1152 THE STRUCTURE OF EVOLUTIONARY THEORY
these putative homologies, if expressive of common ancestry, require that primary
larvae be regarded as the basal anatomical form of bilaterian animals—a hypothesis
supported by some (Peterson and Davidson, 2000, for example), but vigorously
rejected by others (Valentine and Collins, 2000, for example).
We cannot yet determine—if genetic and developmental data of modern
organisms could allow us, in principle, to resolve such questions at all— whether a
hypothetical urbilaterian already possessed highly developed phenotypic structures
(in either larvae or adults) acting like the Corinthian columns of my metaphor on
pages 1134- 114 2, or whether this common ancestor had only expressed its extensive
genetic and developmental pathways, preserved forever after as homologies in all
bilaterian phyla, as phenotypic Pharaonic bricks of limited specification and
extensive flexibility. The solution to such puzzles requires paleontological data (not
yet available, but eminently attainable in principle). In any case, I regard this issue as
a largely speculative sidelight that does not affect—and must not lead us to forget or
put aside—the striking reformulation of evolutionary theory implied by the well-
documented genetic and developmental homologies alone. De Robertis expresses this
key argument in the final line of his 1997 article on the ancestry of segmentation:
"The realization that all Bilateria are derived from a complex ancestor represents a
major change in evolutionary thinking, suggesting that the constraints imposed by the
previous history of species played a greater role in the outcome of animal evolution
than anyone would have predicted until recently."
The Hox genes of diploblasts apparently do not show the intergenic organization
of colinearity that defines the key developmental homology of bilaterians, but
cnidarians do possess Hox genes with some commonality of action to their bilaterian
homologs. Martinez et al. (1998) found four Hox