lation
to the parent, and of the parent in relation to the young. In social
animals it will adapt the structure of each individual for the benefit
of the community; if each in consequence profits by the selected change.
What natural selection cannot do, is to modify the structure of one
species, without giving it any advantage, for the good of another
species; and though statements to this effect may be found in works of
natural history, I cannot find one case which will bear investigation.
A structure used only once in an animal's whole life, if of high
importance to it, might be modified to any extent by natural selection;
for instance, the great jaws possessed by certain insects, and used
exclusively for opening the cocoon--or the hard tip to the beak of
nestling birds, used for breaking the egg. It has been asserted, that
of the best short-beaked tumbler-pigeons more perish in the egg than are
able to get out of it; so that fanciers assist in the act of hatching.
Now, if nature had to make the beak of a full-grown pigeon very short
for the bird's own advantage, the process of modification would be very
slow, and there would be simultaneously the most rigorous selection of
the young birds within the egg, which had the most powerful and hardest
beaks, for all with weak beaks would inevitably perish: or, more
delicate and more easily broken shells might be 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