web, or of the plumes and hooks on a seed may
be safely inferred from their structure, we may with equal safety infer
that many flowers are constructed for the express purpose of ensuring a
cross with a distinct plant. From these various considerations, the
conclusion arrived at in the chapter just referred to--namely, that great
good of some kind is derived from the sexual concourse of distinct
individuals--must be admitted.

To return to our illustration: we have hitherto assumed that the birds were
kept down to the same number by indiscriminate slaughter; but if the least
choice be permitted in their preservation and slaughter, the whole result
will be changed. Should the owner observe any slight variation in one of
his birds, and wish to obtain a breed thus characterised, he would succeed
in a surprisingly short time by carefully selecting and pairing the young.
As any part which has once varied generally goes on varying in the same
direction, it is easy, by continually preserving the most strongly marked
individuals, to increase the amount of difference up to a high,
predetermined standard of excellence. This is methodical selection.

If the owner of the aviary, without any thought of making a new breed,
simply admired, for instance, short-beaked more than long-beaked birds, he
would, when he had to reduce the number, generally kill the latter; and
there can be no doubt that he would thus in the course of time sensibly
modify his {424} stock. It is improbable, if two men were to keep pigeons
and act in this manner, that they would prefer exactly the same characters;
they would, as we know, often prefer directly opposite characters, and the
two lots would ultimately come to differ. This has actually occurred with
strains or families of cattle, sheep, and pigeons, which have been long
kept and carefully attended to by different breeders without any wish on
their part to form new and distinct sub-breeds. This unconscious kind of
selection will more especially come into action with animals which are
highly serviceable to man; for every one tries to get the best dog, horse,
cow, or sheep, and these animals will transmit more or less surely their
good qualities to their offspring. Hardly any one is so careless as to
breed from his worst animals. Even savages, when compelled from extreme
want to kill some of their animals, would destroy the worst and preserve
the best. With animals kept for use and not for mere amusement,