184 SEXUAL SELECTION: BIRDS. [Paet II 



to act on and accumulate; and, as sexual selection is 

 always at work, it would (judging from what we know 

 of the results on domestic animals of man's unintentional 

 selection) he a surprising fact if animals inhabiting sepa- 

 rate districts, which can never cross and thus blend theii 

 newly-acquired characters, were not, after a sufficient 

 lapse of time, differently modified. These remarks like- 

 wise apply to the nuptial or summer plumage, whether 

 confined to the males or common to both sexes. 



Although the females of the above closely-allied spe- 

 cies, together with their young, differ hardly at all from 

 each other, so that the males alone can be distinguished, 

 yet in most cases the females of the species within the 

 same genus obviously differ from each other. The differ- 

 ences, however, are rarely as great as between the mules. 

 "We see this clearly in the whole family of the Gallinaceae : 

 the females, for instance, of the common and Japan pheas- 

 ant, and especially of the gold and Amherst pheasant, 

 of the silver pheasant and the wild-fowl, resemble each 

 other very closely in color, while 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 modified to a less extent than the 

 males. Some few birds, however, offer a singular and 

 inexplicable exception ; thus the females of Paradisea 

 apoda and P. Papuana differ from each other more than 

 do their respective males ; 7 the female of the latter spe- 

 cies having the under surface pure white, while the female 

 P. apoda is deep brown beneath. So, again, as I hear 

 from Prof. Newton, the males of two species of Oxy- 

 notus (shrikes), which represent each other in the islands 

 of Mauritius and Bourbon, 8 differ but little in color, while 



7 Wallace, 'The Malay Archipelago,' vol. ii. 1869, p. 391. 



8 These species are described, with colored figures, by M. F. Pollen, 

 in 'Ibis,' 1866, p. 275. 



