1092 THE STRUCTURE OF EVOLUTIONARY THEORY
so inconceivable in theory that empirical counterclaims hardly seemed necessary.
After all, the notion required extensive genetic homology among phyla, and the
power of natural selection, working on different paths for a minimum of 530 million
years since the origin of distinct phyla in the Cambrian explosion, seemed to
guarantee such thorough change at effectively every nucleotide position that the
requisite common foundation could not possibly have been maintained (see Mayr,
1963, p. 609, as previously discussed on pp. 539 and 1066).
When, in the mid-1980's, initial studies began to discern deep homology
between arthropod and vertebrate Hox genes, I well remember saying to myself
(amidst my astonishment about a result so consonant with the theoretical framework
that I had espoused in 1977 in my first book, Ontogeny and Phylogeny, but had not
dared to view as subject to empirical validation in my lifetime): yes, perhaps for some
functional commonality in the broadest construction of basic body axes (A-P in
particular), but surely not for the more detailed structural homology—particularly
between arthropod metameres and vertebrate somites—demanded by the old
archetypal theories. But, only 15 years later, central nuggets of validity had been
affirmed for nearly all the classical archetypal theories, even the most farfetched.
Needless to say, the archetypes do not function as their inventors claimed. The
differences between leaves and floral parts do not arise by progressive refinement of
sap up the stem; and the abstract vertebra does not function as a generator for all
major features of the axial skeleton (including ribs and appendages) in vertebrates
and arthropods.
Moreover, at least two prominent claims for the vertebral archetype probably
hold little, if any, validity. The distinctive features of the vertebrate skull and
forebrain seem to arise, in large part, under the formative influence of the distinctive
neural crest (see the classical statement of Gans and Northcutt, 1983), and not as a
complex fusion (much like an arthropod tagma) of a definable number of rostral
vertebrae (from 3 to 8 in various formulations). And although some broad
homologies may set the basic axes of limbs in both arthropods and vertebrates (see
pp. 1138-1142), the structures cannot be regarded as basically homologous, even in
underlying developmental pathways; nor can they be derived from any particular
component of a generalized vertebra. Nonetheless, all three major archetypal theories
of Goethe and Geoffroy—the classical sources of ridicule for the general concept—
have now been confirmed in aspects that cannot be dismissed as superficial or
secondary.
MEHR LICHT (MORE LIGHT) ON GOETHE'S ANGIOSPERM ARCHETYPE.
Students of the mustard Arabidopsis have discovered unexpected validity in central
features of Goethe's founding theory of the archetypal leaf (see Pelaz et al., 2000).
Starting at the bottom, Meinke (1992) studied the lee (leafy cotyledon) mutant that
partially transforms cotyledons into leaves. He argued that the wild-type allele (LEC)
activates "a wide range of embryo-specific