The Descent of Man

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, etc., whilst the females
are brown or obscurely 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, etc., 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, 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 nup-