and sometimes in another, is not in most cases known; but the period of
variability seems often to have been the determining cause. When the
two sexes have inherited all characters in common they necessarily
resemble each other; but as the successive variations may be
differently transmitted, every possible gradation may be found, even
within the same genus, from the closest similarity to the widest
dissimilarity between the sexes. With many closely-allied species,
following nearly the same habits of life, the males have come to differ
from each other chiefly through the action of sexual selection; whilst
the females have come to differ chiefly from partaking more or less of
the characters thus acquired by the males. The effects, moreover, of
the definite action of the conditions of life, will not have been
masked in the females, as in the males, by the accumulation through
sexual selection of strongly-pronounced colours and other ornaments.
The individuals of both sexes, however affected, will have been kept at
each successive period nearly uniform by the free intercrossing of many
individuals.
With species, in which the sexes differ in colour, it is possible or
probable that some of the successive variations often tended to be
transmitted equally to both sexes; but that when this occurred the
females were prevented from acquiring the bright colours of the males,
by the destruction which they suffered during incubation. There is no
evidence that it is possible by natural selection to convert one form
of transmission into another. But there would not be the least
difficulty in rendering a female dull-coloured, the male being still
kept bright-coloured, by the selection of successive variations, which
were from the first limited in their transmission to the same sex.
Whether the females of many species have actually been thus modified,
must at present remain doubtful. When, through the law of the equal
transmission of characters to both sexes, the females were rendered as
conspicuously coloured as the males, their instincts appear often to
have been modified so that they were led to build domed or concealed
nests.
In one small and curious class of cases the characters and habits of
the two sexes have been completely transposed, for the females are
larger, stronger, more vociferous and brighter coloured than the males.
They have, also, become so quarrelsome that they often fight together
for the possession of the males, like the males of other pugnacious
525