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. Nor let it be supposed that no differences in the
constitution of seedling kidney-beans ever appear, for an account has
been published how much more hardy some seedlings appeared to be than
others.
On the whole, I think we may conclude that habit,
use, and disuse, have, in some cases, played a considerable part in the
modification of the constitution, and of the structure of various
organs; but that the effects of use and disuse have often been largely
combined with, and sometimes overmastered by, the natural selection of
innate differences.
_Correlation of Growth_. I mean by this expression that the whole
organisation is so tied together during its growth and development,
that when slight variations in any one part occur, and are accumulated
through natural selection, other parts become modified. This is a very
important subject, most imperfectly understood. The most obvious case
is, that modifications accumulated solely for the good of the young or
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