162 THE STRUCTURE OF EVOLUTIONARY THEORY
an absence of environmental change would probably bring evolution to an eventual
halt, as selective pressures for adaptive alteration diminished (see Stenseth and
Maynard Smith, 1984). Purely biotic interaction might drive evolution for some
time following a cessation of environmental change, but probably not indefinitely.
The possibility of too little change has rarely been viewed as a threat to
Darwinism, largely because the geological record seems so clearly to emphasize
potential dangers in the other direction (though see pp. 492-502 on Lord Kelvin).
The specter of "too much" change, on the other hand, has haunted Darwinism from
the start. In particular, if the theory of geological catastrophism were generally
true, or even just sufficiently important in relative frequency, then Darwinism
would be compromised as the primary agent of pattern in the history of life.
By catastrophism, I mean to designate the classical theory of global paroxysm
as a primary agent of geological change—in particular, the idea that mass
extinctions thus engendered might lie largely outside the domain of traditional
Darwinism. Of course, mass extinctions cannot be construed as "undarwinian" per
se. If environment changes so rapidly that organisms cannot adapt fast enough by
natural selection, then many species will die. But, in a conceptual world of relative
frequency, where Darwinism must not only operate, but also dominate as the
creator of change, such formative power for mass extinction constitutes a serious
challenge. If we survey the entire history of life, and find that catastrophic mass
extinction, with non-Darwinian fortuity in causes of change (on either the
"random" or the "different rules" model—see Chapter 12, and Gould, 1985a,
1993c), establishes more features of overall pattern than the ordinary interplay of
taxa during normal times (between such episodes of coordinated death) can build
and maintain, then Darwin's view of life lacks the generality once accorded. In
particular, the key uniformitarian argument will then fail. The adaptive struggles of
immediate moments will not extrapolate to explain the patterns of life's history.
Moreover, if these undarwinian components of fortuity in extinction, and success
for reasons unrelated to the original adaptive basis of traits, also maintain strong
influence at lesser scales of smaller mass extinctions (Raup and Sepkoski, 1984),
and even, in a fractal manner, for some ordinary extinctions in normal times (Raup,
1991), then the challenge may become truly pervasive.
These characterizations of Darwinian requirements cannot be dismissed or
downgraded as conjectures or reconstructions, only inferentially based on
deductions from premises stated by Darwin for different reasons. Darwin devoted
an entire chapter of the Origin, number 10 "on the geological succession of organic
beings," to an exploration of the geological stage and its requirements for natural
selection. He argues that biotic competition, gradualistically expressed through
time as coordinated waxing and waning of interacting clades, marks the overall
pattern of life—and that the apparent fossil evidence for more rapid change, set by
physical environments and leading to mass extinctions, must generally be read as
artifacts of an imperfect record (see Chapter 12 for detailed exegesis of Darwin's
arguments on this subject).