structure of the crystalline lens + + Virginia, pigs of + + Volcanic islands, denudation of + + Vulture, naked skin on head + + Wading-birds + + Wagner, Dr., on Cecidomyia + + Wagner, Moritz, on the importance of isolation + + Wallace, Mr., on origin of species + --on the limit of variation under domestication + --on dimorphic lepidoptera + --on races in the Malay Archipelago + --on the improvement of the eye + --on the walking-stick insect + --on laws of geographical distribution + --on the Malay Archipelago + --on mimetic animals + + Walsh, Mr. B.D., on phytophagic forms + --on equal variability + + Water, fresh, productions of + + Water-hen + + Waterhouse, Mr., on Australian marsupials + --on greatly developed parts being variable + --on the cells of bees + --on general affinities + + Water-ouzel + + Watson, Mr. H.C., on range of varieties of British plants + --on acclimatisation + --on flora of Azores + --on rarity of intermediate varieties + --on Alpine plants + --on convergence + --on the indefinite multiplication of species + + Weale, Mr., on locusts transporting seeds + + Web of feet in water-birds + + Weismann, Prof., on the causes of variability + --on rudimentary organs + + West Indian islands, mammals of + + Westwood, on species in large genera being closely allied to others + --on the tarsi of Engidae + --on the antennae of hymenopterous insects + + Whales + + Wheat, varieties of + + White Mountains, flora of + + Whittaker, Mr., on lines of escarpment + + Wichura, Max, on hybrids + + Wings, reduction of size + --of insects homologous with branchiae + --rudimentary, in insects + + Wolf crossed with dog + --of Falkland Isles + + Wollaston, Mr., on varieties of insects + --on fossil varieties of shells in Madeira + + Wollaston, Mr., on colours of insects on sea-shore + --on wingless beetles + --on rarity of intermediate varieties + --on insular insects + --on land-shells of Madeira naturalised + + Wolves, varieties of + + Woodcock with earth attached to leg + + Woodpecker, habits of + --green colour of + + Woodward, Mr., on the duration of specific forms + --on Pyrgoma + --on the continuous succession of genera + --on the succession of types + + World, species changing simultaneously throughout + + Wright, Mr. Chauncey, on the giraffe + --on abrupt modifications + + Wrens, nest of + + Wyman, Prof., on correlation of colour and effects of poison + --on the cells of the bee + + Youatt, Mr., on selection + --on sub-breeds of sheep + --on rudimentary horns in young cattle + + Zanthoxylon + + Zebra, stripes on + + Zeuglodons + + + + + +End of Project Gutenberg's On the Origin of Species, by Charles Darwin + +*** END OF THIS PROJECT 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FOR PUBLIC DOMAIN ETEXTS*Ver.04.29.93*END* + + + + + +This etext was prepared by Sue Asscher <asschers@aia.net.au> + + + + + +The Origin of Species by means of Natural Selection; + +or, the + +Preservation of Favoured Races in the Struggle for Life. + +By Charles Darwin, M.A., F.R.S., +Author of "The Descent of Man," etc., etc. + +Sixth London Edition, with all Additions and Corrections. +The 6th Edition is often considered the definititive edition. +Also see Project Gutenberg Etext #1228 for an earlier edition. + + + +"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."--Whewell: "Bridgewater Treatise". + +"The only distinct meaning of the word 'natural' is STATED, FIXED or +SETTLED; since what is natural as much requires and presupposes an +intelligent agent to render it so, i.e., to effect it continually or at +stated times, as what is supernatural or miraculous does to effect it for +once."--Butler: "Analogy of Revealed Religion". + + +"To conclude, therefore, let no man out of a weak conceit of sobriety, or +an ill-applied moderation, think or maintain, that a man can search too far +or be too well studied in the book of God's word, or in the book of God's +works; divinity or philosophy; but rather let men endeavour an endless +progress or proficience in both."--Bacon: "Advancement of Learning". + + +AN HISTORICAL SKETCH + +OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES, + +PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK. + +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 (Aristotle, in his "Physicae 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.), 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. + +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 +Vertebres". In these 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. (I have taken +the date of the first publication of Lamarck from Isidore Geoffroy Saint- +Hilaire's ("Hist. Nat. Generale", 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.) + +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 "monde ambiant" 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 +probleme a reserver entierement a l'avenir, suppose meme que l'avenir doive +avoir prise sur lui." + +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. + +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 +"Amaryllidaceae" (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. + +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 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. + +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. + +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. + +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." + +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. + +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, FIRST, 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 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; SECOND, 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. + +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. + +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 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." + +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 palaeontologists 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," etc.(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. + +M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850 (of +which a Resume appeared in the "Revue et Mag. de Zoolog.", Jan., 1851), +briefly gives his reason for believing that specific characters "sont +fixes, pour chaque espece, tant qu'elle se perpetue au milieu des memes +circonstances: ils se modifient, si les circonstances ambiantes viennent a +changer. En resume, L'OBSERVATION des animaux sauvages demontre deja la +variabilite LIMITEE des especes. Les EXPERIENCES sur les animaux sauvages +devenus domestiques, et sur les animaux domestiques redevenus sauvages, la +demontrent plus clairment encore. Ces memes experiences prouvent, de plus, +que les differences produites peuvent etre de VALEUR GENERIQUE." In his +"Hist. Nat. Generale" (tom. ii, page 430, 1859) he amplifies analogous +conclusions. + +>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. + +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. + +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 Museum", tom. i, +page 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 mysterieuse, indeterminee; fatalite pour +les uns; pour les autres volonte providentielle, dont l'action incessante +sur les etres vivantes determine, a toutes les epoques de l'existence du +monde, la forme, le volume, et la duree de chacun d'eux, en raison de sa +destinee dans l'ordre de choses dont il fait partie. C'est cette puissance +qui harmonise chaque membre a l'ensemble, en l'appropriant a la fonction +qu'il doit remplir dans l'organisme general de la nature, fonction qui est +pour lui sa raison d'etre." (From references in Bronn's "Untersuchungen +uber die Entwickelungs-Gesetze", it appears that the celebrated botanist +and palaeontologist 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'Espece", it seems that Bory St. Vincent, Burdach, Poiret and Fries, have +all admitted that new species are continually being produced. 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.) + +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. + +In this same year, 1853, Dr. Schaaffhausen published an excellent pamphlet +("Verhand. des Naturhist. Vereins der Preuss. Rheinlands", etc.), 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." + +A well-known French botanist, M. Lecoq, writes in 1854 ("Etudes sur +Geograph. Bot. tom. i, page 250), "On voit que nos recherches sur la fixite +ou la variation de l'espece, nous conduisent directement aux idees emises +par deux hommes justement celebres, 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. + +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, not a casual phenomenon," or, as +Sir John Herschel expresses it, "a natural in contradistinction to a +miraculous process." + +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. + +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. + +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." + +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. + +The first edition of this work was published on November 24, 1859, and the +second edition on January 7, 1860. + + + +CONTENTS. + + +INTRODUCTION + + +CHAPTER I. + +VARIATION UNDER DOMESTICATION. + +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. + + +CHAPTER II. + +VARIATION UNDER NATURE. + +Variability -- Individual Differences -- Doubtful species -- Wide ranging, +much diffused, and common species, vary most -- Species of the larger +genera in 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. + + +CHAPTER III. + +STRUGGLE FOR EXISTENCE. + +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. + + +CHAPTER IV. + +NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST. + +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. + + +CHAPTER V. + +LAWS OF VARIATION. + +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. + + +CHAPTER VI. + +DIFFICULTIES OF THE THEORY. + +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. + + +CHAPTER VII. + +MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION. + +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. + + +CHAPTER VIII. + +INSTINCT. + +Instincts comparable with habits, but different in their origin -- +Instincts graduated -- Aphides and ants -- Instincts variable -- Domestic +instincts, their origin -- Natural instincts of the cuckoo, 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. + + +CHAPTER IX. + +HYBRIDISM. + +Distinction between the sterility of first crosses and of hybrids -- +Sterility various in degree, not universal, affected by close +interbreeding, removed by domestication -- Laws governing the sterility of +hybrids -- Sterility not a special endowment, but incidental on other +differences, 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. + + +CHAPTER X. + +ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + +On the absence of intermediate varieties at the present day -- On the +nature of extinct intermediate varieties; on their number -- On the 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 +palaeontological 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. + + +CHAPTER XI. + +ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + +On the slow and successive appearance of new species -- On their different +rates of change -- Species once lost do not reappear -- Groups of species +follow the same general rules in their appearance and disappearance as do +single species -- On extinction -- On simultaneous changes in the forms of +life throughout the world -- On the affinities of extinct species to each +other and to living species -- On the state of development of ancient forms +-- On the succession of the same types within the same areas -- Summary of +preceding and present chapter. + + +CHAPTER XII. + +GEOGRAPHICAL DISTRIBUTION. + +Present distribution cannot be accounted for by differences in physical +conditions -- Importance of barriers -- Affinity of the productions of the +same continent -- Centres of creation -- Means of dispersal by changes of +climate and of the level of the land, and by occasional means -- Dispersal +during the Glacial period -- Alternate Glacial periods in the north and +south. + + +CHAPTER XIII. + +GEOGRAPHICAL DISTRIBUTION -- CONTINUED. + +Distribution of fresh-water productions -- On the inhabitants of oceanic +islands -- Absence of Batrachians and of terrestrial Mammals -- On the +relation of the inhabitants of islands to those of the nearest mainland -- +On colonisation from the nearest source with subsequent modification -- +Summary of the last and present chapter. + + +CHAPTER XIV. + +MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- +RUDIMENTARY ORGANS. + +Classification, groups subordinate to groups -- Natural system -- Rules and +difficulties in classification, explained on the theory of descent with +modification -- Classification of varieties -- Descent always used in +classification -- Analogical or adaptive characters -- Affinities, general, +complex and radiating -- Extinction separates and defines groups -- +Morphology, between members of the same class, between parts of the same +individual -- Embryology, laws of, explained by variations not supervening +at an early age, and being inherited at a corresponding age -- Rudimentary +Organs; their origin explained -- Summary. + + +CHAPTER XV. + +RECAPITULATION AND CONCLUSION. + +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. + + +GLOSSARY OF SCIENTIFIC TERMS. + + +INDEX. + + + + +ORIGIN OF SPECIES. + + +INTRODUCTION. + +When on board H.M.S. Beagle, as naturalist, I was much struck with certain +facts in the distribution of the 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. + +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. + +This abstract, which I now publish, must necessarily be imperfect. I +cannot here give references and authorities for my several statements; and +I must trust to the reader reposing some confidence in my accuracy. No +doubt errors 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. + +I much regret that want of space prevents my having the satisfaction of +acknowledging the generous assistance which I have received from very many +naturalists, some of them personally unknown to me. I cannot, however, let +this opportunity pass without expressing my deep obligations to Dr. Hooker, +who, for the last fifteen years, has aided me in every possible way by his +large stores of knowledge and his excellent judgment. + +In considering the origin of species, it is quite conceivable that a +naturalist, reflecting on the mutual affinities of organic beings, on their +embryological relations, their geographical distribution, geological +succession, and other such facts, might come to the conclusion that 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, etc., 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 of external conditions, or of habit, or of +the volition of the plant itself. + +It is, therefore, of the highest importance to gain a clear insight into +the means of modification and coadaptation. At the commencement of my +observations it seemed to me probable that a careful study of domesticated +animals and of cultivated plants would offer the best chance of making out +this obscure problem. Nor have I been disappointed; in this and in all +other perplexing cases I have invariably found that our knowledge, +imperfect though it be, of variation under domestication, afforded the best +and safest clue. I may venture to express my conviction of the high value +of such studies, although they have been very commonly neglected by +naturalists. + +>From these considerations, I shall devote the first chapter of this +abstract to variation under domestication. We shall thus see that a large +amount of hereditary modification is at least possible; and, what is +equally or more important, we shall see how great is the power of man in +accumulating by his selection successive slight variations. I will then +pass on to the variability of species in a state of nature; but I shall, +unfortunately, be compelled to treat this subject far too briefly, as it +can be treated properly only by giving long catalogues of facts. We shall, +however, be enabled to discuss what circumstances are most favourable to +variation. In the next chapter the struggle for existence 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 +NATURALLY SELECTED. From the strong principle of inheritance, any selected +variety will tend to propagate its new and modified form. + +This fundamental subject of natural selection will be treated at some +length in the fourth chapter; and we shall then see how natural selection +almost inevitably causes much extinction of the less improved forms of +life, and 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 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. + +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. + + +CHAPTER I. + +VARIATION UNDER DOMESTICATION. + +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. + +CAUSES OF VARIABILITY. + +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. + +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 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, etc. 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. + +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 of body or constitution, causing coughs or colds, rheumatism, +or inflammation of various organs. + +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 Kolreuter 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 cultivation, and vary very slightly--perhaps hardly more +than in a state of nature. + +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, etc., 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. + +EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED VARIATION; +INHERITANCE. + +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. + +Many laws regulate variation, some few of which can be dimly 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.e. 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. + +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, etc.; 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. + +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 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, etc., +appearing in several members of the same family. If strange and rare +deviations of structure are truly inherited, less strange and commoner +deviations may be freely admitted to be inheritable. Perhaps the correct +way of viewing the whole subject would be, to look at the inheritance of +every character whatever as the rule, and non-inheritance as the anomaly. + +The laws governing inheritance are 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 APPEARANCE of the +peculiarity, and not to the primary cause 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. + +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 DEFINITE 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. + +CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN +VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE +SPECIES. + +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. + +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 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. + +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. + +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 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. + +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 Canidae 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 Rutimeyer. +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. + +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, etc., but that each of these +kingdoms possesses several peculiar breeds of cattle, sheep, etc., we must +admit that many domestic breeds must have originated in Europe; for whence +otherwise could they have been derived? So it is 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, etc.--so unlike all wild +Canidae--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, etc., +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. + +BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN. + +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, 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 oesophagus. The Jacobin has the feathers +so much reversed along the back of the neck that they form a hood, and it +has, proportionally to its size, 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. + +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 vertebrae 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 +oesophagus; 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 scutellae on the toes, the development of skin between +the toes, are all points of structure which are variable. The period at +which the perfect plumage is acquired varies, as does the state of the down +with which the nestling birds are clothed when hatched. The shape and size +of the eggs vary. The manner of flight, 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. + +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. + +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 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. + +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 Columbidae 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. + +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 +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. + +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, inter se, seems to me rash +in the extreme. + +>From these several reasons, namely, the improbability of man having +formerly made seven or eight supposed species of pigeons to 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 Columbidae, +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. + +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 Aegyptian dynasty, about 3000 B.C., as was pointed out to me +by Professor Lepsius; but Mr. Birch informs me that pigeons are given in a +bill of fare in the previous dynasty. In the time of the Romans, as we +hear from Pliny, immense prices were given for pigeons; "nay, they are come +to this pass, that they can reckon up their pedigree and race." Pigeons +were much valued by Akber Khan in India, about the year 1600; never less +than 20,000 pigeons were taken with the court. "The monarchs of Iran and +Turan sent him some very rare birds;" and, continues the courtly historian, +"His Majesty, by crossing the breeds, which method was never practised +before, has improved them astonishingly." About this same period the Dutch +were as eager about pigeons as were the old Romans. The paramount +importance of these considerations in explaining the immense amount of +variation which pigeons have undergone, will likewise be obvious when 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. + +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? + +PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS. + +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. + +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 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. + +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. + +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 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. + +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. + +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 +encyclopaedia. 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. + +UNCONSCIOUS SELECTION. + +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, +etc., 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; but what concerns us is, that the +change has been effected unconsciously and gradually, and yet so +effectually that, though the old Spanish pointer certainly came from Spain, +Mr. Borrow has not seen, as I am informed by him, any native dog in Spain +like our pointer. + +By a similar process of selection, and by careful training, 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. + +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." + +If there exist savages so barbarous as never to think of the inherited +character of the offspring of their domestic animals, yet any one animal +particularly useful to them, for any special purpose, would be carefully +preserved during famines and other accidents, to which savages are so +liable, and such choice animals would thus generally leave more offspring +than the inferior ones; so that in this case there would be a kind of +unconscious selection going on. We see the value set on animals even by +the barbarians of Tierra del Fuego, by their killing and devouring their +old women, in times of dearth, as of less value than their dogs. + +In plants the same gradual process of improvement through the occasional +preservation of the best individuals, whether or not sufficiently distinct +to be ranked at their first appearance as distinct varieties, and whether +or not two or more species or races have become blended together by +crossing, may plainly be recognised in the increased size and beauty which +we now see in the varieties of the heartsease, rose, pelargonium, dahlia, +and other plants, when compared with the older varieties or with their +parent-stocks. No one would ever expect to get a first-rate heartsease or +dahlia from the seed of a wild plant. No one would expect to raise a +first-rate melting pear from the seed of a wild pear, though he might +succeed from a poor seedling growing wild, if it had come from a +garden-stock. The pear, though cultivated in classical times, appears, +from Pliny's description, to have been a fruit of very inferior quality. I +have seen great surprise expressed in horticultural works at the wonderful +skill of gardeners in having produced such splendid results from such poor +materials; but the art 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. + +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. + +In regard to the domestic animals kept by uncivilised man, it should not be +overlooked that they almost always have to struggle for their own food, at +least during certain seasons. And in two countries very differently +circumstanced, individuals of the same species, having slightly different +constitutions or structure, would often succeed better in the one country +than in the other, and thus by a process of "natural selection," as will +hereafter be more fully explained, two sub-breeds might be formed. This, +perhaps, partly explains 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. + +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 oesophagus--a habit which is disregarded by all +fanciers, as it is not one of the points of the breed. + +Nor let it be thought that some great deviation of structure would be +necessary to catch the fancier's eye: he perceives extremely small +differences, and it is in human nature to value any novelty, however +slight, in one's own possession. Nor must the value which would formerly +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 fleeting of +characters, have lately been exhibited as distinct at our poultry-shows. + +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. + +CIRCUMSTANCES FAVOURABLE TO MAN'S POWER OF SELECTION. + +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 IN SMALL LOTS, +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 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. + +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, etc., may be attributed in main part to selection not having been +brought into play: in cats, from the difficulty in pairing them; in +donkeys, from only a few being kept by poor people, and little attention +paid to their breeding; 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 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. + +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. + +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 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, etc., 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. + + +CHAPTER II. + +VARIATION UNDER NATURE. + +Variability -- Individual differences -- Doubtful species -- Wide ranging, +much diffused, and common species, vary most -- Species of the larger +genera in 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. + +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. + +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 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. + +INDIVIDUAL DIFFERENCES. + +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 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 larvae 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. + +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. + +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 Muller has described analogous but more extraordinary +cases 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 antennae 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. + +DOUBTFUL SPECIES. + +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 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. + +Hence, in determining whether a form should be ranked as a species or a +variety, the opinion of naturalists having sound judgment and wide +experience seems the only guide to follow. We must, however, in many +cases, decide by a majority of naturalists, for few well-marked and +well-known varieties can be named which have not been ranked as species by +at least some competent judges. + +That varieties of this doubtful nature are far from uncommon cannot be +disputed. Compare the several floras of Great Britain, of France, or of +the United States, drawn up by different botanists, and see what a +surprising number of forms have been ranked by one botanist as good +species, and by another as mere varieties. Mr. H.C. Watson, to whom I lie +under deep obligation for assistance of all kinds, has marked for me 182 +British plants, which are generally considered as varieties, but which have +all been ranked by botanists as species; and in making this list he has +omitted many trifling varieties, but which nevertheless have been ranked by +some botanists as species, and he has entirely omitted several highly +polymorphic genera. Under genera, including the most polymorphic forms, +Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112--a +difference of 139 doubtful forms! 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 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. + +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? + +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 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. + +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. + +Many of the cases of strongly marked varieties or doubtful species well +deserve consideration; for several interesting lines of argument, from +geographical distribution, analogical variation, hybridism, etc., have been +brought to bear 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 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. + +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 +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 palaeontology, geographical botany and +zoology, of anatomical structure and classification." + +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. + +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. + +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 toward 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. + +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. + +>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. + +WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST. + +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 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. + +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. + +SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN THE +SPECIES OF THE SMALLER GENERA. + +If the plants inhabiting a country as described in any Flora, be divided +into two equal masses, all those in the larger genera (i.e., 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. + +>From looking at species as only strongly marked and well-defined varieties, +I was led to anticipate that the species of the larger genera in each +country would oftener present varieties, than the species of the smaller +genera; for wherever many closely related species (i.e., species of the +same genus) have been formed, many varieties or incipient species ought, as +a general rule, to be now forming. Where many large trees grow, we expect +to find saplings. Where many species of a genus have been formed through +variation, circumstances have been favourable for variation; and hence we +might expect that the circumstances would generally 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. + +To test the truth of this anticipation I have arranged the plants of twelve +countries, and the coleopterous insects of two districts, into two nearly +equal masses, the species of the larger genera on one side, and those of +the smaller genera on the other side, and it has invariably proved to be +the case that a larger proportion of the species on the side of the larger +genera 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. + +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. + +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 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. + +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. + +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; +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. + +SUMMARY. + +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. + +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. + + +CHAPTER III. + +STRUGGLE FOR EXISTENCE. + +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. + +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. + +Again, it may be asked, how is it that varieties, which I have called +incipient species, become ultimately converted into good and distinct +species, which in most cases obviously differ from each other far more than +do the varieties of the same species? How do those groups of species, +which constitute what are called distinct genera and which differ from each +other more than do the species of the same genus, arise? All these +results, as we shall more fully see in the next chapter, follow 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. + +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. + +THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE. + +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. + +GEOMETRICAL RATIO OF INCREASE. + +A struggle for existence inevitably follows from the high rate at which all +organic beings tend to increase. Every being, which during its natural +lifetime produces several eggs or seeds, must suffer destruction during +some period of its life, and during some season or occasional year, +otherwise, on the principle of geometrical increase, its numbers would +quickly become so inordinately great that no country could support the +product. Hence, as more individuals are produced than can possibly +survive, there must in every case be a struggle for existence, either one +individual with another of the same species, or with the individuals of +distinct species, or with the physical conditions of life. It is the +doctrine of Malthus applied with manifold force to the whole animal and +vegetable kingdoms; for in this case there can be no artificial increase of +food, and no prudential restraint from marriage. Although some species may +be now increasing, more or less rapidly, in numbers, all cannot do so, for +the world would not hold them. + +There is no exception to the rule that every organic being naturally +increases at so high a rate, that, if not destroyed, the earth would soon +be covered by the progeny of a single pair. Even slow-breeding man has +doubled in twenty-five years, and at this rate, in less than a thousand +years, there would literally not be standing room for his progeny. +Linnaeus 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. + +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. + +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. + +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. + +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 check, mitigate the destruction ever so +little, and the number of the species will almost instantaneously increase +to any amount. + +NATURE OF THE CHECKS TO INCREASE. + +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. + +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 destroyed by +beasts of prey; for even the tiger in India most rarely dares to attack a +young elephant protected by its dam. + +Climate plays an important part in determining the average numbers of a +species, and periodical seasons of extreme cold or drought 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 DIRECTLY 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. + +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. + +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. + +On the other hand, in many cases, a large stock of individuals of the same +species, relatively to the numbers of its enemies, is absolutely necessary +for its preservation. Thus we can easily raise plenty of corn and +rape-seed, etc., in our fields, because the seeds are in great excess +compared with the number of birds which feed on them; nor can the birds, +though having a superabundance of food at this one season, increase in +number proportionally to the supply of seed, as their numbers are checked +during 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. + +COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE STRUGGLE +FOR EXISTENCE. + +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 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. + +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 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! + +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, 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! + +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! + +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 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. + +STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE SAME +SPECIES. + +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. + +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. + +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. + +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. + +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 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. + +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. + + +CHAPTER IV. + +NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST. + +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. + +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 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. + +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. + +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 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. + +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 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. + +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? + +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 good; silently +and insensibly working, WHENEVER AND WHEREVER OPPORTUNITY OFFERS, at the +improvement of each organic being in relation to its organic and inorganic +conditions of life. We see nothing of these slow changes in progress, +until the hand of time has marked the long lapse of ages, and then so +imperfect is our view into long-past geological ages that we see only that +the forms of life are now different from what they formerly were. + +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. + +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 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. + +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, etc., 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. + +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. + +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 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. + +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 undergo +modification and improvement at the same time in the same area. + +SEXUAL SELECTION. + +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. + +Among birds, the contest is often of a more peaceful character. All those +who have attended to the subject, believe that there is the severest +rivalry between the males of many species to attract, by singing, the +females. The rock-thrush of Guiana, birds of paradise, and some others, +congregate, and successive males display 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. + +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. + +ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE +FITTEST. + +In order to make it clear how, as I believe, natural selection acts, I must +beg permission to give one or two imaginary illustrations. Let us take the +case of a wolf, which preys on various animals, securing some by craft, +some by strength, and some by fleetness; and let us suppose that the +fleetest prey, a deer for instance, had from any change in the country +increased in numbers, or that other prey had decreased in numbers, during +that season of the year when the wolf 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. + +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. + +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 +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. + +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. + +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 +district, competing with and conquering the unchanged individuals on the +margins of an ever-increasing circle. + +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 Leguminosae, 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. + +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. 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. + +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, etc., 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 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. + +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 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. + +ON THE INTERCROSSING OF INDIVIDUALS. + +I must here introduce a short digression. In the case of animals and +plants with separated sexes, it is of course obvious that two individuals +must always (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 Kolreuter. We shall +presently see its importance; but I must here treat the subject with +extreme brevity, though I have the materials prepared for an ample +discussion. All vertebrate animals, all insects and some other large +groups of animals, pair for each birth. Modern research has much +diminished the number of supposed hermaphrodites and of real hermaphrodites +a large number pair; that is, two individuals regularly unite for +reproduction, which is all that concerns us. But still there are many +hermaphrodite animals which certainly do not habitually pair, and a vast +majority of plants are hermaphrodites. What reason, it may be asked, is +there for supposing in these cases that two individuals ever concur in +reproduction? As it is impossible here to enter on details, I must trust +to some general considerations alone. + +In the first place, I have collected so large a body of facts, 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 CLOSE 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. + +On the belief that this is a law of nature, we can, I think, understand +several large classes of facts, such as the following, which on any other +view are inexplicable. Every hybridizer knows how unfavourable exposure to +wet is to the fertilisation of a flower, yet what a multitude of flowers +have their anthers and stigmas fully exposed to the weather! 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 Gartner, the +influence of the foreign pollen. + +When the stamens of a flower suddenly spring towards the pistil, or slowly +move one after the other towards it, the contrivance seems adapted solely +to ensure self-fertilisation; and no doubt it is useful for this end: but +the agency of insects is often required to cause the stamens to spring +forward, as Kolreuter 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 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! + +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 VARIETY 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 SPECIES 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. + +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 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. + +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. + +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. + +>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. + +CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL +SELECTION. + +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? + +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 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. + +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. + +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. + +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 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, etc., 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. + +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. + +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. + +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 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. + +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 Europaeo-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 area, and from having been exposed to less varied, and therefore +less severe, competition. + +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. + +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 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. + +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. + +EXTINCTION CAUSED BY NATURAL SELECTION. + +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. + +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. + +>From these several considerations I think it inevitably follows, that as +new species in the course of time are formed through natural selection, +others will become rarer and rarer, and finally extinct. The forms which +stand in closest competition with those undergoing modification and +improvement, will naturally suffer most. And we have seen in the chapter +on the Struggle for Existence that it is the most closely-allied +forms,--varieties of the same species, and species of the same genus or +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." + +DIVERGENCE OF CHARACTER. + +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. + +As has always been my practice, I have sought light on this 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, etc., +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. + +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. + +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 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. + +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 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. + +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. + +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. + +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 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. + +THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH DIVERGENCE +OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON ANCESTOR. + +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. + +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 +(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. + +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 +a1 and m1. 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. + +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 a2, which will, owing to the principle of +divergence, differ more from (A) than did variety a1. Variety m1 is +supposed to have produced two varieties, namely m2 and s2, 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. + +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 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. + +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 a1 to a10. 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. + +After ten thousand generations, species (A) is supposed to have produced +three forms, a10, f10, and m10, 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 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 a14 and m14, all descended from (A). +Thus, as I believe, species are multiplied and genera are formed. + +In a large genus it is probable that more than one species would vary. In +the diagram I have assumed that a second species (I) has produced, by +analogous steps, after ten thousand generations, either two well-marked +varieties (w10 and z10) 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 n14 to z14, +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. + +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 quite new station, in which +offspring and progenitor do not come into competition, both may continue to +exist. + +If, then, our diagram be assumed to represent a considerable amount of +modification, species (A) and all the earlier varieties will have become +extinct, being replaced by eight new species (a14 to m14); and species (I) +will be replaced by six (n14 to z14) new species. + +But we may go further than this. The original species of our genus were +supposed to resemble each other in unequal degrees, as is so generally the +case in nature; species (A) being more nearly related to B, C, and D than +to the other species; and species (I) more to G, H, K, L, than to the +others. These two species (A and I), were also supposed to be very common +and widely diffused species, so that they must originally have had some +advantage over most of the other species of the genus. Their modified +descendants, fourteen in number at the fourteen-thousandth generation, will +probably have inherited some of the same advantages: they have also been +modified and improved in a diversified manner at each stage of descent, so +as to have become adapted to many related places in the natural economy of +their country. It seems, therefore, 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. + +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 +a14 and z14 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 a14, q14, p14, will be nearly related from having recently +branched off from a10; b14 and f14, from having diverged at an earlier +period from a5, will be in some degree distinct from the three first-named +species; and lastly, o14, e14, and m14, 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. + +The six descendants from (I) will form two sub-genera or genera. But as +the original species (I) differed largely from (A), standing 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. + +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. + +It is worth while to reflect for a moment on the character of the new +species F14, 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 (F14) 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. + +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. + +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 a14 to p14, those marked b14 and f14, and those +marked o14 to m14, 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. + +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 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. + +ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE. + +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, probably with more truth, look at the plants which have their +several organs much modified and reduced in number as the highest. + +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. + +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, 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. + +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 aerial 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 Muller, 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. + +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 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. + +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. + +CONVERGENCE OF CHARACTER. + +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 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. + +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, +etc.; 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 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, etc. 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 VERY 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. + +SUMMARY OF CHAPTER. + +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 +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. + +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. + +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. + +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 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. + +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 occasionally see an animal like the Ornithorhynchus or +Lepidosiren, which in some small degree connects by its affinities two +large branches of life, and which has apparently been saved from fatal +competition by having inhabited a protected station. As buds give rise by +growth to fresh buds, and these, if vigorous, branch out and overtop on all +sides many a feebler branch, so by generation I believe it has been with +the great Tree of Life, which fills with its dead and broken branches the +crust of the earth, and covers the surface with its ever-branching and +beautiful ramifications. + + +CHAPTER V. + +LAWS OF VARIATION. + +Effects of 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. + +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. + +It is very difficult to decide how far changed conditions, such as of +climate, food, etc., have acted in a definite manner. There is 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. + +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. + +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. + +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 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. + +EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED BY NATURAL +SELECTION. + +>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. + +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 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. + +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. + +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 swimmers if +they had not been able to swim at all and had stuck to the wreck. + +The eyes of moles and of some burrowing rodents are rudimentary in size, +and in some cases are quite covered 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. + +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. + +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, Schiodte 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 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 +Schiodte 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 Schiodte'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 antennae 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 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. + +ACCLIMATISATION. + +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, etc., 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, 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. + +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. + +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 Encyclopaedias 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 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. + +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. + +CORRELATED VARIATION. + +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 larvae +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 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. + +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. + +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, etc.), +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. + +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 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 Compositae countenances this idea; but with the Umbelliferae 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 Compositae 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. + +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 Umbelliferae these differences are of such apparent +importance--the seeds being sometimes orthospermous in the exterior flowers +and coelospermous 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. + +We may often falsely attribute to correlated variation structures which are +common to whole groups of species, and which in truth are simply due to +inheritance; for an ancient progenitor may have acquired through natural +selection some one modification in structure, and, after thousands of +generations, some other and independent modification; and these two +modifications, having been transmitted to a whole group of descendants with +diverse habits, would naturally be thought to be 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. + +COMPENSATION AND ECONOMY OF GROWTH. + +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. + +I suspect, also, that some of the cases of compensation which have been +advanced, and likewise some other facts, may be merged under a more general +principle, namely, that natural selection is continually trying to +economise in every part of the organisation. If under changed conditions +of life a structure, before useful, becomes less useful, 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 antennae. 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. + +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. + +MULTIPLE, RUDIMENTARY, AND LOWLY-ORGANISED STRUCTURES ARE VARIABLE. + +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 vertebrae 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, 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. + +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. + +A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN +COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY +VARIABLE. + +Several years ago I was much struck 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 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. + +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. + +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 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, etc., 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. + +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 GENERATIVE VARIABILITY, as it may be called, still +present in a high degree. For in this case the variability will seldom as +yet have been fixed by the continued selection of the individuals varying +in the required manner and degree, and by the continued rejection of those +tending to revert to a former and less modified condition. + +SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS. + +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, DIFFERS considerably in closely-allied species, it +is often VARIABLE 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. + +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 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. + +SECONDARY SEXUAL CHARACTERS VARIABLE. + +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. + +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 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 Engidae, 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 antennae 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. + +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 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. + +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. + +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. + +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 rock-pigeon, I presume that no one will +doubt that this is a case of reversion, and not of a new yet analogous +variation appearing in the several breeds. We may, I think, confidently +come to this conclusion, because, as we have seen, these coloured marks are +eminently liable to appear in the crossed offspring of two distinct and +differently coloured breeds; and in this case there is nothing in the +external conditions of life to cause the reappearance of the slaty-blue, +with the several marks, beyond the influence of the mere act of crossing on +the laws of inheritance. + +No doubt it is a very surprising fact that characters should reappear after +having been lost for many, 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 BOTH +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. + +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 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. + +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. + +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 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 NOT an albino, has been described without either spinal or shoulder +stripe; and these stripes are sometimes very obscure, or actually quite +lost, in dark-coloured asses. The koulan of Pallas is said to have been +seen with a double shoulder-stripe. 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. + +With respect to the horse, I have collected cases in England of the spinal +stripe in horses of the most distinct breeds, and of ALL colours; +transverse bars on the legs are not rare in duns, mouse-duns, and in one +instance in a chestnut; a faint shoulder-stripe may sometimes be seen in +duns, and I have seen a trace in a bay horse. My son made a careful +examination and sketch for me of a dun Belgian cart-horse with a double +stripe on each shoulder and with leg-stripes. I have myself seen a dun +Devonshire pony, and a small dun Welsh pony has been carefully described to +me, both with THREE parallel stripes on each shoulder. + +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 Eastern China; +and from Norway in the north to the Malay Archipelago in the south. In all +parts of the world these stripes occur far oftenest in duns and mouse-duns; +by the term dun a large range of colour is included, from one between brown +and black to a close approach to cream colour. + +I am aware that Colonel Hamilton Smith, who has written on this subject, +believes that the several breeds of the horse 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, etc., inhabiting the most distant parts of the world, should +have all have been crossed with one supposed aboriginal stock. + +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. + +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 TENDENCY in the young of each successive generation to produce the +long-lost character, and that this tendency, from unknown causes, sometimes +prevails. And we have just seen that in several species of the horse genus +the stripes are either plainer or appear more commonly in the young than in +the old. Call the breeds of pigeons, some of which have bred true for +centuries, species; and how exactly parallel is the case with that of the +species of the horse genus! For myself, I venture confidently to look back +thousands on thousands of generations, and I see an animal striped like a +zebra, but perhaps otherwise very differently constructed, the common +parent of our domestic horse (whether or not it be descended from one or +more wild stocks) of the ass, the hemionus, quagga, and zebra. + +He who believes that each equine species was independently created, will, I +presume, assert that each species has been created with a tendency to vary, +both under nature and under domestication, in this particular manner, so as +often to become striped like 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, but had been created in +stone so as to mock the shells now living on the sea-shore. + +SUMMARY. + +Our ignorance of the laws of variation is profound. Not in one case out of +a hundred can we pretend to assign any reason why this or that part 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, 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. + +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. + + +CHAPTER VI. + +DIFFICULTIES OF THE THEORY. + +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. + +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. + +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? + +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? + +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? + +Fourthly, how can we account for species, when crossed, being sterile and +producing sterile offspring, whereas, when varieties are crossed, their +fertility is unimpaired? + +The two first heads will be here discussed; some miscellaneous objections +in the following chapter; Instinct and Hybridism in the two succeeding +chapters. + +ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES. + +As natural selection acts solely by the preservation of profitable +modifications, each new form will tend in a fully-stocked country to take +the place of, and finally to exterminate, its own less improved parent-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. + +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. + +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 +quite confounded me. But I think it can be in large part explained. + +In the first place we should be extremely cautious in inferring, because an +area is now continuous, that it has been continuous during a long period. +Geology would lead us to believe that 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. + +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 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. + +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. + +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 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. + +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. + +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. + +Thirdly, when two or more varieties have been formed in different portions +of a strictly continuous area, intermediate varieties will, it is probable, +at first have been formed in the intermediate zones, but they will +generally have had a short duration. For these intermediate varieties +will, from reasons already assigned (namely from what we know of the actual +distribution of closely allied or representative species, and likewise of +acknowledged varieties), exist in the intermediate zones in lesser numbers +than the varieties which they tend to connect. From this cause alone the +intermediate varieties will be liable to accidental extermination; and +during the process of further modification through natural selection, they +will almost certainly be beaten and supplanted by the forms which they +connect; for these, from existing in greater numbers will, in the +aggregate, present more varieties, and thus be further improved through +natural selection and gain further advantages. + +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. + +ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS AND +STRUCTURE. + +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. + +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 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. + +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. + +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 +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. + +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. + +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? + +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 had come to a +high stage of perfection, so as to have given them a decided advantage over +other animals in the battle for life. Hence the chance of discovering +species with transitional grades of structure in a fossil condition will +always be less, from their having existed in lesser numbers, than in the +case of species with fully developed structures. + +I will now give two or three instances, 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. + +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 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. + +Petrels are the most aerial 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. + +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, 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. + +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. + +ORGANS OF EXTREME PERFECTION AND COMPLICATION. + +To suppose that the eye with all its inimitable contrivances for adjusting +the focus to different distances, for admitting different amounts of light, +and for the correction of spherical and chromatic aberration, could have +been formed by natural selection, seems, I freely confess, absurd in the +highest 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 Vox populi, vox Dei, 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 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. + +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. + +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. + +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 organs in the Articulata are so much diversified that +Muller formerly made three main classes with seven subdivisions, besides a +fourth main class of aggregated simple eyes. + +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. + +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 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. + +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? + +MODES Of TRANSITION. + +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 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. + +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. + +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 branchiae 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 aerial 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. + +The illustration of the swim-bladder in fishes is a good one, because it +shows us clearly the highly important fact that an organ originally +constructed for one purpose, namely flotation, may be converted into one +for a 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. + +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 +branchiae 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 branchiae +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. + +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 branchiae, the whole surface of the body +and of the sack, together with the small frena, serving for respiration. +The Balanidae 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 lacunae of the sack and body, and which have been considered by +all naturalists to act as branchiae. Now I think no one will dispute that +the ovigerous frena in the one family are strictly homologous with the +branchiae of the other family; indeed, 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 branchiae, 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 branchiae in this +latter family had originally existed as organs for preventing the ova from +being washed out of the sack? + +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 Muller, 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. + +SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION. + +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. + +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. + +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 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. + +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 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. + +Fritz Muller, 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 Muller, 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 branchiae, 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, +while all the other important organs were closely similar, or rather, +identical. + +Fritz Muller 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. Muller +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 Muller to accept the views maintained by +me in this volume. + +Another distinguished zoologist, the late Professor Claparede, has argued +in the same manner, and has arrived at the same result. He shows that +there are parasitic mites (Acaridae), 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 maxillae or lips, +and of appendages on the under side of the hind part of the body. + +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 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. + +>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. Cruger 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 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. Cruger 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. Cruger 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. Cruger +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. + +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. + +How, it may be asked, in the foregoing and in innumerable other 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. + +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. + +ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL SELECTION. + +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 great, though of a very different +kind, as in the case of the most perfect and complex organs. + +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. + +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. + +In the second place, we may easily err in attributing importance to +characters, and in believing that they have been developed 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, etc., 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. + +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. + +We are profoundly ignorant of the cause of each slight variation 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. + +UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW ACQUIRED. + +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 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. + +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 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 diatomaceae: 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. + +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 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. + +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. + +Natural selection will never produce in a being any structure more +injurious than beneficial to that being, for natural selection acts solely +by and for the good of each. No organ will be formed, as Paley has +remarked, for the purpose of causing pain or for doing an injury to its +possessor. If a fair balance be struck between the good and evil caused by +each part, each will be found on the whole advantageous. After the lapse +of time, under changing conditions of life, if any part comes to be +injurious, it will be modified; or if it be not so, the being will become +extinct, as myriads have become extinct. + +Natural selection tends only to make each organic being as perfect as, or +slightly more perfect than the other inhabitants of the same country with +which it 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 Muller 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? + +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, 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? + +SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CONDITIONS OF EXISTENCE +EMBRACED BY THE THEORY OF NATURAL SELECTION. + +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. + +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. + +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. + +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. + +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. + +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, 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. + +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. + +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. + +It is generally acknowledged that all organic beings have been formed on +two great laws--Unity of Type, and the Conditions of Existence. By unity +of type is meant that fundamental agreement in structure which we see in +organic beings of the same class, and which is quite independent of their +habits of life. On my theory, unity of type is explained by unity of +descent. The expression of conditions of existence, so often insisted on +by the illustrious Cuvier, is fully embraced by the principle of natural +selection. For natural selection acts by either now adapting the varying +parts of each being to its organic and inorganic conditions of life; or by +having adapted them during 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. + + +CHAPTER VII. + +MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION. + +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. + +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. + +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 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. + +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. + +The celebrated palaeontologist, 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, etc., 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. + +Bronn also insists that distinct species never differ from each other 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. + +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 Nageli 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, etc. + +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. + +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. + +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 be of no special use, I may mention that, according +to Dr. Schobl, 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. + +With respect to plants, to which on account of Nageli'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. + +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. + +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 columnae five of the alternate stamens are +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 +Malpighiaceae 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. + +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 Compositae +and Umbelliferae (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 Compositae the central +achenes alone are furnished with a pappus; and in Hyoseris the same head +yields achenes of three different forms. In certain Umbelliferae the +exterior seeds, according to Tausch, are orthospermous, and the central one +coelospermous, and this is a character which was considered by De Candolle +to be in other species of the highest systematic 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. + +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, etc., 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 aestivation is almost as frequently that of the +Rhinanthideae as of the Antirrhinideae, to which latter tribe the genus +belongs. Aug. St. Hilaire gives the following cases: the genus +Zanthoxylon belongs to a division of the Rutaceae 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 PLUS OU MOINS LARGE, s'etend 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 oleaeformis two forms +which he did not at first doubt were distinct species, but he subsequently +saw them growing on the same bush; and he then adds, "Voila donc dans un +meme individu des loges et un style qui se rattachent tantot a un axe +verticale et tantot a un gynobase." + +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 Nageli'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. + +>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 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, etc., 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. + +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. + +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 +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. + +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. + +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, etc., 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 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. + +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. + +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. + +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 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. + +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. + +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 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, etc.--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. + +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, 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. + +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. + +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 ALL +DIRECTIONS, 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." + +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. + +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. + +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 jaw, of about 300 plates or +laminae, 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 Balaenoptera rostrata only about nine inches in +length. The quality of the whalebone also differs in the different +species. + +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 +Criblatores, 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 lamellae by finely graduated steps, each of service to +its possessor. + +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 lamellae, 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 lamellae. 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 length of the head +of a moderately large Balaenoptera 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 Balaenoptera, the lamellae 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 lamellae +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 lamellae 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 lamellae, 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. + +>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 lamellae 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 lamellae are considerably less developed +than in the common duck; but I do not know whether they use their beaks for +sifting the water. + +Turning to another group of the same family. In the Egyptian goose +(Chenalopex) the beak closely resembles that of the common duck; but the +lamellae 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 lamellae 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 +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 +lamellae are less developed than in the common goose. + +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 lamellae, 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. + +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 lamellae 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 lamellae 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 lamellae would be two-thirds of the length of the +plates of baleen in the Balaenoptera 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 struggle for existence, and +that the structure of every part of its frame must be well adapted to its +conditions of life. + +The Pleuronectidae, 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 Pleuronectidae are admirably adapted by their +flattened and asymmetrical structure for their habits of life, is manifest +from several species, such as soles, flounders, etc., 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 Schiodte 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." + +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 +Pleuronectidae, 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 swim-bladder. 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. + +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 Pleuronectidae, 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. Schiodte believes, in +opposition to some other naturalists, that the Pleuronectidae 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 Pleuronectidae, 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. Gunther, concludes his abstract of Malm's paper, by +remarking that "the author gives a very simple explanation of the abnormal +condition of the Pleuronectoids." + +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 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. + +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 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. +Gunther, 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. + +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 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. + +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 IS 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. + +We will now turn for a short space to the lower divisions of the animal +kingdom. The Echinodermata (star-fishes, sea-urchins, etc.) are furnished +with remarkable organs, called pedicellariae, 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. + +With respect to these organs, Mr. Mivart, as on so many previous occasions, +asks: "What would be the utility of the FIRST RUDIMENTARY BEGINNINGS of +such structures, and how could such insipient buddings have ever preserved +the life of a single Echinus?" He adds, "not even the SUDDEN 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 pedicellariae, 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. + +With respect to the steps by which these curious organs have been evolved, +Mr. Agassiz infers from his own researches and those of Mr. Muller, that +both in star-fishes and sea-urchins the pedicellariae 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 pedicellariae. The gradation +extends even to the manner in which ordinary spines and the pedicellariae, +with their supporting calcareous rods, are articulated to the shell. In +certain genera of star-fishes, "the very combinations needed to show that +the pedicellariae 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 pedicellariae, 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 pedicellariae 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 pedicellariae, would be of service. + +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 +pedicellariae. All possible gradations, as he adds, may likewise be found +between the pedicellariae of the star-fishes and the hooks of the +Ophiurians, another group of the Echinodermata; and again between the +pedicellariae of sea-urchins and the anchors of the Holothuriae, also +belonging to the same great class. + +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 +tremble. When the jaws are touched with a needle they seize it so firmly +that the branch can thus be shaken. + +Mr. Mivart adduces this case, chiefly on account of the supposed difficulty +of organs, namely the avicularia of the Polyzoa and the pedicellariae 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 pedicellariae and avicularia. The latter +resembles somewhat more closely the chelae 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. + +As the chelae 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 chelae of a lobster. And all these gradations can be actually traced. + +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 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. + +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. + +In the vegetable kingdom Mr. Mivart only alludes to two cases, 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 ORIGIN 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. + +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, 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. + +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. + +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 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 Muller 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. + +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. + +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. + +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. + +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. + +With the giraffe, the continued preservation of the individuals of some +extinct high-reaching ruminant, which had the longest necks, legs, etc., +and could browse a little above the average height, and 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 lamellae, like those of the domestic ducks--and then into lamellae, +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 lamellae are first used as teeth, then partly as teeth and +partly as a sifting apparatus, and at last almost exclusively for this +latter purpose. + +With such structures as the above lamellae 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 mammae 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 pedicellariae, 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 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. + +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." + +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. + +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. + +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, etc.; 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. + +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 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. + +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, etc., 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. + +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 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. + +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. + +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 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. + +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. + + +CHAPTER VIII. + +INSTINCT. + +Instincts comparable with habits, but different in their origin -- +Instincts graduated -- Aphides and ants -- Instincts variable -- Domestic +instincts, their origin -- Natural instincts of the cuckoo, 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. + +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. + +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. + +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. +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. + +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. + +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 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. + +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, etc.; 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. + +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 antennae; 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 antennae on the abdomen first of one aphis and then of another; +and each, as soon as it felt the antennae, immediately lifted up its +abdomen and excreted a limpid drop of sweet juice, which was eagerly +devoured by the ant. Even the quite young aphides behaved in this manner, +showing that the action was instinctive, and not the result of experience. +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. + +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 are not more fearful than small; and the magpie, so wary in England, +is tame in Norway, as is the hooded crow in Egypt. + +That the 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. + +INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS. + +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 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. + +How strongly these domestic instincts, habits, and dispositions are +inherited, and how curiously they become mingled, is well shown when +different breeds of dogs are crossed. Thus it is known that a cross with a +bull-dog has affected for many generations the courage and obstinacy of +greyhounds; and a cross with a greyhound has given to a whole family of +shepherd-dogs a tendency to hunt hares. These domestic instincts, when +thus tested by crossing, resemble natural instincts, which in a like manner +become curiously blended together, and for a long period exhibit traces of +the instincts of either parent: for example, Le Roy describes a dog, whose +great-grandfather was a wolf, and this dog showed a trace of its wild +parentage only in one way, by not coming in a straight line to his master, +when called. + +Domestic instincts are sometimes spoken of as actions which have become +inherited solely from long-continued and compulsory habit, but this 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 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. + +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. + +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 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. + +SPECIAL INSTINCTS. + +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. + +INSTINCTS OF THE CUCKOO. + +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 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 Muller, 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. + +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! + +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 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. + +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. + +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?" + +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 +Gallinaceae, 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, 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. + +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 Sphegidae (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 larvae, 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. + +SLAVE-MAKING INSTINCT. + +This remarkable instinct was first discovered in the Formica (Polyerges) +rufescens by Pierre Huber, a better observer even than his celebrated +father. This ant is absolutely dependent on its slaves; without their aid, +the species would certainly become extinct in a single year. The males and +fertile females do no work 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 +larvae. 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 larvae and pupae 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 larvae, 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. + +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 larvae and pupae 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 on the slaves being captured in greater +numbers in Switzerland than in England. + +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 pupae to rear as slaves. I then dug up a small +parcel of the pupae of F. fusca from another nest, and put them down on a +bare spot near the place of combat; they were eagerly seized and carried +off by the tyrants, who perhaps fancied that, after all, they had been +victorious in their late combat. + +At the same time I laid on the same place a small parcel of the pupae 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 pupae 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 pupae of F. fusca, whereas they were much terrified +when they came across the pupae, 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 pupae. + +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 +pupae. I traced a long file of ants burdened 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. + +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 larvae, 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 larvae. So that the masters in this country receive much less +service from their slaves than they do in Switzerland. + +By what steps the instinct of F. sanguinea originated I will not pretend to +conjecture. But as ants which are not slave-makers, will, as I have seen, +carry off pupae of other species, if scattered near their nests, it is +possible that such pupae 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 pupae, 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 instinct--always supposing +each modification to be of use to the species--until an ant was formed as +abjectly dependent on its slaves as is the Formica rufescens. + +CELL-MAKING INSTINCT OF THE HIVE-BEE. + +I will not here enter on minute details on this subject, but will merely +give an outline of the conclusions at which I have arrived. He must be a +dull man who can examine the exquisite structure of a comb, so beautifully +adapted to its end, without enthusiastic admiration. We hear from +mathematicians that bees have practically solved a recondite problem, and +have made their cells of the proper shape to hold the greatest possible +amount of honey, with the least possible consumption of precious wax in +their construction. It has been remarked that a skilful workman, with +fitting tools and measures, would find it very difficult to make cells of +wax of the true form, though this is 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. + +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 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. + +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:- + +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 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. + +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. + +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.e. about the width of an ordinary cell), and were +in depth about one sixth of the diameter of the sphere of which they formed +a part, the rims of the basins intersected or broke into each other. As +soon as this occurred, the bees ceased to excavate, and began to build up +flat walls of wax on the lines of intersection between the basins, so that +each hexagonal prism was built upon the scalloped edge of a smooth basin, +instead of on the straight edges of a three-sided pyramid as in the case of +ordinary cells. + +I then put into the hive, instead of a thick, rectangular piece of wax, a +thin and narrow, knife-edged ridge, coloured with vermilion. The bees +instantly began on both sides to excavate little basins near to each other, +in the same way as before; but the ridge of wax was so thin, that the +bottoms of the basins, if they had been excavated to the same depth as in +the former experiment, would have broken into each other from the opposite +sides. The bees, however, did not suffer this to happen, and they stopped +their excavations in due time; so that the basins, as soon as they had been +a little deepened, came to have flat 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. + +Considering how flexible thin wax is, I do not see that there is any +difficulty in the bees, whilst at work on the two sides of a strip of wax, +perceiving when they have gnawed the wax away to the proper thinness, and +then stopping their work. In ordinary combs it has appeared to me that the +bees do not always succeed in working at exactly the same rate from the +opposite sides; for I have noticed half-completed rhombs at the base of a +just-commenced cell, which were slightly concave on one side, where I +suppose that the bees had excavated too quickly, and convex on the opposed +side where the bees had worked less quickly. In one well-marked instance, +I put the comb back into the hive, and allowed the bees to go on working +for a short time, and again examined the cell, and I found that the rhombic +plate had been completed, and had become PERFECTLY FLAT: it was absolutely +impossible, from the extreme thinness of the little plate, that they could +have effected this by gnawing away the convex side; and I suspect that the +bees in such cases stand in the opposed cells and push and bend the ductile +and warm wax (which as I have tried is easily done) into its proper +intermediate plane, and thus flatten it. + +>From the experiment of the ridge of vermilion wax we can see that, if the +bees were to build for themselves a thin wall of wax, they could make their +cells of the proper shape, by standing at the proper distance from each +other, by excavating at the same rate, and by endeavouring to make equal +spherical hollows, but never allowing the spheres to break into each other. +Now bees, as may be clearly seen by examining the edge of a growing comb, +do make a rough, circumferential wall or rim all round the comb; and they +gnaw 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. + +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, 1/352 of +an 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. + +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. + +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 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. + +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 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. + +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 larvae, 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. + +OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS: +NEUTER AND STERILE INSECTS. + +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; 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. + +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 from +both the males and fertile females, and yet, from being sterile, they +cannot propagate their kind. + +The subject well deserves to be discussed at great length, but I will here +take only a single case, that of working or sterile ants. How the workers +have been rendered sterile is a difficulty; but not much greater than that +of any other striking modification of structure; for it can be shown that +some insects and other articulate animals in a state of nature occasionally +become sterile; and if such insects had been social, and it had been +profitable to the community that a number should have been annually born +capable of work, but incapable of procreation, I can see no 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? + +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 members of insect +communities; the difficulty lies in understanding how such correlated +modifications of structure could have been slowly accumulated by natural +selection. + +This difficulty, though appearing insuperable, is lessened, or, as I +believe, disappears, when it is remembered that selection may be applied to +the family, as well as to the individual, and may thus gain the desired +end. 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. + +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 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. + +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 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. + +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. + +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. + +An analogous explanation has been given by Mr. Wallace, of the equally +complex case, of certain Malayan butterflies regularly appearing under two +or even three distinct female forms; and by Fritz Muller, of certain +Brazilian crustaceans likewise appearing under two widely distinct male +forms. But this subject need not here be discussed. + +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. + +SUMMARY. + +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 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. + +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 larvae of +ichneumonidae feeding within the live bodies of caterpillars, not as +specially endowed or created instincts, but as small consequences of one +general law leading to the advancement of all organic beings--namely, +multiply, vary, let the strongest live and the weakest die. + + +CHAPTER IX. + +HYBRIDISM. + +Distinction between the sterility of first crosses and of hybrids -- +Sterility various in degree, not universal, affected by close +interbreeding, removed by domestication -- Laws governing the sterility of +hybrids -- Sterility not a special endowment, but incidental on other +differences, 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. + +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. + +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. + +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 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. + +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. + +DEGREES OF STERILITY. + +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, Kolreuter and Gartner, who +almost devoted their lives to this subject, without being deeply impressed +with the high generality of some degree of sterility. Kolreuter 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. Gartner, +also, makes the rule equally universal; and he disputes the entire +fertility of Kolreuter's ten cases. But in these and in many other cases, +Gartner 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 Gartner 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 Gartner 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 Leguminosae, in which +there is an acknowledged difficulty in the manipulation) half of these +twenty plants had their fertility in some degree impaired. Moreover, as +Gartner 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. + +It is certain, on the one hand, that the sterility of various species when +crossed is so different in degree and graduates away so insensibly, and, on +the other hand, that the fertility of pure species is so easily affected by +various circumstances, that for all practical purposes it is most difficult +to say where perfect fertility ends and sterility begins. I think no +better evidence of this can be required than that the two most experienced +observers who have ever lived, namely Kolreuter and Gartner, 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. + +In regard to the sterility of hybrids in successive generations; though +Gartner 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 Gartner, +namely, that if even the less fertile hybrids be artificially fertilised +with hybrid pollen of the same kind, their fertility, notwithstanding the +frequent ill effects 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 +Gartner 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 +ARTIFICIALLY FERTILISED hybrids, in contrast with those spontaneously self- +fertilised, may, as I believe, be accounted for by too close interbreeding +having been avoided. + +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 Kolreuter and Gartner 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 Gartner. 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. + +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 Muller, 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 +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. + +The practical experiments of horticulturists, though not made with +scientific precision, deserve some notice. It is notorious in how +complicated a manner the species of Pelargonium, Fuchsia, Calceolaria, +Petunia, Rhododendron, etc., have been crossed, yet many of these hybrids +seed freely. For instance, Herbert asserts that a hybrid from Calceolaria +integrifolia and plantaginea, species most widely dissimilar in general +habit, "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 +Gartner 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. + +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 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. + +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 inter se 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 inter se. +This was effected by Mr. Eyton, who raised two hybrids from the same +parents, but from different hatches; and from these two birds he raised no +less than eight hybrids (grandchildren of the pure geese) from one nest. +In India, however, these cross-bred geese must be far more fertile; for I +am assured by two eminently capable judges, namely Mr. Blyth and 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. + +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 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 Rutimeyer on their important osteological differences, +as well as from those by Mr. Blyth on their differences in habits, voice, +constitution, etc., 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. + +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. + +LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS. + +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 +Gartner'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. + +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 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. + +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. + +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. + +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 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 Gartner 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. + +No one has been able to point out what kind or what amount of difference, +in any recognisable character, is sufficient to prevent two species +crossing. It can be shown that plants most widely different in habit and +general appearance, and having strongly marked differences in every part of +the flower, even in the pollen, in the fruit, and in the cotyledons, can be +crossed. Annual and perennial plants, deciduous and evergreen trees, +plants inhabiting different stations and fitted for extremely different +climates, can often be crossed with ease. + +By a reciprocal cross between two species, I mean the case, for instance, +of a 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 Kolreuter. 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 Kolreuter 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. Gartner, moreover, +found that this difference of 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. + +Several other singular rules could be given from Gartner: 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. + +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, in the +facility of effecting an union. The hybrids, moreover, produced from +reciprocal crosses often differ in fertility. + +Now do these complex and singular rules indicate that species have been +endowed with sterility simply to prevent their becoming confounded in +nature? I think not. For why should the sterility be so extremely +different in degree, when various species are crossed, all of which we must +suppose it would be equally important to keep from blending together? Why +should the degree of sterility be innately variable in the individuals of +the same species? Why should some species cross with facility and yet +produce very sterile hybrids; and other species cross with extreme +difficulty, and yet produce fairly fertile hybrids? Why should there often +be so great a difference in the result of a reciprocal cross between the +same two species? Why, it may even be asked, has the production of hybrids +been permitted? To grant to species the special power of producing +hybrids, and then to stop their further propagation by different degrees of +sterility, not strictly related to the facility of the first union between +their parents, seems a strange arrangement. + +The foregoing rules and facts, on the other hand, appear to me clearly to +indicate that the sterility, both of first crosses and of hybrids, is +simply incidental or dependent on unknown differences 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 SPECIALLY endowed quality, but will admit that it is +incidental on differences in the laws of growth of the two plants. We can +sometimes see the reason why one tree will not take on another from +differences in their rate of growth, in the hardness of their wood, in the +period of the flow or nature of their sap, etc.; but in a multitude of +cases we can assign no reason whatever. Great diversity in the size of two +plants, one being woody and the other herbaceous, one being evergreen and +the other deciduous, and adaptation to widely different climates, does not +always prevent the two grafting together. As in hybridisation, so with +grafting, the capacity is limited by systematic affinity, for no one has +been able to graft together trees belonging to quite distinct families; +and, on the other hand, closely allied species and varieties of the same +species, can usually, but not invariably, be grafted with ease. But this +capacity, as in hybridisation, is by no means absolutely governed by +systematic affinity. Although many distinct genera within the same family +have been grafted together, in other cases species of the same genus will +not take on each other. The pear can be grafted far more readily on the +quince, which is ranked as a distinct genus, than on the apple, which is a +member of the same genus. Even different varieties of the pear take with +different degrees of facility on the quince; so do different varieties of +the apricot and peach on certain varieties of the plum. + +As Gartner found that there was sometimes an innate difference in different +INDIVIDUALS of the same two species in crossing; so Sagaret believes this +to be the case with different individuals of the same two species in being +grafted together. As in reciprocal crosses, the facility of effecting an +union is often very far from equal, so it sometimes is in grafting. The +common gooseberry, for instance, cannot be grafted on the currant, whereas +the currant will take, though with difficulty, on the gooseberry. + +We have seen that the sterility of hybrids which have their reproductive +organs in an imperfect condition, is a 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, etc., which seed much more freely when fertilised +with the pollen of a distinct species than when fertilised with pollen from +the same plant. + +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, +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. + +ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS. + +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. + +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 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. + +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 Gartner and Kolreuter 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. + +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 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 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. + +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. + +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 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 inter se, they transmit +to their offspring from generation to generation the same compounded +organisation, and hence we need not be surprised that their sterility, +though in some degree variable, 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. + +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. + +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, +etc., 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. + +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. + +RECIPROCAL DIMORPHISM AND TRIMORPHISM. + +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 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. + +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. + +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 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 inter se. 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. + +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 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. + +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. + +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, Gartner, likewise concluded that species when crossed +are sterile owing to differences confined to their reproductive systems. + +FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT +UNIVERSAL. + +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 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 Gartner 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. + +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, 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. + +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. + +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. Gartner 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 other; but only a single head produced any seed, +and this one head produced only five grains. Manipulation in this case +could not have been injurious, as the plants have separated sexes. No one, +I believe, has suspected that these varieties of maize are distinct +species; and it is important to notice that the hybrid plants thus raised +were themselves PERFECTLY fertile; so that even Gartner did not venture to +consider the two varieties as specifically distinct. + +Girou de Buzareingues crossed three varieties of gourd, which like the +maize has separated sexes, and he asserts that their mutual fertilisation +is by so much the less easy as their differences are greater. How far +these experiments may be trusted, I know not; but the forms 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. + +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 Gartner: 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 DISTINCT +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 Gartner'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. + +Kolreuter, 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 of this one +variety must have been in some manner and in some degree modified. + +>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. + +HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY. + +Independently of the question of fertility, the offspring of species and of +varieties when crossed may be compared in several other respects. Gartner, +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. + +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 Gartner 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. Gartner 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. + +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 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. + +But to return to our comparison of mongrels and hybrids: Gartner 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, Gartner 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. Gartner 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 +Kolreuter. + +Such alone are the unimportant differences which Gartner 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 Gartner 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 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. + +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. + +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. + +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 having been produced by +secondary laws, this similarity would be an astonishing fact. But it +harmonises perfectly with the view that there is no essential distinction +between species and varieties. + +SUMMARY OF CHAPTER. + +First crosses between forms, sufficiently distinct to be ranked as species, +and their hybrids, are very generally, but not universally, sterile. The +sterility is of all degrees, and is often so slight that the 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. + +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. + +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 +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. + +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. + +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. + + +CHAPTER X. + +ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. + +On the absence of intermediate varieties at the present day -- On the +nature of extinct intermediate varieties; on their number -- On the 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 +palaeontological 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. + +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 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. + +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 DIRECTLY intermediate between them. But this is a wholly +false view; we should always look for forms intermediate between each +species and a common but unknown progenitor; and the progenitor will +generally have differed in some respects from all its modified descendants. +To give a simple illustration: the fantail and pouter pigeons 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. + +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. + +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 DIRECT intermediate links will have existed between them. But such a +case would imply that one form had remained for a very long period +unaltered, whilst its descendants had undergone a vast amount of change; +and the principle of competition between organism and organism, between +child and parent, will render this a very rare event; for in all cases the +new and improved forms of life tend to supplant the old and unimproved +forms. + +By the theory of natural selection all living species have been connected +with the parent-species of each genus, by differences not greater than we +see between the 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. + +ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF DEPOSITION AND EXTENT OF +DENUDATION. + +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. + +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. + +We have, however, recently learned from the observations of Ramsay, in the +van of many excellent observers--of Jukes, Geikie, Croll and others, that +subaerial 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 subaerial 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 subaerial denudation better than the surrounding +surface; this surface consequently has been gradually lowered, with the +lines of harder rock 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 subaerial agencies, which apparently have +so little power, and which seem to work so slowly, have produced great +results. + +When thus impressed with the slow rate at which the land is worn away +through subaerial 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. + +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 DIFFERENT parts +of Great Britain; and this is the result: + Feet +Palaeozoic strata (not including igneous beds)..57,154 +Secondary strata................................13,190 +Tertiary strata..................................2,240 + +--making altogether 72,584 feet; that is, very nearly thirteen and +three-quarters British miles. Some of these formations, which are +represented in England by thin beds, are thousands of feet in thickness on +the Continent. Moreover, between each successive formation we have, in the +opinion of most geologists, 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. + +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 subaerial 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 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. + +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. + +ON THE POORNESS OF PALAEONTOLOGICAL COLLECTIONS. + +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 palaeontologist, 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 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 +Chthamalinae (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. + +With respect to the terrestrial productions which lived during the +Secondary and Palaeozoic 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 palaeozoic formations. + +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 +palaeontologists, 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 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. + +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. + +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 subaerial 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 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. + +I am convinced that nearly all our ancient formations, which are throughout +the greater part of their thickness RICH IN FOSSILS, have thus been formed +during subsidence. Since publishing my views on this subject in 1845, I +have watched the progress of geology, and have been surprised to note how +author after author, in treating of this or that great formation, has come +to the conclusion that it was accumulated during subsidence. I may add, +that the only ancient tertiary formation on the west coast of South +America, which has been bulky enough to resist such degradation as it has +as yet suffered, but which will hardly last to a distant geological age, +was deposited during a downward oscillation of level, and thus gained +considerable thickness. + +All geological facts tell us plainly that each area has undergone numerous +slow oscillations of level, and apparently these oscillations have affected +wide spaces. Consequently, formations rich in fossils and sufficiently +thick and extensive to resist subsequent degradation, 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, THICK 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. + +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 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. + +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, etc., 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") and granite +rocks exceed, in the proportion of 19 to 12.5, the whole of the newer +Palaeozoic 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. + + +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. + +ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION. + +>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. + +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 palaeontologists, whose opinions are worthy +of much deference, namely Bronn and Woodward, have concluded that the +average duration of each formation is twice or thrice as long as the +average duration of specific forms. But insuperable difficulties, as it +seems to me, prevent us 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. + +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 palaeozoic 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, INCLUDING FOSSIL REMAINS, have gone on +accumulating within the same area during the whole of this period. It is +not, for instance, probable that sediment was deposited during the whole of +the glacial period near the mouth of the Mississippi, within that limit of +depth at which marine animals can 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 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. + +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 palaeontologist that +very thick deposits are usually barren of organic remains, except near +their upper or lower limits. + +It would seem that each separate formation, like the whole pile of +formations in any country, has generally been intermittent in its +accumulation. When we see, as is so often the case, a formation composed +of beds of 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 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. + +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. + +It is notorious on what excessively slight differences many +palaeontologists 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 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. + +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. + +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. + +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 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. + +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. + +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! + +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 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. + +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. + +It may be doubted whether the duration of any one great period of +subsidence over the whole or part of the archipelago, together with a +contemporaneous accumulation of sediment, would EXCEED the average duration +of the same specific forms; and these contingencies are indispensable for +the preservation of all the transitional gradations between any two or more +species. If such gradations were not 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. + +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 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 palaeontologists, be +ranked as new and distinct species. + +If then there be some degree of truth in these remarks, we have no right to +expect to find, in our geological formations, an infinite number of those +fine transitional forms, which, on 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 palaeontologists, 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. + +ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES. + +The abrupt manner in which whole groups of species suddenly appear in +certain formations, has been urged by several palaeontologists--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 palaeontological 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, 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. + +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? + +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 Palaeontology, +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 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, palaeontologists 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. + +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 palaeontologist, 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 have abundant evidence of the +existence of this group of animals during the secondary period. + +The case most frequently insisted on by palaeontologists 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 +palaeozoic 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 Palaeontology 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. + +>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 palaeontological 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. + +ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN +FOSSILIFEROUS STRATA. + +There is another and allied difficulty, which is much more serious. I +allude to the manner in which species belonging to several of the 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, +etc., 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. + +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. + +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 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 palaeozoic 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. + +The case at present must remain inexplicable; and may be truly urged as a +valid argument against the views here entertained. To show that it may +hereafter receive some explanation, I will give the following hypothesis. +>From the nature of the organic remains which do not appear to have +inhabited profound depths, in the several formations of Europe and of the +United States; and from the amount of sediment, miles in thickness, of +which the formations are composed, we may infer that from first to last +large islands or tracts of land, whence the sediment was derived, occurred +in the neighbourhood of the 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. + +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 palaeozoic or +secondary formation. Hence, we may perhaps infer, that during the +palaeozoic and secondary periods, neither continents nor continental +islands existed where our oceans now extend; for had they existed, +palaeozoic 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 +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. + +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 +palaeontologists, namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E. +Forbes, etc., and all our greatest geologists, as Lyell, Murchison, +Sedgwick, etc., 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 +palaeontologists 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. + + +CHAPTER XI. + +ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. + +On the slow and successive appearance of new species -- On their different +rates of change -- Species once lost do not reappear -- Groups of species +follow the same general rules in their appearance and disappearance as do +single species -- On extinction -- On simultaneous changes in the forms of +life throughout the world -- On the affinities of extinct species to each +other and to living species -- On the state of development of ancient forms +-- On the succession of the same types within the same areas -- Summary of +preceding and present chapters. + +Let us now see whether the several facts and 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. + +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. + +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 +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. + +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 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. + +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. + +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. + +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 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. + +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. + +ON EXTINCTION. + +We have as yet only spokesn 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, etc., whose general views would naturally lead them to +this conclusion. On the contrary, we have every reason to believe, from +the study of the tertiary formations, that species and groups of species +gradually disappear, one after another, first from one spot, then from +another, and finally from the world. 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 palaeozoic 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. + +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 certain, from the analogy of all other mammals, even of the +slow-breeding elephant, and from the history of the naturalisation of the +domestic horse in South America, that under more favourable conditions it +would in a very few years have stocked the whole continent. But we could +not have told what the unfavourable conditions were which checked its +increase, whether some one or several contingencies, and at what period of +the horse's life, and in what degree they severally acted. If the +conditions had gone on, however slowly, becoming less and less favourable, +we assuredly should not have perceived the fact, yet the fossil horse would +certainly have become rarer and rarer, and finally extinct--its place being +seized on by some more successful competitor. + +It is most difficult always to remember that the increase of every living +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. + +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. + +The theory of natural selection is grounded on the belief that each new +variety and ultimately each new species, is produced and maintained by +having some advantage over those with which it comes into competition; and +the consequent extinction of less-favoured forms almost inevitably follows. +It is the same with our domestic productions: when a new and slightly +improved variety has been raised, it at first supplants the less improved +varieties in the same neighbourhood; when much improved it is transported +far and near, like our short-horn cattle, and takes the place of other +breeds in other countries. Thus the appearance of new forms and the +disappearance of old forms, both 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. + +The competition will generally be most severe, as formerly explained and +illustrated by examples, between the forms which are most like each other +in all respects. Hence the improved and modified descendants of a species +will generally cause the extermination of the parent-species; and if many +new forms have been developed from any one species, the nearest allies of +that species, i.e. the species of the same genus, will be the most liable +to extermination. Thus, as I believe, a number of new species descended +from one species, that is a new genus, comes to supplant an old genus, +belonging to the same family. But it must often have happened that a new +species belonging to some one group 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. + +With respect to the apparently sudden extermination of whole families or +orders, as of Trilobites at the close of the palaeozoic 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. + +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. + +ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE WORLD. + +Scarcely any palaeontological 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, 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 palaeozoic 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 palaeozoic and tertiary stages, would still be manifest, and +the several formations could be easily correlated. + +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. + +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 +MARINE formations, namely, the upper pliocene, the pleistocene and strictly +modern beds of Europe, North and South America, and Australia, from +containing fossil remains in some degree allied, and from not including +those forms which are found only in the older underlying deposits, would be +correctly ranked as simultaneous in a geological sense. + +The fact of the forms of life changing simultaneously in the above large +sense, at distant parts of the world, has greatly struck those admirable +observers, MM. de Verneuil and d'Archiac. After referring to the +parallelism of the palaeozoic forms of life in various parts of Europe, +they add, "If struck by this strange sequence, we turn our attention to +North America, and there discover a series of analogous phenomena, it will +appear certain that all these modifications of species, their extinction, +and the introduction of new ones, cannot be owing to mere changes in marine +currents or other causes more or less local and temporary, but depend on +general laws which govern the whole animal kingdom." M. Barrande has made +forcible remarks to precisely the same effect. It is, indeed, quite futile +to look to changes of currents, climate, or other physical conditions, as +the cause of these great mutations in the forms of life throughout the +world, under the most different climates. We must, as Barrande has +remarked, look to some special law. We shall see this more clearly when we +treat of the present distribution of organic beings, and find how slight is +the relation between the physical conditions of various countries and the +nature of their inhabitants. + +This great fact of the parallel succession of the forms of life throughout +the world, is explicable on the theory of natural selection. New species +are formed by 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 of +parallelism in the succession of the productions of the land than with +those of the sea. + +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. + +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. + +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 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. + +ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS. + +Let us now look to the mutual affinities of extinct and living species. +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 palaeontologist, M. Gaudry, has +shown in the most striking manner that many of the fossil mammals +discovered by him in Attica serve to break 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." + +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 palaeozoic 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. + +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. 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. + +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. + +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, a14, q14, p14, will form +a small family; b14 and f14, a closely allied family or subfamily; and o14, +i14, m14, 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 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 F14. + +All the many forms, extinct and recent, descended from (A), make, as before +remarked, one order; and this order, from the continued effects of +extinction and divergence of character, has become divided into several +sub-families and families, some of which are supposed to have perished at +different periods, and some to have endured to the present day. + +By looking at the diagram we can see that if many of the extinct forms +supposed to be embedded in the successive formations, were discovered at +several points low down in the series, the three existing families on the +uppermost line would be rendered less distinct from each other. If, for +instance, the genera a1, a5, a10, f8, m3, m6, m9, 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 a14, etc., and b14, +etc.) 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 (a14 to m14), 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. + +Under nature the process will be far more complicated than is represented +in the diagram; for the groups will have been more numerous; they will have +endured for extremely unequal lengths of time, and will have been modified +in various degrees. As we possess only the last volume of the geological +record, and that in a very broken condition, we have no right to expect, +except in 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 +palaeontologists is frequently the case. + +Thus, on the theory of descent with modification, the main facts with +respect to the mutual affinities of the extinct forms of life to each other +and to living forms, are explained in a satisfactory manner. And they are +wholly inexplicable on any other view. + +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 palaeontologists 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. + +It is no real objection to the truth of the statement that the fauna of +each period as a whole is nearly intermediate in character between the +preceding and succeeding faunas, that certain genera offer exceptions to +the rule. For instance, 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. + +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 palaeontologists, 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. + +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. + +ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS. + +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 +palaeozoic 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 palaeontologists would answer in the affirmative; and it seems +that this answer must be admitted as true, though difficult of proof. + +It is no valid objection to this conclusion, that certain Brachiopods 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. + +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 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. + +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 most skilful naturalist, from an examination of +the species of the two countries, could not have foreseen this result. + +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. + +ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE LATER +TERTIARY PERIODS. + +Mr. Clift many years ago showed that the fossil mammals from the Australian +caves were closely allied to the living marsupials of that continent. In +South America, a similar relationship is manifest, even to an uneducated +eye, in the gigantic pieces of armour, like those of the armadillo, found +in several parts of La Plata; and Professor Owen has shown in the most +striking manner that most of the fossil mammals, buried there in such +numbers, are related to South American types. This relationship is even +more clearly seen in the wonderful collection of fossil bones made by MM. +Lund and Clausen in the caves of Brazil. I was so much impressed with +these facts that I strongly insisted, in 1839 and 1845, on this "law of the +succession of types,"--on "this wonderful relationship in the same +continent between the dead and the living." Professor Owen has +subsequently extended the same generalisation to the mammals of the Old +World. We see the same law in this author's restorations of the extinct +and gigantic birds of New Zealand. We see it also in the birds of the +caves of Brazil. Mr. Woodward has shown that the same law holds good with +sea-shells, but, from the wide distribution of most molluscs, it is not +well displayed by them. Other cases could be added, as the relation +between the extinct and living land-shells of Madeira; and between the +extinct and living brackish water-shells of the Aralo-Caspian Sea. + +Now, what does this remarkable law of the succession of the same types +within the same areas mean? He would be a bold man who, after comparing +the present climate of Australia and of parts of South America, under the +same latitude, would attempt to account, on the one hand 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. + +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. + +It may be asked in ridicule whether I suppose that the megatherium and +other allied huge monsters, which formerly lived in 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. + +SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS. + +I have attempted to show that the geological record is extremely imperfect; +that only a small portion of the globe has been geologically explored with +care; that only certain classes of organic beings have been largely +preserved in a fossil state; that the number both of specimens and of +species, preserved in our museums, is absolutely as nothing compared with +the 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; 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. + +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. + +Passing from these difficulties, the other great leading facts in +palaeontology 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 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. + +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. + +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. + +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 +palaeontologists, 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. + +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 +palaeontology 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. + + +CHAPTER XII. + +GEOGRAPHICAL DISTRIBUTION. + +Present distribution cannot be accounted for by differences in physical +conditions -- Importance of barriers -- Affinity of the productions of the +same continent -- Centres of creation -- Means of dispersal by changes of +climate and of the level of the land, and by occasional means -- Dispersal +during the Glacial period -- Alternate Glacial periods in the North and +South. + +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! + +In the southern hemisphere, if we compare large tracts of land in +Australia, South Africa, and western South America, between latitudes 25 +and 35 degrees, we shall find parts extremely similar in all their +conditions, yet it would not be possible to point out three faunas and +floras more utterly dissimilar. Or, again, we may compare the productions +of South America south of latitude 35 degrees with those north of 25 +degrees, 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. + +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, etc., are not as impassable, or likely to have endured so long, as +the oceans separating continents, the differences are very inferior in +degree to those characteristic of distinct continents. + +Turning to the sea, we find the same law. 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. +Gunther 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 further westward from the +eastern islands of the tropical parts of the Pacific, we encounter no +impassable barriers, and we have innumerable islands as halting-places, 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. + +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. + +The bond is simply inheritance, that cause which alone, as far as 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. + +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. + +According to these views, it is obvious that the several species of the +same genus, though inhabiting the most distant quarters of the world, must +originally have proceeded from the same source, as they 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. + +SINGLE CENTRES OF SUPPOSED CREATION. + +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 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! + +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 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. + +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. + +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 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. + +Before discussing the three classes of facts, which I have selected as +presenting the greatest amount of difficulty on the theory of "single +centres of creation," I must say a few words on the means of dispersal. + +MEANS OF DISPERSAL. + +Sir C. Lyell and other authors have ably treated this subject. I can give +here only the briefest abstract of the more important facts. Change of +climate must have had a powerful influence on migration. A region 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 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. + +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 Leguminosae were tried, and, with one +exception, they resisted the salt-water badly; seven species of the allied +orders, Hydrophyllaceae and Polemoniaceae, were all killed by a month's +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, +etc., and some of these floated for a long time. It is well known what a +difference there is in the buoyancy of green and seasoned timber; and it +occurred to me that floods 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. + +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 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. + +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 COMPLETELY 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. + +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 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. + +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. + +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 (Motacillae), wheatears, +and whinchats (Saxicolae), 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. + +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 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. + +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 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. + +DISPERSAL DURING THE GLACIAL PERIOD. + +The identity of many plants and animals, on mountain-summits, separated +from each other by hundreds of miles of lowlands, where 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. + +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 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. + +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. + +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. + +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. + +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 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 degrees, lived during the +Pliocene period further north, under the Polar Circle, in latitude 66-67 +degrees; 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. + +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. + +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 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. + +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. + +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. + +ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH. + +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. + +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 degrees, and on the shores of the Pacific, where the +climate is now so different, as far south as latitude 46 degrees. 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 degrees 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 degrees to the southernmost extremity, we have the clearest +evidence of former glacial action, in numerous immense boulders transported +far from their parent source. + +>From these several facts, namely, from the glacial action having 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. + +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 the illustrious +Humboldt long ago found species belonging to genera characteristic of the +Cordillera. + +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. + +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. + +On the southern mountains of Australia, Dr. F. Muller 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 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. + +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, etc., of northern forms of fish. Dr. Hooker informs me that +twenty-five species of Algae are common to New Zealand and to Europe, but +have not been found in the intermediate tropical seas. + +>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." + +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 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. + +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 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. + +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. + +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. + +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. + +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 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. + +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. + + +CHAPTER XIII. + +GEOGRAPHICAL DISTRIBUTION--continued. + +Distribution of fresh-water productions -- On the inhabitants of oceanic +islands -- Absence of Batrachians and of terrestrial Mammals -- On the +relation of the inhabitants of islands to those of the nearest mainland -- +On colonisation from the nearest source with subsequent modification -- +Summary of the last and present chapters. + +FRESH-WATER PRODUCTIONS. + +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, etc., and at the dissimilarity of the surrounding +terrestrial beings, compared with those of Britain. + +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. Gunther 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 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. + +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. Gunther +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. + +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 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. + +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 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. + +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. + +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 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. + +ON THE INHABITANTS OF OCEANIC ISLANDS. + +We now come to the last of the three classes of facts, which I have +selected as presenting the greatest amount of difficulty 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. + +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 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. + +Although in oceanic islands the species are few in number, the proportion +of endemic kinds (i.e. 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, 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 larvae, 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. + +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. + +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 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. + +ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS. + +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. + +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 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, +aerial 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. + +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 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. + +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 aerial bats, the singular proportions of certain orders of +plants, herbaceous forms having been developed into trees, etc., seem to me +to accord better with the 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. + +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 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. + +ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEAREST +MAINLAND. + +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. 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. + +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 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. + +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 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. + +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 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. + +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, etc., 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. 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. + +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 Felidae and Canidae. 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. + +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 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. + +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. + +SUMMARY OF THE LAST AND PRESENT CHAPTERS. + +In these chapters I have endeavoured to show that if we make due allowance +for our ignorance of the full effects of 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. + +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, 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. + +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. + +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 force, some existing in scanty numbers--and this we do +find in the several great geographical provinces of the world. + +On these same principles we can understand, as I have endeavoured to show, +why oceanic islands should have few inhabitants, but 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 aerial 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. + +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 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. + + +CHAPTER XIV. + +MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- +RUDIMENTARY ORGANS. + +Classification, groups subordinate to groups -- Natural system -- Rules and +difficulties in classification, explained on the theory of descent with +modification -- Classification of varieties -- Descent always used in +classification -- Analogical or adaptive characters -- Affinities, general, +complex and radiating -- Extinction separates and defines groups -- +Morphology, between members of the same class, between parts of the same +individual -- Embryology, laws of, explained by variations not supervening +at an early age, and being inherited at a corresponding age -- Rudimentary +organs; their origin explained -- Summary. + +CLASSIFICATION. + +>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 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. + +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. + +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 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 +Linnaeus, 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. + +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 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. + +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 Proteaceae, 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 +Connaraceae "differ in having one or more ovaria, in the existence or +absence of albumen, in the imbricate or valvular aestivation. 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 antennae, 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 antennae in these two divisions of the same order are +of unequal physiological importance. Any number of instances could be +given of the varying importance for classification of the same important +organ within the same group of beings. + +Again, no one will say that rudimentary or atrophied organs are of high +physiological or vital importance; yet, undoubtedly, organs in this +condition are often of 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. + +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 Algae--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. + +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 Linnaeus, 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 +Malpighiaceae, 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 Malpighiaceae. This case well +illustrates the spirit of our classifications. + +Practically, when naturalists are at work, they do not trouble themselves +about the physiological value of the characters which they use in defining +a group or in allocating any particular species. If they find a character +nearly uniform, and common to a great number of forms, and not common to +others, they use it as one of high value; if common to some lesser number, +they use it as of subordinate value. This principle has been broadly +confessed by some naturalists to be the true one; and by none more clearly +than by that excellent botanist, Aug. St. Hilaire. If 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 Muller 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 branchiae, while another species is destitute +of them. + +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 larvae not having been +excluded; in order to show this, Fritz Muller 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. + +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 hitherto been found +impossible. There are crustaceans at the opposite ends of the series, +which have hardly a character in common; yet the species at both ends, from +being plainly allied to others, and these to others, and so onwards, can be +recognised as unequivocally belonging to this, and to no other class of the +Articulata. + +Geographical distribution has often been used, though perhaps not quite +logically, in classification, more especially in very large groups of +closely allied forms. Temminck insists on the utility or even necessity of +this practice in certain groups of birds; and it has been followed by +several entomologists and botanists. + +Finally, with respect to the comparative value of the various groups of +species, such as orders, 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. + +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. + +But I must explain my meaning more fully. I believe that the ARRANGEMENT +of the groups within each class, in due subordination and relation to each +other, must be strictly genealogical in order to be natural; but that the +AMOUNT of difference in the several branches or groups, though allied in +the same degree in blood to their common progenitor, may differ greatly, +being due to the different degrees of modification which they have +undergone; and this is expressed by the forms being ranked under different +genera, families, sections or orders. The reader will best understand what +is meant, if he will take the trouble to refer to the diagram in the fourth +chapter. We will suppose the letters A to L to represent 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 +(a14 to z14) 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 F14 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 ARRANGEMENT remains strictly +true, not only at the present time, but at each successive period of +descent. All the modified descendants from A will have inherited something +in common from their common parent, as will all the descendants from I; so +will it be with each subordinate branch of descendants at each successive +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. + +It may be worth while to illustrate this view of classification, by taking +the case of languages. If we possessed a perfect pedigree of mankind, a +genealogical arrangement of the races of man would afford the best +classification of the various languages now spoken throughout the world; +and if all extinct languages, and all intermediate and slowly changing +dialects, 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. + +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, etc.; +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, without any thought on the subject, these +tumblers are kept in the same group, because allied in blood and alike in +some other respects. + +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. + +As descent has universally been used in classing together the individuals +of the same species, though the males and females and larvae 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 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. + +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. + +ANALOGICAL RESEMBLANCES. + +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. + +Among insects there are innumerable instances; thus Linnaeus, 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. + +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. + +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 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. + +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 ANALOGICAL VARIATION +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. + +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. + +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 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. + +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 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. + +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. + +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 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. + +ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC BEINGS. + +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. + +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. + +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. + +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 extraordinary difficulty which +naturalists have experienced in describing, without the aid of a diagram, +the various affinities which they perceive between the many living and +extinct members of the same great natural class. + +Extinction, as we have seen in the fourth chapter, has played an important +part in defining and widening the intervals between the several groups in +each class. We may thus account 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 the high importance of looking to types, whether or not we can +separate and define the groups to which such types belong. + +Finally, we have seen that natural selection, which 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, etc., we can understand the rules which we are compelled to follow +in our classification. We can understand why we value certain resemblances +far more than others; why we 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. + +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. + +MORPHOLOGY. + +We have seen that the members of the same class, independently of their +habits of life, resemble each other in the general plan of their +organisation. This resemblance is often expressed by the term "unity of +type;" or by saying that the several parts and organs in the different +species of the class are homologous. The whole subject is included under +the general 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?" + +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, 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 maxillae. The same law governs the +construction of the mouths and limbs of crustaceans. So it is with the +flowers of plants. + +Nothing can be more hopeless than to attempt to explain this similarity of +pattern in members of the same class, by utility or by the doctrine of +final causes. The hopelessness of the attempt has been expressly admitted +by Owen in his most interesting work on the "Nature of Limbs." On the +ordinary view of the independent creation of each being, we can only say +that so it is; that it has 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. + +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 maxillae, +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 +suctorial crustaceans, the general pattern seems thus to have become +partially obscured. + +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 vertebrae. 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. + +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 vertebrae? 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? + +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 +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 vertebrae; 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. + +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. + +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, "homogenous"; and the resemblances which cannot thus be +accounted for, he proposes to call "homoplastic". 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 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. + +Naturalists frequently speak of the skull as formed of metamorphosed +vertebrae; 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 +vertebrae, jaws and legs, etc., 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--vertebrae 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. + +DEVELOPMENT AND EMBRYOLOGY. + +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 (Chloeon) 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 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 +larvae, and these others, which finally are developed into mature males and +females, propagating their kind in the ordinary manner by eggs. + +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 larvae 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 Coccidae;" the term +parthenogenesis implying that the mature females of the Coccidae 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. + +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 +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 larvae 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 +leguminosae. + +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. + +The case, however, is different when an animal, during any part of its +embryonic career, is active, and has to provide for itself. The period of +activity may come on earlier or later in life; but whenever it comes on, +the adaptation of the larva to its conditions of life is just as perfect +and as beautiful as in the adult animal. 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 larvae of some insects belonging to very +different orders, and on the dissimilarity of the larvae of other insects +within the same order, according to their habits of life. Owing to such +adaptations the similarity of the larvae 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 larvae of allied +species, or groups of species, differing more from each other than do the +adults. In most cases, however, the larvae, 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 larvae in all their stages barely +distinguishable. + +The embryo in the course of development generally rises in organisation. I +use this expression, though I am aware that it is hardly possible to define +clearly what is meant by 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 larvae 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 antennae; 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 antennae, and their two eyes are +now reconverted into a minute, single, simple eye-spot. In this last and +complete state, cirripedes may be considered as either more highly or more +lowly organised than they were in the larval condition. But in some genera +the larvae 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. + +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 larvae 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. + +Sometimes it is only the earlier developmental stages which fail. Thus, +Fritz Muller 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 zoea-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 zoeas; nevertheless Muller assigns reasons for his +belief, that if there had been no suppression of development, all these +crustaceans would have appeared as nauplii. + +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 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 larvae 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, larvae which have to provide for their +own wants, being perfectly adapted to the surrounding conditions; and +lastly, the fact of certain larvae 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. + +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. + +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. + +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. + +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. + +These facts are explained by the above two principles. Fanciers select +their dogs, horses, pigeons, etc., 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 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. + +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. + +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 +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 Muller 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. + +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 larvae +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, etc., would be useless; and in this case the metamorphosis would be +retrograde. + +>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 with six legs, two long antennae, and four eyes. These larvae +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 larvae 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 larvae of the Sitaris leap on the eggs and +devour them. Afterwards they undergo a complete change; their eyes +disappear; their legs and antennae become rudimentary, and they feed on +honey; so that they now more closely resemble the ordinary larvae 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. + +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 larvae under the nauplius-form; and as these larvae live +and feed in the open sea, and are not adapted for any peculiar habits of +life, and from other reasons assigned by Fritz Muller, 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 branchiae, a swim- +bladder, four fin-like limbs, and a long tail, all fitted for an aquatic +life. + +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 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 larvae; we have seen, for instance, that cirripedes, +though externally so like shell-fish, are at once known by their larvae 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. + +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. + +RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS. + +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 mammae; 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? + +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 mammae 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 +Kolreuter 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 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." + +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 Compositae, 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. + +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 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. Gunther, 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 branchiae. + +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 Scrophulariaceae 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. + +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. + +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 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. + +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. + +It appears probable that disuse has been the main agent in 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. + +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. + +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 +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. + +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. + +SUMMARY. + +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 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. + +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. + +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. Larvae 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. + +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. + + +CHAPTER XV. + +RECAPITULATION AND CONCLUSION. + +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. + +As this whole volume is one long argument, it may be convenient to the +reader to have the leading facts and inferences briefly recapitulated. + +That many and 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. + +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 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. + +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. + +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 Gartner and Kolreuter. 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. + +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 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. + +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 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. + +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 DIRECTLY connecting links between +them, but only between each and some extinct and supplanted form. Even on +a wide area, which has during a long period remained continuous, and of +which the 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. + +On this doctrine of the extermination of an infinitude of connecting links, +between the living and extinct inhabitants of the world, and at each +successive period between the extinct and still older species, why is not +every geological formation charged with such links? Why does not every +collection of fossil remains afford plain evidence of the gradation and +mutation of the forms of life? 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. + +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 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. + +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. + +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 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. + +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. + +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. + +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 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. + +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. + +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. + +As geology plainly proclaims that each land has undergone great 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! + +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. + +On the view that species are only strongly marked and permanent varieties, +and that each species first existed as a variety, we can see why it is that +no line of demarcation can be drawn between species, commonly supposed to +have been produced by special acts of creation, and varieties which are +acknowledged to have been produced by secondary laws. On this same view we +can understand how it is that in 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. + +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. + +As natural selection acts solely by accumulating slight, successive, +favourable variations, it can produce no great or sudden 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. + +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. + +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. + +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 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 ichneumonidae 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. + +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! + +On the ordinary view of each species having been independently created, why +should specific characters, or those by which the species of the same genus +differ from each other, be more variable than the generic characters in +which they all agree? Why, for instance, should the colour of a flower be +more likely to vary in any one species of a genus, if the other species +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. + +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 at some +instincts being not perfect and liable to mistakes, and at many instincts +causing other animals to suffer. + +If species be only well-marked and permanent varieties, we can at once see +why their crossed offspring should follow the same complex laws in their +degrees and kinds of resemblance to their parents--in being absorbed into +each other by successive crosses, and in other such points--as do the +crossed offspring of acknowledged varieties. This similarity would be a +strange fact, if species had been independently created and varieties had +been produced through secondary laws. + +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 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. + +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. + +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 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. + +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. + +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, +etc.; and we have to discover the lines of descent by the most permanent +characters, whatever they may be, and of however slight vital importance. + +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 vertebrae 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 branchiae. + +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 embryological and homologous structures, but we are +too blind to understand her meaning. + +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. + +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." + +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 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." + +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. + +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. + +Although I am fully convinced of the truth of the views given in this +volume under the form of an abstract, I by no means expect to convince +experienced naturalists whose minds are stocked with a multitude of facts +all viewed, during a long course of years, from a point of view directly +opposite to mine. It is so easy to hide our ignorance under such +expressions as the "plan of creation," "unity of design," etc., and to +think that we give an explanation when we only restate a fact. Any one +whose disposition leads him to attach more weight to unexplained +difficulties than to the explanation of a certain number of facts will +certainly reject 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. + +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. + +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. + +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. + +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. + +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 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 algae 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, etc., we have distinct +evidence in their embryological, homologous, and rudimentary structures, +that within each kingdom all the members are descended from a single +progenitor. + +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, to be +capable of definition; and if definable, whether the differences be +sufficiently important to deserve a specific name. This latter point will +become a far more essential consideration than it is at present; for +differences, however slight, between any two forms, if not blended by +intermediate gradations, are looked at by most naturalists as sufficient to +raise both forms to the rank of species. + +Hereafter we shall be compelled to acknowledge that the only distinction +between species and well-marked varieties is, that the latter are known, or +believed to be connected at the present day by intermediate gradations, +whereas species were formerly thus connected. Hence, without 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. + +The other and more general departments of natural history will rise greatly +in interest. The terms used by naturalists, of affinity, relationship, +community of type, paternity, morphology, adaptive characters, rudimentary +and aborted organs, etc., will cease to be metaphorical and will have a +plain signification. When we no longer look at an organic being as a +savage looks at a ship, as 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! + +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 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. + +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. + +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 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. + +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. + +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. + +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. + + +GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME. + +(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.) + +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. + +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 SPHERICAL ABERRATION; 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 +CHROMATIC ABERRATION. + +ABNORMAL.--Contrary to the general rule. + +ABORTED.--An organ is said to be aborted, when its development has been +arrested at a very early stage. + +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. + +ALGAE.--A class of plants including the ordinary sea-weeds and the +filamentous fresh-water weeds. + +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. + +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. + +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 ANALOGOUS, and to be ANALOGUES of each other. + +ANIMALCULE.--A minute animal: generally applied to those visible only by +the microscope. + +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. + +ANTENNAE.--Jointed organs appended to the head in Insects, Crustacea and +Centipedes, and not belonging to the mouth. + +ANTHERS.--The summits of the stamens of flowers, in which the pollen or +fertilising dust is produced. + +APLACENTALIA, APLACENTATA or APLACENTAL MAMMALS.--See MAMMALIA. + +ARCHETYPAL.--Of or belonging to the Archetype, or ideal primitive form upon +which all the beings of a group seem to be organised. + +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). + +ASYMMETRICAL.--Having the two sides unlike. + +ATROPHIED.--Arrested in development at a very early stage. + +BALANUS.--The genus including the common Acorn-shells which live in +abundance on the rocks of the sea-coast. + +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. (Examples, Frogs, Toads, and Newts.) + +BOULDERS.--Large transported blocks of stone generally embedded in clays or +gravels. + +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. + +BRANCHIAE.--Gills or organs for respiration in water. + +BRANCHIAL.--Pertaining to gills or branchiae. + +CAMBRIAN SYSTEM.--A series of very ancient Palaeozoic rocks, between the +Laurentian and the Silurian. Until recently these were regarded as the +oldest fossiliferous rocks. + +CANIDAE.--The Dog-family, including the Dog, Wolf, Fox, Jackal, etc. + +CARAPACE.--The shell enveloping the anterior part of the body in +Crustaceans generally; applied also to the hard shelly pieces of the +Cirripedes. + +CARBONIFEROUS.--This term is applied to the great formation which includes, +among other rocks, the coal-measures. It belongs to the oldest, or +Palaeozoic, system of formations. + +CAUDAL.--Of or belonging to the tail. + +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. (Examples, Cuttle-fish, Nautilus.) + +CETACEA.--An order of Mammalia, including the Whales, Dolphins, etc., +having the form of the body fish-like, the skin naked, and only the fore +limbs developed. + +CHELONIA.--An order of Reptiles including the Turtles, Tortoises, etc. + +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. + +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. + +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." + +COELOSPERMOUS.--A term applied to those fruits of the Umbelliferae which +have the seed hollowed on the inner face. + +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. + +COLUMN.--A peculiar organ in the flowers of Orchids, in which the stamens, +style and stigma (or the reproductive parts) are united. + +COMPOSITAE 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. (Examples, the +Daisy, Dandelion, etc.) + +CONFERVAE.--The filamentous weeds of fresh water. + +CONGLOMERATE.--A rock made up of fragments of rock or pebbles, cemented +together by some other material. + +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. + +CORRELATION.--The normal coincidence of one phenomenon, character, etc., +with another. + +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. + +COTYLEDONS.--The first or seed-leaves of plants. + +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. (Examples, Crab, Lobster, Shrimp, etc.) + +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 antennae are inserted. + +CUTANEOUS.--Of or belonging to the skin. + +DEGRADATION.--The wearing down of land by the action of the sea or of +meteoric agencies. + +DENUDATION.--The wearing away of the surface of the land by water. + +DEVONIAN SYSTEM or FORMATION.--A series of Palaeozoic rocks, including the +Old Red Sandstone. + +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. + +DIFFERENTATION.--The separation or discrimination of parts or organs which +in simpler forms of life are more or less united. + +DIMORPHIC.--Having two distinct forms.--DIMORPHISM is the condition of the +appearance of the same species under two dissimilar forms. + +DIOECIOUS.--Having the organs of the sexes upon distinct individuals. + +DIORITE.--A peculiar form of Greenstone. + +DORSAL.--Of or belonging to the back. + +EDENTATA.--A peculiar order of Quadrupeds, characterised by the absence of +at least the middle incisor (front) teeth in both jaws. (Examples, the +Sloths and Armadillos.) + +ELYTRA.--The hardened fore-wings of Beetles, serving as sheaths for the +membranous hind-wings, which constitute the true organs of flight. + +EMBRYO.--The young animal undergoing development within the egg or womb. + +EMBRYOLOGY.--The study of the development of the embryo. + +ENDEMIC.--Peculiar to a given locality. + +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. + +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. + +EPHEMEROUS INSECTS.--Insects allied to the May-fly. + +FAUNA.--The totality of the animals naturally inhabiting a certain country +or region, or which have lived during a given geological period. + +FELIDAE.--The Cat-family. + +FERAL.--Having become wild from a state of cultivation or domestication. + +FLORA.--The totality of the plants growing naturally in a country, or +during a given geological period. + +FLORETS.--Flowers imperfectly developed in some respects, and collected +into a dense spike or head, as in the Grasses, the Dandelion, etc. + +FOETAL.--Of or belonging to the foetus, or embryo in course of development. + +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. + +FOSSILIFEROUS.--Containing fossils. + +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. + +FRENUM (pl. FRENA).--A small band or fold of skin. + +FUNGI (sing. FUNGUS).--A class of cellular plants, of which Mushrooms, +Toadstools, and Moulds, are familiar examples. + +FURCULA.--The forked bone formed by the union of the collar-bones in many +birds, such as the common Fowl. + +GALLINACEOUS BIRDS.--An order of birds of which the common Fowl, Turkey, +and Pheasant, are well-known examples. + +GALLUS.--The genus of birds which includes the common Fowl. + +GANGLION.--A swelling or knot from which nerves are given off as from a +centre. + +GANOID FISHES.--Fishes covered with peculiar enamelled bony scales. Most +of them are extinct. + +GERMINAL VESICLE.--A minute vesicle in the eggs of animals, from which the +development of the embryo proceeds. + +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. + +GLAND.--An organ which secretes or separates some peculiar product from the +blood or sap of animals or plants. + +GLOTTIS.--The opening of the windpipe into the oesophagus or gullet. + +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. + +GRALLATORES.--The so-called wading-birds (storks, cranes, snipes, etc.), +which are generally furnished with long legs, bare of feathers above the +heel, and have no membranes between the toes. + +GRANITE.--A rock consisting essentially of crystals of felspar and mica in +a mass of quartz. + +HABITAT.--The locality in which a plant or animal naturally lives. + +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. + +HERMAPHRODITE.--Possessing the organs of both sexes. + +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, etc., is composed. The latter is called +serial homology. The parts which stand in such a relation to each other +are said to be homologous, and one such part or organ is called the +homologue of the other. In different plants the parts of the flower are +homologous, and in general these parts are regarded as homologous with +leaves. + +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 Cicadae, frog-hoppers, and Aphides, are well-known +examples. + +HYBRID.--The offspring of the union of two distinct species. + +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. + +HYPERTROPHIED.--Excessively developed. + +ICHNEUMONIDAE.--A family of hymenopterous insects, the members of which lay +their eggs in the bodies or eggs of other insects. + +IMAGO.--The perfect (generally winged) reproductive state of an insect. + +INDIGENES.--The aboriginal animal or vegetable inhabitants of a country or +region. + +INFLORESCENCE.--The mode of arrangement of the flowers of plants. + +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. + +INSECTIVOROUS.--Feeding on insects. + +INVERTEBRATA, or INVERTEBRATE ANIMALS.--Those animals which do not possess +a backbone or spinal column. + +LACUNAE.--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. + +LAMELLATED.--Furnished with lamellae or little plates. + +LARVA (pl. LARVAE).--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. + +LARYNX.--The upper part of the windpipe opening into the gullet. + +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. + +LEGUMINOSAE.--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). + +LEMURIDAE.--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. + +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. + +LITTORAL.--Inhabiting the seashore. + +LOESS.--A marly deposit of recent (Post-Tertiary) date, which occupies a +great part of the valley of the Rhine. + +MALACOSTRACA.--The higher division of the Crustacea, including the ordinary +crabs, lobsters, shrimps, etc., together with the woodlice and +sand-hoppers. + +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 +(MAMMAE, MAMMARY GLANDS) 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 PLACENTA, 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 PLACENTAL MAMMALS; the latter, or APLACENTAL MAMMALS, include the +Marsupials and Monotremes (ORNITHORHYNCHUS). + +MAMMIFEROUS.--Having mammae or teats (see MAMMALIA). + +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. + +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, etc. (see +MAMMALIA). + +MAXILLAE.--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. + +MELANISM.--The opposite of albinism; an undue development of colouring +material in the skin and its appendages. + +METAMORPHIC ROCKS.--Sedimentary rocks which have undergone alteration, +generally by the action of heat, subsequently to their deposition and +consolidation. + +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. + +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. (Examples, grasses, lilies, orchids, +palms, etc.) + +MORAINES.--The accumulations of fragments of rock brought down by glaciers. + +MORPHOLOGY.--The law of form or structure independent of function. + +MYSIS-STAGE.--A stage in the development of certain crustaceans (prawns), +in which they closely resemble the adults of a genus (Mysis) belonging to a +slightly lower group. + +NASCENT.--Commencing development. + +NATATORY.--Adapted for the purpose of swimming. + +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 CYCLOPS +was described as a distinct genus under the name of NAUPLIUS. + +NEURATION.--The arrangement of the veins or nervures in the wings of +insects. + +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 WORKERS. + +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, etc., from the surface of the +eye. + +OCELLI.--The simple eyes or stemmata of insects, usually situated on the +crown of the head between the great compound eyes. + +OESOPHAGUS.--The gullet. + +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. + +OPERCULUM.--A calcareous plate employed by many Molluscae to close the +aperture of their shell. The OPERCULAR VALVES of Cirripedes are those +which close the aperture of the shell. + +ORBIT.--The bony cavity for the reception of the eye. + +ORGANISM.--An organised being, whether plant or animal. + +ORTHOSPERMOUS.--A term applied to those fruits of the Umbelliferae which +have the seed straight. + +OSCULANT.--Forms or groups apparently intermediate between and connecting +other groups are said to be osculant. + +OVA.--Eggs. + +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. + +OVIGEROUS.--Egg-bearing. + +OVULES (of plants).--The seeds in the earliest condition. + +PACHYDERMS.--A group of Mammalia, so called from their thick skins, and +including the elephant, rhinoceros, hippopotamus, etc. + +PALAEOZOIC.--The oldest system of fossiliferous rocks. + +PALPI.--Jointed appendages to some of the organs of the mouth in insects +and Crustacea. + +PAPILIONACEAE.--An order of plants (see LEGUMINOSAE), The flowers of these +plants are called PAPILIONACEOUS, or butterfly-like, from the fancied +resemblance of the expanded superior petals to the wings of a butterfly. + +PARASITE.--An animal or plant living upon or in, and at the expense of, +another organism. + +PARTHENOGENESIS.--The production of living organisms from unimpregnated +eggs or seeds. + +PEDUNCULATED.--Supported upon a stem or stalk. The pedunculated oak has +its acorns borne upon a footstool. + +PELORIA or PELORISM.--The appearance of regularity of structure in the +flowers of plants which normally bear irregular flowers. + +PELVIS.--The bony arch to which the hind limbs of vertebrate animals are +articulated. + +PETALS.--The leaves of the corolla, or second circle of organs in a flower. +They are usually of delicate texture and brightly coloured. + +PHYLLODINEOUS.--Having flattened, leaf-like twigs or leafstalks instead of +true leaves. + +PIGMENT.--The colouring material produced generally in the superficial +parts of animals. The cells secreting it are called PIGMENT-CELLS. + +PINNATE.--Bearing leaflets on each side of a central stalk. + +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. + +PLACENTALIA, PLACENTATA.--or PLACENTAL MAMMALS, See MAMMALIA. + +PLANTIGRADES.--Quadrupeds which walk upon the whole sole of the foot, like +the bears. + +PLASTIC.--Readily capable of change. + +PLEISTOCENE PERIOD.--The latest portion of the Tertiary epoch. + +PLUMULE (in plants).--The minute bud between the seed-leaves of +newly-germinated plants. + +PLUTONIC ROCKS.--Rocks supposed to have been produced by igneous action in +the depths of the earth. + +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 +(POLLEN-TUBES) which issue from the pollen-grains adhering to the stigma, +and penetrate through the tissues until they reach the ovary. + +POLYANDROUS (flowers).--Flowers having many stamens. + +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. + +POLYMORPHIC.--Presenting many forms. + +POLYZOARY.--The common structure formed by the cells of the Polyzoa, such +as the well-known seamats. + +PREHENSILE.--Capable of grasping. + +PREPOTENT.--Having a superiority of power. + +PRIMARIES.--The feathers forming the tip of the wing of a bird, and +inserted upon that part which represents the hand of man. + +PROCESSES.--Projecting portions of bones, usually for the attachment of +muscles, ligaments, etc. + +PROPOLIS.--A resinous material collected by the hivebees from the opening +buds of various trees. + +PROTEAN.--Exceedingly variable. + +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. + +PUPA (pl. PUPAE).--The second stage in the development of an insect, from +which it emerges in the perfect (winged) reproductive form. In most +insects the PUPAL STAGE is passed in perfect repose. The CHRYSALIS is the +pupal state of butterflies. + +RADICLE.--The minute root of an embryo plant. + +RAMUS.--One half of the lower jaw in the Mammalia. The portion which rises +to articulate with the skull is called the ASCENDING RAMUS. + +RANGE.--The extent of country over which a plant or animal is naturally +spread. RANGE IN TIME expresses the distribution of a species or group +through the fossiliferous beds of the earth's crust. + +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. + +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 RETROGRADE +DEVELOPMENT or METAMORPHOSIS. + +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. + +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. + +RUBUS.--The bramble genus. + +RUDIMENTARY.--Very imperfectly developed. + +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. + +SACRAL.--Belonging to the sacrum, or the bone composed usually of two or +more united vertebrae to which the sides of the pelvis in vertebrate +animals are attached. + +SARCODE.--The gelatinous material of which the bodies of the lowest animals +(Protozoa) are composed. + +SCUTELLAE.--The horny plates with which the feet of birds are generally +more or less covered, especially in front. + +SEDIMENTARY FORMATIONS.--Rocks deposited as sediments from water. + +SEGMENTS.--The transverse rings of which the body of an articulate animal +or annelid is composed. + +SEPALS.--The leaves or segments of the calyx, or outermost envelope of an +ordinary flower. They are usually green, but sometimes brightly coloured. + +SERRATURES.--Teeth like those of a saw. + +SESSILE.--Not supported on a stem or footstalk. + +SILURIAN SYSTEM.--A very ancient system of fossiliferous rocks belonging to +the earlier part of the Palaeozoic series. + +SPECIALISATION.--The setting apart of a particular organ for the +performance of a particular function. + +SPINAL CORD.--The central portion of the nervous system in the Vertebrata, +which descends from the brain through the arches of the vertebrae, and +gives off nearly all the nerves to the various organs of the body. + +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. + +STERNUM.--The breast-bone. + +STIGMA.--The apical portion of the pistil in flowering plants. + +STIPULES.--Small leafy organs placed at the base of the footstalks of the +leaves in many plants. + +STYLE.--The middle portion of the perfect pistil, which rises like a column +from the ovary and supports the stigma at its summit. + +SUBCUTANEOUS.--Situated beneath the skin. + +SUCTORIAL.--Adapted for sucking. + +SUTURES (in the skull).--The lines of junction of the bones of which the +skull is composed. + +TARSUS (pl. TARSI).--The jointed feet of articulate animals, such as +insects. + +TELEOSTEAN FISHES.--Fishes of the kind familiar to us in the present day, +having the skeleton usually completely ossified and the scales horny. + +TENTACULA or TENTACLES.--Delicate fleshy organs of prehension or touch +possessed by many of the lower animals. + +TERTIARY.--The latest geological epoch, immediately preceding the +establishment of the present order of things. + +TRACHEA.--The windpipe or passage for the admission of air to the lungs. + +TRIDACTYLE.--Three-fingered, or composed of three movable parts attached to +a common base. + +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 Palaeozoic +rocks, and most abundantly in those of Silurian age. + +TRIMORPHIC.--Presenting three distinct forms. + +UMBELLIFERAE.--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 (UMBEL) nearly to +the same level. (Examples, parsley and carrot.) + +UNGULATA.--Hoofed quadrupeds. + +UNICELLULAR.--Consisting of a single cell. + +VASCULAR.--Containing blood-vessels. + +VERMIFORM.--Like a worm. + +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 VERTEBRAE, which constitutes the centre of the +skeleton and at the same time supports and protects the central parts of +the nervous system. + +WHORLS.--The circles or spiral lines in which the parts of plants are +arranged upon the axis of growth. + +WORKERS.--See neuters. + +ZOEA-STAGE.--The earliest stage in the development of many of the higher +Crustacea, so called from the name of ZOEA applied to these young animals +when they were supposed to constitute a peculiar genus. + +ZOOIDS.--In many of the lower animals (such as the Corals, Medusae, etc.) +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 ZOOIDE. + + +INDEX. + +Aberrant groups + +Abyssinia, plants of + +Acclimatisation + +Adoxa + +Affinities of extinct species +--of organic beings + +Agassiz on Amblyopsis +--on groups of species suddenly appearing +--on prophetic forms +--on embryological succession +--on the Glacial period +--on embryological characters +--on the latest tertiary forms +--on parallelism of embryological development and geological succession +--Alex., on pedicellariae + +Algae of New Zealand + +Alligators, males, fighting + +Alternate generations + +Amblyopsis, blind fish + +America, North, productions allied to those of Europe +--boulders and glaciers of +--South, no modern formations on west coast + +Ammonites, sudden extinction of + +Anagallis, sterility of + +Analogy of variations + +Andaman Islands inhabited by a toad + +Ancylus + +Animals, not domesticated from being variable +--domestic; descended from several stocks +--acclimatisation of + +Animals of Australia +--with thicker fur in cold climates +--blind, in caves +--extinct, of Australia + +Anomma + +Antarctic islands, ancient flora of + +Antechinus + +Ants attending aphides +--slave-making instinct +--neuters, structure of + +Apes, not having acquired intellectual powers + +Aphides attended by ants + +Aphis, development of + +Apteryx + +Arab horses + +Aralo-Caspian Sea + +Archeopteryx + +Archiac, M. de, on the succession of species + +Artichoke, Jerusalem + +Ascension, plants of + +Asclepias, pollen of + +Asparagus + +Aspicarpa + +Asses, striped +--improved by selection + +Ateuchus + +Aucapitaine, on land-shells + +Audubon, on habits of frigate-bird +--on variation in birds' nests +--on heron eating seeds + +Australia, animals of +--dogs of +--extinct animals of +--European plants in +--glaciers of + +Azara, on flies destroying cattle + +Azores, flora of + +Babington, Mr., on British plants + +Baer, Von, standard of Highness +--comparison of bee and fish +--embryonic similarity of the Vertebrata + +Baker, Sir S., on the giraffe + +Balancement of growth + +Baleen + +Barberry, flowers of + +Barrande, M., on Silurian colonies +--on the succession of species +--on parallelism of palaeozoic formations +--on affinities of ancient species + +Barriers, importance of + +Bates, Mr., on mimetic butterflies + +Batrachians on islands + +Bats, how structure acquired +--distribution of + +Bear, catching water-insects + +Beauty, how acquired + +Bee, sting of +--queen, killing rivals +--Australian, extermination of + +Bees, fertilizing flowers +--hive, not sucking the red clover +--hive, cell-making instinct +--Ligurian +--variation in habits + +Bees, parasitic +--humble, cells of + +Beetles, wingless, in Madeira +--with deficient tarsi + +Bentham, Mr., on British plants +--on classification + +Berkeley, Mr., on seeds in salt-water + +Bermuda, birds of + +Birds acquiring fear +--beauty of +--annually cross the Atlantic +--colour of, on continents +--footsteps, and remains of, in secondary rocks +--fossil, in caves of Brazil +--of Madeira, Bermuda, and Galapagos +--song of males +--transporting seeds +--waders +--wingless + +Bizcacha, affinities of + +Bladder for swimming, in fish + +Blindness of cave animals + +Blyth, Mr., on distinctness of Indian cattle +--on striped Hemionus +--on crossed geese + +Borrow, Mr., on the Spanish pointer + +Bory St. Vincent, on Batrachians + +Bosquet, M., on fossil Chthamalus + +Boulders, erratic, on the Azores + +Branchiae +--of crustaceans + +Braun, Prof., on the seeds of Fumariaceae + +Brent, Mr., on house-tumblers + +Britain, mammals of + +Broca, Prof., on Natural Selection + +Bronn, Prof., on duration of specific forms +--various objections by + +Brown, Robert, on classification + +Brown-Sequard, on inherited mutilations + +Busk, Mr., on the Polyzoa + +Butterflies, mimetic + +Buzareingues, on sterility of varieties + +Cabbage, varieties of, crossed + +Calceolaria + +Canary-birds, sterility of hybrids + +Cape de Verde Islands, productions of +--plants of, on mountains + +Cape of Good Hope, plants of + +Carpenter, Dr., on foraminifera + +Carthemus + +Catasetum + +Cats, with blue eyes, deaf +--variation in habits of +--curling tail when going to spring + +Cattle destroying fir-trees +--destroyed by flies in Paraguay +--breeds of, locally extinct +--fertility of Indian and European breeds +--Indian + +Cave, inhabitants of, blind + +Cecidomyia + +Celts, proving antiquity of man + +Centres of creation + +Cephalopodae, structures of eyes +--development of + +Cercopithecus, tail of + +Ceroxylus laceratus + +Cervulus + +Cetacea, teeth and hair +--development of the whalebone + +Cetaceans + +Ceylon, plants of + +Chalk formation + +Characters, divergence of +--sexual, variable +--adaptive or analogical + +Charlock + +Checks to increase +--mutual + +Chelae of Crustaceans + +Chickens, instinctive tameness of + +Chironomus, its asexual reproduction + +Chthamalinae + +Chthamalus, cretacean species of + +Circumstances favourable to selection of domestic products +--to natural selection + +Cirripedes capable of crossing +--carapace aborted +--their ovigerous frena +--fossil +--larvae of + +Claparede, Prof., on the hair-claspers of the Acaridae + +Clarke, Rev. W.B., on old glaciers in Australia + +Classification + +Clift, Mr., on the succession of types + +Climate, effects of, in checking increase of beings +--adaptation of, to organisms + +Climbing plants +--development of + +Clover visited by bees + +Cobites, intestine of + +Cockroach + +Collections, palaeontological, poor + +Colour, influenced by climate +--in relation to attacks by flies + +Columba livia, parent of domestic pigeons + +Colymbetes + +Compensation of growth + +Compositae, flowers and seeds of +--outer and inner florets of +--male flowers of + +Conclusion, general + +Conditions, slight changes in, favourable to fertility + +Convergence of genera + +Coot + +Cope, Prof., on the acceleration or retardation of the period of +reproduction + +Coral-islands, seeds drifted to +--reefs, indicating movements of earth + +Corn-crake + +Correlated variation in domestic productions + +Coryanthes + +Creation, single centres of + +Crinum + +Croll, Mr., on subaerial denudation +--on the age of our oldest formations +--on alternate Glacial periods in the North and South + +Crosses, reciprocal + +Crossing of domestic animals, importance in altering breeds +--advantages of +--unfavourable to selection + +Cruger, Dr., on Coryanthes + +Crustacea of New Zealand + +Crustacean, blind +air-breathers + +Crustaceans, their chelae + +Cryptocerus + +Ctenomys, blind + +Cuckoo, instinct of + +Cunningham, Mr., on the flight of the logger-headed duck + +Currants, grafts of + +Currents of sea, rate of + +Cuvier on conditions of existence +--on fossil monkeys + +Cuvier, Fred., on instinct + +Cyclostoma, resisting salt water + +Dana, Prof., on blind cave-animals +--on relations of crustaceans of Japan +--on crustaceans of New Zealand + +Dawson, Dr., on eozoon + +De Candolle, Aug. Pyr., on struggle for existence +--on umbelliferae +--on general affinities + +De Candolle, Alph., on the variability of oaks +--on low plants, widely dispersed +--on widely-ranging plants being variable +--on naturalisation +--on winged seeds +--on Alpine species suddenly becoming rare +--on distribution of plants with large seeds +--on vegetation of Australia +--on fresh-water plants +--on insular plants + +Degradation of rocks + +Denudation, rate of +--of oldest rocks +--of granite areas + +Development of ancient forms + +Devonian system + +Dianthus, fertility of crosses + +Dimorphism in plants + +Dirt on feet of birds + +Dispersal, means of +--during Glacial period + +Distribution, geographical +--means of + +Disuse, effect of, under nature + +Diversification of means for same general purpose + +Division, physiological, of labour + +Divergence of character + +Dog, resemblance of jaw to that of the Thylacinus + +Dogs, hairless, with imperfect teeth +--descended from several wild stocks +--domestic instincts of +--inherited civilisation of +--fertility of breeds together +--of crosses +--proportions of body in different breeds, when young + +Domestication, variation under + +Double flowers + +Downing, Mr., on fruit-trees in America + +Dragon-flies, intestines of + +Drift-timber + +Driver-ant + +Drones killed by other bees + +Duck, domestic, wings of, reduced +--beak of +--logger-headed + +Duckweed + +Dugong, affinities of + +Dung-beetles with deficient tarsi + +Dyticus + +Earl, Mr., W., on the Malay Archipelago + +Ears, drooping, in domestic animals +--rudimentary + +Earth, seeds in roots of trees +--charged with seeds + +Echinodermata, their pedicellariae + +Eciton + +Economy of organisation + +Edentata, teeth and hair +--fossil species of + +Edwards, Milne, on physiological division of labour +--on gradations of structure + +Edwards, on embryological characters + +Eggs, young birds escaping from + +Egypt, productions of, not modified + +Electric organs + +Elephant, rate of increase +--of Glacial period + +Embryology + +Eozoon Canadense + +Epilipsy inherited + +Existence, struggle for +--condition of + +Extinction, as bearing on natural selection +--of domestic varieties + +Eye, structure of +--correction for aberration + +Eyes, reduced, in moles + +Fabre, M., on hymenoptera fighting +--on parasitic sphex +--on Sitaris + +Falconer, Dr., on naturalisation of plants in India +--on elephants and mastodons +--and Cautley on mammals of sub-Himalayan beds + +Falkland Islands, wolf of + +Faults + +Faunas, marine + +Fear, instinctive, in birds + +Feet of birds, young molluscs adhering to + +Fertilisation variously effected + +Fertility of hybrids +--from slight changes in conditions +--of crossed varieties + +Fir-trees destroyed by cattle +--pollen of + +Fish, flying +--teleostean, sudden appearance of +--eating seeds +--fresh-water, distribution of + +Fishes, ganoid, now confined to fresh water +--ganoid, living in fresh water +--electric organs of +--of southern hemisphere + +Flight, powers of, how acquired + +Flint-tools, proving antiquity of man + +Flower, Prof., on the larynx +--on Halitherium +--on the resemblance between the jaws of the dog and Thylacinus +--on the homology of the feet of certain marsupials + +Flowers, structure of +--in relation to crossing +--of composite and umbelliferae +--beauty of +--double + +Flysch formation, destitute of organic remains + +Forbes, Mr. D., on glacial action in the Andes + +Forbes, E., on colours of shells +--on abrupt range of shells in depth +--on poorness of palaeontological collections +--on continuous succession of genera +--on continental extensions +--on distribution during Glacial period +--on parallelism in time and space + +Forests, changes in, in America + +Formation, Devonian +--Cambrian +--intermittent +--thickness of, in Britain + +Formica +--rufescens +--sanguinea +--flava, neuter of + +Forms, lowly organised, long enduring + +Frena, ovigerous, of cirripedes + +Fresh-water productions, dispersal of + +Fries on species in large genera being closely allied to other species + +Frigate-bird + +Frogs on islands + +Fruit-trees, gradual improvement of +--in United States +--varieties of, acclimatised in United States + +Fuci, crossed + +Fur, thicker in cold climates + +Furze + +Galapagos Archipelago, birds of +--productions of + +Galaxias, its wide range + +Galeopithecus + +Game, increase of, checked by vermin + +Gartner on sterility of hybrids +--on reciprocal crosses +--on crossed maize and verbascum +--on comparison of hybrids and mongrels + +Gaudry, Prof., on intermediate genera of fossil mammals in Attica + +Geese, fertility when crossed +--upland + +Geikie, Mr., on subaerial denudation + +Genealogy, important in classification + +Generations, alternate + +Geoffroy St. Hilaire, on balancement +--on homologous organs + +Geoffroy St. Hilaire, Isidore, on variability of repeated parts +--on correlation, in monstrosities +--on correlation +--on variable parts being often monstrous + +Geographical distribution + +Geography, ancient + +Geology, future progress of +--imperfection of the record + +Gervais, Prof., on Typotherium + +Giraffe, tail of +--structure of + +Glacial period +--affecting the North and South + +Glands, mammary + +Gmelin, on distribution + +Godwin-Austin, Mr., on the Malay Archipelago + +Goethe, on compensation of growth + +Gomphia + +Gooseberry, grafts of + +Gould, Dr. Aug. A., on land-shells + +Gould, Mr., on colours of birds +--on instincts of cuckoo +--on distribution of genera of birds + +Gourds, crossed + +Graba, on the Uria lacrymans + +Grafting, capacity of + +Granite, areas of denuded + +Grasses, varieties of + +Gray, Dr. Asa, on the variability of oaks +--on man not causing variability +--on sexes of the holly +--on trees of the United States +--on naturalised plants in the United States +--on aestivation +--on rarity of intermediate varieties +--on Alpine plants + +Gray, Dr. J.E., on striped mule + +Grebe + +Grimm, on asexual reproduction + +Groups, aberrant + +Grouse, colours of +--red, a doubtful species + +Growth, compensation of + +Gunther, Dr., on flat-fish +--on prehensile tails +--on the fishes of Panama +--on the range of fresh-water fishes +--on the limbs of Lepidosiren + +Haast, Dr., on glaciers of New Zealand + +Habit, effect of, under domestication +--effect of, under nature +--diversified, of same species + +Hackel, Prof., on classification and the lines of descent + +Hair and teeth, correlated + +Halitherium + +Harcourt, Mr. E.V., on the birds of Madeira + +Hartung, M., on boulders in the Azores + +Hazel-nuts + +Hearne, on habits of bears + +Heath, changes in vegetation + +Hector, Dr., on glaciers of New Zealand + +Heer, Oswald, on ancient cultivated plants +--on plants of Madeira + +Helianthemum + +Helix pomatia, resisting salt water + +Helmholtz, M., on the imperfection of the human eye + +Helosciadium + +Hemionus, striped + +Hensen, Dr., on the eyes of Cephalopods + +Herbert, W., on struggle for existence +--on sterility of hybrids + +Hermaphrodites crossing + +Heron eating seed + +Heron, Sir R., on peacocks + +Heusinger, on white animals poisoned by certain plants + +Hewitt, Mr., on sterility of first crosses + +Hildebrand, Prof., on the self-sterility of Corydalis + +Hilgendorf, on intermediate varieties + +Himalaya, glaciers of +--plants of + +Hippeastrum + +Hippocampus + +Hofmeister, Prof., on the movements of plants + +Holly-trees, sexes of + +Hooker, Dr., on trees of New Zealand +--on acclimatisation of Himalayan trees +--on flowers of umbelliferae +--on the position of ovules +--on glaciers of Himalaya +--on algae of New Zealand +--on vegetation at the base of the Himalaya +--on plants of Tierra del Fuego +--on Australian plants +--on relations of flora of America +--on flora of the Antarctic lands +--on the plants of the Galapagos +--on glaciers of the Lebanon +--on man not causing variability +--on plants of mountains of Fernando Po + +Hooks on palms +--on seeds, on islands + +Hopkins, Mr., on denudation + +Hornbill, remarkable instinct of + +Horns, rudimentary + +Horse, fossil in La Plata +--proportions of, when young + +Horses destroyed by flies in Paraguay +--striped + +Horticulturists, selection applied by + +Huber on cells of bees + +Huber, P., on reason blended with instinct +--on habitual nature of instincts +--on slave-making ants +--on Melipona domestica + +Hudson, Mr., on the Ground-woodpecker of La Plata +--on the Molothrus + +Humble-bees, cells of + +Hunter, J., on secondary sexual characters + +Hutton, Captain, on crossed geese + +Huxley, Prof., on structure of hermaphrodites +--on the affinities of the Sirenia +--on forms connecting birds and reptiles +--on homologous organs +--on the development of aphis + +Hybrids and mongrels compared + +Hybridism + +Hydra, structure of + +Hymenoptera, fighting + +Hymenopterous insect, diving + +Hyoseris + +Ibla + +Icebergs transporting seeds + +Increase, rate of + +Individuals, numbers favourable to selection +--many, whether simultaneously created + +Inheritance, laws of +--at corresponding ages + +Insects, colour of, fitted for their stations +--sea-side, colours of +--blind, in caves +--luminous +--their resemblance to certain objects +--neuter + +Instinct, not varying simultaneously with structure + +Instincts, domestic + +Intercrossing, advantages of + +Islands, oceanic + +Isolation favourable to selection + +Japan, productions of + +Java, plants of + +Jones, Mr. J.M., on the birds of Bermuda + +Jordain, M., on the eye-spots of star fishes + +Jukes, Prof., on subaerial denudation + +Jussieu on classification + +Kentucky, caves of + +Kerguelen-land, flora of + +Kidney-bean, acclimatisation of + +Kidneys of birds + +Kirby, on tarsi deficient in beetles + +Knight, Andrew, on cause of variation + +Kolreuter, on intercrossing +--on the barberry +--on sterility of hybrids +--on reciprocal crosses +--on crossed varieties of nicotiana +--on crossing male and hermaphrodite flowers + +Lamarck, on adaptive characters + +Lancelet, eyes of + +Landois, on the development of the wings of insects + +Land-shells, distribution of +--of Madeira, naturalised +--resisting salt water + +Languages, classification of + +Lankester, Mr. E. Ray, on longevity +--on homologies + +Lapse, great, of time + +Larvae + +Laurel, nectar secreted by the leaves + +Laurentian formation + +Laws of variation + +Leech, varieties of + +Leguminosae, nectar secreted by glands + +Leibnitz, attack on Newton + +Lepidosiren, limbs in a nascent condition + +Lewes, Mr. G.H., on species not having changed in Egypt +--on the Salamandra atra +--on many forms of life having been at first evolved + +Life, struggle for + +Lingula, Silurian + +Linnaeus, aphorism of + +Lion, mane of +--young of, striped + +Lobelia fulgens + +Lobelia, sterility of crosses + +Lockwood, Mr., on the ova of the Hippocampus + +Locusts transporting seeds + +Logan, Sir W., on Laurentian formation + +Lowe, Rev. R.T., on locusts visiting Madeira + +Lowness, of structure connected with variability +--related to wide distribution + +Lubbock, Sir J., on the nerves of coccus +--on secondary sexual characters +--on a diving hymenopterous insect +--on affinities +--on metamorphoses + +Lucas, Dr. P., on inheritance +--on resemblance of child to parent + +Lund and Clausen, on fossils of Brazil + +Lyell, Sir C., on the struggle for existence +--on modern changes of the earth +--on terrestrial animals not having been developed on islands +--on a carboniferous land-shell +--on strata beneath Silurian system +--on the imperfection of the geological record +--on the appearance of species +--on Barrande's colonies +--on tertiary formations of Europe and North America +--on parallelism of tertiary formations +--on transport of seeds by icebergs +--on great alternations of climate +--on the distribution of fresh-water shells +--on land-shells of Madeira + +Lyell and Dawson, on fossilized trees in Nova Scotia + +Lythrum salicaria, trimorphic + +Macleay, on analogical characters + +Macrauchenia + +McDonnell, Dr., on electric organs + +Madeira, plants of +--beetles of, wingless +--fossil land-shells of +--birds of + +Magpie tame in Norway + +Males, fighting + +Maize, crossed + +Malay Archipelago, compared with Europe +--mammals of + +Malm, on flat-fish + +Malpighiaceae, small imperfect flowers of + +Mammae, their development +--rudimentary + +Mammals, fossil, in secondary formation +--insular + +Man, origin of + +Manatee, rudimentary nails of + +Marsupials, fossil species of + +Marsupials of Australia, structure of their feet + +Martens, M., experiment on seeds + +Martin, Mr. W.C., on striped mules + +Masters, Dr., on Saponaria + +Matteucci, on the electric organs of rays + +Matthiola, reciprocal crosses of + +Maurandia + +Means of dispersal + +Melipona domestica + +Merrill, Dr., on the American cuckoo + +Metamorphism of oldest rocks + +Mice destroying bees +--acclimatisation of +--tails of + +Miller, Prof., on the cells of bees + +Mirabilis, crosses of + +Missel-thrush + +Mistletoe, complex relations of + +Mivart, Mr., on the relation of hair and teeth +--on the eyes of cephalopods +--various objections to Natural Selection +--on abrupt modifications +--on the resemblance of the mouse and antechinus + +Mocking-thrush of the Galapagos + +Modification of species, not abrupt + +Moles, blind + +Molothrus, habits of + +Mongrels, fertility and sterility of +--and hybrids compared + +Monkeys, fossil + +Monachanthus + +Mons, Van, on the origin of fruit-trees + +Monstrosities + +Moquin-Tandon, on sea-side plants + +Morphology + +Morren, on the leaves of Oxalis + +Moths, hybrid + +Mozart, musical powers of + +Mud, seeds in + +Mules, striped + +Muller, Adolph, on the instincts of the cuckoo + +Muller, Dr. Ferdinand, on Alpine Australian plants + +Muller, Fritz, on dimorphic crustaceans +--on the lancelet +--on air-breathing crustaceans +--on the self-sterility of orchids +--on embryology in relation to classification +--on the metamorphoses of crustaceans +--on terrestrial and fresh-water organisms not undergoing any metamorphosis +--on climbing plants + +Multiplication of species not indefinite + +Murchison, Sir, R., on the formations of Russia +--on azoic formations +--on extinction + +Murie, Dr., on the modification of the skull in old age + +Murray, Mr. A., on cave-insects + +Mustela vison + +Myanthus + +Myrmecocystus + +Myrmica, eyes of + +Nageli, on morphological characters + +Nails, rudimentary + +Nathusius, Von, on pigs + +Natural history, future progress of +--selection +--system + +Naturalisation of forms distinct from the indigenous species +--in New Zealand + +Naudin, on analagous variations in gourds +--on hybrid gourds +--on reversion + +Nautilus, Silurian + +Nectar of plants + +Nectaries, how formed + +Nelumbium luteum + +Nests, variation in + +Neuter insects + +New Zealand, productions of, not perfect +--naturalised products of +--fossil birds of +--glaciers of +--crustaceans of +--algae of +--number of plants of +--flora of + +Newman, Col., on humble-bees + +Newton, Prof., on earth attached to a partridge's foot + +Newton, Sir I., attacked for irreligion + +Nicotiana, crossed varieties of +--certain species very sterile + +Nitsche, Dr., on the Polyzoa + +Noble, Mr., on fertility of Rhododendron + +Nodules, phosphatic, in azoic rocks + +Oak, varieties of + +Onites apelles + +Orchids, fertilisation of +--the development of their flowers +--forms of + +Orchis, pollen of + +Organisation, tendency to advance + +Organs of extreme perfection +--electric, of fishes +--of little importance +--homologous +--rudiments of, and nascent + +Ornithorhynchus, mammae of + +Ostrich not capable of flight +--habit of laying eggs together +--American, two species of + +Otter, habits of, how acquired + +Ouzel, water + +Owen, Prof., on birds not flying +--on vegetative repetition +--on variability of unusually developed parts +--on the eyes of fishes +--on the swim-bladder of fishes +--on fossil horse of La Plata +--on generalised form +--on relation of ruminants and pachyderms +--on fossil birds of New Zealand +--on succession of types +--on affinities of the dugong +--on homologous organs +--on the metamorphosis of cephalopods + +Pacific Ocean, faunas of + +Pacini, on electric organs + +Paley, on no organ formed to give pain + +Pallas, on the fertility of the domesticated descendants of wild stocks + +Palm with hooks + +Papaver bracteatum + +Paraguay, cattle destroyed by flies + +Parasites + +Partridge, with ball of dirt attached to foot + +Parts greatly developed, variable + +Parus major + +Passiflora + +Peaches in United States + +Pear, grafts of + +Pedicellariae + +Pelargonium, flowers of +--sterility of + +Peloria + +Pelvis of women + +Period, glacial + +Petrels, habits of + +Phasianus, fertility of hybrids + +Pheasant, young, wild + +Pictet, Prof., on groups of species suddenly appearing +--on rate of organic change +--on continuous succession of genera +--on close alliance of fossils in consecutive formations +--on change in latest tertiary forms +--on early transitional links + +Pierce, Mr., on varieties of wolves + +Pigeons with feathered feet and skin between toes +--breeds described, and origin of +--breeds of, how produced +--tumbler, not being able to get out of egg +--reverting to blue colour +--instinct of tumbling +--young of + +Pigs, black, not affected by the paint-root +--modified by want of exercise + +Pistil, rudimentary + +Plants, poisonous, not affecting certain coloured animals +--selection, applied to +--gradual improvement of +--not improved in barbarous countries +--dimorphic +--destroyed by insects +--in midst of range, have to struggle with other plants +--nectar of +--fleshy, on sea-shores +--climbing +--fresh-water, distribution of +--low in scale, widely distributed + +Pleuronectidae, their structure + +Plumage, laws of change in sexes of birds + +Plums in the United States + +Pointer dog, origin of +--habits of + +Poison not affecting certain coloured animals + +Poison, similar effect of, on animals and plants + +Pollen of fir-trees +--transported by various means + +Pollinia, their development + +Polyzoa, their avicularia + +Poole, Col., on striped hemionus + +Potemogeton + +Pouchet, on the colours of flat-fish + +Prestwich, Mr., on English and French eocene formations + +Proctotrupes + +Proteolepas + +Proteus + +Psychology, future progress of + +Pyrgoma, found in the chalk + +Quagga, striped + +Quatrefages, M., on hybrid moths + +Quercus, variability of + +Quince, grafts of + +Rabbits, disposition of young + +Races, domestic, characters of + +Race-horses, Arab +--English + +Radcliffe, Dr., the electrical organs of the torpedo + +Ramond, on plants of Pyrenees + +Ramsay, Prof., on subaerial denudation +--on thickness of the British formations +--on faults + +Ramsay, Mr., on instincts of cuckoo + +Ratio of increase + +Rats, supplanting each other +--acclimatisation of +--blind, in cave + +Rattle-snake + +Reason and instinct + +Recapitulation, general + +Reciprocity of crosses + +Record, geological, imperfect + +Rengger, on flies destroying cattle + +Reproduction, rate of + +Resemblance, protective, of insects +--to parents in mongrels and hybrids + +Reversion, law of inheritance +--in pigeons, to blue colour + +Rhododendron, sterility of + +Richard, Prof., on Aspicarpa + +Richardson, Sir J., on structure of squirrels +--on fishes of the southern hemisphere + +Robinia, grafts of + +Rodents, blind + +Rogers, Prof., Map of N. America + +Rudimentary organs + +Rudiments important for classification + +Rutimeyer, on Indian cattle + +Sageret, on grafts + +Salamandra atra + +Saliva used in nests + +Salmons, males fighting, and hooked jaws of + +Salt-water, how far injurious to seeds +--not destructive to land-shells + +Salter, Mr., on early death of hybrid embryos + +Salvin, Mr., on the beaks of ducks + +Saurophagus sulphuratus + +Schacht, Prof., on Phyllotaxy + +Schiodte, on blind insects +--on flat-fish + +Schlegel, on snakes + +Schobl, Dr., on the ears of mice + +Scott, Mr. J., on the self-sterility of orchids +--on the crossing of varieties of verbascum + +Sea-water, how far injurious to seeds +--not destructive to land-shells + +Sebright, Sir J., on crossed animals + +Sedgwick, Prof., on groups of species suddenly appearing + +Seedlings destroyed by insects + +Seeds, nutriment in +--winged +--means of dissemination +--power of resisting salt-water +--in crops and intestines of birds +--eaten by fish +--in mud +--hooked, on islands + +Selection of domestic products +--principle not of recent origin +--unconscious +--natural +--sexual +--objections to term +--natural, has not induced sterility + +Sexes, relations of + +Sexual characters variable +--selection + +Sheep, Merino, their selection +--two sub-breeds, unintentionally produced +--mountain, varieties of + +Shells, colours of, littoral +--hinges of +--seldom embedded + +Shells, fresh-water, long retain the same forms +--fresh-water, dispersal of +--of Madeira +--land, distribution of +--land, resisting salt water + +Shrew-mouse + +Silene, infertility of crosses + +Silliman, Prof., on blind rat + +Sirenia, their affinities + +Sitaris, metamorphosis of + +Skulls of young mammals + +Slave-making instinct + +Smith, Col. Hamilton, on striped horses + +Smith, Dr., on the Polyzoa + +Smith, Mr. Fred., on slave-making ants +--on neuter ants + +Snake with tooth for cutting through egg-shell + +Somerville, Lord, on selection of sheep + +Sorbus, grafts of + +Sorex + +Spaniel, King Charles' breed + +Specialisation of organs + +Species, polymorphic +--dominant +--common, variable +--in large genera variable +--groups of, suddenly appearing +--beneath Silurian formations +--successively appearing +--changing simultaneously throughout the world + +Spencer, Lord, on increase in size of cattle + +Spencer, Mr. Herbert, on the first steps in differentiation +--on the tendency to an equilibrium in all forces + +Sphex, parasitic + +Spiders, development of + +Sports in plants + +Sprengel, C.C., on crossing +--on ray-florets + +Squalodon + +Squirrels, gradations in structure + +Staffordshire, heath, changes in + +Stag-beetles, fighting + +Star fishes, eyes of +--their pedicellariae + +Sterility from changed conditions of life +--of hybrids +--laws of +--causes of +--from unfavourable conditions +--not induced through natural selection + +St. Helena, productions of + +St. Hilaire, Aug., on variability of certain plants +--on classification + +St. John, Mr., on habits of cats + +Sting of bee + +Stocks, aboriginal, of domestic animals + +Strata, thickness of, in Britain + +Stripes on horses + +Structure, degrees of utility of + +Struggle for existence + +Succession, geological +--of types in same areas + +Swallow, one species supplanting another + +Swaysland, Mr., on earth adhering to the feet of migratory birds + +Swifts, nests of + +Swim-bladder + +Switzerland, lake inhabitants of + +System, natural + +Tail of giraffe +--of aquatic animals +--prehensile +--rudimentary + +Tanais, dimorphic + +Tarsi deficient + +Tausch, Dr., on umbelliferae + +Teeth and hair correlated +--rudimentary, in embryonic calf + +Tegetmeier, Mr., on cells of bees + +Temminck, on distribution aiding classification + +Tendrils, their development + +Thompson, Sir W., on the age of the habitable world +--on the consolidation of the crust of the earth + +Thouin, on grafts + +Thrush, aquatic species of +--mocking, of the Galapagos +--young of, spotted +--nest of + +Thuret, M., on crossed fuci + +Thwaites, Mr., on acclimatisation + +Thylacinus + +Tierra del Fuego, dogs of +--plants of + +Timber-drift + +Time, lapse of +--by itself not causing modification + +Titmouse + +Toads on islands + +Tobacco, crossed varieties of + +Tomes, Mr., on the distribution of bats + +Transitions in varieties rare + +Traquair, Dr., on flat-fish + +Trautschold, on intermediate varieties + +Trees on islands belong to peculiar orders +--with separated sexes + +Trifolium pratense +--incarnatum + +Trigonia + +Trilobites +--sudden extinction of + +Trimen, Mr., on imitating-insects + +Trimorphism in plants + +Troglodytes + +Tuco-tuco, blind + +Tumbler pigeons, habits of, hereditary +--young of + +Turkey-cock, tuft of hair on breast + +Turkey, naked skin on head +--young of, instinctively wild + +Turnip and cabbage, analogous variations of + +Type, unity of + +Types, succession of, in same areas + +Typotherium + +Udders enlarged by use +--rudimentary + +Ulex, young leaves of + +Umbelliferae, flowers and seeds of +--outer and inner florets of + +Unity of type + +Uria lacrymans + +Use, effects of +--under domestication +--in a state of nature + +Utility, how far important in the construction of each part + +Valenciennes, on fresh-water fish + +Variability of mongrels and hybrids + +Variation, under domestication +--caused by reproductive system being affected by conditions of life +--under nature +--laws of +--correlated + +Variations appear at corresponding ages +--analogous in distinct species + +Varieties, natural +--struggle between +--domestic, extinction of +--transitional, rarity of + +Varieties, when crossed +--fertile +--sterile +--classification of + +Verbascum, sterility of +--varieties of, crossed + +Verlot, M., on double stocks + +Verneuil, M. de, on the succession of species + +Vibracula of the Polyzoa + +Viola, small imperfect flowers of +--tricolor + +Virchow, on the structure of the crystalline lens + +Virginia, pigs of + +Volcanic islands, denudation of + +Vulture, naked skin on head + +Wading-birds + +Wagner, Dr., on Cecidomyia + +Wagner, Moritz, on the importance of isolation + +Wallace, Mr., on origin of species +--on the limit of variation under domestication +--on dimorphic lepidoptera +--on races in the Malay Archipelago +--on the improvement of the eye +--on the walking-stick insect +--on laws of geographical distribution +--on the Malay Archipelago +--on mimetic animals + +Walsh, Mr. B.D., on phytophagic forms +--on equal variability + +Water, fresh, productions of + +Water-hen + +Waterhouse, Mr., on Australian marsupials +--on greatly developed parts being variable +--on the cells of bees +--on general affinities + +Water-ouzel + +Watson, Mr. H.C., on range of varieties of British plants +--on acclimatisation +--on flora of Azores +--on rarity of intermediate varieties +--on Alpine plants +--on convergence +--on the indefinite multiplication of species + +Weale, Mr., on locusts transporting seeds + +Web of feet in water-birds + +Weismann, Prof., on the causes of variability +--on rudimentary organs + +West Indian islands, mammals of + +Westwood, on species in large genera being closely allied to others +--on the tarsi of Engidae +--on the antennae of hymenopterous insects + +Whales + +Wheat, varieties of + +White Mountains, flora of + +Whittaker, Mr., on lines of escarpment + +Wichura, Max, on hybrids + +Wings, reduction of size +--of insects homologous with branchiae +--rudimentary, in insects + +Wolf crossed with dog +--of Falkland Isles + +Wollaston, Mr., on varieties of insects +--on fossil varieties of shells in Madeira + +Wollaston, Mr., on colours of insects on sea-shore +--on wingless beetles +--on rarity of intermediate varieties +--on insular insects +--on land-shells of Madeira naturalised + +Wolves, varieties of + +Woodcock with earth attached to leg + +Woodpecker, habits of +--green colour of + +Woodward, Mr., on the duration of specific forms +--on Pyrgoma +--on the continuous succession of genera +--on the succession of types + +World, species changing simultaneously throughout + +Wright, Mr. Chauncey, on the giraffe +--on abrupt modifications + +Wrens, nest of + +Wyman, Prof., on correlation of colour and effects of poison +--on the cells of the bee + +Youatt, Mr., on selection +--on sub-breeds of sheep +--on rudimentary horns in young cattle + +Zanthoxylon + +Zebra, stripes on + +Zeuglodons + + + + + +End of Project Gutenberg Etext Origin of Species, 6th Edition, by Darwin + diff --git a/old/otoos610.zip b/old/otoos610.zip Binary files differnew file mode 100644 index 0000000..c79aaef --- /dev/null +++ b/old/otoos610.zip |