Having so fully discussed variation under nature it will be unnecessary
to devote so much space to domesticated animals and cultivated plants,
especially as Mr. Darwin has published two remarkable volumes on the
subject where those who desire it may obtain ample information. A
general sketch of the more important facts will, however, be given, for
the purpose of showing how closely they correspond with those described
in the preceding chapter, and also to point out the general principles
which they illustrate. It will also be necessary to explain how these
variations have been increased and accumulated by artificial selection,
since we are thereby better enabled to understand the action 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.
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