1300 THE STRUCTURE OF EVOLUTIONARY THEORY
markedly increased extinction entirely. Rather, he need only "spread out" the
appearance of true simultaneity into a period long enough to permit explanation by
biotic competition, perhaps intensified by tough physical times that make organismal
battles even more stringent than usual, while remaining within the ordinary range and
mode. After all, and in the anachronism of modern slang, "when the going gets tough,
the tough get going."
Thus, high rates of change in physical environments, so long as they stay within
permissible uniformitarian limits, will enhance extinction by "turning up the gain" on
the dial of input from the geological stage. But, to emphasize the key point of
Darwin's efforts, a world of conceptual difference separates a false appearance of
catastrophe that can, by invoking the imperfection of geological records, be spread
over sufficient time to remain within the uniformitarian range (leading to
intensification of evolutionary rates by ordinary modes), and a true catastrophe that
must impose its burden of extinction by direct environmental impress under rules
different from those regulating the primarily biotic competition of normal times and
ordinary ecology (and generating a macroevolutionary vector of progress as a result).
Darwin would therefore turn to his old standby of an imperfect geological record to
disperse this third and greatest challenge to his extrapolationist vision.
Darwin presents two basic arguments—the first more theoretical and biological,
and the second far more practical, crucial, operational and geological—to buttress his
claim that a threatening appearance of simultaneity in mass extinction and origination
should be "spread out" to occupy enough time for explanation on uniformitarian
premises by the ordinary operation of natural selection. First, theory dictates that old
species generally become extinct by biotic competition with new and improved
forms, not by direct extirpation through marked changes in the physical environment.
"The extinction of old forms is the almost inevitable consequence of the production
of new forms" (p. 343). Darwin even denies a possible "escape route" for the less fit
by asserting that mean global diversity has remained fairly constant through time—so
the poorly adapted must go to the wall in clearing limited space for improved forms,
and cannot hang on at the peripheries of a general expansion that welcomes the new
without necessarily destroying the old: "Thus the appearance of new forms and the
disappearance of old forms, both natural and artificial, are bound together... We
know that the number of species has not gone on indefinitely increasing, at least
during the later geological periods, so that looking to later times we may believe that
the production of new forms has caused the extinction of about the same number of
old forms" (p. 320). Moreover, to enhance the implausibility of truly catastrophic
mass dying, Darwin holds that "the complete extinction of the species of a group is
generally a slower process than their production" (p. 318).
In a long discussion on pages 325-327, Darwin collates all aspects of his
biological argument that ordinary competition will explain the literal appearance of
simultaneous global extinction and origination. The final paragraph summarizes his
extrapolationist convictions (p. 327):