CHAPTER V. LAWS OF VARIATION.
4. CORRELATED VARIATION.
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, and no doubt wholly different classes of facts may be here
easily confounded together. We shall presently see that simple inheritance
often gives the false appearance of correlation. One of the most obvious
real cases is, that variations of structure arising in the young or larvae
naturally tend to affect the structure of the mature animal. The several
parts which are homologous, and which, at an early embryonic period, are
identical in structure, and which are necessarily exposed to similar
conditions, seem eminently liable to vary in a like 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 by some anatomists 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 in the union of the
petals into a tube. Hard parts seem to affect the form of adjoining soft
parts; it is believed by some authors that with birds the diversity in the
shape of the pelvis causes the remarkable diversity in the shape of the
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 and form of several of the most important viscera.