Vickipedia

DARWINIAN THEORY. Before attempting- to discuss the theory of Evolution of Plants and Animals by Natural Selection, as promulgated by Darwin (q. v.), it is necessary briefly to consider first, the scope and aim of biological science; and secondly, the influence exerted upon biology by the progress of other departments of knowledge.

1. Nature of Biology.—The primary labors of the botanist and zoologist are, of course, to collect and preserve, to describe and figure the innumerable and varied forms occurring in nature; and in this task, therefore, naturalists have been occupied since the earliest times. The increase of such knowledge necessitated the attempt at orderly arrangement and intelligible cataloguing, problems solved by Linnaeus, whose Systema Naturæ first satisfactorily organized the natural history sciences.

The detailed study of internal structure, as well as of external form, commenced by Hunter and Haller, was enormously extended by Cuvier (q. v.), whose labors resulted in the conception that the multitudinous forms of animal life were all organized upon a few distinct plans, of which he defined the vertebrate, molluscan, articulate, and radiate; while Geoffrey St. Hilaire and Goethe were principally instrumental in introducing the idea of homology (seeMETAMORPHOSIS).But it is not sufficient to analyze the organism into its constituent organs, and to describe and compare these; we must inquire into their minute structure. These organs were analyzed into tissues by the Bichât, and these again into their component protoplasmic units—cells—by Schleiden and Schwann, and thus anatomy acquired the subordinate province of Histology. Finally, the mode of origin of the adult organism from the germ or egg comes to be investigated, and after thus adding to our previous knowledge that of Embryology, we are in a position to complete our summary of the structural aspects of an organism by defining its relation to its fellows—in other words, by fixing its position in the natural system of classification. These subjects of anatomy, histology, embryology, and Taxonomy or classification constitute the science of Morphology.

But an organism has yet other aspects, functional as well as structural, dynamical as well as statical; its organs have activities, and for the study of these, a new department of biology must be constituted—Physiology, which (although, by reason of the urgent needs of the practitioner and the student of medicine, as yet mainly concentrated upon the study of the functions of the human body) has a field co-extensive with that of morphology.

To the consideration of the forms and the activities of organisms, a new line of inquiry has been much more recently added, that referring to the position in time and space in which the organism occurs and the answer to this comes under a new head, that of Distribution, chronological (geological) or geographical, as the case may be.

These three great divisions of biological knowledge, morphological, physiological, and distributional, being constituted, the questions what, how, and where being approximately answered (and since the search for final causes—for the why—is outside the field of science), only one more possible inquiry remains—namely, whence these organisms, with their particular structures, functions, and positions in space and time ? In other words, how did all these phenomena arise—what is their origin or Ætiology ?

The necessity for a theory of the origin of plants and animals thus coming to be felt, only two hypotheses present themselves, since the suggestion that they may have existed in their present state from infinite time, is not only incapable of support by positive evidence, but absolutely negatived by geology. The first and historically earlier hypothesis is that of Special Creation, which assumes the sudden origin of the existing species, without reference to previously existing species, by the intervention of supernatural causes; the second is that of Evolution, and assumes the gradual origin of the existing species from pre-existing species by ordinary descent, with modification by the action of natural causes. A little reflection will show (1) that the idea of cause, although presented in different forms by the two rival hypotheses, and at different degrees of remoteness, is not excluded by one more than the other; and (2) that just as the hypothesis of the origin of solar and stellar systems from nebulæ is considered on its own merits, without confusion with any hypotheses which may subsequently arise as to the origin of the nebulas themselves, so we must separate the inquiry as to the origin of species, with which the Darwinian theory is alone concerned, from all subsequent hypotheses as to the origin or the nature of life (seeLIFE; GENERATION, SPONTANEOUS),thus keeping clear of the misunderstandings and misrepresentations with which the subject has too frequently been encumbered.

First in order, therefore, the arguments for and against the theory of the origin of species by special creation demands our examination, of course on scientific grounds alone. From the nature of the case no positive arguments are, or can be presented; and consequently, from the naturalist’s point of view, it is urged that not only is no evidence forthcoming, but that the hypothesis fails to explain the existing facts, much less to act as an instrument of research; while on philosophical grounds, it is objected that besides being in no respect a scientific hypothesis, but one which necessarily excludes all scientific hypotheses, it stands discredited a priori as the last survivor of a series of once universally diffused pre-scientific beliefs in the irregular and arbitrary occurrence of phenomena, and so is destitute of support from analogy; that it neither satisfies the intellectual wants, nor meets the moral difficulties of the explanation of nature; and worst of all, that it is a purely verbal hypothesis, incapable of any definite representation in thought—in short, inconceivable.

Passing to the second theory, we find it strongly urged in the first place, that not only is much evidence forthcoming, but that it does plausibly explain the known facts, and is even serviceable in the search for new ones; that it belongs to that class of explanations in terms of the natural order of things which have now superseded the system of catastrophic and supernatural hypotheses in every other field of knowledge; that it is capable of clear representation in thought; and that it satisfies not merely the intellectual wants, but meets the moral difficulties. For a full development of this most general form of the discussion, the reader may consult Spencer’s Principles of Biology, vol. i.

1. Influence on Biology of Progress in other Sciences.—The enormous progress of every department of knowledge during the past few generations has not lain merely, as is too commonly supposed, in ever-increasing minuteness of specialization upon ever-multiplying details, but has rather consisted in the concentration of innumerable previously unrelated phenomena into few groups, and of these again into fewer; through the construction of far-reaching hypotheses, which, if surviving and satisfying scrutiny and criticism, observation and experiment, have passed through stages of .possibility and likelihood, to that of overwhelming or practically infinite (more rarely absolute) probability, and are then termed generalizations, or more figuratively, laws. A rich harvest of such general conceptions has been garnered by astronomy, and such successive labors as those of Copernicus, Kepler, and Newton, in widening our knowledge of the universe, have widened not a little, the theoretic range and grasp of the scientific intellect. And it is important to bear in mind, first, that each of these advances consisted, as every such advance must do, in the substitution of a verified scientific hypothesis for a provisional, though “•hue-honored explanation, in terms of the mysterious and supernatural; and, secondly, that a theory of the evolution of solar and stellar systems (see NEBULAR HYPOTHESIS)is largely maintained by modern astronomers.

In chemistry, such conceptions as those of molecular constitution, and of the indestructibility of matter, of the similarity in composition of our planet with sun and stars, and of the intimate relation between inorganic and organic compounds, are highly instructive; while the actually observed genesis of many species of minerals by the action of natural causes, and the frequent transmutation of one species into another, when some definite change takes place in the surrounding conditions, are not without interest. Moreover, to the theory of the conservation of energy (seeFORCE)unifying as it has done, not only all the physical sciences, but these with physiology, a far vaster influence upon biology is due.

But the most important of all influences whatever on the organic sciences has come from geology. The discovery that our earth dates from an almost incalculably remote antiquity, together with the establishment as the fundamental axiom of the science that the present is the key to the past, and consequently that the present phenomena of the earth’s crust do not result from catastrophe .and deluge, still less from special creation, but are the product of a slow and progressive evolution (by natural causes still in operation) from a widely different previously existing state of things, furnish the evolutionist with the most primary of his data. To the establishment of this new theory of geologic evolution, revolutionary yet uniformitarian, of which the theory of organic evolution is but the complement and corollary, it is interesting to note that after Hutton and Lyell, perhaps no move important service has been rendered by any geological works than by the series (Geological Observations, 1844; Coral Beefs, 1842; Earthworms, 1881; and the essay on the Imperfection of the Geological Record, summarized below), which we owe to Darwin.

Nor is it only the preliminary sciences which have influenced biologists, and have aided them in their inquiries as to the origin of their set of phenomena. The human and social sciences— psychology and philology, anthropology and history, have all contributed their ætiological example and results, so that it might almost be debated whether the biological evolutionist has not been more indebted to all the other sciences for his theory, than they to him for theirs.

Origin of the Idea of Evolution in Biology.—No doubt largely influenced by such as existed of the scientific conceptions outlined above, as well as by the Cartesian doctrine, that the universe is a mechanism, and is therefore to be explained on mechanical principles, the evolutionary hypothesis made its first distinct appearance in the work of De Maillet (Telliamed, written 1735, published 1758), and was expounded in more or less varying form by more than thirty writers before Darwin, among whom the most notable were Erasmus Darwin, Goethe, Lamarck, and Geoffroy St. Hilaire.

Their hypotheses, although based on masses of biological evidence drawn from homologies and rudimentary organs, from classification and development, from geological and geographical distribution, and so on. never succeeded in gaining general acceptance among naturalists—a failure largely attributable to established prejudice, aided as it \vas by the authority of Cuvier. Yet, while rendering it extremely probable that modification had occurred, they all came short, as Darwin has pointed out, in one most important particular, that of showing how the modification of one species from another could take place, ‘ so as to acquire that perfection of structure and co-adaptation which justly excites our admiration;’ since the hypotheses of the potency of external conditions, of habit, or of the volition of the organism itself, alike successively broke down.

Darwin, in his turn, struck especially by the distributional phenomena he witnessed during his ‘ Naturalists’ Voyage,’ devoted himself to the solution of the problem of the origin of species, specially concentrating himself upon this weakest point of the preceding theories. After twenty-one years’ continuous work, he was compelled, on receiving a paper by Mr. A. R. Wallace (then exploring the Malay Archipelago), in which views identical with his own were expressed, to proceed to the publication of his results, first in brief outline (Journ. Linn. Soc., 1858), and the following year in that fuller abstract, ‘ The Origin of Species by means of Natural Selection,’ which may now be briefly summarised, in so far as further compression of such ‘ intellectual pemmican ‘ is possible. For details and explanations, the reader must consult the original work (sixth edition, 1875).

Outline of the ‘ Origin of Species.’—In order to gain insight, then, into the means of modification, Darwin commences with a study of the variation of plants and animals under domestication (later expanded into a separate work; second edition. 1876).

Variation and Heredity.—While all plants and animals exhibit some degree of variation, this is greatest among domesticated species, owing to their new and less uniform conditions of life. These may act directly on the whole organization, or on separate parts, and the variation, though rarely, is sometimes definite, as when size increases with quantity of food, or color changes with its quality; or the conditions may act indirectly by influencing the reproductive system, which is peculiarly sensitive. Changed habits produce an inherited effect, e. g., the leg-bones of the common duck weigh proportionally more, and its wing-bones less, than in the wild variety, because it flies less and walks more. So, too, tame mammals acquire drooping ears, since these are rarely pricked in alarm. One variation is usually correlated with others, thus long-beaked pigeons have small feet, and conversely. All variations tend to be inherited. The popular belief that domestic races revert to the aboriginal stock is unsupported by facts.

Save that domestic varieties are less uniform than wild species, often differ more widely in some single part, and are fertile when crossed, there is no well-marked distinction between these and so-called true species. If, therefore, such varieties as of the dog can be shown to be descended from a single wild species, there necessarily arises great doubt as to immutability of closely allied natural species, such as the foxes. While the many breeds of clog appear to have arisen from several wild species, and those of cattle also from two or three, fowls, ducks, rabbits, &c., all certainly arise from a single ancestral species. The case of pigeons is of peculiar importance, since pouter, carrier, fan tail, and tumbler differ so thoroughly, externally and internally, that any ornithologist would be compelled to assign to them, not merely specific but generic distinctness, if he had discovered them in the wild state. There is at least as much difficulty in believing that such breeds can have proceeded from a common ancestor, as in the case of any group of birds in nature; and every breeder of these and other domestic animals has been firmly convinced of their descent from distinct species.

Yet these are proven to arise from the common rock-dove (Columba lima) (see COLUMBIDÆ;),and thus those who admit the unity of domestic races should be cautious in deriding the unity of wild ones.

Domestic races all exhibit adaptations to man’s use or fancy, rather than their own good. The key to this is man’s power of selection; nature gives successive variations, man accumulates them, so making for himself useful breeds, and often (e.g., sheep, cattle, roses, dahlias) profoundly modifying their character even in a single lifetime; so that in all characters to which he attends, they may differ more than the distinct species of the same genera. Again more even than conscious, that unconscious selection which results from every one trying to possess and breed the best animals, is important. Two flocks of Leicester sheep, equally kept pure, appeared of quite different varieties after fifty years. Such slowly accumulated change explains why we know so little of the origin of domestic races; and its absence in regions inhabited by Uncivilized man, explains why these yield no plants worth immediate culture. Human selection is facilitated by the keeping of large numbers, since variations will be more frequent, and by preventing crosses; some species vary, however, more than ethers.

Variation under Nature.—All like organisms in nature present individual differences, more considerable than is usually supposed; no two blades of grass are alike, and far more marked differences often occur, several castes or varieties sometimes existing in the same sex. Between these castes, and much more frequently between forms which systematic botanists and zoologists rank as true species, perfectly intermediate forms may occur. No agreement about the definition of species (the amount of difference necessary to give any two forms specific rank), has ever been come to; thus, in the British flora alone, there are nearly two hundred disputed forms, and individual opinion is in these cases the only criterion. As long as a genus is imperfectly known, and its species founded upon few specimens, they appear clearly limited. But with better knowledge, intermediate forms flow in, and doubts as to specific limits augment. The terms species and variety are thus arbitrarily given to sets of individuals more or less closely resembling each other. SeeVARIETY, SPECIES, GENUS.

Individual differences are thus of the highest importance, as the first steps towards the slightest varieties worth recording, these towards more distinct and permanent varieties, and these again towards sub-species, and these to species; though extinction may often stop the progress.

The species which present most varieties are those which have the greatest geographical range, or the widest diffusion in their own territory, or possess the greater number of individuals; and in the larger genera of each country the species vary more frequently than in the smaller genera; and in many respects the species of large genera present a strong analogy with varieties, which analogy is alone intelligible on the view that they once existed as such.

Struggle for Existence.—All organic beings tend to increase with extreme rapidity, so that if not destroyed, the earth would soon be covered by the progeny of a single pair. This is evidenced not merely by calculation, but by actual observation of the extraordinary rapidity with which plants and animals have spread, when introduced into new and favorable circumstances.

Since organisms then are reproducing themselves so rapidly, and since all their offspring cannot escape their enemies, get food and live, much less leave progeny in turn—since, in other words, the doctrine of Malthus applies to animals and plants with manifold force (for these can have no artificial increase of food, and no prudential restraints on marriage)—there must in every case be a struggle for existence, either of one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life; often, indeed, with all these at once, and that more or less intensely throughout the whole of life.

The checks to increase are most obscure, and vary in each case. In all cases the amount of food, of course, gives the extreme limit. The youngest organisms generally suffer most; seedlings, for instance, are destroyed in vast numbers, thus, even in a patch of ground purposely dug and cleared, where no choking from other plants could take place, 295 out of 357 seedling-weeds were destroyed, chiefly by slugs and insects. So, too, the stock of game on an estate depends chiefly upon the destruction of vermin. Climate, however, is highly important, and periodic seasons of extreme cold and drought seem the most effective of all checks—a severe winter sometimes destroying four-fifths or more of the birds of a locality. Epidemics, too, may occur, especially where numbers have inordinately increased. On the other hand, a large stock of individuals of the same species is essential for its preservation.

The complex relations of all animals and plants to each other require illustration. The plantation of part of a heath with Scotch fir leads to the profound alteration of its flora and fauna, while the growth of these firs again is wholly dependent upon the exclusion of cattle. Many flowers depend for fertilization on the visit of a special insect, e.g., red clover on humble-bees. But bees are destroyed by field-mice, and consequently protected by cats; hence, not only no bees, no clover, but also the more cats, the more clover! The struggle for life is most severe between individuals and varieties of the same species, and between the species of the same genus, since these tend to fill the same place in the economy of nature; hence we see the brown rat supplanting the black, and the hive-bee supplanting its Australian congener. The structure of every being is related to that of the others with which it competes, or from which it seeks to escape, or on which it preys; as is alike evident in the structure of the tiger, and of the parasite which clings to his hair. So, too. the albumen of a seed is chiefly useful in favoring the young plant’s struggle for light and air against the adult plants around.

Natural Selection.—But how will the struggle for existence act with regard to variation? Can the principle of selection, so potent in the hands of man, apply under nature? Most efficiently so. Let us bear in mind (1) the constant occurrence of variation; (2) the infinite complexity of the relations in which organisms stand to each other, and to the physical conditions of life; and consequently (3) what infinitely varied diversities of structure might be useful 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 also occur in the course of many generations? And 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, would have the best chance of surviving and procreating their kind, while injurious variations would be destroyed ? This preservation of favorable variations, and destruction of injurious ones, is termed Natural Selection, or less figuratively, the Survival of the Fittest.

Taking the case of a country undergoing a change of climate, the proportional number of its inhabitants would change, some species probably also becoming extinct—and these changes would in many ways affect the survivors. A further disturbance would come from the immigration of new forms; or if that were prevented, we should have places in the economy of nature which might be better filled up. Any slight favorable modification of the old species would tend to be preserved, and we have seen that changed conditions increase variability.

Nor are such changes, often though they have occurred, necessary in order to leave places for natural selection to fill by improving some of the varying forms. No country can be named where the native inhabitants are perfectly adapted to their conditions and competitors, for as some foreigners have taken firm possession in every country, we may safely conclude that the natives might have been modified with advantage to resist them.

And when human selection has produced such great results, why may not natural? The former act» only for man’s own good, on mere external and visible characters, and irregularly throughout a short period; the latter acts for the good of the being itself, on the whole machinery of its life, and incessantly throughout almost infinite time. (It is important here to remember that the objection to this agency on the ground of its presumed insignificance, is identical with that so long and unsuccessfully employed against Lyell’s explanation of the origin of the physical features of the globe by summing up the existing natural changes.)

Natural selection thus 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 organization. Nevertheless, low and simple forms will long endure, if well fitted for their simple conditions.

Natural selection may modify the egg, seed, or young, as easily as the adult, and these modifications may effect through correlation the structure of the latter, and conversely.

Besides Natural, we have to consider Sexual Selection, i.e., not merely do individuals struggle for existence, but the males struggle for the females, and the most vigorous thus tend to leave most progeny. Special weapons, offensive and defensive, like the cock’s spurs, the stag’s horns, or the lion’s mane, are used in this struggle, and the most useful variations are thus those which are transmitted. Again, just as man can in a short time give beauty to his domestic birds, so there is no good reason to doubt that female birds in thousands of generations, by selecting, as they are observed to do, the most melodious or beautiful males might produce a marked effect, and many sexual differences are thus explained.

The theory of natural selection may be applied in special cases, e. g. (1) to explain the evolution of swift greyhound-like varieties of wolves; (2) the origin and excretion of nectar in flowers, its use to insects, and their action in transferring pollen from flower to flower, and its advantage in intercrossing; and the resultant modification and adaptation of flower and insect to each other by the preservation of advantageous variations.

The circumstances favorable to the production of new forms through natural selection are also reviewed. These are chiefly, great variability; large numbers of individuals; the complex effects of intercrossing; isolation in small areas, yet also extension over continental ones, especially if these oscillate in level; and considerable lapse of time. Rare species are shown to be in process of extinction. The divergence of character in domestic breeds, largely due to the fact that ‘ fanciers do not, and will not, admire a medium standard, but like extremes,’ applies throughout nature, from the 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 nature, and so to increase In numbers. Thus, taking a carnivorous animal, which has reached the average numbers which its territory will support, it is

•evident that it can succeed in increasing only by its varying descendants seizing places hitherto occupied by other animals, thus changing their food or habitat. This must hold equally of all species, and is separately demonstrated for plants. The greatest amount of life can be supported by great diversification of structure; hence in small areas where competition is severe, the inhabitants are extremely varied.

The probable effects of the action of Natural Selection, through divergence of character and extinction, on the descendants of a common ancestor are then discussed in detail with an illustrative diagram. This takes the form of a genealogical tree—’ 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.’

Laws of Variation.—(These can only be very briefly treated.) Of the cause of most variations we are still ignorant, but the same laws appear to have acted in producing the lesser differences between varieties of the same species, arid the greater differences between species of the same genus. Changed conditions sometimes induce definite and permanent effects : habit, use, and disuse are potent in their effects. Specific characters are more variable than generic, and varietal than either. Rudimentary organs and secondary sexual characters are highly variable. Species closely related, of similar constitution and similarly influenced, present analogous variations, and frequently exhibit characters which can only be explained as reversions to those of their ancient progenitors; e.g., zebra-like stripes on horses, or wood-pigeon’s markings on fan tails, tumblers, &c.

Difficulties and Objections.—In four chapters all the miscellaneous objections raised against the theory between 1859, and the appearance of the latest edition, are successfully stated, weighed, discussed, and met, as well as the much more serious difficulties pointed out by Darwin himself. These latter are,

(1) the definiteness of species and the rarity of transitional forms;

(2) the enormous degree of modification in habits and structure which the theory assumes, and the power of Natural Selection to produce on the one hand an organ of such trifling importance as the tail of a giraffe, and on the other, an organ so wonderful as the eye; (3) the acquirement and modification by Natural Selection of such marvelous instincts as those of the bee; (4) the sterility of crossed species, and the fertility of crossed varieties. For these discussions, however, the reader must consult the original work.

Imperfection of the Geological Record.—On the doctrine of the extermination of an enormous number of intermediate varieties, the links between existing and remote ancestral forms—why is not every geological formation charged with such links ? Why does not every collection of fossils afford plain evidence of the gradation and mutation of the forms of life ? Geology, assuredly, does not reveal any such finely graduated organic chain, and this is one of the most obvious and plausible objections to the theory. The explanation lies in the extreme—the almost incredible—imperfection of the geological record. Only a small portion of the globe has been geologically explored with care, only certain classes of beings have been fossilized, and the number, both of specimens and species yet discovered, is absolutely as nothing compared with the number which must have passed away during even a single formation.

The Malay Archipelago is about the size of Europe, and, therefore, equals in area the formations best known to us; its present condition represents that of Europe, while its strata were being deposited; its fauna and flora are among the richest on the globe, yet, even if all the species were to be collected which ever lived there, how imperfectly would they represent the natural history of the world! Only few of these are preserved at all, and most of these in an imperfect manner; moreover, subsidence being almost necessary for the accumulation of rich deposits, great intervals of time must have elapsed between successive formations, so that, during periods of elevation, when variation would be most frequent, the record is least perfect. Moreover, single formations have not been continuously deposited; the duration of specific forms probably exceeds that of each formation; migrations have largely taken place; widely ranging species are most variable, and oftenest give rise to new species; varieties have been at first local; and finally, it is probable that periods of modification are short as

compared with periods of permanence. Hence we cannot find interminable varieties, and any linking variety between two forms is, of course, ranked as a distinct species, for the whole chain cannot be permanently restored. Thus the geological record is a history of the world indeed, but one imperfectly kept, and written in a changing dialect; of this history we possess the last volume only, relating 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.

Geological Succession of Organic Beings (Distribution in Time).— The preceding difficulties excepted, the facts of palæontology agree admirably with the theory. New species come in slowly and successively; they change in different rates and degrees; old forms pass through rarity to extinction, and never reappear; dominant forms spread and vary, their descendants displacing the inferior groups, so that after long intervals of time the productions of the world appear to have changed simultaneously. The most ancient forms differ most widely from those now living, yet frequently present characters intermediate between groups now widely divergent, and they resemble to a remarkable extent the embryos of the more recent and more highly specialized animals belonging to the same classes. These laws, and above all, the important law of the succession of the same types within the same areas during the later geological periods, and most notably between the Tertiary period and the present time (e.g., fossil and recent marsupials in Australia, and edentates in South America), cease to be mysterious, and become at once intelligible on the principle of inheritance, and on that alone.

[Since the publication of the Origin of Species (1859), palæontological research has been constantly furnishing the most triumphant verification of these views. The imperfection of the geological record was so far from over-estimated, that Huxley (Science and Culture, 1880), in comparing our present knowledge of the mammalian Tertiary fauna with that of 1859, states that the results of the investigations of Gaudry, Marsh, and Filhol, are ‘ as if zoologists were to become acquainted with a country hitherto unknown, as rich in novel forms of life as Brazil or South Africa once were to Europeans.’

Gaudry found the intermediate stages by which civets passed into hyænas; Filhol disinterred still more remote ancestral carnivores; while Marsh obtained a complete series of forms intermediate between that, in some respects, most anomalous of mammals, the horse, and the simplest five-toed ungulates (seeMAMMALIA).Again, the belief of Darwin that the distinctness of birds from all other vertebrates was to be accounted for by the extinction of a long line of progenitors connecting them with reptiles, was in 1859 a mere assumption; but in 1862, the long-tailed and intensely reptilian bird Archæopteryx (q. v.) was discovered, while in 1875 the researches of Marsh brought to light certain cretaceous birds, one (Hesperornis} with teeth set in a groove, the other (Ichthyornis) with teeth in sockets, and with bi-concave vertebras. Besides these reptilian birds, bird-like reptiles have similarly been forthcoming, and the hypothesis of Darwin is thus admirably verified. Considerable light, too, has been thrown on the pedigree of crocodiles; ammonites, trilobites, and other invertebrates have been arranged in series, while important collateral evidence is also furnished by ‘persistent types’ such as Ceratodus, Beryx, Nautilus, Lingula, &c., which have survived—we must assume by ordinary generation—almost completely unchanged since remote geological periods. On such grounds, therefore, Huxley asserts (op. cit.) that ‘on the evidence of palæontology, the evolution of many existing forms of animal life from their predecessors is no longer an hypothesis, but an historical fact; it is only the nature of the physiological factors which is still open to discussion.’]

Geographical Distribution.—Neither the similarity nor the dissimilarity of the inhabitants of various regions, whether of land or sea, can be accounted for by differences in climate, or other physical conditions, but are related, in the most striking degree, to the absence or presence of barriers to migration between those regions. Within the same area there exists the most marked affinity among the species, though these differ from point to point. Species appear to have arisen in separate definite centers, the few apparent exceptions being accounted for by migration and dispersal, followed by climatal and geographical changes. But for a summary of our knowledge of the existing mode of distribution of organic life, and of the way in which that distribution has been effected, as well as of the very important bearing of these facts upon the theory of evolution, which they may be said indeed, more than any other class of facts, to have suggested, se^ the articleGEOGRAPHICAL DISTRIBUTION.

Morphological Arguments.—The physiological and distributional lines of argument being summarized, those furnished by morphology, although not less numerous and highly important, can only be very briefly outlined. These are mainly four, and are derived from (a) Classification, (5) Homologies, (c) Embryology, (d) Rudimentary Organs. (a) Classification,—Naturalists arrange the species, genera, and families in each class, on what is called the Natural System. But what is meant by this system ? Is it, after all, merely an artificial scheme for enunciating general propositions, and of placing together the forms most like each other—or does it, as many believe, reveal the plan of creation ? The grand fact of classification is, that organic beings, throughout all time, are arranged in groups subordinated under other groups, individuals under varieties, and these again under species; species under genera; those under sub-families, families, and orders; and all under a few fraud classes. The nature of all these relationships—the rules followed and the difficulties met by naturalists in their classifications—the high value set upon constant and prevalent structures, whether these be of great or little use, or, as with rudimentary organs, of none at all; the wide opposition in value between such misleading resemblances of adaptation, as for instance the fish-like form of whales, and such characters of true affinity as are afforded by the structure of their circulatory or respiratory system—all these receive a simple and natural explanation on the view of the common descent of allied forms with modification through variation and natural selection; while it is to be noted that no other explanation has ever even been attempted. The element of descent, too, is already used in linking all the sexes, ages, forms, and varieties of the same species, widely though these (e. g., Cirripedes, &c.) may differ from each other in structure : and we have only to extend it to understand the meaning and origin of the Natural System.

(5) Homology.—The members of the same class, independently of their habits of life, resemble each other in their general plan of organization; thus, the hand of man, the digging-paw of the mole, the leg of the horse, the paddle of the porpoise, and the wing of the bat, are all constructed on the same pattern, bone corresponding to bone, and similarly with the hind limb. Again, the mouths of insects are of innumerable varieties of form and use —witness the long spiral trunk of a moth, and the great jaws of a beetle—yet these are formed by modifications of an upper lip, mandibles, and two pairs of maxillæ. And so it is with the limbs of crustaceans, or the flowers of plants; in fact, with the organs of every class of beings.

This conformity to type is ‘ powerfully suggestive of true relation ship, of inheritance from a common ancestor;’ it admits, in short, as no one indeed denies, of a simple explanation in terms of the evolutionary theory, and thus strengthens that theory not a little. It has been attempted to explain this unity of plan in two other ways—first, by assuming it due to utility, which is negatived by the facts, since organs of identical use (e. g., the wings of a bird and those of a butterfly) very frequently do not conform to the same type at all; secondly, by attributing it to a unity of design, which, however, (a) instead of being always maintained, as it should be, on the theory, is not unfrequently quite lost in highly specialized forms; and which, even if it always existed, (&) would directly suggest the unity of descent, the design thus serving only to mislead “the anatomist.

Serial Homology, too, has to be accounted for—that unity of type which is found on comparing the different parts and organs in the same individual, so that the wonderfully complex and varied jaws and legs of a lobster; or the widely different leaves—sepals, petals, stamens, and pistils of a flower, are all found to be modifications of a simple limb, and a simple leaf-organ respectively. Not only are such metamorphoses apparent on comparison, but they can be actually observed to occur during the development of each individual; is then the term metamorphosis to have a more metaphorical meaning when applied to the species, or has it not actually arisen in past time, through the natural selection and transmission of advantageous variations ?

(c) Development.—It has been already indicated that the serially homologous parts in the same individual are alike during an early embryonic period, as also are the homologous organs in animals which, like bat, horse, and porpoise, may be widely differentiated in adult life. So closely, too, do the embryos of the most distinct species belonging to the same class resemble each other, that even Von Baer was unable to distinguish whether two unlabelled specimens were lizards, birds, or mammals. This law of embryonic resemblance holds very widely, e.g., young crustaceans. The embryo often retains within the egg or womb, structures which are of no service to it, either at that or at a later period of life, like the transitory gill-arches of birds or mammals; while on the other hand, larvas which, like those of insects, have to provide for their own wants, undergo complete secondary adaptation to the surrounding conditions. The process of development goes from the general to the special, thus there is generally an advance in organization. In peculiar conditions, however, degeneration may occur. All these facts are readily explained on the principle of successive slight variations not necessarily or generally supervening very early in life, and being inherited at a corresponding period; and it is thus in the highest degree probable that most embryonic stages show us more or less completely the progenitor

of the group in its adult state; and embryology thus rises greatly in interest (see DEVELOPMENT OF THE EMBRYO).

(d) Rudimentary Organs.—Rudimentary, atrophied, and aborted organs, bearing the plain stamp of inutility, are so extremely common that it is impossible to name a higher animal in which none occurs. The mammas of male mammals, the hind-legs of boas, the wings of many birds, or the teeth of fœtal whales, and the upper incisors of unborn calves, are familiar instances. Such organs are intelligible on the evolutionary theory, and on that theory alone.

Recapitulation and Conclusion.—After tersely summing up the preceding mass of evidence, Darwin concludes by pointing out (a) that the theory of evolution by natural selection is no more inimical to religion than that of gravitation, to which the same objection was strongly raised; (5) its revolutionary influence on the study of all departments of natural history; (c) on psychology (q. v.); (d) on the origin of man and his history (seeDESCENT OF MAN);(e} on our theories of future progress.

Envoy.—‘ 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 on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance, which is almost implied by reproduction; Variability from the indirect and direct action of the 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 cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.’