4 o8 THE POPULAR SCIENCE MONTHLY 



Arguments of this kind can not, of course, lead to a definite settle- 

 ment 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 

 Ovocytes rhinoceros, recently described by Doane 8 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 in- 

 stances 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 prob- 

 ably 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 ortho- 

 genesis. 



It is true that external characters or purely morphological char- 

 acters 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 inter- 

 nal coordination — the nervous and the chemical — have been concerned 

 in evolution. Both kinds of mechanisms are of selective value. Re- 

 garding 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 hered- 

 ity of chemical characteristics lies a whole field of experiment on char- 

 acters susceptible of accurate quantitative measurement as yet barely 

 touched by the hand of the Mendelian. 



s Science, 1913, N. S., XXXVIII., p. 883. 



