488 THE DESCENT OP MAN. 



males alone are either wholly or partially white, as with the 

 several bell-birds of South America (Chasmorhynchus), the Ant- 

 arctic goose (Berniola antarctica), the silver-pheasant, &c., whilst 

 the females are brown or oi)scurely mottled. Therefore, on the 

 same principle as before, it is probable 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 white plumage through sexual selection. In 

 some of these cases the plumage becomes white only at maturity. 

 This is the case with certain gannets, tropic-birds, &c., and with 

 the snow-goose (Anse*- hyperboreus). As the latter breeds on the 

 "barren grounds," when not covered with snow, and as it mi- 

 grates southward during the winter, there is no reason to suppose 

 that its snow-white adult plumage serves as a protection. In the 

 Anastomus oscitans, 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 gray and black. With many kinds 

 of gulls (Larus), the head and neck become pure white during 

 the summer, being gray or mottled during the winter and in the 

 young state. On the other hand, with the smaller gulls, or sea- 

 mews (Gavia), and with some terns (Sterna), exactly the reverse 

 occurs; for the heads of the young birds during the first year, 

 and of the adults during the winter, are either pure white, or 

 much paler colored than during the breeding-season. These latter 

 cases offer another instance of the capricious manner in which 

 sexual selection appears often to have acted."' 



That aquatic birds have acquired a white plumage so much 

 oftener than terrestrial birds, probably depends on their large 

 size and strong powers of flight, so that they can easily defend 

 themselves or escape from birds of prey, to which moreover they 

 are not much exposed. Consequently, sexual selection has not 

 here been interfered with or guided for the sake of protection. 

 No doubt with birds which roam over the open ocean, the males 

 and females could find each other much more easily, when made 

 conspicuous either by being perfectly white or intensely black; 

 so that these colors may possibly serve the same end as the 

 call-notes of many land-birds."* A white or black bird when it 



"s On Larus, Gavia, and. Sterna, see Macgillivray, 'Hist. Brit. Birds,' 

 vol. V. pp. 515, 584, 626. On the Anser hyperboreus, Aubudon, 'Ornith. 

 Biography," vol. iv. p. 562. On the Anastomus, Mr. Blyth, in 'Ibis,' 

 1S67, -x 173. 



^ It may be noticed that with vultures, which roam far and wide 

 high in the air, like marine birds over the ocean, three or four species 

 are almost wholly or largely white, and that many others are black. 

 So that here again conspicuous colors may possibly aid th*" sexes In 

 finding each other during the breeding seaein, 



