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 larva,
will, it may safely be concluded, affect the structure of the adult;
in the same manner as any malconformation affecting the early embryo,
seriously affects the whole organisation of the adult. The several
parts of the body which are homologous, and which, at an early
embryonic period, are alike, seem liable to vary in an allied manner:
we see this in the right and left sides of the body varying in the
same manner; in the front and hind legs, and even in the jaws and
limbs, varying together, for the lower jaw is believed to be
homologous with the limbs.
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