1118 THE STRUCTURE OF EVOLUTIONARY THEORY
that the original mouth, which would have opened dorsally in vertebrates, simply
closed up, while a separate ventral mouth originated as a neomorph.
This history, discussed extensively in Chapter 4, becomes crucially relevant to the
modern validation of Geoffroy's own centerpiece for his theory of dorsoventral
inversion. I must also address this question in the largely non-historical second half
of this book because later exegetes have seriously misrepresented Geoffroy's intent
by substituting a later phyletic version that modern research in evo-devo rightly
rejects—and that, if conflated with Geoffroy's actual theory, will lead to continued
and unjustified dismissal of his second, and remarkably ingenious, archetypal
formulation.
In short, Geoffroy never advanced (and, I suspect, never even conceptualized) a
historical argument about direct evolutionary transformation: the claim that
vertebrates evolved from arthropods when an ancestral trilobite or merostome
literally flipped upside down during its phyletic ascent. However, the American
morphologist William Patten did popularize such an evolutionary account in arguing
that the first prominent group of putative fossil vertebrates, the jawless
"ostracoderms," did not belong to a completed vertebrate line, but represented an
intermediate stage between arthropods and fishes along the "great highway of organic
evolution" (his words, see Patten, 1912, 1920), with the transition literally achieved
by anatomical inversion, as an arthropod that swam on its back settled to the bottom,
thus converting its original dorsal side to a new belly. (The ostracoderms, with their
external plates,
10 - 19. An unintentionally amusing illustration from Gaskell, 1908, showing the inverted
topology of vertebrates and arthropods—with major nerve cord above the gut in vertebrates
and below in arthropods.