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 can be more singular than the relation
between blue eyes and deafness in cats, and the tortoise-shell colour with the female sex; the
feathered feet and skin between the outer toes in pigeons, and the presence of more or less down
on the young birds when first hatched, with the future colour of their plumage; or, again, the
relation between the hair and teeth in the naked Turkish dog, though here probably homology
comes into play? With respect to this latter case of correlation, I think it can hardly be accidental,
that if we pick out the two orders of mammalia which are most abnormal in their dermal
coverings, viz. Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, &c.), that these are
likewise the most abnormal in their teeth.