558 THE STRUCTURE OF EVOLUTIONARY THEORY
working only by a struggle among organisms for personal reproductive success.
Strict Darwinians must defend extrapolation as crucial at both poles— for
observable events at small scale cannot generate the full panoply of phylogeny
without such a principle; while daily happenings cannot accumulate into totalities
if the background setting does not "behave" properly—if, for example, the
ecological stage explodes every once in a geological while, fortuitously dumping
most of the improved and accumulated inhabitants into a vat of extinction.
For these two reasons—to validate the entire methodological pole and to
uphold the third leg of the Darwinian tripod supporting the ideological pole—the
principle of extrapolation represents the key to the validity of the Synthesis as a
fully general theory of evolution. And extrapolation—the essential Lyellian
postulate that Darwin imbibed from his most important mentor (see p. 94)—
embodies two central aspects of what Lyell and his school called
"uniformitarianism": (i) the complete theoretical sufficiency of currently acting
small-scale changes to produce, by successive and imperceptible increments, the
entire panoply of large-scale phenomena; and (ii) the proper "behavior" of the
earth, with geological change sufficiently slow and steady that trends produced by
gradualistic, accumulative natural selection will not be derailed often enough to
yield a history of life patterned more by these geological upsets than by biological
accumulations.
For this last leg of the tripod, the Synthesis did not so much harden as become
emboldened during its ontogeny. We have seen (pp. 514-518) how Haldane and
Huxley, in the early days of the Synthesis, still respected (even feared) apparent
paleontological exceptions to natural selection as the cause of trends. But the
balance of power had shifted by the 1960's. Simpson (1944, 1953) and others had
forged their "consistency argument"—the principle that all known phenomena of
the fossil record can, in principle, be explained by modern mechanisms of genetics
and selection (even though no direct proof of sufficiency can be derived from
paleontological evidence). Paleontology had been tamed, taken in by the synthesis,
and told to behave. (And I do intend "taken in" in the metaphorical sense as well,
as I shall argue more explicitly in Chapter 9.) Paleontology could retain the
archives of actual phenomenology as its particular bailiwick in exchange for giving
up the conceit of believing that the fossil record could say anything distinctive
about the causes of evolutionary change.
The distinguished panel on "the evolution of life" at the Chicago centennial
celebration of 1959 included Julian Huxley as chairman, Th. Dobzhansky, E. B.
Ford, Ernst Mayr, Ledyard Stebbins, and Sewall Wright. After an orthodox
discussion of mechanisms, Huxley shifted the topic to "the course, the process, of
evolution as shown in the fossils" (Tax and Callender, 1960, volume 3, p. 127)—
and paleobotanist D. I. Axelrod rose to present a summary in advance,
characterizing the history of life as a stately process of unfolding to more and
better: "I think most of us are in full agreement about the gradual change in time:
increasing diversification; then, gradual transformation,