1322 THE STRUCTURE OF EVOLUTIONARY THEORY
generality to macroevolutionary pattern, but denies that microevolution works in
Darwin's required manner. The opposite refutation of the second end-member fully
accepts Darwin's argument for the operation of microevolution, but denies the
extrapolationist premise of scaling in continuity to impart the same theme of progress
to higher levels, and thus to life's broadest history.
If only for the obvious reason that Darwinian selection has been so
overwhelmingly validated, both empirically and theoretically, as a dominant
mechanism of evolutionary change in populations at generational time scales, the first
end-member refutation has not garnered much consideration or support, although
Raup's principled exploration (1991b, 1992, 1996), to be discussed just below,
deserves considerable respect and attention, if only to remind us that apparently
absurd propositions may hold far more plausibility than our knee-jerk reactions
allow. But the second end-member, based on the opposite premise of
nonextrapolatability for a Darwinian mechanism fully valid at its primary, smallest
scale, tier of time, has been—correctly in my judgment—the standard, if usually
inchoate or inarticulated, view of paleontologists uncomfortable with the full
sufficiency of microevolutionary principles to explain the entire history of life. The
scaling of time's tiers, in this second position (and to cite a pair of metaphors), is
neither fractal nor isometric.
Before defending this second position as the key to solving the paradox of the
first tier, I should say that, despite some initial enthusiasm as our profession first
embraced the renewed respectability of catastrophism, I doubt that any paleontologist
would now defend a dichotomous division of time into two tiers—an ordinary or
"background" world, granted entirely to Darwinian mechanisms, between
catastrophic episodes; and a few, but markedly effective, momentary disruptions of
this generality in episodes of mass extinction. This model of an alternation between
background and wipeout regimes presents far too simple a picture, while admittedly
capturing the central principle of higher and rarer modes that must be titrated with
ordinary Darwinism to generate the full pattern. As with the cognate theme of
hierarchical levels of selection, explored and defended throughout this book, we must
try to render time as a series of rising tiers, each featuring distinctive modes of evo-
lution, and each functioning as a gatekeeper to bar full passage, a ringmaster to add
new acts to the mix, and a facilitator to alter, in interesting ways, the expression of
conventional Darwinism in its domain.
Time's higher tiers, in other words, introduce causes and phenomena to expand
the modalities of evolution, not to restrict or refute the powerful Darwinian forces
that rise from the organismal level in ecological time, but do not maintain their
pervasive sway in these broader realms. Again, as with rising hierarchical levels of
structure, our increasing understanding of evolution at time's upper tiers establishes
the architecture of a larger, sturdier, and interestingly different Darwinian edifice, and
does not operate as a demolition team for razing old domiciles and then building
some hurried heap of superficial appeal, thrown up without a foundation, and
therefore destined to topple as soon as the inevitable winds of fashion change their
capricious course.