176 SEXUAL selection: birds. PartH. 



plumage between the sexes are of the same general 

 nature as the occasionally greater differences. A good 

 illustration 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 differences 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 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 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 com- 

 mencing to become obscure for the sake of protection. 

 Even if we consider somewhat more marked sexual dif- 

 ferences, 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 



