4 Serres attempted an
explanation of the nervous system of Invertebrates. Invertebrates, he
considered, lacked the cerebrospinal axis of Vertebrates, and their
nervous system was the homologue of the sympathetic system of
Vertebrates. The relation of the invertebrate to the vertebrate nervous
system being thus fixed, can the nervous system of Invertebrates be
reduced to one plan? It does not seem possible to establish a common
plan for the adult nervous systems. But apply the principle of
parallelism, which has proved so valuable within the limits of the
vertebrate series. Taking insects as the highest class, we find that
there are three stages in the development of their nervous system; in
the first the nervous system is composed of two separate strands, in the
second the strands unite round the oesophagus, in the third they unite
also behind. Now in _Bulla aperta_, stage (1) is permanent; in _Clio_,
_Doris_, _Aplysia_, _Tritonia_, _Sepia_, _Helix_, stage (2) is
permanent, and in _Unio_ stage (3). In fact, all the varieties of the
nervous system of molluscs fall into one or other of these three
classes. "It follows, then, that as regards their nervous system, the
Mollusca are more or less advanced larvae of insects" (p. 380). The law
of parallelism is here applied to single organ-systems, but in later
years Serres applied it to whole organisations also, saying that the
lower Invertebrates were permanent embryos of the higher.

In the paper of 1834, already referred to, Serres pushed his
speculations further and attempted to establish the unity of type of all
animals, Vertebrates and Invertebrates alike--a favourite pastime of the
transcendentalists. It is incontestable, he admits, that adult
Invertebrates are quite different in structure from adult Vertebrates,
"but if one regards them as what I take them to be, namely, _permanent
embryos_, and if one compares their organisation with the embryogeny of
Vertebrates, one sees the differences disappear, and from their
analogies arise a crowd of unsuspected resemblances" (_loc. cit._, p.
247).

The last point of Serres' doctrine which calls for remark is his
interpretation of abnormalities as being often comparable to grades of
structure permanent in the lower animals. Thus the double aorta which
may occur as an abnormality in man is the normal and permanent state in
reptiles. This idea, of course, he got from Etienne Geoffroy St Hilaire.
It is further developed in his "_