Chap. Y. 
COKRELATIOK OF GROWTH. 
143 
use, and disuse, have, in some cases, played a consider¬ 
able 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 selec¬ 
tion of innate variations. 
Correlation of Growth,—! 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 organisa¬ 
tion 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 
