The Descent of Man

CORRELATED VARIATION.
In man, as in the lower animals, many structures
are so intimately related, that when one
part varies so does another, without our being
able, in most cases, to assign any reason. We
cannot say whether the one part governs the
other, or whether both are governed by some
earlier developed part. Various monstrosities,
as I. Geoffroy repeatedly insists, are thus intimately
connected. Homologous structures are
particularly liable to change together, as we see
on the opposite sides of the body, and in the
upper and lower extremities. Meckel long ago
remarked, that when the muscles of the arm
depart from their proper type, they almost always
imitate those of the leg; and so, conversely,
with the muscles of the legs. The organs
of sight and hearing, the teeth and hair,
the colour of the skin and of the hair, colour
and constitution, are more or less correlated.
(55. The authorities for these several statements
are given in my 'Variation of Animals under