o the tracheal system. Radiates
approach the homogeneity of plants; they appear to lack a distinct
nervous system and sense organs, and the lowest of them show only a
homogeneous pulp which is mobile and sensitive. All four classes are
principally distinguished from one another by the broad structural
relations of their neuromuscular system, of the organs of the animal
functions. Vertebrates have a spinal cord and brain, an internal
skeleton built on a definite plan, with an axis and appendages; in
Molluscs the muscles are attached to the skin and the shell, and the
nervous system consists of separate masses; Articulates have a hard
external skeleton and jointed limbs, and their nervous system consists
of two long ventral cords; Radiates have ill-defined nervous and
muscular systems, and in their lowest forms possess the animal
functions without the animal organs.

This well-rounded classification of animal forms is in a sense the
crown of Cuvier's work, for the principle of the subordination of
characters, in the interpretation which he gives to it, is a direct
application of his principle of functional correlation. Each of the
great groups is built upon one plan. The idea of the unity of plan has
become for Cuvier a commonplace of his thought, and it is tacitly
recognised in all his anatomical work. But he never takes it as a
hard-and-fast principle which must at all costs be imposed upon the
facts.

Cuvier has become known as the greatest champion of the fixity of
species, but it is not often recognised that his attitude to this
problem is at least as scientific as that of the evolutionists of his
own and later times. No doubt he became dogmatic in his rejection of
evolution-theory, but he was on sure ground in maintaining that the
evolutionists of his day went beyond their facts. He considered that
certain forms (species) have reproduced themselves from the origin of
things without exceeding the limits of variation. His definition of a
species was, "the individuals descended from one another or from
common parents, together with those that resemble them as much as they
resemble one another."[62] "These forms are neither produced nor do
they change of themselves; life presupposes their existence, for it
cannot arise save in organisations ready prepared for it."[63]

He based his rejection of all theories of descent upon the absence of
definite evidence for evolution. If species have gradually changed, he
argued,