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SEXUAL selection: bikds. 
Pakt II. 
pheasant and some other birds should be encumbered 
with plumes 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), the 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 white plumage through sexual 
selection. The species 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 ease with certain gannets, tropic-birds, 
&c., and Avith 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 
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 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 re- 
verse occurs ; for the heads of the young birds during 
