150 THE STRUCTURE OF EVOLUTIONARY THEORY
validates evolution itself (vs. creationism), but not Darwin's, or anyone else's,
proposed mechanism of evolutionary change.
INSENSIBILITY OF INTERMEDIACY. We now come to the heart of what natural
selection requires. This second, "just right," statement does not advance a claim
about how much time a transition must take, or how variable a rate of change
might be. The second meaning simply asserts that, in going from A to a
substantially different B, evolution must pass through a long and insensible
sequence of intermediary steps—in other words, that ancestor and descendant must
be linked by a series of changes, each within the range of what natural selection
might construct from ordinary variability. Without gradualism in this form, large
variations of discontinuous morphological import—rather than natural selection—
might provide the creative force of evolutionary change. But if the tiny increment
of each step remains inconsequential in itself, then creativity must reside in the
summation of these steps into something substantial—and natural selection, in
Darwin's theory, acts as the agent of accumulation.
This meaning of gradualism underlies Darwin's frequent invocation of the old
Leibnizian and Linnaean aphorism, Natura non facit saltum (nature does not
proceed by leaps). Darwin's commitment to this postulate can only strike us as
fierce and, by modern standards, overly drawn. Thus, Darwin writes (p. 189): "If it
could be demonstrated that any complex organ existed, which could not possibly
have been formed by numerous, successive, slight modifications, my theory would
absolutely break down." And lest we doubt that "my theory" refers specifically to
the mechanism of natural selection (and not simply to the assertion of evolution),
Darwin often draws an explicit link between selection as a creative force and
gradualism as an implied necessity: "Undoubtedly nothing can be effected through
Natural Selection except by the addition of infinitesimally small changes; and if it
could be shown that... transitional states were impossible, the theory would be
overthrown" (in Natural Selection—see Stauffer, 1975, p. 250). And in the
concluding chapter of the Origin: "As natural selection acts solely by accumulating
slight, successive, favorable variations, it can produce no great or sudden
modification; it can act only by very short and slow steps. Hence the canon of
'Natura non facit saltum'... is on this theory simply intelligible" (p. 471).
But would the theory of natural selection "absolutely break down" if even a single
organ—not to mention an entire organism—could arise by large and discontinuous
changes? Does Darwinism truly require the following extreme
puzzled by Gingerich's definition until I realized the source of his confusion. He had
switched definitions from the empirical issue of rates (meaning iii of this discussion)—a
lively and testable argument opposing stasis to gradualism defined as a rate of change—
to the completely settled question of historical continuity. Does anyone seriously think
that supporters of punctuated equilibrium, or any scientist for that matter, would deny
historical continuity? His argument therefore dissolves into the empty linguistic effort of
trying to win a debate by shifting a definition. The question of punctuated equilibrium
will be resolved by empirical testing under the third definition of gradualism. (See
Chapter 9 for a full discussion of this issue.)