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
transporting animals from one district to another. And as 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, natural selection would
inevitably tend to preserve those individuals which were born with
constitutions best adapted to any country which they inhabited. In
treatises on many kinds of cultivated plants, certain varieties are said to
withstand certain climates better than others; this is 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 in England by seed, and of
which, consequently, new varieties have not been produced, has even been
advanced, as proving that acclimatisation cannot be effected, for it is now
as tender as ever it was! 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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