Chap. XVI. THE YOUNG LIKE THE ADULT FEMALES. 193 
males ; the female of the latter species having the 
under surface pure white, wdiilst the female P. apoda is 
deep brown beneath. So, again, as I hear from Professor 
Newton, the males of two species 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 Mauritian 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., we 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/ 1866, p. 275. 
^ ‘Variation of Animals, &c., under Domestication,’ vol. i. p. 251. 
VOL. II. O 
