THE DESCENT OF MAN 



This is the case with certain gaanets, tropic-birds, etc., and 

 with the snow-goose {Anser hyperboreus). As the latter 

 breeds on the "barren grounds" when not covered with 

 snow, and as it migrates 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 sum- 

 mer; 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 dur- 

 ing 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 (Q-avia), and with so'me 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, go that they 

 can easily defend themselves or escape from birds of prey, 

 to which, moreover, they are not much exposed. Conse- 

 quently, 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 con- 

 spicuous 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 



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

 vol. V. pp. 515, 584, 626. On tlie Anser hyperboreus, Audubon, "Ornlth. 

 Biography," vol. iv. p. 562. On the Anastomus, Mr. Blyth, In "Ibis," 1867, 

 p. Its. 



" It may be noticed that with vultures, which roam tar and wide high In 

 the air, like marine birds over the ocean, three or four species are almost wholly 



