168 SEXUAL SELECTION: BIRDS. [Part IL 



general nature as the oceasionally greater difFerences. A 

 good illustration of this fact has already been afforded by 

 those kingfisliers in which either the tail alone or the 

 whole upjier surface of the plumage differs in the same 

 manner in the two sexes. Similar cases may be observed 

 with parrots and pigeons. The differences in color be- 

 tween the sexes of the same species are, also, of the same 

 general nature as the differences in color between the dis- 

 tinct species of the same group. For, when, in a group in 

 which the sexes are usually alike, the male diffbrs consid- 

 erably from the female, he is not colored in a quite rrtw 

 style. Hence we may infer that, within the same group, 

 the special colors of both sexes, when they are alike, and 

 the colors of the male, when he differs slightly or even 

 considerably from the female, have in most cases been 

 determined by the same general cause ; this being sexual 

 selection. 



It is not probable, as has already been remarked, that 

 differences in color between the sexes, when very slight, 

 can be of service to the female as a protection. Assum- 

 ing, however, that they are of service, they might be 

 thought to be cases of transition ; but we have no reason 

 to believe that many species at any one time are under- 

 going change. Therefore, we can hardly admit that the 

 numerous females which differ very slightly in color from 

 their males are now all commencing to become obscure 

 for the sake of protection. Even if we consider somewhat 

 more marked sexual differences, is it probable, for in- 

 stance, that the head of the female chaffinch, the crimson 

 on the breast of the female bullfinch, the green of the 

 female greenfinch, the crest of the female golden-crested 

 wren, have all been rendered less bright by the slow pro- 

 cess of selection for the sake of protection ? I cannot 

 think so; and still less Avith the slight differences between 

 the sexes of those birds which build concealed nests. On 



