SEXUAL SELECTION 591 



intensified in the adult male. Nevertheless the following 

 considerations render this view extremely doubtful; the 

 male takes a fair share in incubation," and would be thus 

 almost equally exposed to danger ; both sexes of many spe- 

 cies have their heads of an equally bright crimson; in other 

 species 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 Australian 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. Simi- 

 lar 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 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 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. 



2« Audubon's "Ornithological Biography," vol. ii. p. 15; see, also, the 

 'Ibis," vol. i. p. 268. 



" aould's "Handbook to the Birds of Australia," vol. ii. pp. 109-149. 



