carlotta; namely, that the young females, like the young males, have
some crimson about their heads, but that this colour disappears in the
adult female, whilst it is intensified in the adult male. Nevertheless
the following considerations render this view extremely doubtful: the
male takes a fair share in incubation (26. Audubon s Ornithological
Biography, vol. ii. p. 75; see also the Ibis, vol. i. p. 268.), and
would be thus almost equally exposed to danger; both sexes of many
species have their heads of an equally bright crimson; in other species
the difference between the sexes in the amount of scarlet is so slight
that it can hardly make any appreciable difference in the danger
incurred; and lastly, the colouring of the head in the two sexes often
differs slightly in other ways.
The cases, as yet given, of slight and graduated differences in colour
between the males and females in the groups, in which as a general rule
the sexes resemble each other, all relate to species which build domed
or concealed nests. But similar gradations may likewise be observed in
groups in which the sexes as a general rule resemble each other, but
which build open nests.
As I have before instanced the Australian parrots, so I may here
instance, without giving any details, the Australian pigeons. (27.
Gould s Handbook to the Birds of Australia, vol. ii. pp. 109-149.) It
deserves especial notice that in all these cases the slight differences
in plumage between the sexes are of the same general nature as the
occasionally greater differences. A good illustration of this fact has
already been afforded by those kingfishers in which either the tail
alone or the whole upper surface of the plumage differs in the same
manner in the two sexes. Similar cases may be observed with parrots and
pigeons. The differences in colour between the sexes of the same
species are, also, of the same general nature as the differences in
colour between the distinct species of the same group. For when in a
group in which the sexes are usually alike, the male differs
considerably from the female, he is not coloured in a quite new style.
Hence we may infer that within the same group the special colours of
both sexes when they are alike, and the colours of the male, when he
differs slightly or even considerably from the female, have been in
most cases determined by the same general cause; this being sexual
selection.
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