males during the breeding-season; the odoriferous glands having been
acquired through sexual selection. Whether the same view can be
extended to the voice is doubtful, for the vocal organs of the males
must have been strengthened by use during maturity, under the powerful
excitements of love, jealousy or rage, and will consequently have been
transmitted to the same sex. Various crests, tufts, and mantles of
hair, which are either confined to the male, or are more developed in
this sex than in the female, seem in most cases to be merely
ornamental, though they sometimes serve as a defence against rival
males. There is even reason to suspect that the branching horns of
stags, and the elegant horns of certain antelopes, though properly
serving as weapons of offence or defence, have been partly modified for
ornament.
When the male differs in colour from the female, he generally exhibits
darker and more strongly-contrasted tints. We do not in this class meet
with the splendid red, blue, yellow, and green tints, so common with
male birds and many other animals. The naked parts, however, of certain
Quadrumana must be excepted; for such parts, often oddly situated, are
brilliantly coloured in some species. The colours of the male in other
cases may be due to simple variation, without the aid of selection. But
when the colours are diversified and strongly pronounced, when they are
not developed until near maturity, and when they are lost after
emasculation, we can hardly avoid the conclusion that they have been
acquired through sexual selection for the sake of ornament, and have
been transmitted exclusively, or almost exclusively, to the same sex.
When both sexes are coloured in the same manner, and the colours are
conspicuous or curiously arranged, without being of the least apparent
use as a protection, and especially when they are associated with
various other ornamental appendages, we are led by analogy to the same
conclusion, namely, that they have been acquired through sexual
selection, although transmitted to both sexes. That conspicuous and
diversified colours, whether confined to the males or common to both
sexes, are as a general rule associated in the same groups and
sub-groups with other secondary sexual characters serving for war or
for ornament, will be found to hold good, if we look back to the
various cases given in this and the last chapter.
The law of the equal transmission of characters to both sexes, as far
as colour and other ornaments are concerned, has prevailed far more
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