by his disciple E. Serres, the law that
the higher animals repeat during their development the main features of
the adult organisation of animals lower in the scale. Thus he compared
fish as regards certain parts of their structure with the foetus of
mammals. He compared also Articulates with embryonic Vertebrates in
respect of their vertebrae, for in the higher Vertebrates the body of the
vertebra is tubular at an early stage of development, and in Articulates
the body of the vertebra remains tubular permanently (_supra_, p. 61).
As regards their vertebrae, "insects occupy a place in the series of the
ages and developments of the vertebrate animals, that is to say, they
realise one of the states of their embryo, as fishes do one of the
states of their foetal condition."[107]

This idea was destined to exercise a great influence upon the
development of morphology. A further development of the thought is that
certain abnormalities in the higher animals, resulting from arrest of
development, represent states of organisation which are permanent in the
lower animals.[108]

So far we have considered Geoffroy's theories in their application to
the facts. We go on to discuss the theories themselves, and the general
conception of living things which underlies them.

The principle of unity of plan and composition is the keynote of
Geoffroy's work. It states that the same materials of organisation are
to be found in all animals, and that these materials stand always in the
same general spatial relations to one another. The "materials of
organisation" are not necessarily organs in the physiological sense, and
indeed the principle of the unity of plan cannot be upheld if the unity
has reference to organs only. This became clear to Geoffroy, especially
in his later years. In 1835 he wrote, speaking of the principle of the
unity of plan, "I have, moreover, regenerated this principle, and
obtained for it universality of application, by showing that it is not
always the organs as a whole, but merely the materials composing each
organ, that can be reduced to unity."[109] Even in the _Philosophie
anatomique_ he deals rather with parts than with organs; he deals, for
instance, with the elementary parts of the sternum, not with the organ
"sternum" in its totality. The functions of the sternum vary, and the
primary protective function of the sternum may be assumed by quite other
parts, _e.g._, by the clavicles in fish, which protect the