Chap. XVL THE YOUNG LIKE THE ADULT FEMALES. 193 



males ;^ the female of the latter species having the 

 under surface pure white, wliilst the female P. apoda is 

 deep brown beneath. So, again, as I hear from Professor 

 Newton, the males of two sj^ecies of Oxynotus (shrikes), 

 which represent each other in the islands of Mauritius 

 and Bourbon,* differ but little in colour, whilst the 

 females differ much. In the Bourbon species the female 

 appears to have partially retained an immature condition 

 of plumage, for at first sight she " might be taken for 

 " the young of the IMauritian species." These differences 

 may be compared with those which occur, independently 

 of selection by man, and which we cannot explain, in 

 certain sub-breeds of the game-fowl, in which the females 

 are very different, whilst the males can hardly be dis- 

 tinguished.^ 



As I account so largely by sexual selection for the 

 differences between the males of allied species, how can 

 the differences between the females be accounted for 

 in all ordinary cases? We need not here consider the 

 species which belong to distinct genera ; for with these, 

 adaptation to different habits of life, and other agencies, 

 will have come into play. In regard to the differences 

 between the females within the same genus, it appears to 

 me almost certain, after looking through various large 

 groups, that the chief agent has been the transference, 

 in a greater or less degree, to the female of the cha- 

 racters acquired by the males through sexual selection. 

 In the several British finches, the two sexes differ either 

 very slightly or considerably ; and if we compare the 

 females of the greenfinch, chaffinch, goldfinch, bull- 

 finch, crossbill, sparrow, &c., w^e shall see that they 



' Wallace, ' The Malay Archipelago,' vol. ii. 1869, p. 394. 

 * These species are described, with coloured figures, by M. F. Pollen, 

 in ' Ibis,' 186tJ, p. 275. 



^ 'Variation of Animals, &c., under Domestication,' vol. i. p. 251. 



VOL. II. O 



