480 THE STRUCTURE OF EVOLUTIONARY THEORY
emerged as a serious option in a metaphorical sense, as embodied in Kelvin's
argument for the restriction of geological time (see pp. 492-502). But too much
change characterized the core of a geological system—catastrophism— that, if
generally valid, would severely compromise Darwinism by the fundamental
criterion of relative frequency.
Darwin's need for a "golden mean" of geological change flows from his
extrapolationist premise that observable and small-scale natural selection can
provide, by extension, the causal basis for life's history at grand scales of
morphological transformation through geological time. Darwin rooted his defense
of this premise in the validity of uniformitarianism, as preached by his guru,
Charles Lyell. The uniformitarian defense of extrapolationism therefore undergirds
the third leg of my proposed tripod for an "essence" of Darwinian theory. This
Lyellian assumption buttresses the ordinary operation of natural selection in the
immediacy of any ecological moment, but the theme of the first section of this
chapter raises the ante by including Darwin's treatment of pattern on a geological
stage. The raw mechanism of natural selection provides no direction for organic
change, and yields no predictable order for life's history through time. However, by
adding a set of distinctive ecological arguments to the bare-bones mechanics—
notably the domination of overt biotic competition as a primary mode of struggle
within perpetually crowded communities—Darwin could validate the central belief
of his surrounding culture, the concept of progress, as a primary signal of life's
history.
Thus, the "golden mean" of geological change became doubly important to
Darwin, because both the general operation of natural selection, and his particular
rationale for progress in macroevolution, require a Lyellian geological world. The
specter of catastrophism also became much more potent in the light of Darwin's
stipulation that biotic competition acts as the chief agent of direction in life's
history. For if mass extinction (and other phenomena of "too much" environmental
change) establish patterns in the history of life at too high a relative frequency,
then biotic competition will be demoted, if not replaced, by an ordering force of
opposite meaning—for mass extinctions introduce a powerfully confusing and
potentially confuting new actor: the tumbling, whimsical wheel of fortune rather
than the slow and steady wedge of progress.
The norms of science dictate that major works be presented as objective
explorations of data, with general conclusions derived from empirical evidence and
devised late in the process of discovery. But most seminal books in the history of
science can be read as briefs for passionately held, elegantly articulated, brilliantly
advocated (and, to be sure, well-defended) views of nature. As a premier example,
Charles Lyell, a lawyer by profession, may have presented his epochal Principles
of Geology in the conventional style of humble factual documentation. But this
great work must be understood as perhaps the most explicit and most able brief
ever presented in the guise of a major scientific treatise.
The sources of Lyell's success in promoting his uniformitarian view—which
later emerged as such a fitting solution to Darwin's Goldilocks problem by