hap. XV. Birds — Colour and Nidification. 459 



differences in plumage between the sexes are of the same general 

 nature as the occasionally greater differences. A good illustra- 

 tion of this fact has already been afforded by those kingfishers in 

 which either the tail alone or the whole upper surface of the 

 plumage differs in the same manner in the two sexes. Similar 

 cases may be observed with parrots and pigeons. The differ- 

 ences in colour between the sexes of the same species are, also, 

 of the same general nature as the differences in colour between 

 the distinct species of the same group. For when in a group in 

 which the sexes are usually alike, the male differs considerably 

 from the female, he is not coloured in a quite new style. Hence 

 we may infer that within the same group the special colours of 

 both sexes when they are alike, and the colours of the male, when 

 he differs slightly or even considerably from the female, have 

 been in most cases determined by the same general cause ; this 

 being sexual selection. 



It is not probaUe, as has already been remarked, that differ- 

 ences in colour 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 cases of 

 transition ; but we have no reason to believe that many species 

 at any one time are undergoing change. Therefore we can 

 hardly admit that the numerous females which differ very 

 slightly in colour 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 

 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 ? I cannot think so ; and still less with the 

 slight differences between the sexes of those birds which build 

 concealed nests. On the other hand, the differences in colour be- 

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

 explained on the principle of the successive variations, acquired 

 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. 28 



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

 which all the species have both sexes alike and brilliantly 



2 ' See remarks to this effect in my work on ' Variation under Domestic 

 catkn,' vol. ii. chap. xii. 



