1898] SPECIES, SEX, AND THE INDIVIDUAL 191 



Every one knows that there are enormous differences among 

 different species in the degree of development of secondary sexual 

 characters. In many cases there are no such characters, the males 

 and females are similar ; and in these cases not only are sexual 

 differences wanting, but we do not find variations which if increased 

 would lead to them. Secondary sexual characters in the plumage 

 and in other structures are very conspicuous in the Class of birds, 

 but there are numerous species of birds in which the male and 

 female are scarcely distinguishable. It is an important fact that in 

 the latter cases the birds are monogamous, pairing either for a whole 

 season or for life, while birds in which the male plumage is in 

 gorgeous contrast to that of the female are frequently polygamous. 

 This fact has been emphasised by Darwin, but the significance of it 

 in his view was that polygamy involved a relative excess in the 

 number of males, so that those which obtain a plurality of wives 

 have been selected from a large number, leaving a remnant which 

 obtain no wives at all. I believe that the correct interpretation of 

 the matter is very different. 



It is well known that, in the most familiar cases of special plum- 

 age in male birds, this plumage is elaborately displayed in courtship 

 in a definite manner peculiar to each species. As Darwin states, 

 " Ornaments of all kinds, whether permanently or temporarily gained, 

 are sedulously displayed by the males, and apparently serve to at- 

 tract or fascinate the females." Now this display is an erection of 

 the feathers by the muscles in the skin, and a movement, an agita- 

 tion or vibration, of the feathers. "We have every reason to believe 

 that a mechanical movement of the feathers must irritate the papillae 

 from which they are produced, and stimulate the proliferation of 

 epidermis to which the growth of the feathers is due. Thus, if we 

 consider only the increased size of the feathers apart from their 

 colour or markings, we may conclude that the display and erection 

 of certain feathers is the exciting cause of their excessive develop- 

 ment in the males. This theory is supported at any rate by the 

 fact that the degree of development of special plumage corresponds 

 to the proportion of his life and activities which the male devotes to 

 courtship. This may, in the present tendency of biological doctrines, 

 be considered absurd, but it will be found, if the facts are examined, 

 that it is literally and scientifically true. The pigeon, for example, 

 pairs for life. He performs gestures of courtship it is true, but he 

 also takes an equal share with the female in the duties of incuba- 

 tion and care of the young, and consequently his courtship consumes 

 only a small portion of his time. A polygamous male bird on the 

 other hand performs no part of the work of incubation or feeding 

 the young, and in the breeding season spends a very large part of 

 his time in displaying his plumage to his numerous partners. 



