of the unity of plan and composition is the true base of natural
history,[117] and that this unity limits the possible transformations of
the organism. Thus, speaking of the influence of the respiratory medium,
he says, "All the same this influence of the external world, if it has
ever become a cause which disturbed organisation, must necessarily have
been confined within fairly narrow limits; animals must have opposed to
it certain conditions inherent to their nature, the existence of the
same materials composing them, and a manifest tendency to resemble one
another, and to reproduce invariably the same primordial type."[118] Unity
of plan and composition is, on this view, prior to adaptation and limits
adaptation. Cuvier's view, on the contrary, is that the necessity of
functional and ecological adaptation accounts for the repetition of the
same types of structure. There are, of all the possible combinations of
organs, only a few viable types--those whose structure is adapted to
their life. Therefore it is reasonable that these few types should be
repeated in innumerable exemplars. One must remember, in order to
appreciate Cuvier's view, that he was not obsessed, as we are, by the
idea of evolution.

Cuvier thought in terms of organs, not in terms of "materials of
organisation." He held that the resemblances between the organs of one
class of animals and the organs of another were due to the similarity of
their functions. "Let us conclude, then, that if there are resemblances
between the organs of fish and those of other classes, it is only in the
measure that there is a resemblance between their functions."[119] There
are only a few kinds of organs, each adapted for a particular function,
and these organs are necessarily repeated from class to class.--"As the
animal kingdom has received only a limited number of organs, it is
inevitable that some at least of these organs should be common to
several classes."[120]

Geoffroy thought in terms of "materials," of parts of indefinite
function, parts which might take on any function. He insists upon the
necessity of disregarding function when tracing out the unity of
composition. He considers, in direct opposition to Cuvier's
interpretation of structural resemblance as due to similarity of
function, that unity of composition is the primary fact, and similarity
of function subsidiary. In his reply in the _Mammiferes_ (1829) to
Cuvier's criticisms in the _Histoire naturelle d