146 THE STRUCTURE OF EVOLUTIONARY THEORY
modal form.) Only under these stringent conditions can natural selection—a force
that makes nothing directly, and must rely upon variation for all raw material—be
legitimately regarded as creative.
Gradualism
Darwinism, like most comprehensive and complex concepts, defies easy definition.
Darwinism cannot be analogized to an object, like the Parthenon, with a clear
criterion of membership for each potential slab (whether now resident in the
British Museum or in Athens). Moreover, the various propositions of Darwinism
cannot be regarded as either independent or of equal force. Darwinism cannot be
construed as a deductive system, with some defining axioms and a set of logical
entailments tied together like a classical proof in plane geometry. But neither can
Darwinism be viewed as a set of separate stones, all of similar size, and each
ejectable from a bag without great disturbance to the others.
As discussed at length in Chapter 1 (pp. 12-24), I view the conceptual
structure of Darwinism much like the metaphor that Darwin himself first used (see
Barrett et al., 1987) for depicting evolution (in the "B Notebook" on transmutation
kept during the 1830's)—the "coral of life" (later superseded, in Chapter 4 of the
Origin, and in other writings, by the tree of life). The central trunk (the theory of
natural selection) cannot be severed, or the creature dies (see Fig. 1-4, p. 18). The
first-order branches are also so fundamental that any severing of a complete branch
converts the theory into something essentially different that must be newly named.
(I have suggested that the theory of natural selection includes three major branches,
discussed in sections B-D of this subchapter.) Each major branch then divides into
smaller sub-branches. (In the present section C, I argue that the second major
branch, the claim for "creativity of natural selection," divides into three important
sub-branches of "requirements for variation," "gradualism," and "the adaptationist
program.")
As further argued in Chapter 1, this model allows us to address the important
question of dispensability. At some level above the base, we may excise a sub
branch, deny its premises, and still consider ourselves Darwinians. I envision the
central trunk and first-order branches as indispensable. Along the continuum from
necessary to avoidable, we may begin to make selective negations at the level of
sub-branches, but not without severe stress to the entire structure. Thus, T. H.
Huxley could oppose gradualism and still consider himself a supporter of natural
selection (though his approbation remained ambiguous and indifferent at best, and
his role as "Darwin's bulldog" rested upon his defense of evolution itself, not his
explication of natural selection). And a modern developmental saltationist might
call himself a Darwinian, though not without an array of "buts" and qualifications.
One other feature of the model requires explicit commentary. I have chosen a
coral in preference to the more conventional tree, because the branches of many
corals form a network by lateral anastomoses (while each limb of a tree stands free,
and may be chopped off without necessarily affecting the others).