n of natural
selection, to be discussed in the succeeding chapter.

_The facts of Variation and Artificial Selection._

Every one knows that in each litter of kittens or of puppies no two are
alike. Even in the case in which several are exactly alike in colours,
other differences are always perceptible to those who observe them
closely. They will differ in size, in the proportions of their bodies
and limbs, in the length or texture of their hairy covering, and notably
in their disposition. They each possess, too, an individual
countenance, almost as varied when closely studied as that of a human
being; not only can a shepherd distinguish every sheep in his flock, but
we all know that each kitten in the successive families of our old
favourite cat has a face of its own, with an expression and
individuality distinct from all its brothers and sisters. Now this
individual variability exists among all creatures whatever, which we can
closely observe, even when the two parents are very much alike and have
been matched in order to preserve some special breed. The same thing
occurs in the vegetable kingdom. All plants raised from seed differ more
or less from each other. In every bed of flowers or of vegetables we
shall find, if we look closely, that there are countless small
differences, in the size, in the mode of growth, in the shape or colour
of the leaves, in the form, colour, or markings of the flowers, or in
the size, form, colour, or flavour of the fruit. These differences are
usually small, but are yet easily seen, and in their extremes are very
considerable; and they have this important quality, that they have a
tendency to be reproduced, and thus by careful breeding any particular
variation or group of variations can be increased to an enormous
extent--apparently to any extent not incompatible with the life, growth,
and reproduction of the plant or animal.

The way this is done is by artificial selection, and it is very
important to understand this process and its results. Suppose we have a
plant with a small edible seed, and we want to increase the size of that
seed. We grow as large a quantity of it as possible, and when the crop
is ripe we carefully choose a few of the very largest seeds, or we may
by means of a sieve sort out a quantity of the largest seeds. Next year
we sow only these large seeds, taking care to give them suitable soil
and manure, and the result is found to be that the _average_ size of the