1298 THE STRUCTURE OF EVOLUTIONARY THEORY
now relying on discontinuity of deposition and environment in single regions through
geological intervals. Strata may seem continuous, he correctly argues, but most times
contributed no sediment because accumulation only occurs during gentle subsidence
of basins, and such conditions do not generally prevail. By the same token, the local
environment occupied by a species in anagenetic transformation will not generally
persist long enough in any single place, and organisms will track their moving
habitats. For example, the shallow marine habitat of so many invertebrate species can
only continue (under conditions that also accumulate sediments with fossils) when
rates of deposition for strata evenly match rates of subsidence for substrates—and
how often, and for how long, can such a fine balance be maintained in any one place?
Having ascribed, to his satisfaction, both global and local absences of
transitional forms to the general imperfection of geological records, Darwin must
finally, in the closing sections of Chapter 9 (and spilling over into substantial parts of
Chapter 10), rebut a third geological challenge to evolution, and especially to
gradualistic explanations framed in terms of natural selection. To overcome this last
obstacle, Darwin must tackle the harder problem of an apparently positive signal
against his expectations, rather than (as in the first two cases) a negative result of
failure to locate an anticipated confirmation. To complete his argument, Darwin must
now explain away the evidence for global episodes of apparently sudden mass
extinction or origin of entire faunas.
We should first pause to ask why Darwin even considered this signal from the
fossil record as such a problem, especially for episodes of mass extinction. Why did
he view the prospect of simultaneous extirpation as an issue at all, either for evolution
or for natural selection? Natural selection does not guarantee the power of adaptation
in all circumstances—and if environments change rapidly and profoundly enough,
these alterations may exceed the power of adaptation by natural selection, with
extinction of most forms as the expected result, even in the most strictly Darwinian of
circumstances.
As a general answer—and as the primary reason for treating this subject within a
chapter on modern critiques of the third leg, or extrapolationist premise, of Darwinian
central logic—Darwin's hostility to catastrophic mass extinction does not arise
primarily from threats posed to the mechanism of natural selection itself, but more
from the challenges raised by the prospect of sudden global change to the key
uniformitarian, or extrapolationist, assumption that observable processes at work in
modern populations can, given the amplitude of geological time, render the full
panoply of macroevolutionary results by prolonged accretion and accumulation.
The problem of mass extinction became acute for Darwin because geological
paroxysm threatened something quite particular, vitally important, and therefore of
much greater immediate pith and moment than his general methodological preference
for locating all causality in the palpable observation of microevolution (see Chapter
2 ). Global catastrophe could undermine the ecological