selected, the thickness of the shell being known to vary like every
other structure.
_Sexual Selection_. Inasmuch as peculiarities often appear under
domestication in one sex and become hereditarily attached to that sex,
the same fact probably occurs under nature, and if so, natural
selection will be able to modify one sex in its functional relations to
the other sex, or in relation to wholly different habits of life in the
two sexes, as is sometimes the case
with insects. And this leads me to say a few words on what I call
Sexual Selection. This depends, not on a struggle for existence, but on
a struggle between the males for possession of the females; the result
is not death to the unsuccessful competitor, but few or no offspring.
Sexual selection is, therefore, less rigorous than natural selection.
Generally, the most vigorous males, those which are best fitted for
their places in nature, will leave most progeny. But in many cases,
victory will depend not on general vigour, but on having special
weapons, confined to the male sex. A hornless stag or spurless cock
would have a poor chance of leaving offspring. Sexual selection by
always allowing the victor to breed might surely give indomitable
courage, length to the spur, and strength to the wing to strike in the
spurred leg, as well as the brutal cock-fighter, who knows well that he
can improve his breed by careful selection of the best cocks. How low
in the scale of nature this law of battle descends, I know not; male
alligators have been described as fighting, bellowing, and whirling
round, like Indians in a war-dance, for the possession of the females;
male salmons have been seen fighting all day long; male stag-beetles
often bear wounds from the huge mandibles of other males. The war is,
perhaps, severest between the males of polygamous animals, and these
seem oftenest provided with special weapons. The males of carnivorous
animals are already well armed; though to them and to others, special
means of defence may be given through means of sexual selection, as the
mane to the lion, the shoulder-pad to the boar, and the hooked jaw to
the male salmon; for the shield may be as important for victory, as the
sword or spear.
Amongst birds, the contest is often of a more peaceful character. All
those who have attended to the subject,
believe that there is the severest rivalry between the males of many
species to attract by singing the females. The rock-thrush of Guiana,
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