mature females are less active and less rapid in their movements than
the males, and could not escape so well from danger. Hence, with
animals in a state of nature, we must rely on mere estimation, in order
to judge of the proportions of the sexes at maturity; and this is but
little trustworthy, except when the inequality is strongly marked.
Nevertheless, as far as a judgment can be formed, we may conclude from
the facts given in the supplement, that the males of some few mammals,
of many birds, of some fish and insects, are considerably more numerous
than the females.
The proportion between the sexes fluctuates slightly during successive
years: thus with race-horses, for every 100 mares born the stallions
varied from 107.1 in one year to 92.6 in another year, and with
greyhounds from 116.3 to 95.3. But had larger numbers been tabulated
throughout an area more extensive than England, these fluctuations
would probably have disappeared; and such as they are, would hardly
suffice to lead to effective sexual selection in a state of nature.
Nevertheless, in the cases of some few wild animals, as shewn in the
supplement, the proportions seem to fluctuate either during different
seasons or in different localities in a sufficient degree to lead to
such selection. For it should be observed that any advantage, gained
during certain years or in certain localities by those males which were
able to conquer their rivals, or were the most attractive to the
females, would probably be transmitted to the offspring, and would not
subsequently be eliminated. During the succeeding seasons, when, from
the equality of the sexes, every male was able to procure a female, the
stronger or more attractive males previously produced would still have
at least as good a chance of leaving offspring as the weaker or less
attractive.
POLYGAMY.
The practice of polygamy leads to the same results as would follow from
an actual inequality in the number of the sexes; for if each male
secures two or more females, many males cannot pair; and the latter
assuredly will be the weaker or less attractive individuals. Many
mammals and some few birds are polygamous, but with animals belonging
to the lower classes I have found no evidence of this habit. The
intellectual powers of such animals are, perhaps, not sufficient to
lead them to collect and guard a harem of females. That some relation
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