256 THE STRUCTURE OF EVOLUTIONARY THEORY
secondary modifications upon this fundamental action. In the closing paragraph of
Chapter 6, all forces other than selection become sequelae to its primary action.
Thus, Darwin invoked relative frequency to uphold his evolutionary world
view: a theory of trial-and-error externalism, with natural selection as the only
major creative force for change, and with internal variation restricted to the role of
generating raw material for selection's perusal, and not of supplying important or
consistent direction. Why, then, do many evolutionary biologists continue to
demur? Darwin's basic argument in closing Chapter 6 can only be judged both
brilliant and undoubtedly correct. Most homologies of Unity of Type are, indeed,
adaptations inherited from a distant past, not fodder for constraint theorists who
wish to demote the relative frequency and importance of natural selection.
(Homologies of Unity of Type do act as phyletic constraints upon present
possibilities—elephants will never fly—but such current limitations exist as
consequences of initial adaptations, and therefore cannot stand against natural
selection in any toting of relative frequencies.)
Modern constraint theorists, myself included, balk at Darwin's resolution
because his argument demotes a large chunk of biology to a chink in a corner. The
old Unity of Type theorists, lacking the alternative of "just history," did falsely
assume that deep homology must stand against adaptation. But much validity still
attends their cardinal insight that principles of design, laws of growth, rules of
architecture, nature of materials—generalities transcending the particulars of
specific genealogical pathways—work as important interior channels of constraint
in the positive sense of that undervalued word: for constraints not only prevent
evolutionary motion by failing to supply variation; they also act positively to set
preferred channels of change. Internal forces do not only present isotropic raw
material to the fully creative externality of natural selection. Constraint does not
exist in subservience to adaptation under the nooks-and-crannies and sequelae
arguments of relative frequency. Constraint may never again (and rightly so) be
able to claim primacy,
I don't wish to jump the gun towards modern incarnations in this historical chapter,
but I can't resist noting that this style of treating adaptation vs. constraint has been upheld by
Darwinians ever since, thus generating the extreme frustrations of iconoclasts and reformers
who wish to assert the importance of constraint in setting—not merely limiting—
evolutionary pathways. Later Darwinians (often much more extreme than Darwin himself,
and much more inclined to demote constraint even further) would frequently deny that they
ignored constraint, pointing to a footnote or side comment that made concession by lip
service to the possibility of such influence. But such purposefully restricted acknowledgment
cannot count as balance, pluralism, or fairness. Rather, such treatment amounts to classical
dismissal by the proper criterion of relative frequency! —and the frustrations of a C. H.
Waddington or an E. C. Olson (at the Chicago centennial meetings of 1959) must be
understood as deep and justified (see Chapter 7). Most people don't appreciate the style and
power of relative frequency, and they will misread the strategy of dismissal by footnote as
adequate pluralism rather than sharp rejection. I will never forget George Oster's reply to
John Maynard Smith at the Chicago macroevolution meeting in 1980 (cited in full on p.
102 3), when John insisted that he had (as an adaptationist) always acknowledged constraint,
and George, recognizing footnote and side commentary as dismissal, replied: "Yes John, you
may have had the bicycle, but you didn't ride it."