or
dog would be driven to try to catch more hares, and his numbers would
tend to decrease; his organization, however, being slightly plastic,
those individuals with the lightest forms, longest limbs, and best
eye-sight (though perhaps with less cunning or scent) would be slightly
favoured, let the difference be ever so small, and would tend to live
longer and to survive during that time of the year when food was
shortest; they would also rear more young, which young would tend to
inherit these slight peculiarities. The less fleet ones would be rigidly
destroyed. I can see no more reason to doubt but that these causes in a
thousand generations would produce a marked effect, and adapt the form
of the fox to catching hares instead of rabbits, than that greyhounds
can be improved by selection and careful breeding. So would it be with
plants under similar circumstances; if the number of individuals of a
species with plumed seeds could be increased by greater powers of
dissemination within its own area (that is if the check to increase fell
chiefly on the seeds), those seeds which were provided with ever so
little more down, or with a plume placed so as to be slightly more acted
on by the winds, would in the long run tend to be most disseminated; and
hence a greater number of seeds thus formed would germinate, and would
tend to produce plants inheriting this slightly better adapted down.

{234} See _Origin_, Ed. i. p. 90, vi. p. 110.

Besides this natural means of selection, by which those individuals are
preserved, whether in their egg or seed or in their mature state, which
are best adapted to the place they fill in nature, there is a second
agency at work in most bisexual animals tending to produce the same
effect, namely the struggle of the males for the females. These
struggles are generally decided by the law of battle; but in the case
of birds, apparently, by the charms of their song{235}, by their beauty
or their power of courtship, as in the dancing rock-thrush of Guiana.
Even in the animals which pair there seems to be an excess of males
which would aid in causing a struggle: in the polygamous animals{236},
however, as in deer, oxen, poultry, we might expect there would be
severest struggle: is it not in the polygamous animals that the males
are best formed for mutual war? The most vigorous males, implying
perfect adaptation, must generally gain the victory in their several
contests. This kind of selection, ho