692 THE DESCENT OF MAN 



and the colors of the male, when he differs slightly or evea 

 considerably from the female, have been in most cases de- 

 termined 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. Assuming, 

 however, that they are of service, they might be thought to 

 be eases of transition ; but we have no reason to believe that 

 many species at any one time are undergoing change. There- 

 fore 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 differ- 

 ences, is it probable, for instance, 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 process of selection for the sake of protection ? 1 

 cannot think so ; and still less with the slight differences be- 

 tween the sexes of those birds which build concealed nests. 

 On the other hand, the differences in color between the 

 sexes, whether great or small, may to a large extent be 

 explained on the principle of the successive variations ac- 

 quired by the males through sexual selection having been 

 from the first more or less limited in their transmission to 

 the females. That the degree of limitation should differ 

 in different species of the same group will not surprise any 

 one who has studied the laws of inheritance, for they are 

 so complex that they appear to us in our ignorance to be 

 capricious in their action." 



As far as I can discover, there are few large groups of 

 birds in which all the species have both sexes alike and 

 brilliantly colored, but I hear from Mr. Sclater that this 



'8 See remarks to this effect in my work on "Variation under Domestica- 

 tion, " vol. ii. chap. xii. 



