1178 THE STRUCTURE OF EVOLUTIONARY THEORY
in other words the discontinuity of ways and means by which organisms that
inhabit the world derive their livelihood from the environment.But the striking discontinuities in morphospace, and their ordering into
taxonomic hierarchies, surely don't, at least primarily, "reflect the objectively
ascertainable discontinuity of adaptive niches"—and Dobzhansky certainly
understood the unstated major reason for such inhomogeneity, even though the strict
adaptationism so favored at this time had momentarily clouded his excellent
judgment, thus explaining his curious omission. Cats, lions and tigers work admirably
well, with each species displaying excellent adaptation to its immediate environment.
But the set of all feline species does not clump closely together in morphospace
because the summits of an underlying topography now happen to lie at such close
mutual proximity in an external world "out there." Felines form a tight cluster
because they share, by historical constraints of ordinary genealogy, a large set of
distinctive traits, unique to them alone by virtue of "propinquity of descent," to cite
Darwin's own description of the phenomenon—although each of these traits probably
arose for good and conventional adaptationist reasons in a common ancestor. And the
larger gap between felines and canines also records, as its primary raison d'etre, a
greater separation in history, not the architecture of spacing between two groups of
peaks in the current mountain range of worldly ecology.
Sometimes we need the press of new data to inspire the recollection of old
truths. I do not doubt that many discontinuities in morphospace represent the
colonization by optimal phenotypes of widely dispersed peaks in maximal
biomechanical efficiency. But I am equally confident that more of nature's evidently
nonrandom, and oddly dispersed, clusters in morphospace, bearing such enormously
different weights ranging from single "outliers" to millions of species, primarily
record the historical constraints imposed by workable solutions with adaptive
origins—developmental designs that then congealed, enforcing reiteration and change
within their internally directed channels forever after. Five, for all I know, may be
optimal for the radial symmetry of echinoderms, and therefore predictable for any
phylum in their domain. But can we argue that the sixfold way of a much, much
larger clump marks an optimal and inevitable number for walking, and that elytra
represent the only possible design for joint excellence in flight and protection? God,
as one of our most celebrated colleagues famously exclaimed, must have an
inordinate fondness for these particular creatures if he allowed one design among so
many conceivable alternatives to congeal so hard, and then to iterate so often, in
nature's wondrous interplay of constraint and adaptation.