4.70 The Descent of Man. Part II. 



from one another, so that the males alone can be distinguished, 

 yet the females of most species within the same genus obviously 

 differ from each other. The differences, however, are rarely 

 as great as between the males. We see this clearly in the 

 whole family of the Gallinacese: the females, for instance, of 

 the common and Japan pheasant, and especially of the Gold and 

 Amherst pheasant— of the silver pheasant and the wild fowl 

 —resemble one another very closely in colour, whilst the males 

 differ to an extraordinary degree. So it is with the females of 

 most of the Cotingidse, Fringillidse, and many other families. 

 There can indeed be no doubt that, as a general rule, the females 

 have been less modified than the males. Some few birds, 

 however, offer a singular and inexplicable exception ; thus the 

 females of Paradisea apoda and f. papuana differ from each 

 other more than do their respective males; 7 the female of the 

 latter species having the under surface pure white, whilst 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, 8 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 

 inexplicable ones, which occur independently of man's selection 

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

 very different, whilst the males can hardly be distinguished. 9 



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 greater or less transference to 

 the female of the characters 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, bullfinch, 

 crossbill, sparrow, &c, we shall see that they differ from one 



7 Wallace, <The Malay Arclii- ' Ibis,' 1866, p. 275. 



pelago,' vol. ii. 1869, p. 394. * ' Variation of Animals, &<•., 



8 These species are deserved, with under Domestication,' vol. i. p. 251 

 coloured figures, by M. F Polio, ir 



