ETWEEN THE
HEAD AND THE EXTREMITIES--BETWEEN THE SKIN AND DERMAL
APPENDAGES--BETWEEN THE ORGANS OF SIGHT AND HEARING--CORRELATED
MODIFICATIONS IN THE ORGANS OF PLANTS--CORRELATED
MONSTROSITIES--CORRELATION BETWEEN THE SKULL AND EARS--SKULL AND CREST
OF FEATHERS--SKULL AND HORNS--CORRELATION OF GROWTH COMPLICATED BY THE
ACCUMULATED EFFECTS OF NATURAL SELECTION--COLOUR AS CORRELATED WITH
CONSTITUTIONAL PECULIARITIES.

All the parts of the organisation are to a certain extent connected or
correlated together; but the connexion may be so slight that it hardly
exists, as with compound animals or the buds on the same tree. Even in the
higher animals various parts are not at all closely related; for one part
may be wholly suppressed or rendered monstrous without any other part of
the body being affected. But in some cases, when one part varies, certain
other parts always, or nearly always, simultaneously vary; they are then
subject to the law of correlated variation. Formerly I used the somewhat
vague expression of correlation of growth, which may be applied to many
large classes of facts. Thus, all the parts of the body are admirably
coordinated for the peculiar habits of life of each organic being, and they
may be said, as the Duke of Argyll insists in his 'Reign of Law,' to be
correlated for this purpose. Again, in large groups of animals certain
structures always co-exist; for instance, a peculiar form of stomach with
teeth of peculiar form, and such structures may in one sense be said to be
correlated. But these cases have no necessary connexion with the law to be
discussed in the present chapter; for we do not know that {320} the initial
or primary variations of the several parts were in any way related; slight
modifications or individual differences may have been preserved, first in
one and then in another part, until the final and perfectly co-adapted
structure was acquired; but to this subject I shall presently recur. Again,
in many groups of animals the males alone are furnished with weapons, or
are ornamented with gay colours; and these characters manifestly stand in
some sort of correlation with the male reproductive organs, for when the
latter are destroyed these characters disappear. But it was shown in the
twelfth chapter that the very same peculiarity may become attached at any
age to either sex, and afterwards be exclusively transmitted by the same
sex at a corresponding age. In thes