the fact remains that many correlations are not
explicable in terms of function, and the substitution of correlation
as an empirical principle for correlation as a rational principle
marks for Cuvier a step away from his functional comparative anatomy
towards a pure morphology. It is significant that in later times the
term correlation has come to be applied more especially to the purely
empirical constancies of relation, and has lost most of its functional
significance. But the correlation of the parts of an organism is no
mere mathematical concept, to be expressed by a coefficient, but
something deeper and more vital.

Cuvier interpreted the functional dependence of the parts in terms of
what we now call the general metabolism. He had a clear vision of the
constant movement of molecules in the living tissue, combining and
recombining, of the organism taking in and intercalating molecules
from outside from the food and rejecting molecules in the excretions,
a ceaseless _tourbillon vital_. "This general movement, universal in
every part, is so unmistakably the very essence of life that parts
separated from a living body straightway die."[51] The organisation of
the body, the arrangement of its solids and liquids, is adapted to
further the _tourbillon vital_. "Each part contributes to this general
movement its own particular action and is affected by it in particular
ways, with the result that, in every being, life is a unity which
results from the mutual action and reaction of all its parts."[52]

Cuvier, however, did not resolve life into metabolism, nor reduce
vital happenings to the chemical level. The form of organised bodies
is more essential than the matter of which they are composed, for the
matter changes ceaselessly while the form remains unchanged. It is in
form that we must seek the differences between species, and not in the
combinations of matter, which are almost the same in all.[53] The
differences are to be sought at the level of the second and third
degrees of composition.

The existence of differences of form introduces a new problem, the
problem of diversity. There are only a few possible combinations of
the principal organs, but as you get down to less important parts the
possible scope of variation is greatly increased, and most of the
possible variations do exist. Nature seems prodigal of form, of form
which needs not to be useful in order to exist. "It needs only to be
possible, _i.e._, of suc