Historical Constraints and the Evolution of Development 1119
do resemble, but by convergence, the eurypterid arthropods in several features of
external form and function.)
For Geoffroy, however, the inversion of axes between vertebrates and
arthropods does not denote an evolutionary transition in either direction, but
represents instead (as the archetypal mode of thinking would imply) two opposite
specializations upon a shared abstract groundplan that generated both great phyla
along predictable pathways of internally specified laws of form and their permissible
transformations.
As a structuralist thinker, committed to a formal, rather than a functional,
approach to the explanation of organic design and variation, Geoffroy argued that the
apparently fundamental difference in disposition of organs between arthropods and
vertebrates should be reconceptualized as both secondary and superficial—a
consequence of opposite ecological orientations for the same archetypal structure.
The shared and constraining pattern specifies a central gut and a peripheral major
channel for the nervous system, both oriented parallel to the body's A-P axis.
Vertebrates, so to speak (and befitting their higher status and dignity), have oriented
their main nerve tract upwards toward the sun and surface, while the humbler
arthropods have directed the same peripheral aspect of archetypal form downward
towards the earth and ocean bottom.
A functional theory, like Darwinian natural selection, would tend to interpret
this ecological correlation as a primary impetus for the later evolutionary fixation of
these two opposite arrangements. But to a structuralist thinker like Geoffroy, the
same ecological situation becomes both derivative and temporally consequential (not
to mention ideologically inconsequential as well). The established differences
represent a realized subset of possible transformations for an archetypal form under
structurally determined rules of geometric constraint and possibility. The
happenstance of opposite ecological orientation for a common archetypal design only
records a later adaptive overlay—a diversity of form arising for structural reasons and
then finding both an appropriate suite of functions and the right environments for
their realization. Thus, in Geoffroy's view, the inversion of dorsoventral axes in
arthropods vs. vertebrates does not validate a direct flip of evolutionary
transformation, but rather represents two separately developed expressions of a
common archetypal structure, one oriented up towards the sun, the other down
towards the earth, in a secondary ecological specialization that can only obscure an
underlying, and truly ruling, unity of constrained design.
The modern version of Geoffroy's vision—so different in genetic and evo-
lutionary (as opposed to formal and archetypal) evidence, yet so eerily similar in
philosophical style (as a structuralist account based on internal channels of
transformational constraint)—originated in the mid 1990's based on unanticipated
discoveries of genetic homology in genes that operate in patterning dorsal and ventral
surfaces and structures in Drosophila and Xenopus. (See Sasai, et al., 1994; Holley et
al., 1995; and De Robertis and Sasai, 1996, for the pioneering work of De Robertis's
lab at UCLA, and Francois et al., 1994; and Francois and Bier, 1995, for studies of
similar import from Bier's lab at