Historical Constraints and the Evolution of Development 1117
phylogeny of life along broad and fruitful (but still limited) routes of wonderfully
diverse, but historically rule-bound, adaptive designs.
GEOFFROY'S SECOND ARCHETYPAL THEORY OF DORSO-VENTRAL IN-
VERSION OF THE COMMON BILATERIAN GROUNDPLAN. The ridicule
heaped upon Geoffroy's second archetypal theory for homologizing arthropods and
vertebrates did not descend entirely from his genuine and original argument, but from
an explicitly phyletic version championed by later evolutionists, but never intended
by Geoffroy himself. (Geoffroy maintained a generally supportive attitude towards an
evolutionary world view, and even gave his name to a theory of causation that he did
advocate in passing, but never really developed in extenso—the idea that soft
inheritance could operate by immediate impress of external conditions upon parts of
organisms, yielding inheritable changes directly, rather than by the more indirect
route of Lamarckian organic response to "felt needs." In fact, late 19th century
discussions of evolutionary mechanisms often listed three primary contenders:
Darwinism for the theory of natural selection, Lamarckism for soft inheritance by
organic response, and so-called "Geoffroyism" for soft inheritance following direct
imposition. In this light of Geoffroy's positive attitude to evolution, his failure to cite,
as support for transmutation, his archetypal theory of dorsoventral inversion between
insects and vertebrates provides strong evidence that he did not intend this anatomical
comparison to be read in a phylogenetic context.)
After resolving (to his satisfaction) the common structure of vertebrate and
arthropod segmentation, and dealing with the inconvenient fact, for a hypothesis of
homology, that vertebrates grow internal hard parts and arthropods an exoskeleton
(see pp. 304-306 for Geoffroy's ingenious, and gloriously wrong, resolution),
Geoffroy moved to a second archetypal theory for the outstanding remaining
difference in basic anatomy between the phyla: their apparently reversed dorsoventral
orientations—for the two main nerve cords of arthropods run along the ventral
surface below the central gut, while the single nerve tube of vertebrates runs along
the dorsal surface, above the gut. As we all learned in Biology 1 (but usually not with
proper respect or understanding for Geoffroy's interesting conjecture), Geoffroy
resolved this striking difference by suggesting that the same groundplan underlay the
development of both phyla, but that this common design appeared in reversed
orientation, with arthropods interpreted as, essentially, vertebrates turned on their
backs (see Fig. 10-19).
We also learned, quite correctly—for Geoffroy's supporters never resolved this
issue with any plausibility—that such a reversal scarcely brings the two phyla into
complete correspondence. In the knottiest remaining difficulty, the front end of the
arthropod gut passes between the nerve cords and emerges on the lower surface as a
ventral mouth. In an overturned position, the vertebrate mouth should therefore pass
by the dorsal nerve cord and emerge on the upper surface. But vertebrate mouths are
also ventral. So Geoffroy weakly argued that the mouths of the two phyla are not
homologous—and