species (52. Rhamphastos carinatus, Gould s Monograph of
Ramphastidae. ), says that the colours of the beak are doubtless in
the finest and most brilliant state during the time of pairing. There
is no greater improbability that toucans should be encumbered with
immense beaks, though rendered as light as possible by their
cancellated structure, for the display of fine colours (an object
falsely appearing to us unimportant), than that the male Argus 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, 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 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 white 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 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 coloured than during the breeding-season.
These latter cases offer another instance of the capricious manner in
which sexual selection appears often to have acted. (53. On Larus,
Gavia, and Sterna, see Macgillivray, History of British Birds, vol.
v. pp. 515, 584, 626. On the Anser hyperboreus, Audubon,
Ornithological Biography, vol. iv. p. 562. On the Anastomus, Mr.
519