Factors of Evolution.—The causes of change or adaptive modification, or the factors of evolution, are at least four well known, and probably many more still unknown: 1. The physical environment—heat and cold, dryness and moisture—affects function of organs, and function affects structure, and both changed function and changed structure are inherited by offspring, and so increased from generation to generation, becoming greater without limit. 2. Increased use or disuse of organs enforced or permitted by change in the environment, physical or organic, or both, induces change in form, size, and structure of the organs; and this change is inherited by the offspring, and so from generation to generation small differences are integrated until they become great without limit. These two factors were recognized by Lamarck. 3. “Natural selection,” or “survival of the fittest,” among divergent varieties of offspring. This is the distinctive Darwinian factor. In the two preceding factors the change is during the individual lifetime, and reproduction is supposed to transmit it unchanged to the offspring. In this factor, on the contrary, the form and structure are supposed to remain unchanged during the individual life, but for some unknown cause there are slight variations in different directions (divergent) in the offspring from the same parents. Now, when we remember that by reproduction the number of individuals tends to increase by geometrical progression, and that in each generation only a very few (on an average only two from all the offspring of one pair) can survive, it is evident that among these divergent varieties those will most likely survive which are most in harmony with the external environment, and which possess the most efficient organs of defense or of escape, or for food-taking. The surviving offspring, therefore, will be on the average better in these respects than their parents. It matters not how little better, for the integration of even infinitesimal improvements from generation to generation will eventually produce any required amount of change. 4. To the above Darwin has added also “sexual selection.” In natural selection there is struggle of all for food, or means of living. In sexual selection there is a struggle among the males for possession of the female, and the means of procreation. The one is connected with the nutritive appetite, the other with the reproductive appetite. This mode of selection acts in two ways, by the law of battle and the law of attractiveness. The strongest or the most attractive males alone, or mainly, leave offspring, which, of course, inherit their peculiarities; and these are increased indefinitely by integration through successive generations, thus increasing the strength or the beauty. Of these two laws, the law of battle is most conspicuous among mammals, and the law of attractiveness among birds. It is evident that this factor can not operate among many lower animals which are hermaphroditic, nor among plants.
Of these acknowledged factors of evolution, the first two were known to Lamarck and the older evolutionists. The third and fourth are distinctively Darwinian. According to Darwin, while all these are operative, the third is the most powerful; but Spencer accords this distinction to the Lamarckian factors. Many American zoölogists take the same view.
Such until very recently were all the recognized factors of evolution. But, within the past year (1886) has taken place, it seems to us, the most important advance in the theory of evolution since Darwin. It is the suggestion by Mr. Catchpool,[14] and afterward the more full elaboration by Dr. Romanes, of another factor, which he calls “physiological selection.”[15]
The great objections to the sufficiency of the theory of evolution, as left by Darwin, were twofold: 1. While natural selection accounts completely for the formation of useful structures or adaptive modifications, and therefore for differences characterizing classes, orders, families, and even genera—for these are all adaptive—it can not so completely account for those constituting species; for these consist mostly of trivial differences in coloration, relative proportion of parts, which are of no perceivable use in the struggle for life, and therefore could not be preserved and integrated by natural selection. Therefore, according to Romanes, natural selection is a theory of origin of adaptive structures rather than of origin of species. Comparing to a growing tree, once admit lateral buds started, and natural selection completely accounts for the growth in different directions, and therefore for the profuse ramification; but the origin of the lateral buds is not explained.
2. The second difficulty is as follows: Such commencing differences as constitute varieties and species not only would not be preserved and integrated by natural selection unless useful, but would immediately be swamped by cross-breeding with the parental form. But, as the whole divergence commences in varieties, evidently it could not commence at all unless this cross-breeding be in some way prevented. This may, indeed, be done, without the assumption of any new factor of evolution, by migration; and, hence, migration must be regarded as an important agent in the creation of new forms, not only by the effect of a new environment, but also by prevention of the swamping of commencing species by cross-breeding with the parental form; but in a crowded locality, without outlet for migration (the very conditions most favorable for severe competitive struggle, and therefore for most potent operation of natural selection; and therefore, also, according to Darwin, for profuse diversification), commencing varieties could not pass into species, because swamped by cross-breeding. Once the divergence reaches the point of cross-sterility—i. e., of species—then, indeed, by true breeding, characters, even though not useful, may be preserved. But how is it to commence?
This difficulty has been severely felt by all Darwinists. It seems to us that it is largely met by Dr. Romanes. According to Romanes, no organ is so subject to varietal changes as the reproductive, and these in no respect so much as in degrees of fertility. Unfortunately, these changes are not visible, and must be judged of only by the results. It is not uncommon, for example, to find sterility between individuals (sexual incompatibility) who are both of them perfectly fertile with other individuals. Similarly, cross-sterility, partial or complete, is not uncommon between varieties or races, as Mr. Darwin has long ago noticed. It very generally, as we know, occurs between, and, in fact, is constantly used as a test of, species. Now, this cross-sterility with parent stock, which we find so constant a character of species, and which, therefore, must have commenced as a partial cross-sterility in varieties, is it antecedent or consequent to other variations? It has been usual to suppose it consequent to a certain amount of divergence, viz., that which constitutes, or at least approaches, species. But, according to Romanes, it is antecedent. Among many other variations, this is that one which originates species, because it prevents reversion by cross-breeding with the parent stock, and insures true breeding with its own kind. In a word, it sexually isolates the species. Suppose, then, a species multiplying indefinitely in one locality: trivial variations of many kinds, and in many directions, occur among the offspring. These are merged by cross-breeding into the original type, which, therefore, remains unchanged. But, from time to time, among these variations there occur some affecting the reproductive organs in such wise as to produce partial or complete cross-sterility with the parent form. This is the beginning of a new species. It breeds true with its own kind, and therefore all the associated variations external and visible, and therefore constituting species, although trivial and of no use in the struggle for life, are preserved.
This view completely accounts for the cross-fertility of artificial breeds equivalent in other respects to species; for cross-sterility is not an end aimed at by the breeder, it being easy to prevent cross-breeding, if desired, by artificial isolation. But, if this view be true, species from widely-different geographical regions ought also to be often cross-fertile, because, having been formed by geographical isolation, sexual isolation was not a necessary factor in their formation. This point deserves testing by careful observation.
It may be, and has been, objected to Dr. Romanes’s claims, that this is no new factor; that physiological selection is only a form of natural selection. This objection, it seems to us, is little more than a play upon words. It certainly is selection, and by a natural process, and therefore in some sense a natural selection, but not in the sense of Darwin. It is not a selection of individuals fittest to survive; for cross-fertile individuals are as fit to survive as individuals, though not as species, as are cross-sterile. Natural selection is intent only on preserving the best individuals; physiological selection on preserving the kind. Natural selection continues the direction of progress unchanged; physiological makes new directions.
In addition to all these factors of organic evolution, there is still another far higher factor characteristic of man alone. This is the conscious, voluntary co-operation of the thing evolving—the spirit of man—in the work of its own evolution. This may be called the rational factor. This, the most important factor of human evolution, is usually ignored by writers on evolution—either as non-existent, or else as lying beyond the domain of science. We will emphasize its importance by taking it up more fully in the next chapter.
It will be observed that Darwin and his followers take divergent variations of offspring simply as a known fact, upon which natural selection operates to produce progressive modification; and, as the cause of variation in offspring is wholly unknown, such variations are often spoken of as fortuitous. But, of course, it is well understood that nothing in Nature is really fortuitous. They may, however, for all purposes of natural selection be thus regarded until we know their cause. It is evident, then, that if we, with Darwin, take natural selection, as the most important known factor, the really most important cause of evolution is the cause of varieties. This is the unknown fundamental factor. As Darwin reduced Agassiz’s three formal laws of succession to more general laws of life, and thus made one important step in the advance of biological science, so he who shall explain the cause of divergent variation will make another important step by reducing the phenomena to still more general and fundamental laws of life.