CHAPTER IV

FACTORS AND MECHANISM OF EVOLUTION

Even in the latest and maturest formulations of scientific research, the dramatic tone is never lost. The causes at work are conceived in a highly impersonal way, but hitherto no science has been content to do its work in terms of inert magnitude alone. Activity continues to be imputed to the phenomena with which science deals, and activity is, of course, not a fact of observation but is imputed to the phenomena by the observer. Epistemologically speaking, activity is imputed to phenomena for the purpose of organizing them into a dramatically consistent system."-THORSTEIN VEBLEN.

THERE is to-day no doubt in our minds of the truth, the actuality, of descent. It is not the theory of descent: it is the fact, the law, of descent, of which we talk and write. Organisms are blood-related: they are transformed, descended from one another. This, which is the common knowledge of presentday post-Darwinian science, was the belief of many naturalists even before the days of Darwin. "From the Greeks to Darwin" was not all darkness nor complete freedom from taint of the "pernicious evolution doctrine." Goethe, Erasmus Darwin, Lamarck, to mention only familiar names, were evolutionists: they believed in the transmutation of species, believed in descent. But it was Darwin who gave the waiting naturalists substantial and satisfactory reasons for the beliefs that were in them; who gave them strength to have the courage of their convictions.

While Darwinism, in our present-day use of the name, is not synonymous with descent and evolution, but is the name of a causomechanical explanation of it, or a group of causal factors, yet it might justifiably be more broadly used, and held still to mean, what it certainly did to the world generally for a good many years after the "Origin of Species" appeared, the

general theory of organic evolution and the particular doctrine of the descent of man from the lower animals. For it was Darwin who really proved these things to be realities. But in biology to-day Darwinism is the name which refers to certain particular causal factors or determining agents in the actual production and control of the transmutation of species and the progress and direction of the lines of descent. And the modern scientific adverse criticism of Darwinism which is beginning to find its echoes in popular literature must never be mistaken to be disparagement or adverse criticism of the doctrine of descent, the law of organic evolution. So we are not in this book discussing the probabilities of the truth or untruth of evolution nor presenting evidences or argument to justify a belief in the doctrine. As the days have long passed when the shape of the earth, or the behavior of the members of the solar system, was a fit subject for debate, so the days are now by when the truth or falsity of the law of organic descent is a debatable thesis. The earth is subspherical, the planets revolve about the sun, and species of organisms descend from other species.

But in what particular way, or as the effect of what particular causal factors, this descent or transformation of species, that is, kinds of organisms, comes about,-here there is unlimited field for debate and polemic, for hypothesis and investigation, for deduction and determination. It is the factors of organic evolution, the factors of each of the particular phases or aspects of evolution phenomena, that are the subject of present-day biological study and discussion. The mechanism and method of evolution is a subject, with its score of moot questions, its enormous opportunity and inspiration, for fact gathering, fact arranging, and fact interpreting. So in biological science today, no less but even more than in those first exciting days after the "Origin of Species," the subject and problems of evolution are the inspiriting and absorbing matters which chiefly occupy the attention of biologists. What these factors are that compose the chief subject of present-day evolution study and discussion may be summarily set out in the present chapter.

In the first place it is obvious that there can be no transformation or change of species unless there is an ever-present actual variation. By variation is simply meant, in the larger sense, that no two individual organisms in the world, nor for that matter any two that have ever been in the world, are exactly

alike. This refers not only to individuals of different species of plants and animals, but to individuals of the same species and even (and this in a way is most important of all) to individuals born of the same parents. It is indeed this last condition that is the actual basis and fundamental beginning for species change. That this variation does exist is absolute fact, and there is no discussion of it.

To what extent or degree, what parts of an organism are chiefly affected, whether or no this variation shows a regularity in its occurrence or a determinateness of tendency or direction, whether or no this variation is based on inheritance and if so in what degree of similarity or identity-all these and a dozen other questions are the moot problems in connection with the great factor variation. These are undecided things, which means, on the whole, that variation, apart from the observed and admitted actuality of the occurrence, is itself a great evolution problem.

The variation alone, however, presumably does not make new species nor maintain lines of descent. If this variation is, as it seems to be, almost unlimited in its range of appearance, then as species are of definite character and number and as lines of descent are even more definite and more limited as to number, there must be some factor which determines what kinds or lines of variation may or shall persist and what shall be extinguished. Is there something incident to the causes of variation that determines what lines of descent shall be established by it or based on it, or is there some added factor which, having no control over the initial appearance of variation, has absolute control over its persistence and headway? Darwin's factors of selection, more particularly natural selection, is the explanation of this control offered in the famous "Origin of Species." And natural selection has been in the minds of biologists until to-day, at least, undoubtedly that factor in evolution which has been believed to have the chief control in the forming of species and the direction of descent lines.

But in reference to this particular factor three schools of biologists have gradually grown up; namely, first the school headed by Weismann, who has believed and contended that natural selection is almost the only factor which, on a basis of fortuitous, that is, uncontrolled, variation, has produced the species and lines of descent as we know them; second, the

school which holds that natural selection has practically nothing to do with species-forming but only, and in a large general way, with the control of descent; and, third, the compromise school, which attributes to natural selection an important part in both species-forming and control of general descent lines, but recognizes the simultaneous existence and the considerable importance of several other species-forming and descent-modifying factors. In addition to these three schools one must note that a number of active working biologists repudiate the factor of natural selection entirely, holding it to be a vagary and an artifact of logic.

Associated with natural selection in the general theory of selective action is Darwin's conception of sexual selection. This factor was presumed by Darwin to play a part only in the formation and control of those often very obvious but never wellunderstood characteristics of a secondary sexual character which distinguish the sexes in many species of animals. Let one recall these characters in the pea fowl, the bird of paradise, the pheasant, some of the butterflies, the lamellicorn beetles, many fishes, and so on. According to the theory of sexual selection the females have chosen for their consorts those males best endowed by variation with these ornamental characteristics, so that by this selection there has come about a gradual cumulation of the characteristics culminating in such bizarrerie as we are familiar with in numerous living animals.

The word selection will certainly bring to the mind of the reader also a third kind of selective process, namely, that called artificial selection, and this kind of selection is, of course, a factor, and an important one, and has been such for some eighty centuries, in the modification of plant and animal forms. But however widely differing and extraordinarily modified cultivated and domesticated kinds of animals and plants may be, these different kinds are not looked on by biologists as having the validity, that is, the stability and characteristics of origin, that the different species of animals and plants found in nature have.

All the different kinds of pigeons, for example, are known to be due primarily to the artificial modification of a single wild kind, the rock dove of Europe, and all of these different artificially produced kinds agree in an important physiological characteristic, namely, that of being able to mate freely with each other and with their common ancestor. As this physiological char

acteristic is precisely one of the criteria largely used in determining species limits in nature, naturalists call the artificially produced kinds by another name than species; they call them races or varieties, meaning by this to indicate obvious structural and functional differences. Thus artificial selection, while a factor in determining the extent and character of the modification of many kinds of animals and plants, is not considered a factor in the determination of natural lines of descent. Its value in this regard lies in the clew it gives to natural processes of the same kind.

Selection by nature among the variations which appear is made possible only by several other factors or actually existent conditions. One is the "prodigality of production" or the constant tendency to overpopulation due to reproduction by multiplication or in a geometrically progressive ratio. Every mature female or hermaphroditic plant or animal produces, at least in the condition of eggs or germ cells, more than one new individual like itself. (There are a very few exceptional cases, compensated for, however, in other ways.) Most produce many new individuals and some reproduce enormously. Certain fishes lay millions of eggs; so do certain oysters; many insects produce thousands of young; many plants produce myriads of seeds. But not all can grow up: there is neither room nor food for all. There must inevitably be a selection by active or passive, guided or fortuitous, means.

It is a necessary assumption, for the effectiveness of the natural selection factor, that this selection is actually based on the fitness or advantage of some of the variations as compared with others. The trying out or determination of the advantage of these variations comes about as an inevitable active or passive competition for life among the overabundantly appearing new individuals. This is the "struggle for existence," and the "survival of the fittest" is the expression of the assumed fact of the success of the individuals advantageously (i. e., most fitly) varying. The unfit and the less fit are assumed to compose the thousands and hundreds of thousands who must die where only tens or hundreds can live at one time.

But if natural selection, which is, so far, obviously one of but individuals alone, is to produce new species and control descent lines, it has to depend on a further factor, one named by a familiar word, but not at all explained by it, namely, the factor