1116 THE STRUCTURE OF EVOLUTIONARY THEORY
phylogenetic precursor of the vertebrate neural crest" (p. 648). Nonetheless,
emphasizing the novelty of vertebrate usage, whatever the homology of development
with amphioxus, "the migrating cells of amphioxus do not differentiate into the wide
variety of cell types known to originate from the vertebrate neural crest, but
eventually remain part of the epidermis" (Holland and Holland, 1998, p. 654).
Despite these caveats, we can only conclude that the first fruits of evo-devo have
revealed some remarkable, and extensive, homology in both genetic structure and
action among animal phyla (particularly between arthropods and chordates, the
former prototypes of separation in our traditions and literature), and that these data
have confirmed some important aspects of the most ridiculed formalist theory of
constraint in the history of morphological and evolutionary thought: Geoffroy's claim
for homology between vertebrate and arthropod segments, with the idealized segment
itself regarded as the basic unit of generation.
The history of our dawning realization—as expressed in an acknowledgment
that this heresy of homology between phyla must be reclothed in modern genetic
language as a partial reality—also followed an interesting pathway of strong
reluctance gradually yielding to bemused acceptance of ever widening scope. From
the initial Mayrian stance of near theoretical impossibility for recognizable genetic
homology, we first admitted (with the discovery of vertebrate Hox genes) that
footprints of common ancestry could be preserved during more than 500 million
years and such substantial anatomical divergence. But we still doubted that such
genetic resemblances could continue to encode phenotypic homologies in these
disparate phyla. Then, in a second step, we acknowledged the potential homology of
Hox action in general spatial organization along the A-P axis, but still declined to
accept any common basis for segmentation in arthropods and vertebrates (the key to
Geoffroy's hypothesis).
In a third stage, the similarity of Hox action in patterning vertebrate
rhombomeres and arthropod segments demonstrated at least some homology between
modes of differentiation in arthropod segments and in the compartmentalized
organization of the vertebrate hindbrain and its extensive derivatives. In a fourth
stage, researchers then discovered some partial and limited homologies in earlier
determinants of segmentation itself (pair-rule and segment-polarity genes) between
arthropods and some aspects of segmental development in the main vertebrate body
axis posterior to the rhombomeres.
In short, and in a story to be, no doubt, extensively continued (expanded,
contracted, changed, reinterpreted, etc.), Geoffroy's theory of complete and
overarching homology based on a common vertebral archetype surely will not prevail
in anything like its original form. But this most ridiculed of all heresies, so contrary
in principle to strict Darwinian expectations of the Modern Synthesis, and so widely
dismissed as a romantic delusion until just a few years ago, has now resurfaced, in
appropriately revised terms, as a primary, and initially surprising example of the
unanticipated durability of ancient genetic pathways, and of their continuing power to
constrain the subsequent