fundamental conception that in every Vertebrate there are found
the same "organic materials," or units of construction. This conception,
which Geoffroy calls the _Theorie des analogues_ (p. xxxii.), is clearly
one part of the old idea of the unity of type; it teaches the _unity of
composition_ of organic beings, while the _Principe des connexions_ adds
the _unity of plan_.

Both conceptions are logically implicit in the vague notion of unity of
type; Geoffroy disengaged them, and pushed each to its logical extreme.

Most of the ordinary homologies of structure in air-breathing
Vertebrates have already been seized, he continues, for they are more or
less obvious, and many intermediate states exist (p. xxxiv.). But
ordinary methods of comparison fail when the attempt is made to
homologise the structure of fishes with that of air-breathing
Vertebrates, for the homologies are anything but obvious and no
intermediate organs are found.

Most air-breathing Vertebrates have a larynx, a trachea, and bronchi,
which are absent in fish; and fish have many parts which seem to be
absent in higher Vertebrates. But apply the "Theory of Analogues"; it
teaches that there can be no organ peculiar to fish and not found in
other Vertebrates; apply the "Principle of Connections," it will show
which organs are homologous in the two types (p. xxxv.).

Comparative anatomists, with few exceptions, had hitherto taken man as
the type, and referred all structure to his; Geoffroy's principles led
him to give preference to no one animal in particular, but to seize upon
each part in the species in which it reaches the maximum of its
development (p. xxxvi.). He is thus led to refer all structures to a
generalised abstract type. In this abstract type each organ exists at
the maximum of its development, each organ shows all its potentialities
realised. In a way, therefore, this type, this abstraction, gives the
scheme of the possible transformations of each organ.

It is true Geoffroy does not refer to this "Archetype" in so many words,
but it must always have been vaguely present in his mind. He has this
idea in his head when he says in one of his later works, "There is,
philosophically speaking, only a single animal."[88] The "single animal"
is simply the generalised type.

Having laid down his two principles Geoffroy goes on to apply them to
the difficult case of the comparison of the skeleton of fish with the
skeleton of the higher Vertebrates.