How much of the acclimatisation of species to any peculiar climate is
due to mere habit, and how much to the natural selection of varieties
having different innate constitutions, and how much to both means
combined, is a very obscure question. That habit or custom has some
influence I must believe, both from analogy, and from the incessant
advice given in agricultural works, even in the ancient Encyclopaedias
of China, to be very cautious in transposing animals from one district
to another; for it is not likely that man should have succeeded in
selecting so many breeds and sub-breeds with constitutions specially
fitted for their own districts: the result must, I think, be due to
habit. On the other hand, I can see no reason to doubt that natural
selection will continually tend to preserve those individuals which
are born with constitutions best adapted to their native countries. In
treatises on many kinds of cultivated plants, certain varieties are
said to withstand certain climates better than others: this is very
strikingly shown in works on fruit trees published in the United
States, in which certain varieties are habitually recommended for the
northern, and others for the southern States; and as most of these
varieties are of recent origin, they cannot owe their constitutional
differences to habit. The case of the Jerusalem artichoke, which is
never propagated by seed, and of which consequently new varieties have
not been produced, has even been advanced--for it is now as tender as
ever it was--as proving that acclimatisation cannot be effected! The
case, also, of the kidney-bean has been often cited for a similar
purpose, and with much greater weight; but until some one will sow,
during a score of generations, his kidney-beans so early that a very
large proportion are destroyed by frost, and then collect seed from
the few survivors, with care to prevent accidental crosses, and then
again get seed from these seedlings, with the same precautions, the
experiment cannot be said to have been even tried.
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