There is much
difficulty in ascertaining how largely our domestic productions have been
modified; but we may safely infer that the amount has been large, and that
modifications can be inherited for long periods. As long as the conditions
of life remain the same, we have reason to believe that a modification,
which has already been inherited for many generations, may continue to be
inherited for an almost infinite number of generations. On the other hand
we have evidence that variability, when it has once come into play, does
not cease under domestication for a very long period; nor do we know that
it ever ceases, for new varieties are still occasionally produced by our
oldest domesticated productions.
Variability is not actually caused by man; he only unintentionally exposes
organic beings to new conditions of life and then nature acts on the
organisation and causes it to vary. But man can and does select the
variations given to him by nature, and thus accumulates them in any desired
manner. He thus adapts animals and plants for his own benefit or pleasure.
He may do this methodically, or he may do it unconsciously by preserving
the individuals most useful or pleasing to him without any intention of
altering the breed. It is certain that he can largely influence the
character of a breed by selecting, in each successive generation,
individual differences so slight as to be inappreciable except by an
educated eye.
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