e cases we have inheritance limited by,
or correlated with, both sex and age; but we have no reason for supposing
that the original cause of the variation was necessarily connected with the
reproductive organs, or with the age of the affected being.

In cases of true correlated variation, we are sometimes able to see the
nature of the connexion; but in most cases the bond is hidden from us, and
certainly differs in different cases. We can seldom say which of two
correlated parts first varies, and induces a change in the other; or
whether the two are simultaneously produced by some distinct cause.
Correlated variation is an important subject for us; for when one part is
modified through continued selection, either by man or under nature, other
parts of the organisation will be unavoidably modified. From this
correlation it apparently follows that, with our domesticated animals and
plants, varieties rarely or never differ from each other by some single
character alone.

One of the simplest cases of correlation is that a modification which
arises during an early stage of growth tends to influence the subsequent
development of the same part, as well as of other and intimately connected
parts. Isidore Geoffroy St. Hilaire states[804] that this may constantly be
observed with monstrosities {321} in the animal kingdom; and
Moquin-Tandon[805] remarks, that, as with plants the axis cannot become
monstrous without in some way affecting the organs subsequently produced
from it, so axial anomalies are almost always accompanied by deviations of
structure in the appended parts. We shall presently see that with
short-muzzled races of the dog certain histological changes in the basal
elements of the bones arrest their development and shorten them, and this
affects the position of the subsequently developed molar teeth. It is
probable that certain modifications in the larvae of insects would affect
the structure of the mature insects. But we must be very careful not to
extend this view too far, for, during the normal course of development,
certain members in the same group of animals are known to pass through an
extraordinary course of change, whilst other and closely allied members
arrive at maturity with little change of structure.

Another simple case of correlation is that with the increased or decreased
dimensions of the whole body, or of any particular part, certain organs are
increased or diminished in number, or are otherwise m