176 
SEXUAL SELECTION: BIRDS. 
Paet IL 
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 
