1316 THE STRUCTURE OF EVOLUTIONARY THEORY
to Darwin's own emphasis on the importance of "spreading out" the timing of
apparent "mass extinction" into sufficient spans for explanation by ordinary
Darwinian competition, with sequential and individual deaths of species occurring in
conventional realms of natural selection, at most against a background of unusual
environmental perturbation that may "turn up the gain" of intensity for standard
causes, but will not change the usual rules or reasons. I also showed, in particular,
how Darwin needed this extrapolationist argument to validate a concept of progress
through life's history that his cultural context demanded (and to which he personally
assented), but that he recognized (and, in his philosophical radicalism, greatly
appreciated) as underivable from the "pure" operation of natural selection, and
therefore recoverable only by an additional ecological postulate about the
predominance of biotic competition in a perpetually crowded biota (the "metaphor of
the wedge").
The key phenomenon for this entire discussion—whether explanation be offered
by Darwinian extrapolation, or by the random or different rules alternatives—has
always resided in the observed selectivity of mass extinctions: why do some taxa
flourish and others die, especially since the observed patterns of mass extinctions do
not simply intensify the tendencies of normal times (that is, mass dyings do not
preferentially remove those groups already on the wane by competition with superior
forms during "background" times. Mammals were not expanding, with dinosaur
retreating, during the long span of Cretacous life).
Under the "different rules" model, extirpated groups die for definable reasons of
conventional anatomy, physiology, behavior, or population structure. But their death
follows from the unpredictable, and suddenly instituted, "different rules" of
catastrophically altered environments in episodes of mass extinction, and does not
occur because these taxa had evolved properties that would have doomed them in the
same manner (albeit more slowly and sequentially) for ordinary reasons of Darwinian
competition during normal times. In fact, and even worse for conventional arguments
about progress, the traits that spell doom in catastrophically altered circumstances
may just as well have originated as the adaptive features that secured success, and
competitive superiority in the normal Darwinian times just preceding. In this
important sense, if the previous Darwinian "best" often die for unpredictable but
deterministic reasons in the suddenly altered worlds of catastrophic mass extinction,
then Darwin's crucial argument for progress (always weak and suspect because it
could not flow from the abstract logic of natural selection itself, and required an
additional ecological belief in plenitude and biotic struggle) collapses through the
disruption, or even the reversal of its vector, as imposed by these dramatic episodes
with their different rules for who flourishes and who goes to the wall. And if these
episodes are sufficiently numerous, profound, rapid and different (my four criteria of
p. 1313), then their accumulated impact may balance, or even reverse, the Darwinian
accumulation during much longer stretches of normal times, thereby imbuing the full
pattern