Historical Constraints and the Evolution of Development 1091
However, in Raff's larger sense, both modes express the common, cardinal
feature of nonisotropic, or channeled, variation—thus imposing a preferred structure,
from the "inside" of organic development, upon the raw material that external forces
of Darwinian selection must utilize. Both modes also delighted (or disturbed)
evolutionary biologists with the greatest surprises in the last generation of our
science—based on results that were actively unexpected in theory, not merely
unsuspected for lack of imagination. I shall, in this first section, discuss deep
homology as the more general and fundamental of the two modes. My second section
shall then extend Raff's theme of directed variation as the focus of constraint within
evolutionary theory, this time through the positive channel of unanticipated
parallelisms.
In a famous line from the prologue of Faust, Goethe wrote: Es irrt der Mensch,
so lang er strebt—we err, so long as our struggle lasts. Goethe probably intended this
celebrated statement as a romantic effusion about human striving in general, but we
may apply his words to the nearly universal attitude of fellow biologists, at least since
Darwin's watershed of 1859, towards Goethe's own brainchild in developmental
biology, and towards the general approach to morphology—a word of Goethe's own
invention—embodied within such theories.
As discussed in extenso in Chapter 4, Goethe's theory of the leaf as a botanical
archetype for all lateral structures off the angiosperm stem (including cotyledons and
all flower parts) presented the most famous botanical proposal among a set of
archetypal theories that would soon sweep the world of animal morphology as well,
culminating in the vertebral archetype advocated by Owen for all major parts of the
vertebrate skeleton (including the skull and limbs) and, most extensively, by Etienne
Geoffroy Saint-Hilaire for the generative basis of all animal form (first for all
vertebrates, then adding arthropods, then mollusks, and no doubt proceeding further
had he not then encountered the wrath and active opposition of Cuvier and his
functionalist theory of adaptive form—see Chapter 4, pp. 291-312).
The argument that structural and morphological archetypes underlie, and
actively generate, a basic and common architecture in taxonomically distant groups
defines—both as a fact of our profession's actual history and as a dictate of the logic
of our explanatory theories—the strongest kind of claim for developmental constraint
as a major factor in patterns of evolutionary change and the occupation of
morphospace. I suspect that the depth of this challenge has always been recognized,
but the empirical case for such constraining archetypes has remained so weak, since
the heyday of Geoffroy and Owen some 150 years ago, that the issue simply didn't
generate much serious concern—and rightly so.
The concept of interphylum archetypes, deemed too bizarre to warrant active
refutation, experienced the curt and derisive dismissal reserved for crackpot ideas in
science. (Goldschmidt's saltational apostasy, on the other hand, inspired voluminous
and impassioned denial because his ideas did seem sufficiently and dangerously
plausible to the Modern Synthesis—see pp. 451- 466.). Indeed, the notion of
interphylum archetypes struck most biologists as