228 SEXUAL selection: birds. PartIL 



l^heasant and some otlier birds should be encumbered 

 with phimes so long as to impede their flight. 



In the same manner, as the males alone of various 

 species are black, the females being dull-coloured ; 

 so in a few cases the males alone are either wholly 

 or partially white, as with the several Bell-birds 

 of South America (Chasmorhynchus), the Antarctic 

 goose (Bernicla antarctica), tlie silver-pheasant, &c., 

 whilst the females are brown or obscurely mottled. 

 Therefore, on the same principle as before, it is pro- 

 bable that both sexes of many birds, such as white 

 cockatoos, several egrets with their beautiful plumes, 

 certain ibises, gulls, terns, &c,, have acquired their 

 more or less completely wdiite plumage through sexual 

 selection. The sj)ecies which inhabit snowy regions of 

 course come under a different head. The white plum- 

 age of some of the above-named birds appears in 

 both sexes only when they are mature. This is 

 likewise the case with certain gannets, tropic-birds, 

 &c., and with the snow-goose (Anser liyperhoreus). As 

 the latter breeds on the *' barren grounds," when not 

 covered with snow, and as it mio-rates southward durino: 

 the winter, there is no reason to suppose that its snow- 

 white adult plumage serves as a protection. In the 

 case of the Anastomus oscitans previously alluded to, 

 we have still better evidence that the white plumage 

 is a nuptial character, for it is developed only during 

 the summer ; the young in their immature state, and 

 ' the adults in their winter dress, being grey and black. 

 With many kinds of gulls (Larus), the head and neck 

 become pure Avhite during the summer, being grey 

 or mottled during the Avinter and in the young state. 

 On the other hand, v»ith the smaller gulls, or sea-mews 

 (Gavia), and with some terns (Sterna), exactly the re- 

 verse occurs ; for the heads of the young birds during 



