ous
and muscular, the digestive, the circulatory, and the respiratory.
Each organ or system of organs may have many forms. If any form of any
organ could exist in combination with any form of all the others there
would be an enormous number of combinations theoretically possible.
But these combinations do not all exist in Nature, for organs are not
merely assembled (_rapproche's_), but act upon one another, and act
all together for a common end. Accordingly only the combinations that
fulfil these conditions exist in Nature. Cuvier thus dismisses the
question of a science of possible organic forms and considers only the
forms or combinations actually existing. This question of the
possibility of a "theoretical" morphology of living things, after the
fashion of the morphology of crystals with their sixteen possible
types, was raised in later years by K. G. Carus, Bronn, and Haeckel.

Organisms, then, are harmonious combinations of organs, and the
harmony is primarily a harmony of functions. Every function depends
upon every other, and all are necessary. The harmony of organs and
their mutual dependence are the results of the interdependence of
function. This thought, the recognition of the functional unity of the
organism, is the fundamental one at the base of all Cuvier's work.
Before him men had recognised more or less clearly the harmony of
structure and function, and had based much of their work upon this
unanalysed assumption. Cuvier was the first naturalist to raise this
thought to the level of a principle peculiar to natural history. "It
is on this mutual dependence of the functions and the assistance which
they lend one to another that are founded the laws that determine the
relations of their organs; these laws are as inevitable as the laws of
metaphysics and mathematics, for it is evident that a proper harmony
between organs that act one upon another is a necessary condition of
the existence of the being to which they belong."[45]

This rational principle, peculiar to natural history, Cuvier calls the
principle of the conditions of existence, for the following
reason:--"Since nothing can exist that does not fulfil the conditions
which render its existence possible, the different parts of each being
must be co-ordinated in such a way as to render possible the existence
of the being as a whole, not only in itself, but also in its relations
with other beings, and the analysis of these conditions often leads to
ge