that is by the advantage acquired by certain individuals over their
rivals. But in most cases of this kind it is impossible to distinguish
between the effects of natural and sexual selection. Whole chapters
could be filled with details on the differences between the sexes in
their sensory, locomotive, and prehensile organs. As, however, these
structures are not more interesting than others adapted for the
ordinary purposes of life I shall pass them over almost entirely,
giving only a few instances under each class.
There are many other structures and instincts which must have been
developed through sexual selection such as the weapons of offence and
the means of defence of the males for fighting with and driving away
their rivals their courage and pugnacity their various ornaments their
contrivances for producing vocal or instrumental music and their glands
for emitting odours, most of these latter structures serving only to
allure or excite the female. It is clear that these characters are the
result of sexual and not of ordinary selection, since unarmed,
unornamented, or unattractive males would succeed equally well in the
battle for life and in leaving a numerous progeny, but for the presence
of better endowed males. We may infer that this would be the case,
because the females, which are unarmed and unornamented, are able to
survive and procreate their kind. Secondary sexual characters of the
kind just referred to, will be fully discussed in the following
chapters, as being in many respects interesting, but especially as
depending on the will, choice, and rivalry of the individuals of either
sex. When we behold two males fighting for the possession of the
female, or several male birds displaying their gorgeous plumage, and
performing strange antics before an assembled body of females, we
cannot doubt that, though led by instinct, they know what they are
about, and consciously exert their mental and bodily powers.
Just as man can improve the breeds of his game-cocks by the selection
of those birds which are victorious in the cockpit, so it appears that
the strongest and most vigorous males, or those provided with the best
weapons, have prevailed under nature, and have led to the improvement
of the natural breed or species. A slight degree of variability leading
to some advantage, however slight, in reiterated deadly contests would
suffice for the work of sexual selection; and it is certain that
secondary sexual characters are eminently variable. Just as man can
give beauty, according to his standard of taste, to his male poultry,
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