Tiers of Time and Trials of Extrapolationism 1313
respectability (after 150 years of successful Lyellian anathematization) by stating an
emerging consensus about four crucial and general features of such events, each
strongly negative (and, in their ensemble, probably fatal) for the key extrapolationist
premise needed to maintain a claim of exclusivity for a strictly Darwinian theory of
evolutionary process (with descriptive aspects of life's pageant still left for
paleontological documentation, because a general theory must underpredict an actual
outcome in a historically contingent world): mass extinctions are more frequent, more
rapid, more intense, and more different in their effects than paleontologists had
suspected, and that Lyellian geology and Darwinian biology could permit.
In terms of the broad categories of pattern in life's history that will now require
at least partial explanation from catastrophic theories of mass extinction, and will
probably not be rendered on Darwinian assumptions of extrapolation from
microevolutionary theory, * I would summarize the most important
- May I, for one last time, repeat the rationale, from the internal logic of this book, that
leads me to the otherwise odd strategy of treating the refutation of the third, or extrapolationist,
leg of central Darwinian logic at reduced length (in Chapter 6 in the historical half and the
present Chapter 12 in the modern half) relative to the other two legs of hierarchy and levels of
selection (Chapter 3 in the first half, and 8-9 in the second half) and structuralist thinking and
constraint vs. strict selectionism (Chapters 4-5 in the first half and 10-11 in the second half). My
reasons rest upon four arguments, the first three intertwined and intellectual, the fourth separate
and personal. First, this book treats the biological structure of evolutionary, particularly
Darwinian, theory. Of the three central components designated as legs on a tripod of conceptual
definition for Darwinism—(1) levels of selection (Darwinian organismal vs. modern notions of
a full hierarchy); (2) functional vs. structural approaches (externalist selection as virtually the
sole creative force in evolutionary change vs. the importance of internal constraint from several
sources, not all selectionist); and (3) uniformitarian extrapolation of microevolutionary styles of
natural selection to explain the full panoply of life's changes through geological time—the
biological aspects of the critique for the third theme of extrapolationism lie mainly within
expansions and revisions of Darwinism on the first two legs. Second, I therefore decided to
confine my explicit discussion of the third leg (since the biological critiques had already been
treated under the first two legs) to the surrogate theme of geological requirements to potentiate
the biological mechanism (the kind of stage needed to display the proposed play). The surrogate
theme of a different profession does not require such detailed treatment as the central biological
critiques. Third, and almost as a historical footnote (but a truly intriguing point missed in most
discussions of Darwinism and the 19th century debates about evolution), Darwin himself
became uncomfortable with his need to call upon such surrogate themes from other disciplines,
and he tried (in later years) to avoid such ancillaries, and to devise a theory that would render
his conclusions about life's history entirely by biological principles. Most importantly, he
abandoned his early satisfaction with allopatric modes of speciation (that called upon
geographic surrogacy to ensure a world with sufficient opportunity for isolation of populations),
and moved towards the sympatric theory embodied in his "principle of divergence" (see Chapter
4 of the Origin, including the book's sole figure), and considered by him (in the famous
"eureka" statement of his carriage ride in 1854, see p. 224) as second in importance only to his
formulation of natural selection itself in devising his full theory. Because this sympatric theory,
relying upon the generality of selective benefits for extreme forms in any environment, proved
ultimately quite unsatisfactory, and even illogical, we can grasp, with even greater clarity, the
importance of this issue in Darwin's mind—as he so rarely allowed himself to place such weight
upon a patently dubious argument (and I do mean patent, for he fretted overtly about the