that culminated in the Darwinian centennial celebrations of 1959). On the first
branch of agency, the cut labeled Rl (see Fig. 1-4) expanded Darwin's unilevel
theory of organismal selection into a hierarchical model of selection acting
simultaneously on several legitimate levels of Darwinian individuality (genes, cell-
lineages, organisms, demes, species, and clades). I shall show in Chapters 3, 8, and
9 how the logic of this pronounced expansion builds a theory fascinatingly
different from, and not just a smooth extension of, Darwin's single level
mechanism of agency—my reason for portraying the hierarchical model as a
deeply interesting R-cut rather than a more superficial S-cut.
On the second branch of efficacy, the cut labeled R2 accepts the validity of
Darwin's argument for creativity (by leaving the base of the branch intact), but
introduces a sufficient weight of formalist thinking—via renewed appreciation for
the enormous importance of structural, historical, and developmental constraint in
channeling the pathways of evolution, often in highly positive ways—that the pure
functionalism of a strictly Darwinian (and externalist) approach to adaptation no
longer suffices to explain the channeling of phyletic directions, and the clumping
and inhomogeneous population of organic morphospace. The strict Darwinian
form of explanation has thereby been greatly changed and enriched, but in no way
defeated. I shall discuss the historical aspect of this branch in Chapters 4 and 5, and
modern reformulations of this R2 cut in Chapters 10 and 11.
On the final branch of scope, the cut labeled R3 accepts the Darwinian
contention that microevolutionary modes and principles can build grand patterns
by cumulation through geological immensity, but rejects the argument that such
extrapolations can render the entire panoply of phenomena in life's history without
adding explicitly macroevolutionary modes for distinctive expression of these
processes at higher tiers of time—as in the explanation of cladal trends by species
sorting under punctuated equilibrium, rather than by extended adaptive anagenesis
of purely organismal selection, and in the necessity of titrating adaptive
microevolutionary accumulation with occasional resetting of rules and patterns by
catastrophically triggered mass extinctions at time's highest tier. Chapters 6 and 12
discuss historical and modern critiques of Darwinian extrapolationism.
For now, I will say little about the even higher and more superficial S-cuts of
subbranches, but I will at least indicate how I construe this category by stating a
hypothetical example for each branch: an SI cut, for example, might accept the
selective basis of evolutionary change in a purely mechanical sense, but then deny
full force to Darwin's deliciously radical philosophical claim that all apparent
"higher level" harmony arises consequentially, through the invisible hand of lower
levels acting for personal reproductive success. One might, in principle, propose
such a revision by arguing that a higher force, operating by an overarching
principle of order, "employs" natural selection as its mechanical agent. (I speak
only hypothetically here, for no such defend-able scientific hypothesis now exists,
although the concept certainly remains intelligible. Explicitly theological versions
don't count as science, whatever their kind or form of potential validity.) An S2 cut
might be assayed by a
22 THE STRUCTURE OF EVOLUTIONARY THEORY