228 
sexual selection: birds. 
Par ! 1 
pheasant and some other birds should be enciunbei' et 
with plumes so long as to impede their flight. 
In the same manner, as the males alone of varied 
species are black, the females being dull-coloui' eC ’ 
so in a few cases the males alone are either wholly 
or partially white, as with the several Bell-bk'^ 
of South America (Chasmorhynchus), the Antai ctl ‘ 
goose ( Bernicla aniardica), the silver-pheasant, 
whilst the females are brown or obscurely ' 
Therefore, on the same principle as before, it is l’ 1 .' 0 ” 
bable that both sexes of many birds, such as wb lte 
cockatoos, several egrets with their beautiful p>lun ,eS ’ 
certain ibises, gulls, terns, &c., have acquired th e 'j 
more or less completely white plumage through seS Ui \ 
selection. The species which iidiabit snowy region' 1 0 
course come under a different head. The white ph 1 ". 1 
age of some of the above-named birds appears 
both sexes only when they are mature. This ’ 
likewise the case with certain gannets, tropic-h' 1 '^ 
&c‘., and with the snow-goose {Ans&r hyperboreus )• ' 
the latter breeds on the “ barren grounds,” when ^ 
covered with snow, and as it migrates southward din' 111 '’ 
the winter, there is no reason to suppose that its s&°' 
white adult plumage serves as a protection. In t *' 
case ot the Anas! omits oscitans previously alluded ^ 
we have still better evidence that the white plu 
is a nuptial character, for it is developed only dm 11 '^ 
the summer; the young in their immature state, 
the adults in their winter dress, being grey and bh lC ^ 
With many kinds of gulls (Larus), the bead and 
become pure white during the summer, being 
or mottled during the winter and in the young st “ 
On the other hand, with the smaller gulls, or sea-n^'' 
(Gavia), and with some terns (Sterna), exacilv the ’ 
verse occurs; for the heads of the young birds du 1,1 ' t ’ 
