the majority of the
constituent materials are employed, or even as these elements come to
change their respective dimensions or proportions" (p. 134). As to the
elementary constituents, "they give proof of individuality, and
sometimes even, in certain abnormalities, of independence, and rise to
the level of primary organisatory materials" (p. 132). What holds good
for the sternum holds good for other organs--and accordingly the unity
of plan and composition can be demonstrated for all the organs of
Vertebrates.

Soon after the publication of the _Philosophie anatomique_ (1818)
Geoffroy went further in his search for unity, and maintained that the
structure of insects and Crustacea could be reduced to the vertebrate
type.

He proposed to replace Cuvier's classification of the animal kingdom
into the four large groups, Vertebrata, Mollusca, Articulata, and
Radiata by the following classification:--[90]

Hauts-Vertebres (Vertebrata, Cuv.).
Vertebres /
\
Dermo-Vertebres (Articulata, Cuv.).

Mollusques (Mollusca, Cuv.).
Invertebres /
\
Rayonnes (Radiata, Cuv.).

The idea upon which is based the comparison of Articulates with
Vertebrates is that each skeletal segment of Articulates is a vertebra.
In the Hauts-vertebres the vertebrae are internal; in the
Dermo-vertebres they are external. "_Every animal lives either outside
or inside its vertebral column_."[91] The essence of a vertebra is not
its form, nor its function, but its composition from four elementary
pieces which unite round a central space (_Isis, loc. cit._, p. 532).
Serres had shown that in the higher animals every vertebra is formed
from four centres of ossification, that the body of the vertebra is at
first tubular, and that afterwards it becomes filled up. In lobsters and
crabs each segment is composed of four elementary pieces, as may be seen
most easily in young ones. "Accordingly each segment corresponds to a
true vertebra in composition: there is the same number of 'materials,'
the same order in the course of ossification, the same kind of
articulation, the same annular arrangement, the same empty space in the
middle" (p. 534). The only difference is that in Articulates the central
space is very great and contains all the organs of the body, whereas in
the higher Vertebrates the body of the vertebra becomes completely
filled up. In the thoraci