tution, 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. These tendencies,
I do not doubt, may be mastered more or less completely by natural
selection: thus a family of stags once existed with an antler only on
one side; and if this had been of any great use to the breed it might
probably have been rendered permanent by natural selection.

Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants; and nothing is more common than
the union of homologous parts in normal structures, as the union of the
petals of the corolla into a tube. Hard parts seem to affect the form of
adjoining soft parts; it is believed by some authors that the diversity
in the shape of the pelvis in birds causes the remarkable diversity in
the shape of their kidneys. Others believe that the shape of the pelvis
in the human mother influences by pressure the shape of the head of the
child. In snakes, according to Schlegel, the shape of the body and
the manner of swallowing determine the position of several of the most
important viscera.

The nature of the bond of correlation is very frequently quite obscure.
M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain
malconformations very frequently, and that others rarely coexist,
without our being able to assign any reason. What ca