e relations of the framework to the rest of the
skeleton are the same in fish and air-breathing Vertebrates, but simply
because gills are considered the equivalents of lungs--a comparison
which is purely physiological.

Even with these concessions to the functional view of living things,
Geoffroy was unable to make good his contention that all animals are
built upon the same plan. His arguments failed to carry conviction to
his contemporaries, and Cuvier in particular subjected them to
destructive, and indeed final, criticism.

The paper, already referred to, in which Cuvier disposed of the
transcendentalists' comparison of Cephalopods and Vertebrates is of
great significance, for it states in the clearest way the radical
opposition between the functional and the formal attitudes to living
things.

Cuvier points out that if by unity of composition is meant identity,
then the statement that all animals show the same composition is simply
not true--compare a polyp with a man!--on the other hand, if by unity is
meant simply resemblance or homology, the statement is true within
certain limits, but it has been employed as a principle since the days
of Aristotle, and the theory of unity of composition is original only in
so far as it is false. He admits, however, that Geoffroy has seized upon
many hidden homologies, especially by his valuable discovery of the
importance of foetal structure. In all this Cuvier is undoubtedly right.
Unity of plan and composition, as Geoffroy conceived it, simply does not
exist. Cuvier goes on to say that this principle of Geoffroy's, in the
greatly modified form in which it can be accepted, and has been accepted
from the dawn of zoology, is not the sole and unique principle of the
science. On the contrary, it is merely a subordinate principle,
subordinate to a higher and more fruitful principle, that, namely, of
the conditions of existence, of the adaptation (_convenance_) of the
parts, of the co-ordination of the parts for the role which the animal
is to play in Nature. "That is the true philosophical principle," he
says, "whence may be deduced the possibility of certain resemblances,
the impossibility of certain others; it is the rational principle from
which follows the principle of the unity of plan and composition, and in
which at the same time it finds those limits, which some would like to
disregard" (p. 248).

Geoffroy's position is the direct contrary. He holds that the principle