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+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
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