VARIATION AND SELECTION. is not proposed in the present article to trace the successive steps by which the general doctrine of the origin of species by descent with modification has come to gain acceptance among naturalists (see EVOLUTION). The present problem is concerned solely with the deter-minant factors of evolution, with searching out the mechanism of the evolutionary process, and of discerning if possible such order as may lie under the apparent flux of change. Yet a brief retrospect of the essential views of successive evolutionists is still necessary, but only in so far as these have been concerned, not simply with the empirical evidence for descent with modification, but with the deeper attempt to explain the rationale of this process, to show how it is that modifications come to arise, persist, and increase at all. In other words, just as there are these two essential aspects of the doctrine of evolution, which must on pain of immediate confusion be clearly kept apart, so it is necessary to distinguish between the corre-sponding sides of its historical development, and, passing beyond the simple hypothesis and evidences of descent, consider the attempts to explain the actual process of modification.
Theories of Variation. If we pass over the speculations of the earliest evolu-tionistsDe Maillet, Maupertuis, Robinet, Bonnet, &c. as too vague for rapid summary, and note that Linnaeus was not quite a consistent creationist, since he admitted that many species may be simply fertile hybrids, the his-tory of definite speculation as to the factors of variation Butfou. may be fairly said to begin with Buffon, whose distinct though covert suggestion of the doctrine of descent with modification was supported by emphatic insistence upon the importance of external conditions. He endeavoured to show how "natures, instincts, and most inward quali-ties" are modified through bodily habits, how new functions seem to arise in response to new conditions, and how changes in climate, food, and other conditions of life bring about direct modifications in organisms exposed to them. Xttot, Kant, on the other hand, viewed the evolution of species as related essentially to the mechanical laws of the organ-ism itself, although in his latest writings he allowed for the influence of geographical distribution, food, &c, noted the importance of selection in artificial breeding, and even hinted at the notion of struggle for existence, which was soon afterwards more clearly emphasized by Herder (Ideen Herder. z. Phil. d. Geschichte, 1790) in a striking passage, which may be briefly quoted in free translation: " All is in struggle, each one for himself. . . . Space is too small for the number. . . . Each genus looks after itself as if it were the only one. ... In this way the whole was pre-served. . . . Thus were forces cradled, and limbs counted, and tendencies determined, and the earth came to. bear what it could."
Erasmus Darwin (Zoonomia, 1794) believed that the Erasmus organism has the faculty of improving by its own inherent Darwin, activity, that it has the power of attaining new parts attended with new propensities. Yet the strongest and most active animals are those which propagate and hand on improvements. Transformations, too, may be produced in part by the exertions of the organism in consequence of its desires for food, security, and reproduction. Changed conditions, such as climate, have an indirect influence in changing desires, and thus actions, and so finally struc-ture ; and they may also operate directly. Treviranus Trevir-(1802-1831) assumed an indefinite variability of the organ- auus. ism, with considerable power of adaptation to surroundings, and even anticipated much recent speculation in his sug-gestion of a possible factor of modification in the union of sexual elements. The well-known theory of Lamarck Lamarck (1801-1809) laid special emphasis on function and en-vironment ; for, though the sense of need in association with suitable environment calls out a succession of efforts, and so originates incipient structural modifications, it is to increased functioning that the increase of these modifi-cations must be ascribed, while similarly disuse explains degeneration. Changed conditions produce new wants, nutritive and reproductive; hence changes in climate, or the like, change the organism by changing its habits. Rapid increase is checked by other organisms: the strongest and best-armed for attack devour the weaker, and the less perfect genera are kept down by the more perfect. The less definite view of Goethe included, besides recognition of the conservative or centripetal force of heredity, that of a progressive or centrifugal tendency to adaptation to environment. Oken (1809) similarly regarded all progress from his primeval " Urschleim" as having been in terms of its interaction with the external conditions of life. In 1813 Wells made his now well-known suggestion of the importance of natural selection in determining the varieties of the human species; and in 1831 Patrick Matthew published his much more developed, yet equally disregarded, statement of the same doctrine in its more general applications. In 1828-30 Geoffroy St-Hilaire, afterwards ably succeeded by his son Isidore, denied indefinite variation, regarding function as of secondary importance, and laying special stress on the direct influence of the environment: for instance, it was not so much the effort to fly as the (supposed) diminished proportion of carbonic acid in the atmosphere which determined the evolution of birds from saurians. The veteran geographer Yon Buch naturally inclined to emphasize the influence of geographical isolation (locality, climate, soil, food, &c), and laid stress on the restriction of the area of possible sexual union as bearing upon the origin of varieties. The embryologist Von Baer (1834) dwelt especially upon the organismal nature of variation, on the unfolding, as it were, of new structures; and Schleiden (1838) and other naturalists more or less distinctly advanced similar opinions. In 1844 appeared the Vestiges of Creation, which in its later editions (1853) formulated an hypothesis of progress
(1) by rhythmic impulse through grades of organization,
(2) by another impulse tending to modify organic structures in accordance with external circumstances. In 1852 Naudin argued for the formation of new species in nature in a similar way to that of varieties under cultivation, further attaching great importance to an assumed " principle of finality," apparently a kind of organismal fate. Herbert Spencer, whose weighty arguments for evolution date from 1852, laid special stress upon the modifying influences of environment, this involving changes of function, and so ultimately of structure. Finally, in 1853 Yictor Carus argued for a progressive adaptation to changing external conditions. An undefined hypothesis of internal modifiability appears also to have floated before the mind of Owen.
In this succession of variational theories we recognize the repeated general insistence upon every separate factor in the problem,organism, function, and environment, the successive authors, in fact, almost falling into as many schools. Yet these are singularly unprogressive : the same hypothesis is stated again and again, but always fails to carry conviction; many employ the merest abstractional explanations, in terms of what is simply the old pre-physiological " vital force" (see BIOLOGY), or attempt to cut the knot by the more or less open introduction of a deus ex machina. Even the more definite and scientific theories of Erasmus Darwin, Lamarck, Geoffroy, and Spencer fail adequately to establish in a sufficient number of concrete cases the supposed potency of environment or function, much less of internal or volitional moods, in determining extensive changes; moreover, they are constantly confronted by cases in which any one set of changes seems to take place independently of the other. Hence, despite the deep and unanimous conviction of so many biological thinkers as to the origin of species by evolution, the essential problem remained unsolved. For, until some explanation, rational and concrete, detailed yet universal, of the origin of species should be reached, the cautious naturalist could not feel justified in accepting as a scientific certainty a doctrine which rested so largely upon undemonstrable grounds. Any really new attempt at establishing the doctrine of evolution had thus, not simply to reorganize and strengthen the empirical evidence, by marshalling beside the generalizations of morphological unity of type the facts of more strictly historic bearing, distributional, palaeontological, and embryological,but also to meet this deeper theoretic want by showing not only that evolution has taken place but how. Hence the importance and rapid acceptance of the Origin of Species (1859), Charles for which the time was fully ripe. For it is evident from Darwin, the preceding historical outline that the view still frequently promulgated of the exclusive and, as it were, catastrophic importance of this work in the establishment of the doctrine of evolution must be replaced by a more uniformitarian and, so to speak, less creationist view. It should not seriously be supposed that the theory of descent which had been held in one form or another by the leading biological thinkers of that and the two preceding generations, with almost the individual exception of Cuvier, had been decisively abandoned by men of science, or that the public which had purchased ten editions of the Vestiges of Creation in as many years, and which took up the first edition of the Origin of Species on the day of publication, could be quite unprepared for the acceptance of the major thesis of both works. Yet that a lull had taken place is also unquestionable, for the failure of wave after wave of speculative effort had produced a widespread feeling of discouragement, and a tendency to concentration upon more concrete and smaller (for the most part Linnsean or Cuvierean) problems alone, since of these the solution was comparatively sure. The dramatic coincidence between Darwin's and Wallace's conclusions soon helped to gain immediate attention to the new theory; and this was thoroughly developed and retained by the prompt appearance of the first and second editions of the Origin of Species. Without entering upon a review of this classic treatise, much less a retrospect of the controversy it provoked, it is evident that it was upon almost all sides so much stronger than preceding attempts, its central doctrine so lucid and obvious a statement of the everyday facts of life, its mode of presentation so temperate yet so forcible, its self-criticism so frank and unconcealed, that its marvellous success was owing far more to intrinsic merit than favourable circumstances. The required re-statement of the evidence for the historic fact of evolution was at last learnedly and forcibly done, and the salient difficulties in its way, like that of the imperfection of the geological record, admirably met; yet this portion of the work sinks into comparative ineffectiveness beside the long-sought modal explanation of the process proffered in the theory of natural selection, which was soon seen to throw a new and searching light upon the smallest details of structure, function, and distribution,in fact, to afford the instrument of a new and systematic interpretation of organic nature. Of this doctrine, then, a brief account is needed, and this may be best given by following as closely as possible upon the lines of the magnum opus itself, although the full title of thisThe Origin of Species by means of Natural Selection, or the Preservation of Favoured Paces in the Struggle for Lifeis its own best and briefest summary.
THEORY OF NATURAL SELECTION.
After mentioning that his first light upon the origin of Analysis species was derived from his early distributional studies, of Origin Darwin points out that "a naturalist, reflecting on the^^^ mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succes-sion, and such other 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 co-adaptation which justly excites our admiration." Again, " It is therefore of the highest importance to gain a clear insight into the means of modification and co-adaptation. At the commence-ment of my observations it seemed to me probable that a careful study of domesticated animals and cultivated plants would offer the best chance of making out this obscure problem. Nor have I been disappointed : in this and all other perplexing cases I have invariably found that our knowledge, imperfect though it be, of variation under domestication affords the best and safest clue." Hence the first chapter is devoted to an account of those ex-tensive studies of variation under domestication which later formed the subject of a separate work.
Variation. A comparison between individuals of the same variety of cultivated plants or animals shows a greater degree of variation than under between individuals of any one species or variety in nature. This domesti- higher variability of domestic productions is to be ascribed to the cation, less uniform conditions of their upbringing, perhaps in part also to excess of food. Exposure to new conditions must be continued for generations to set up any great variation ; but this, once set up, con-tinues indefinitely. Changed conditions may directly influence the whole organization or certain parts alone, or act indirectly through the reproductive system. With respect to the direct action, the nature of the organization seems more important than that of the conditions. The effect on offspring maybe definite : e.g., size may depend upon the amount of food, colour upon quality of food, thick-ness of skin and hair upon climate, &c. But indefinite variability is a much commoner result of changed conditions, and has probably played a much more important part in the formation of our domestic races. The reproductive system is peculiarly sensitive to very slight external changes. Many plants and animals will not reproduce in domestication, even though individually vigorous ; others, though weak and sickly, breed freely. Hence we need not be surprised at the reproductive system acting irregularly and producing variations. But that variation is not exclusively associated with sexual repro-duction is demonstrated by the case of plants " sporting " through bud - variations. Such cases, moreover, prove the nature of the organism to be of more importance than the conditions.
Changed habits and use and disuse produce an inherited effect; witness the lighter wing- and heavier leg-bones of the domestic duck, the enlarged udders of milch-cows, or the drooping (unpricked) ears of domestic mammals. Variations are often definitely correlated : short-beaked pigeons have small feet; hairless dogs have imperfect teeth ; and blue-eyed white tom-cats are deaf. Hence selection of any one character will probably modify others indirectly. Although the laws governing inheritance are mostly unknown, probably most, if not all, characters tend to be inherited. The popular statement that domestic varieties, when they run wild, revert to the primitive stock is unsupported by satisfactory evidence ; and, although rever-sions occasionally occur in domestication, there is no sign of any general tendency to the loss of acquired characters. Except in being less uniform than natural species, in often differing more widely in some single part, and in being fertile when crossed, there are no well-marked distinctions between our domestic races and the so-called true species of a genus. The many breeds of dogs and cattle may arise from more than one species ; but probably those of horses and fowls, and clearly those of rabbits, ducks, and pigeons, are each descended from a single species. In this respect the breeds of pigeons are of peculiar importance, since not only carrier and tumbler, runt and barb, pouter and fantail, but at least a score of varieties might be chosen which differ so thoroughly, internally as well as externally, that an ornithologist, treating them as wild birds, would be compelled to grant them specific, and even distinct generic rank. Yet, since all these have indisputably arisen from the wild rock-dove (Oolumba lima), it is clear that naturalists who admit a unity to such domestic races, which professed breeders have often laughed to scorn, should in turn be cautious before de-riding the unity of wild ones.
How then have domestic races been produced ? By external con-ditions or habits alone ? One of their most remarkable features is in exhibiting adaptation, not to their own good, but to man's use or fancy. We know that all the breeds were not produced as per-fect as we now see them, and the key is man's power of accumu-lative selection : nature gives successive variations ; man adds them up, making for himself useful breeds. Skilful breeders speak of the organization as plastic and under control, and have effected ex-tensive modifications within our own generation. Unconscious selection, which results from every one trying to possess and breed the best individuals, is even more important. This accumulated change explains why we so often cannot recognize the wild parent stocks of our cultivated plants, while its absence in countries in-habited by uncivilized man explains why these never yield plants worth immediate culture. Man's power of selection is facilitated by keeping large numbers, in which variations are more likely to occur. Facility in preventing crosses is also of importance, as of pigeons as contrasted with cats ; some species are, however, less variable than others, e.g., the goose.
Individual differences arise even in the offspring of the same Varia-parents and tend to be inherited ; hence they afford material for tion natural selection to act on and accumulate, precisely as they would under for human selection. (Polymorphic generae.g., Rosa, Rubus, nature. Hieracium, &c.may perhaps owe their protean character to their variations being of no service or disservice, and consequently not being acted on by natural selection.) In determining whether a form should be ranked as species or variety, the opinion of natural-ists of sound judgment and wide experience is the only guide, yet this lacks unanimity (see SPECIES) : for example, of the polymorphic genera of the British flora alone Bentham reckons 112 species but Babington 251. Wallace has shown that no certain criterion can possibly be given by which to define his convenient working cate-gories of Malayan Lepidoptera, &c, viz., variable forms, local forms, sub-species, and representative species. As De Candolle concluded from his monograph on oaks (in which he shows at least two-thirds of his 300 species to be provisional), "so long as a genus is imper-fectly known and its species founded upon a few specimens " they seem clearly limited ; but, "just as we come to know them better, intermediate forms flow in and doubts as to specific limits augment." The terms variety and species are thus arbitrarily applied to inde-finable groups of more or less closely similar individuals. Common species that range wide and are much diffused are those which vary most. The species of the larger genera in each country vary more frequently than the species of the smaller genera. The species of large genera present strong analogies with varieties, which we can only understand if they originated as such.
Struggle for Existence. The term " struggle for existence" is used in a wide sense, including dependence of one being upon another, and embracing (which is more important), not only the life of the individual, but success in leaving progeny. From the high (geometrical) rate of increase of all organic beings (slow breeders only requiring a few more years to people a whole district) struggle inevitably follows, either one individual with another of the same species, or with the individuals of a distinct species, or with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to the entire animal and vegetable kingdoms, for in this case there can be no artificial increase of food and no prudential restraints from marriage. The checks to increase are most obscure. Eggs or young animals generally suffer most, and plants, mostly as seed-lings, both from germinating on ground already occupied and from animals. The amount of food, of course, gives the extreme limit of numbers, very frequently, however, the attacks of enemies, as of game by vermin. Climate plays an important part, and periodi-cal seasons of extreme cold have destroyed as many as four-fifths of the birds of an observed area. Epidemics, too, may occur. In many a species a large stock of individuals is often essential to its preservation. Complex and unexpected checks and relations exist between organic beings which have to struggle together; witness the profound alteration of the flora and fauna of a heath when planted with Scotch fir, these again being wholly dependent upon the exclusion of cattle. But in several parts of the world insects determine the existence of cattle. Again, red clover depends for fertilization upon the humble-bees, these upon immunity from the attacks of field-mice, and thus indirectly upon the numbers of cats ; hence no bees, no clover, and the more cats, the more clover ! The struggle will almost invariably be most severe between the indi-viduals 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 wdll generally be almost equally severe, and we sometimes see the contest soon de-cided (as in the case of varieties of wheat or sweet pea, of the mountain-sheep or medicinal leech). Similarly, the struggle be-tween species of the same genus wdll generally be more severe than between the species of distinct genera, e.g., the replacement of the black rat by the brown or of the large cockroach by the small. The structure of every being is related to that of all the others with which it competes, from which it escapes, or on which it preys ; witness alike the teeth and talons of the tiger, or the legs and claws of the parasite clinging to his hair. The albumen of a seedling favours its struggle with plants already growing around it.
How will this struggle for existence act in regard to variation ? Natural Can the principle of selection, so potent in the hands of man, apply selection, under nature ? Most efficiently ; for, when we bear in mind the constant occurrence of variation, with the strength of the heredi-tary tendency, also how infinitely close and complex are the mutual relations of 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 condi-tions of life, can it 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 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, 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 in-jurious would be inevitably destroyed. This presentation of favour-able and this destruction of injurious variations are called natural selection, or, less metaphorically, the survival of the fittest. The probable coarse of natural selection may be understood from the case of a country undergoing change of climate. The proportional numbers of its species will be changed ; some will probably become extinct; and these changes would seriously affect the others. Im-migration of new forms might occur, wdth further serious disturb-ance, or, where this is impossible, we should have places in the economy of nature which might be better filled up. In such cases slight modifications which in any way favoured the individuals of any species, by adapting them better to their altered conditions, would tend to be preserved, and natural selection would have free scope for the work of improvement. Moreover, changed conditions increase variability. As man produces a great result, what may not natural selection effect ? The former acts only for his own good, nature for that of the being itself; man on mere external characters, nature on the whole machinery of life ; man irregularly and imperfectly for a short time, nature by accumulation during whole geological periods. Natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations, reject-ing 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. It may operate on char-acters which we are apt to consider of very trifling importance, and its accumulation of small variations may set up unexpected cor-relative changes. It may modify the egg, seed, or young as easily as the adult, the structure of young to parent and of parent to young, and in social animals it will adapt the structure of each for the benefit of all. The theory of natural selection is next illus-trated (1) by supposing the formation of swift varieties of wolves, much as greyhounds have been developed byman ; (2) byreference to the excretion of nectar by flowers, its use to insects, the action of these in carrying pollen, its advantage in intercrossing, and the resultant modification and adaptation of flower and insect to each other through the preservation of their advantageous variations. Circumstances favourable for the production of new forms through natural selection are great variability, large numbers of individuals, the complex effects of intercrossing, isolation in confined areas (yet probably still more extension over continental ones, especially if oscillating in level), and considerable lapse of time, although this by itself must not be supposed to do anything (as if the forms of life were undergoing change by some innate law), but merely to afford increased opportunity for variation and environmental change. Extinction, to which rare species are on the way, is caused by natural selection.
Divergence. The divergence of character brought about by artificial selection of in domestic breeds is efficiently paralleled in nature, since the more char- diversified the offspring of each species, the more they wdll seize acter. on diverse places in the economy of nature, and so increase in numbers. The greatest amount of life can be supported by great diversification of structure. This divergence of character, with extinction of intermediate forms, explains the difficulties of tax-onomy, which are then discussed in detail wdth the aid of a diagram. This, of course, takes the form of a genealogical tree, and suggests that of " 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."
Natural Selection Controversy. The preceding summary of the classical statement of the doctrine of natural selection should be supplemented by reference not only to the original work, to the corroborators of its author, and to the able independent treatise (Natural Selection) of Wallace, but to the enormous mass of exposition, argument, and illustration accumulated oy subsequent writers, commencing with Hooker and Asa Gray, Huxley and Haeckel, but soon becoming too numer-ous even for mention. At the same time the history of the controversy which has arisen out of that statement _should also be treated, in so far as it has kept within scientific bounds. The leading objections which have been brought against natural selection, together with the replies to them, should all be summarized, as, for instance, Fleem-ing Jenkin's important criticism as to the swamping of individual variations by crossing, Mivart's vigorous and detailed polemic against natural selection as capable of accounting for the incipient stages of useful structures, &c., and so on. But this could not be done to the satis-faction of either party without far exceeding the present limits; happily, however, two chapters of the Origin of Species (v. and vi.) are generally admitted to have at least made the attempt up to 1872, the date of the final edition, with great candour and fairness. The various constructive efforts to supplement the hypothesis of natural selection by introducing additional factors of evolutionary change should also be mentioned, headed of course by Darwin's own accessory hypothesis of sexual selection.
We must not, however, leave natural selection without Huxley's a fuller statement of its services and claims, and this may advocacy, most appropriately be taken from the recent and, as it were, judicial deliverance of their veteran advocate, Prof. Huxley. He first points out the grounds of his agnostic position (up to 1858) with respect to evolution as promul-gated by Lamarck, Chambers, and even Spencer : " Firstly, that up to that time the evidence in favour of transmuta-tion was wholly insufficient; and, secondly, no suggestion respecting the causes of the transmutation assumed, which had been made, was in any way adequate to explain the phenomena." He then goes on to say
"The suggestion that new species may result from the selective action of external conditions upon the variations from their specific type which individuals presentand which we call " spontaneous " because we are ignorant of their causationis as wholly unknown to the historian of scientific ideas as it was to biological specialists before 1858. But that suggestion is the central idea of the Origin of Spiecies, and contains the quintessence of Darwinism. . . . That which we were looking for, and could not find, was an hypothesis respecting the origin of known organic forms which assumed the operation of no causes but such as could be proved to be actually at work. We wanted, not to pin our faith to that or any other speculation, but to get hold of clear and definite con-ceptions which could be brought face to face with facts and have their validity tested. The Origin provided us wdth the working hypothesis we sought. . . . The facts of variability, of struggle for existence, of adaptation to conditions were notorious enough, but none of us had suspected that the road to the heart of the species problem lay through them till Darwin and Wallace dispelled the darkness, and the beacon-fire of the Origin guided the be-nighted. . . . The only rational course for those who had no other object than the attainment of truth was to accept " Darwinism " as a working hypothesis and see wdiat could be made of it. Either it would prove its capacity to elucidate the facts of organic life or it would break down under the strain. . . . Whatever may be the ultimate fate of the particular theory put forward by Darwin, I venture to affirm that, so far as my knowledge goes, all the ingenuity and all the learning of hostile critics have not enabled them to adduce a solitary fact of which it can be said, this is irreconcilable wdth the Darwinian theory."
Darwin's own case for natural selection has also been Darwin's tersely put. " The belief in natural selection must at reasons, present be grounded entirely on general considerations, (1) on its being a vera causa, from the struggle for exist-ence and the certain geological fact that species do somehow change; (2) from the analogy of change under domestication by man's selection; (3) and chiefly from this view connecting under an intelligible point of view a host of facts."
Applying to natural selection the accepted tests of a good theory(1) power of explaining all phenomena and meeting all objections, (2) power of meeting new facts as they occur, (3) applicability as an instrument of research, and (4) power of prediction,it seems difficult for the most sceptical of critics to avoid ranking it very high, while to those who regard it as triumphantly surviving this assay the " law of parsimony " appears to preclude the necessity of seeking any further explanation of evolution. Survival As no serious critic wholly denies the fact of a struggle of fittest for existence, nor even a certain preponderance among the causa survivors in any given area of those best fitted for the struggle, however much he may consider the importance of these facts to have been overstrained in certain direc-tions, we are fairly entitled at the outset to regard natural selection, not only as provisionally superseding earlier attempts at solving the problems of geological and geo-graphical distribution, but as adequately,it might almost seem finally,explaining them (see BIOLOGY, DISTRIBUTION). This claim at any rate must be allowed even by those who deny the importance of natural selection as a substantial factor in evolution in the other sub-sciences of physiology and morphology, and this not simply because it was as experts in distribution that Darwin and Wallace themselves arrived at their hypothesis, nor because all subsequent workers in that field of science have adopted it, but because no valid objection, or even supplementary hypothesis, has ever, as regards this important department of science, been so much as proposed. The survival of the fittest, then, is unquestionably a vera causa, and one of no small importance in the study of nature (in fact, in undisputed possession of at least one entire field), as also in the study of social life, from which both Darwin and Wallace were directly inspired. The importance of breed-ing and artificial selection is also undisputed; nor can some small scope for natural selection in the formation of varieties of wild races be denied even by the most grudging of critics who may be versed in the facts of natural history at all. But this amounts to admitting that natural selec-tion, already the undisputed explanation of distributional changes, is at any rate a partial cause in structural and functional ones; while older explanations of these facts, even if not melting into merely abstractional ones, have not succeeded in gaining acceptance. Uni- At this point comes in with ponderous weight the con-tarían cePti°n by which Lyell had for nearly thirty years been ism in preparing the way for Darwin, that of uniformitarianism natural in natural change. For, although it was perhaps almost chauge. as much owing to the unfavourable opinion of Lyell as to the hostility of Cuvier that the doctrine of Lamarck had fallen into such disesteem, and although Lyell himself was accustomed " to keep the name of creation for a natural process which he imagined to be incomprehensible," he was, notwithstanding, as he claimed and as Huxley frankly allows, "strongly disposed to account for the origination of all past and present species of living things by natural causes," and his influence was, therefore, for Huxley, as no doubt for others, " perhaps more potent than any other in keeping alive a sort of pious conviction that evolution after all would turn out true." For, given the principle of Lyell's work," the principle that the past must be ex-plained by the present, unless good cause be shown to the contrary, and the fact that, so far as our knowledge of the past history of life on our globe goes, no other cause can be shown,"it is evident that "consistent uniformi-tarianism postulates evolution as much in the organic as in the inorganic world. The origin of a new species by other than ordinary agencies would be a vastly greater ' catastrophe' than any of those which Lyell successfully eliminated from geological speculation."
Bearing in mind, then, the lesson of Lyell, that any efficient cause, even though slow or at first sight trifling, can be shown, if acting steadily for vast periods of time, to effect enormous results, the apparent difficulty of adding up varietal differences to specific ones, these to generic, and these again to still larger distinctions, becomes only the inevitable continuation of that summation of individual into varietal differences which no one can deny to be happening around us. Thus the onus p>robandi that species have not arisen in this way comes fairly to be thrown by the advocates of natural selection upon its adversaries. And, as these come individually to realize, through those concrete studies which necessarily underlie all active con-viction on one side or the other, the importance of the masses of concrete evidence which have been steadily accumulating, it is little wonder that their majority should not only have so rapidly dwindled, but actually have changed sides so generally as has been the case. Thus, to cite only a single case,(1) we are now in actual possession of an unbroken and geologically successive series of fossil horses, perfectly gradated through some forty-five "species" back to a five-toed ancestor; (2) we can trace the con-tinued evolution of the existing species in the bones, teeth, and finest peculiarities of the specimens around us, as well as the increasing differentiation of well-marked varieties through artificial selection; (3) we can see that such con-tinuous improvement of skeleton, teeth, <fcc, as is now and has all along been in progress, must have been profitable to the animals possessing them in the wild state, just as they now are in the tame (since implying better locomo-tion, digestion, &c). Hence we soon come to feel a real difficulty in understanding (1) how the continuous sum-mation of small individual advantages through varietal characters into specific, and even generic ones, can be doubted, much less denied, and consequently (2) how the general applicability of the theory of natural selection to interpret the gradual process of historic change-can any longer be called in question.
At this point it is right to notice that several evolu- Varia-tionists have suggested or argued for the progress of tion variation along definite lines; but this return upon pre- Darwinian standpoints has been energetically and, it must lmes_ be admitted, on the whole as yet successfully combated. Thus Asa Gray (1861-76) has argued for variation along definite and useful lines. Nageli (1865) regarded the ac-quisition of certain characters which appear to be of no service to plants as offering a difficulty to natural selection, and as affording a proof of some innate tendency in plants towards perfection, corresponding to what Kolliker entitles the "law of creation." Mivart (1871) has maintained that variations are definite, and frequently sudden and considerable. With such positions must also be noticed the continuous and increasing insistence of Spencer (1852-86) upon environmental action and functional change, although spontaneous variation is not excluded nor considerable importance to natural selection denied, Butler in Eng-land, and Cope, Hyatt, and other " Neo-Lamarckians" in America, have more fully developed this reaction, for which experimental evidence has been especially sought in Germany by Semper. And, while Darwin himself long continued an uncompromising opponent of all such heresies, a distinct, though partial, change of opinion was admitted by himself in later years, and has been fully insisted upon both by Mivart and Spencer. While in the Origin of Species up to 1869 variations are spoken of as "accidental" (no doubt only in that sense in which a scientific man may fairly use the word), and there is ample proof of Darwin's originally even contemptuous attitude towards Lamarck, and of his substantial rejection of the comparative importance of function or environment in determining variations, these views are unquestionably modified in the well-known pero-ration to the Origin of Species. The further diminishing importance of natural selection is admitted in subsequent editions, as Spencer has pointed out, in the preface to the Descent of Man, and indeed elsewhere; while a recent pungent criticism cites evidence of a certain measure of apparently contradictory and indefinite fluctuation of opinion between increased insistence upon use and disuse on the one hand and upon spontaneous variability on the other. Pleas for Since, however, Darwin never developed the grounds of hide- tjjjs diminishing reliance upon natural selection, which still remains the central theme of his works, and that upon which their originality, as well as their importance to the theory of evolution, essentially depends (of course as dis-tinguished from their services in summing up the concrete evidence for the fact of evolution and their vast utility in diffusing it), too much weight must not be attached to such isolated expressions of opinion as those referred to above, the more so as these are nowhere borne out by any general change in the tenor of his works. In the absence of any theory of definite and progressive change, and in the presence of multitudinous variations under domestication and in nature which we can neither analyse, rationalize, nor hardly even classify, we are not only justified but logically compelled to regard variation as spontaneous or indefinite, i.e., practically indeterminate in direction, and "therefore unimportant, except as the groundwork for selection to act on." Conversely, variation must be in-definite, else the paramount importance of natural selec-tion must be proportionally impaired as this becomes de-finite (cf. EVOLUTION, vol. viii. p. 751); for we cannot speak of selecting a course from one line of variation, nor even for that matter in the old sense of "variation" at all. Thus, "if it should hereafter be shown that variability is definite and determined in certain directions rather than others" (in short "fated" rather than " spontaneous"), "the importance of natural selection would not be impaired," since it would still have to be regarded as a vera causa in the history of species, yet the function ascribed to it would be practically reversed. It would exchange its former supremacy as the supposed determinant among the indefinite possibilities of structure and function for that of simply accelerating, retarding, or terminating the process of otherwise determined change. It would furnish the brake rather than the steam or rails upon the journey of life ; or, in other words, instead of guiding the ramifica-tions of the tree of life, it would, in Mivart's phrase, do little more than apply the pruning knife to them. In fact, its functions would be restricted to those of the third Fate, and would no longer, as at present, be supposed to include those of the second. Under these circumstances it is unnecessary to appeal at length to the unanimity with which the later generation of Darwin's exponents (e.g., Lankester, Romanes, Allen, &c.) concur with all preceding ones in the necessary proposition, that " natural selection trusts to the chapter of accidents in the matter of variation." The thesis thus summarized by Lankester thoroughly permeates the Variation of Animals and Plants under Domestication, as may be seen from the brief sum-mary of it given below (p. 82). Special reference must, however, be made to the important researches of Weis-mann, since these were devised with the deliberate intention of testing " whether, besides natural selection and the direct action of external conditions, together with the correlative results of these two factors, there might not lie concealed in the organism some other transforming power," such as the " perfecting principle " of Nageli or the like. The results of these investigations have tended entirely to confirm the theory of natural selection in its classic form; moreover, the same naturalist has since ad-duced weighty arguments against the transmissibility of individual variations, whether acquired by habit or im-pressed by environment, and thus proportionally weakened the arguments of Spencer and others in favour of the revival of the Lamarckian factors of change. Variation seems therefore to be driven in more and more upon the reproductive function, and thus an ultra-Darwinian in-sistence upon the obscure and as yet wholly indefinite factor of variation is strongly forced upon us, and natural selection seems more than ever to be our only possible clue.
Armed, however, with this, we have a consistent means Darwin's of re-investigation of the whole organic world, in the course Wo-of which, not only our existing accumulations, as of tax- ^j^ onomy and of comparative anatomy, acquire a new interest, and all existing lines of research receive a potent stimulus, but the marvellous variety of adaptation comes into in-telligibility and order,a new teleology thus replacing the old. Of this profound change in the standpoint and interest of modern biology we can form an idea most simply, yet most representatively, by glancing at Darwin's own biological works. Before the Origin of Species we have the slow and laborious production of the substantial, yet conventional and comparatively arid, Zoology of the Beagle and Manual of the Cirripedia, while afterwards the well-known and fascinating new series of zoological and botanical discoveries and generalizations were poured out with remarkable rapidity. The many bold and ingenious applications of the theory of natural selection which we owe to Darwin and his school should here be reviewed seriatim; but it is impossible to do more than mention some of them, e.g., the renascence and development of Sprengel's discovery of the relations of flowers to insects, the elaborate studies on Fertilization in Orchids, the Forms of Flowers, Insectivorous Plants, Climbing Plants, Movement in Plants, &C. Again, the interpretation of the problems presented by bee and ant society led him to grapple with the problems of mind and language, in the Descent of Man and the Expression of the Emotions; and the systematic application of the conceptions of biological science to those of psychology, sociology, and ethics, which had been com-menced by Comte and continued by Spencer, rapidly passed from its comparative philosophic isolation into wide diffu-sion through the movement of literature and science.
Application. This passage from the inductive verification of the theory of natural selection to its deductive application of an engine of research is conspicuously associated with the ^J^u labours of Haeckel, whose Generelle Morphologie (1866) to bio-and other minor works have a central historic place amid logical the first fruit and seed of the new movement in German science ; biology, which was henceforward almost completely Dar-winian ; and they have equally strong claims to be con-sidered as the starting-point of those logical and unflinch-ing attempts to view all problems in the light of natural selection which have since become more common. By transcending the limits of ordinary biological specialism independently of Darwin, and dispensing with his initial reservation as to the possible creation of primeval forms and with Wallace's final caveat as to the origin of man, a thoroughly " monistic" view of biology and its kindred sciences is obtained. The systematic exclusion of the traditional teleology and of that operation of the consciousness of the organism assumed by Lamarck (since indefinite structural variations cannot be associated with, much less developed by, definite psychical ones, and still less by any permanent psychical bias or character) also necessarily follows ; and along with this goes the conception of the operation of an extra-organismal mind. The struggle for existence is the sole condition of human progress ; and the conception of prevalent or predominant altruism, as systematized by past philosophy and tradition, is thus an alloy of sentimental illusion with baser elements. The doctrine of natural selection is in this way fully developed into a substantially Neo-Lucretian philosophy, and, in justice to the logical clearness of the author to whom we especially owe this systematization, it must be admitted that those who grant his postulates, without endorsing his conclusions, have in all cases either refrained from continuing their development or fallen back upon the insertion of some caveat or clem ex machina at an arbitrary point.
To In the same way we have this view of all things in stnulture s*ruSS^e applied to the explanation of the internal structure an(j and function of the organism itself. Once more to quote function. Huxley, "It is a probable hypothesis that what the world is to organisms in general each organism is to the mole-cules of which it is composed. Multitudes of these, having diverse tendencies, are competing with one another for opportunity to exist and multiply, and the organism as a whole is as much the product of the molecules which are victorious as the fauna or flora of a country is the product of the victorious organic beings in it." This view has more recently been considerably developed by Roux,2 and from its logical continuity with the major theory of which it is a corollary is gaining widened acceptance.
With regard to the destructive criticism of the old teleo-logy, which has been led by Haeckel, and aided or ac-quiesced in by other natural selectionists, it is important to note that the obvious proposition has also repeatedly been urged (e.g., by Huxley and Weismann no less than by their critics, such as Von Hartmann and Lotze), that, just in proportion as our explanation of the origin of organic structures, functions, and adaptations becomes increasingly mechanical, so does an increasingly rigorous teleological view of this mechanism reappear. At most, therefore, we should have exchanged an external and mechanical view of teleology for an (at least relatively) internal and dynamical one. Sexual The later hypotheses accessory to natural selection can here be selection, only very briefly outlined. Partly in course of his inquiries into the descent of man, partly also to account for many remarkable phenomena of sexual differentiation in animals (see SEX), Darwin was led to develop the hypothesis of sexual selection which he had already suggested in the Origin of Species. Not merely do individuals struggle for existence, but the males struggle for the females; thus the more vigorous tend to obtain mates, and so leave most progeny to repeat the struggle. Special weapons are used in the struggle, both offensive, like the cock's spurs or the stag's horns, and defensive, like the lion's mane ; and advantageous variations of these tend to be accumulated. But, as the more beautiful or melodious males appear often to obtain the preference of the females, there is also ground for the conception that the aesthetic advantages frequent in the male sex have similarly been acquired, the more so as we have common experience of how rapidly artificial selection can accumulate superiorities of plumage or song. After the opera-tion of sexual selection has been traced as far as possible through the animal kingdom, the doctrine is applied to man, with the con-clusion " that of all the causes which have led to the differences in external appearance between the races of man, and to a certain extent between man and the lower animals, sexual selection had been the most efficient." Acceptance of this hypothesis has been much less general than of natural selection. Criticism has been busy, and this ranges from the serious yet partial dissent of Wallace to fundamental contrast, such as is involved in the theory of the nature and origin of sex suggested in the article SEX, which views the degree of development of the external characters of sex as essen-tially the outward expression of a proportional stage of the evolu-tion of constitutional "maleness" or "femaleness" (i.e., katabolic and anabolic diathesis respectively).
Under the title of " physiological selection " G. J. Romanes has Physio-lately proposed " an additional suggestion as to the origin of species," logical of which the more critical than constructive essence may be briefly selection, given in the author's own words : " As a theory of origin of species natural selection has in its way three cardinal difficulties,(1) the difference between species and varieties in respect of mutual fertility, (2) the swamping effects of free intercrosthig upon an individual variation, (3) the inutility to species of so large a proportion of specific distinctions. . . . Natural selection is not, properly speak-ing, a theory of the origin of species: it is a theory of the origin or rather of the cumulative developmentof adaptations. Whenever any variation in the highly variable reproductive system occurs, tending to sterility wdth the parent form, without entailing fertility with the varietal form, a physiological barrier must inter-pose, dividing the species into two parts, free to develop distinct histories, without mutual intercrossing, or by independent varia-tion. By regarding mutually sterile species as records of variation in reproductive systems, we are at work, so to speak, on the foun-dation of the matter."
LAWS OF VARIATION.
Even those who attach most importance to the doctrine Origin of of natural selection as affording a rationale of the process varia-of organic evolution will not seriously dispute that, even tlons-were this explanation completely extended to all the de-tails of plant and animal life, another and deeper explana-tion would still be necessary. That is, the theory of natural selection, being from the external standpoint only that of the adaptation of the organism to survive the pressures of the environmentwould all the more urgently stand in need of a complementary internal explanation, which should elucidate the physiological process of change through which the organism has actually been enabled to adapt itself. As a recent writer tersely expresses it, " The survival of variations is a matter secondary to their origin, and it is becoming realized more and more that this is the point to be explained." How, in short, do we get the variations without which natural selection would have nothing on which to operate ? Here we can no longer Darwin's remain satisfied with the merely general conception of theory, variation as essentially indefinite, with which we have be-come familiar as the postulate (p. 81) of the theory of natural selection, but must add to it a retrospect of the theories of earlier authors such as has been already out-lined,(a) a summary and discussion of the extensive labours of Darwin upon the problems of variation from which this conception of indefiniteness was generalized, followed by (6) an account of subsequent progress. Un-fortunately the voluminousness of this literature makes such a task practically impossible : instead of the enormous wealth of concrete details accumulated with respect to the Variation of Animals and Plants under Domestication, a reference to the volumes under that title must almost suffice. Some acquaintance with this work is, of course, necessary adequately to appreciate the force of its author's general con-clusions ; these may, however, be briefly outlined from the chapter of the Origin of Species (chap, v.) devoted to the laws of variation, and from those (xxii.-xxvi.) of the larger work which contain an ample yet similar discussion.
Environment. '' 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." Changed conditions generally induce mere fluctuating variability in individuals, yet sometimes direct and definite effects upon the mass. The facts and considera-tions for and against the belief that the conditions of existence aet in a potent manner in causing definite modifications of structure are confronted in detail. In some few instances a marked effect has been rapidly produced, e.g., on European men in the United States, European dogs in India, horses in the Falkland Islands, oysters in the Mediterranean, &c. The chemical compounds of some plants and the state of their tissues are readily affected by changed conditions. The production of galls, &c, shows how great changes in structure and colour may result from chemical changes. We almost certainly know that organic beings in a state of nature maybe modified in various definite ways by the conditions to which they have been long exposed ; but it is difficult to distinguish be-tween the definite result of changed conditions and the accumula-tion through natural selection of indefinite variations which have proved serviceable. But, even granting the utmost weight to con-ditions, we can rarely see the precise relation between cause and effect. Moreover, many animals and plants of wide range and great diversity of experience yet remain uniform in character. Again, the degree to which domesticated birds, &c, have varied does not stand in any close relation to the amount of change to which they have been subjected. In fact, we may have similar modifications under different conditions, different modifications under similar change of conditions, or no modifications at all. Closely paraliel varieties are often produced from distinct races or even species without ascertainable unity of conditions. Bird-varia-tions, too, seem conspicuously independent of circumstances. All these considerations tend to force on our minds the conviction that what we call the external conditions of life are in many cases quite insignificant in comparison with the organization or constitution of the being which varies. No doubt each variation may have its efficient cause, but it is as hopeless to search for the cause of each as to say why a chill or a poison affects one man differently from another. With respect to acclimatization, although habit does something towards the success of the process, yet the appearance of constitu-tionally different individuals is a far more effective agent. Use and Increased use adds to the size of muscles, together with the disuse. blood-vessels, nerves, bony crests of origin, and even the whole bones ; it also increases glands and strengthens sense-organs. In-creased and intermittent pressure thickens the epidermis ; change of food modifies the coats of the stomaeh and alters the length of the intestine. Disuse weakens and diminishes all parts of the organization,lungs and chest, wings and their associated bones, &c. Although in domesticated animals this never goes so far that a mere rudiment is left, it seems often to have occurred in nature, the effects of disuse being aided by economy of growth with inter-crossing. Changed habits may lead to use or disuse of organs, and consequently to their modification ; yet the effects of habit, use, and disuse have often been largely combined with the natural selec-tion of innate variations and sometimes overmastered by it. <Jorre- Correlated variation means that the whole organization is so tied lated together during its growth and development that, when slight variation, variations in any part occur, and are accumulated through natural selection, other parts become modified, apparently irrespective of advantage in the change. Variations of structure in the young often affect those of the mature animal; the influence of hard parts, mechanical pressure, the relative position of parts, and the size of the whole body all have important influences. Homologous tissues may exhibit associated variations, e.g., hoofs, hair, and teeth. In most cases the correlation is, however, quite obscure, and may seem to be of no utility to the species, as with various monstrosities and diseases. Colour may be associated with other constitutional pecu-liarities. Although correlation is of much importance, we may alse falsely attribute to it structures which are simply due to in-heritance or natural selection, or its effects may be inextricably commingled with those of increased use and of accumulation by natural selection, e.g., the gigantic horns of the Irish elk with the changes necessarily associated with the acquirement of them. Varia- - Homologous and multiple parts are peculiarly variable, and often bility of tend to cohere. Rudimentary and lowly organized structures are homo- variable. The law of compensation of Goethe and Geoffroy, "that logous in order to spend on one side nature is forced to economize on the parts. other," holds true to a certain extent with domestic production, but more doubtfully in nature.
Mechanical Pressure, &c. Mechanical pressure and relative position of parts seem to be of some importance in determining variations ; but such changes are often due to reversion to long-lost characters, which may frequently occur. A part extraordinarily developed in any species tends to be highly variable. Specific characters are more variable than generic characters. Distinct species present analogous variations ; but this may arise either from analogous constitution or from re-version. Secondary sexual characters are highly variable. " What-ever the cause maybe of each slight difference between the offspring and their parentsand a cause for each must existwe 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."
The preceding outline of Darwin's main positions (which Results are in harmony with his essential doctrine of indefinite01'recent variability) prepares us for the discussion of more recent ^aruh research and opinion. But for our present concrete know- opinion, ledge of the influence of environment, use and disuse, in-cluding all such researches as those of Semper,1 or the peculiarly brilliant and luminous investigations of Poulton,2 the recent valuable summary of Arthur Thomson3 may conveniently be referred to. The corresponding theoretic argument for the definite causation of most variations by these agencies has been recently re-stated by Spencer,* along with his proposed limitation of natural selection. This should be taken along with the testimony of the American Neo-Lamarckian school, among which the learned and suggestive, though too undigested, essays of Cope5 are especially prominent. The views of Nageli, Mivart, and other advocates of internal variation here present themselves anew, along with the criticisms and replies to them, as also Weismann's doctrine of variability as being ultimately germinal. But space precludes the survey of this voluminous and unfinished controversy, which, more-over, would not at present yield any general result, since neither the various inductive and deductive arguments, nor the organismal, functional, <fec, and environmental explanations which these variously favour, have been as yet exhaustively stated, still less properly confronted, and least of all reconciled, by any author.
It may be more profitable to attempt, though necessarily in barest outline, a fresh re-examination of the entire field. fresh This may be most appropriately introduced, and the initial theorJT °f conception of the present part of the discussion re-stated, tional by a passage from Weismann, whose substantial acceptance laws, of the doctrine of natural selection has already been noticed (p. 81). "We certainly cannot remain at the purely em-pirical conception of variability and heredity as laid down by Darwin in his admirable work. If the theory of selec-tion is to furnish a method of mechanical explanation, it is essential that its factors should be formulated in a precise mechanical sense. But, as soon as we attempt to do this, it is seen that, in the first enthusiasm over the newly dis-covered principle of selection, the one factor of transform-ation contained in this principle has been unduly pushed into the background to make way for the other more ap-parent and better known factors. The first indispensable factor, and perhaps the most important in any case, in every transformation is the physical nature of the organism itself." Let us briefly summarize, therefore, the main results of a fresh survey of the leading variations presented by plants and animals, i.e., no longer commencing with the analogy of human selection upon the smallest varietal and specific distinctions, and arguing on Lyell's principle for the cumu-lative origin of the characters of larger groups, but consider-ing these larger differences from the standpoint of general physiology, without any hypothesis at all.
The physiological principle invoked is simply that antithesis ho- tween reproductive functions and individual ones which has been familiar since the dawn of physiology, and which, when reduced species to its physical terms, is obviously deducible from the principle of conservation of energy. Instead of the generally received doctrine, summarized above, that of indefinite variation, with progress by ends over means of struggle for existence among individuals, its systematic application furnishes a detailed re-interpretation of the forms pre- vidual sented by plants and animals comparable to that afforded by the struggle received hypothesis, but with an essentially altered view of the and process and factors of organic evolution as a whole. Briefly stated, velop-
the view of evolution thus reached is that of definite variation, with progress essentially through the subordination of individual struggle and development to species-maintaining ends. Arrest of Let us commence with the origin of the flower, which all botanists vegeta- agree in regarding as a shortened branch. The apparent explanation tioii by of natural selection (from two other alternatives, one lengthened, repro- the other unshortened), although morphologically reasonable, is at duction. once excluded by the physiological one of inevitable shortening, since the expense of the reproductive functions necessarily checks the vegetative ones, and since we cannot speak of selection where the imaginable alternatives are physically impossible. Similarly, the shortening of the inflorescence from raceme to spike or flower-head, or its hollowing into a fig, with the corresponding reduction in the size of the flowers, receives the same explanationgrowth of axis and of appendages checked by reproductionsuperseding that current, viz., by mere convenient adaptation to fertilization by insects. The internal structure of the flower is similarly modified, as may be shown in detail in the passage from hypogyny through perigyny to epigyny (these being simply stages of the progressive arrest of the growth of the axis), in the reduction of the floral envelopes and stamens and the number of carpels and ovules, and even in the transition from perispermic to endospermic and finally exalbuminous seeds. But these are the most important of floral variations, and furnish the essentially distinguishing characters of the natural orders ; hence, when these are seen, instead of being indefinite, to be parallel and definite (i.e., determined through the continuous checking of vegetation by reproduction along what is thus a single and definite groove of progressive change), the import-ance of natural selection changes wholly from that of selecting and accumulating supposed indefinite variations to that of retarding definite ones after the stage of maximum utility has beeii in-dependently reached. The same simple conception unlocks in-numerable problems of floral morphology, large and small alike : e.g., it interprets with equal ease the inevitable development of gymnosperm into angiosperm (by continuous subordination of the reproductive carpellary leaf) and the origin of refined minor adapta-tions, like the splitting fruit of the geranium or the cupped stigma of the pansy, not as achievements of natural selection from among fortuitous variations, but as naturally traceable to the checked vege-tation of their respective types of leaf organ, just as in the familiar case of the pinnately-lobed outer sepals of the rose bud. The origin of floral colour as primarily an inevitable consequence of the same principle of vegetative subordination through reproductive pre-ponderance was long ago pointed out by Spencer, who unfortunately, however, also accepted without scrutiny the orthodox hypothesis that this incipient floral colour would revert to the ordinary green vegetation but for the accident of its attractiveness to insects, and consequently must have owed its accumulation simply to the repeated preservation of the offspring of the accidentally brightest coloured; whereas we now see that the arrest of vegetative greenness in the flower and the exclusive appearance of those colour-ing matters associated with the imperfect vegetation of spring and autumn are a continuous process of its subordination as a vegeta-tive, and its development as a reproductive, apparatus. Adapta-tion to insects thus takes a quite secondary place ; similarly with the leafy arrest and floral coloration of bracts ; wdrile a detailed examination of thorny plants practically excludes the hypothesis of mammalian selection altogether, and shows spines to arise as an expression of the diminishing vegetativenessin fact the ebbing vitalityof a shoot or even of an entire species. Evergreens tend to arise in all orders among the forms of more vegetative habit, and in such forms the amount of flowering is usually diminished rela-tively to the deciduous members of the same group, whilst such con-stitutional vegetativeness checks the progress of floral evolution. Hence it is that evergreens, and even orders in which they largely occur, are usually less differentiated than their deciduous congeners. The apparently indefinite variations presented by domesticated plants, e.g., the cabbage tribe, are at once classified and interpreted as so many stages along the same course (from leafy kale, through Brussels sprouts with many leaf buds, to cabbage with apical flower bud, and finally to cauliflower). The importance of this line of evi-dence for the Darwinian view of variation and selection becomes still further reduced, although also broadly settled, when we note that, since the subordination of vegetative to reproductive pre-ponderance gave us the origin of ordinal characters, the converse improvement of individual characters at the expense of reproduc-tive ones (on which domestication usually insists and artificial selec-tion depends) can only give us minor ones, for the most part varietal or specific, if so much. The Lyellian analogy for the ultimate accumulation of varietal differences into ordinal characters has thus been misleading. The preceding transition of both organography and physiology from empirical to rational carries with it a similar transition from empirical to rational taxonomy as well: the antagon-ism of vegetative and reproductive habit is seen to be also general and constitutional. Just as the liliaceous type ranges on one side towards the characteristically vegetative grass or reproductive orchid, so is it with the main variations of every natural alliance, be this order, genus, or even species. Thus the Panunculacem have their grassy and their orchid-like types in meadow-rue and larkspur, yet the species of these very genera show examples of the opposite swing of variation ; again, the two species or varieties of British oak (Q. pedunculated and Q. sessiliftora) are thus, so to speak, the incipient grass and orchid forms of their common ancestor (Q. Robur). What we call higher or lower species or orders are thus the leaders or the laggards along one or other of these two lines of variation, the repre-sentatives of some stage of the predominance on one side or other of that oscillating balance between vegetative and reproductive pro-cesses which have long been known as the essential functions of organic life.
The results of a similar survey of the animal world, despite the Working greater intricacy of the problem, are scarcely less definite or com- of the prehensive, and at once lead to a deeper interpretation of the whole, law in Commencing with an ascending survey, we recognize such essential animal types of Protozoa as the rhizopods, gregarines, and infusors, not as world, the empirically selected products of spontaneous variation among indefinite possibilities, but simply as the predominatingly amoeboid, encysted, and motile phases of the primeval cell-cycle (see MORPHOLOGY), these three forms of which are fixed by the properties of proto-plasm itself (see SEX, vol. xxi. pp. 720-724), each particular phase being fixed by the constitutional bias (diathesis) of its type towards anabolism or katabolism (see PHYSIOLOGY, PROTOPLASM). This rationale of variation in ultimate terms, i.e., both cellular and proto-plasmic, both morphological and physiological, may be continued through all fields, e.g., embryological or pathological, for some diseases are coming to be interpreted as the progressive variation of some func-tion and organ which only disturbs the general balance in its progress towards a new and higher equilibrium. This conception of physio-logical as well as merely structural life-history rationalizes both animal and vegetable taxonomy. Thus the greatest of all steps in morphological progress, that from the Protozoa to the Metazoa, is not due to the selection of the more individuated and highly adapted forms, but to the union of relatively unindividuated cells into an aggregate in which each becomes diminishingly competitive and increasingly subordinated to the social whole. Passing to trifling variations, such as the internal deposition of spicules, we interpret these neither as mere accidents, nor as pure advantages for support or defence, but as arising like plant-crystals or gouty deposits as unremoved waste products, accumulating with local or constitu-tional passivity. How the inevitable constitutional preponderance of anabolism or katabolism excludes spontaneous variation and subordinates external selection in the formation of types would, become clear could we pass in review the animal kingdom, and note how its various alliances all range from passive to more active forms. We should compare the coral or sponge at one end of the scale with the similarly passive (and consequently aiso skeletal) tortoise and glyptodon at the other. We should read in their anabolic diathesis the secret of accumulating size and of inevitable extinction (nay, even unravel the apparent contradictions with re-spect to the influence of the environment, as being specially strong upon the passive types, which hence are far richer in species than the corresponding less readily influenced active forms).
In short, while no more denying the existence of com-petition in nature than the fact of organic progress, we deny the assumed relation of these as cause and effect,
Finally, the preponderating importance of the species-maintaining over the individual functions should again be noted in animals as in plants, since the conviction thus becomes inevitable that, in stat-ing the process of evolution essentially in terms of the survival of the fittest in competition, the centre of gravity of the subject has been misplaced. The constant primary insistence upon individual competition for food, and the very subordinate recognition of the importance of sexual and social co-operation for well-being, which are characteristic of the prevalent theory (witness their proportion in the preceding summary), are traceable at once to the confusion of putting the nutritive factors "in the first place" because they precede the reproductive in time ; whereas the organism inevitably enters upon reproduction and so cedes the preponderance"the first place " to the other-regarding functions. That increase of reproductive sacrifice which at once makes the mammal and marks its essential stages of further progress (monotreme, marsupial, placental), that increase of parental care, that frequent appearance of sociality and co-operation which even in its rudest forms so surely secures the success of the species attaining it, be it mammal or bird, insect or even worm,all these phenomena of survival of the truly fittest, through love, sacrifice, and co-operationneed far other promi-nence than they could possibly receive on the hypothesis of the essential progress of the species through internecine struggle of its individuals at the margin of subsistence. Yet these cases are only the supreme outcomes of that continuous and definite process of variation through individual subordination and species-maintain-ing preponderance which we have seen modelling even the vegetable world.
In short, while no more denying the existence of com-petition in nature than the fact of organic progress, we deny the assumed relation of these as cause and effect,
and propose an interpretation in essential respects directly-converse, -without denying the minor agency of use and dis-use, environment, &c. Each of the greater steps of progress is definitely associated with an increased measure of sub-ordination of individual competition to reproductive or social ends, and of interspecific competition to co-operative adaptation. The ideal of evolution is thus an Eden; and, although competition can never be wholly eliminated, and progress must thus be asymptotic, it is much for our pure natural history to see no longer struggle, but love, as "creation's final law." While ceasing to speak of inde-finite variation, we may of course still conveniently retain the rest of the established phraseology, and continue to speak of "natural selection " and of "survival of the fit-test," always provided that, in passing from the explana-tion of the distributional survival of individuals or species in contest within a given area to the interpretation of the main line of their morphological and physiological progress, we make the transition from the self-regarding to the other-regarding (in ethical language, from the egoistic to the altruistic) sense of these terms which has above been outlined.
It would be premature to enter upon the extended or deductive application of these considerations, since, pending their acceptance, the preceding statement of the received
doctrine of natural selection from spontaneous variations, with all its logical consequences (pp. 81-82), remains valid. Conversely, it is of course obvious that their adoption would involve the extensive modification of the received doctrine, as well as the complementing of new constructive attempts by a re-examination of earlier views of both the process and the philosophy of evolution. (p. GE.)
Jotirn. and Proc. Linn. Soc, 1858.
In the later editions of the Origin a brief account of sexual selec-tion is given at this point, vide infra, p. 82.
North British Review, 1867.
2 Genesis of Species, 1871.
Life of Darwin, vol. ii., "On the Beception of the Origin of Species."
ty.j however, op. cit, p. 129, note 4.
Letter to Bentham, 1863, in Life of Darwin, iii. p. 25.
Romanes, Scientific Evidence of Organic Evolution, 1886.
Through reading Malthas, On Population (see Origin, p. 3 sg.). It is worthy of notice also that Herder's early suggestion (vide supra) owas made in his Philosophy of History, Wells's early statement with respect to the races of man, and Patrick Matthew's in a treatise for practical purposes, while an early statement of the doctrine, hitherto apparently overlooked, was made with respect to machines by James Watt (cf. Catalogue of Industrial Exhibition, 1851).
Butler, Luck or Cunning, London, 1887, chap. xii. ; see also his Evolution, Old and New, London, 1879.
"On the Theory of Individuality," in Proc. Roy. Soc, 1886.
See, however, chapter xxi. of Variation, &o., conclusion.
Studies in the Theory of Descent, Eng. ed., London, 1882.
The reservations with respect to mind and language made by evolutionists so contrasted in many respects as Wallace and Mivart should here be mentioned ; see Mivart's Lessons from Nature, also his "Limit to Evolution," in Nineteenth Century, 1884.
Butler, Luck or Cunning, London, 1887, chap. xii. ; see also his Evolution, Old and New, London, 1879.
Die Continuitdt d. Keimplasmas, Jena, 1885, and Die Bedeutung d. sexuellen Fortpjlanzung, Jena, 1886.
A frank and uncompromising application of this doctrine to the interpretation of nature and society will also be found in Huxley's "'Straggle lor Existence," in Nineteenth Century, February 1888.
a Der Kampfd. Theile im Organismus, Leipsic, 1876.
Journ. Linn, Soc, xix., 1886. For the criticisms of Wallace, Argyll, Meldola, Catchpool, and others, see Nature, vol. xxxiv.
Comp. Variation under Domestication, chap, xxi., conclusion.
1 Animal Life, Int. Sci. Series, 1881.
2 Proe. Roy. Soc., 1884-88.
3 "A Synthetic Summary of the Influence of Environment," in Trans. Roy. Phys. Soc. Edin., 1887-88.
4 Factors of Organic Evolution, London, 1886 (reprinted from Nine-teenth Century).
5 The Origin of the Fittest, London and New York, 1887.
Of. Sutton, General Pathology, London, 1886.
Geddes, " On Variation in Plants," " On the Origin of Thorns and Prickles," and " On the Origin of Evergreens," in Trans. Bot. Soc. Edin., 1886, 1887, 1888.
Geddes, "A Restatement of the Theory of Organic Evolution," Roy. Soc. Edin., 1888.