transposition of the instincts, habits, disposition, colour, size, and
of some points of structure, has been effected between the two sexes.
Now if we might assume that the males in the present class have lost
some of that ardour which is usual to their sex, so that they no longer
search eagerly for the females; or, if we might assume that the females
have become much more numerous than the males and in the case of one
Indian Turnix the females are said to be much more commonly met with
than the males (26. Jerdon, Birds of India, vol. iii. p. 598.) then
it is not improbable that the females would have been led to court the
males, instead of being courted by them. This indeed is the case to a
certain extent with some birds, as we have seen with the peahen, wild
turkey, and certain kinds of grouse. Taking as our guide the habits of
most male birds, the greater size and strength as well as the
extraordinary pugnacity of the females of the Turnix and emu, must mean
that they endeavour to drive away rival females, in order to gain
possession of the male; and on this view all the facts become clear;
for the males would probably be most charmed or excited by the females
which were the most attractive to them by their bright colours, other
ornaments, or vocal powers. Sexual selection would then do its work,
steadily adding to the attractions of the females; the males and the
young being left not at all, or but little modified.
CLASS III. WHEN THE ADULT MALE RESEMBLES THE ADULT FEMALE, THE YOUNG
OF BOTH SEXES HAVE A PECULIAR FIRST PLUMAGE OF THEIR OWN.
In this class the sexes when adult resemble each other, and differ from
the young. This occurs with many birds of many kinds. The male robin
can hardly be distinguished from the female, but the young are widely
different, with their mottled dusky-olive and brown plumage. The male
and female of the splendid scarlet ibis are alike, whilst the young are
brown; and the scarlet colour, though common to both sexes, is
apparently a sexual character, for it is not well developed in either
sex under confinement; and a loss of colour often occurs with brilliant
males when they are confined. With many species of herons the young
differ greatly from the adults; and the summer plumage of the latter,
though common to both sexes, clearly has a nuptial character. Young
swans are slate-coloured, whilst the mature birds are pure white; but
it would be superfluous to give additional instances. These differences
between the young and the old apparently depend, as in the last two
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