1106 THE STRUCTURE OF EVOLUTIONARY THEORY
assumed the name of HOM-C. When researchers discovered vertebrate homologs,
they did not want to use the same names at first, for they had not yet affirmed the
corresponding similarities of colinearity and action (and were probably still reeling
from the basic shock of the discovery itself). So the vertebrate homologs became Hox
genes. This potentially anarchic situation deteriorated further when, after finding four
Hox complexes in vertebrates, researchers started naming the genes in each complex
by their order of discovery, and not by their invariant spatial positions along the
chromosome. (Perhaps they did not yet believe that colinearity could prevail here as
well.) Thus, Hox-1.1 denoted the first discovered, not the most 3', gene of the first
Hox series.
Happily, these discrepancies and illogicalities have now been sorted out and—
like the standardization of railroad gauges, or the choice of an internal combustion
engine for all cars (thus abandoning a host of other early and workable devices)—a
common and integrated terminology has developed, not by the official fiat of any
particular meeting or official commission, but by obvious advantages in daily use.
The four vertebrate complexes have been renamed Hoxa to Hoxd and the genes
within each have been numbered from 1 to 13 in their proper A-P, or 3' to 5', order.
Meanwhile, acknowledging the proven homologies of gene structure, position and
action, the fly folks have dropped their different name for the complex, and now also
denote their sequence as Hox, rather than HOM-C. This congelation of a simple and
unified taxonomy, replacing the previous promiscuity of different and uncoordinated
names for each gene, marks the coherence and maturation of an important field from
an initiating chaos of uncoordinated empirical promise.
SEGMENTAL HOMOLOGIES OF ARTHROPODS AND VERTEBRATES:
GEOFFROY'S VINDICATION. The discovery of these deep homologies in genetic
structure and action among phyla (particularly between vertebrates and arthropods)
brings us back to Geoffroy's daring theory of the vertebral archetype. Researchers
have documented homology in key regulatory genes of development, and have also
shown the conservation of basic developmental patterns between the two phyla,
particularly in differentiation of structures along the A-P axis under the influence of
homologous Hox genes and their principles of colinearity. But Geoffroy's formalist
theory rests upon an additional and crucial premise—one that continued to strike
most researchers as unlikely, even after the first discovery of these broad
commonalities in development. For Geoffroy postulated that the segment (the
vertebra in Geoffroy's terminology) represents a fundamental—and truly
homological— unit of construction in both phyla. Therefore, to validate the basic
premise of Geoffroy's theory, the vertebrate somite must also be homologous with the
insect metamere (similar patterns of differentiation along the A-P axis cannot
suffice), and such a close comparison seemed exceedingly unlikely, if not
anathematic, to most biologists. In the classic pre evo-devo book on the origin of the
coelom and segmentation, Clark (1964) described the independent origin of arthropod
and vertebrate segments as "universally accepted." And