Arguments of this kind can not, of course, lead to a definite settlement of the question of the potency of sexual selection, but they appear to increase the probability that sexual selection may be a reality, and a more potent agent in evolution than we have realized or been inclined to admit. Many peculiar characters have a function that is at present unknown, and many of these unknowns have been supposed to act as factors in sexual selection. More accurate observation has, however, shown their true use in the animal's activities. The horns of the Orocytes rhinoceros, recently described by Doane[1] afford an example of a character, once supposed to be effective in sexual selection, which has proved to be of direct use to the animal in getting its food, and hence, an agent in natural selection. Undoubtedly, further studies of animals and plants in their natural environment will lead to still further instances in which characters apparently useless, so far as their relation to getting food or resisting enemies goes, will prove to have some direct use in the life of the animal or plant. Even at present, characters supposed to be active through sexual selection alone should be scrutinized with some care before their case is admitted to serious consideration.
The view that the secondary sexual characters are the expression of internal metabolic changes, internal secretions, and the like, does not exclude a selective value. Darwin did not explain the origin of such variations, but supposed that they might be of selective value, and if secondary sexual characters are related to the activity of organs of internal secretion, such an occurrence need not disturb us. Physiologists generally regard all an organism's responses as due to an interaction of internal and external factors, and some such similar relationship is probably at the bottom of orthogenesis. As I hope to point out in a later paper, there is much evidence of a chemical nature in favor of orthogenesis.
It is true that external characters or purely morphological characters such as muscles and bones have, heretofore, formed the greater part of the subject matter of discussions on evolution. But the idea of the potency of internal functional factors in evolution has crept in, and Gaskell has pointed out that two general kinds of mechanisms of internal coordination—the nervous and the chemical—have been concerned in evolution. Both kinds of mechanisms are of selective value. Regarding lactation as a secondary sexual character, great changes in such a character, dependent partly upon morphological, but largely upon chemical organization, have been brought about in the various breeds of dairy cattle by artificial selection. And in this domain of the heredity of chemical characteristics lies a whole field of experiment on characters susceptible of accurate quantitative measurement as yet barely touched by the hand of the Mendelian.
- ↑ Science, 1913, N. S., XXXVIII., p. 883.
