306 THE STRUCTURE OF EVOLUTIONARY THEORY
(especially for later, evolutionary versions in the "worm that turned" theory of
vertebrate origins), the mouth lies below the brain and spinal cord in vertebrates,
but above the nerve cords in arthropods, with the esophagus piercing through the
cords. Turn an insect over, and the mouth should lie on top, above the brain.
Geoffroy and later supporters of this homology generally argued that the old
arthropod mouth and nerve-piercing esophagus simply closed, while the digestive
tube formed an entirely new ventral "mouth" (therefore not homologous with the
arthropod orifice). In any case, and with an almost wondrous and partly humorous
irony as I shall show in Chapter 10, Geoffroy's fundamental homology has been
validated (in modern guise) after more than a century of calumny. The genetic
determinants of dorsoventral patterning may well be homologous but reversed in
expression in arthropods and vertebrates (see pp. 1117-1123).
If Geoffroy belittled Cuvier's functionalism in his argument about orientation,
he also attacked the deeper postulate of adaptational primacy by arguing, once
again, that the archetypal vertebra comes first, with any use of the resulting
structures developing only later as a consequence. Why, he asks, do arthropods use
their "ribs" for locomotion? And he answers with the old cliché about mountain
climbing—because they are there. Geoffroy wrote in 1820 (quoted in Russell,
1916, p. 77): "From the circumstance that the vertebra is external, it results that the
ribs must be so too; and, as it is impossible that organs of such a size can remain
passive and absolutely functionless, these great arms, hanging there continually at
the disposition of the animal, are pressed into the service of progression, and
become its efficient instruments."
Cuvier may have been offended by the arthropod connection, but he was too
smart a rhetorician, and too much a figure of the establishment, to be drawn into
the limelight of public debate, thus giving even more publicity to Geoffroy's
apostasy. Geoffroy goaded him throughout the 1820's, but with no public response.
Finally, the dam burst in 1830. Meyranx and Laurencet, two young provincial
naturalists with an eye on advancement, presented a monograph, "Some
considerations on the organization of mollusks," to the Academie des sciences—
the standard path for career building at the time. They suggested that the anatomy
of a squid might be homologized with a vertebrate bent back upon itself at the
middle of the vertebral column, so that the buttocks and base of the spine lined up
with the nape of the neck.
Their original paper has been lost, and we do not know how far they meant to
carry the comparison, or how much they had intended to inject themselves into the
formalist-functionalist controversy. But we do know that Geoffroy, appointed as
one of two commissioners to prepare a public report for the Academie, rejoiced at
this entering wedge for a second imperialistic raid upon Cuvier's embranchements.
The inclusion of arthropods had once seemed radical enough, but if mollusks could
also be brought under the vertebral archetype, then three of four phyla would be
reduced to common design, and a final absorption of the Radiata could not lag far
behind. The dream of total unification now seemed within Geoffroy's grasp.