522 THE DESCENT OF MAN. 



the difference between the sexes in the amount of scarlet is 

 so slight that it can hardly make any appreciable difference 

 in the danger incurred; and lastly, the coloring of the 

 head in the two sexes often differs slightly in other ways. 



The cases, as yet given, of slight and graduated differ- 

 ences in color between the males and females in the groups, 

 in which, as a general rule, the sexes resemble each other, 

 all relate to species which build domed or concealed nests. 

 But similar gradations may likewise be observed in groups 

 in which the sexes as a general rule resemble each other, 

 but which build open nests. 



As I have before instanced the Australian parrots, so I 

 may here instance, without giving any details, the Austra- 

 lian pigeons.* It deserves especial notice that in all these 

 cases the slight differences in 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 color 

 between 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 differs considera- 

 bly from the female he is not colored in a quite new 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 been in most cases 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. 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 color from their males are now 

 all commencing to become obscure for the sake of protection. 



* Gould's " Hand-book to the Birds of Australia," vol. ii, pp. 

 J09-149. 



