498 THE STRUCTURE OF EVOLUTIONARY THEORY
same vector also challenged Darwin by stressing the importance of early rapidity
itself. Restricted time implied too little geology when considered in terms of
Darwin's need for an intermediate amount of environmental change. But the rapid
change on Kelvin's early earth implied too much geology on the flip side of
impermissibility. Intense heat flows and high rates of volcanism and erosion on an
early earth raised the specter of traditional catastrophism, with its paroxysms of
mass extinction. This argument about rapidity opposed Darwin at both essential
poles of practice and theory (see Chapter 2). Could a process like natural selection
really operate effectively on a planet engulfed in such perpetual turmoil? Must
other causes of biological change be postulated for the early earth—a particularly
distressing prospect for Darwin, who yearned, above all, to establish a temporally
invariant and fully general account of evolution. Moreover, the prospect of such a
prominent vector clashed with Darwin's Lyellian vision of an earth operating with
sufficient constancy of change that the study of modern causes would suffice for
explaining the past as well. Kelvin, astute as ever, had explicitly proclaimed the
victory of "catastrophism" more for its explanation of a vector of diminishing
intensity than for its theory of paroxysms (1868, pp. 231-232). And Darwin, who
had so assiduously explored the ramifications of all major ideas in natural history,
rejected the directionalist aspect of catastrophism as firmly as he dismissed the
paroxysmal claim.
Yet Darwin could not escape the directionalist implication, and he eventually
compromised on this single point alone. Putting his best face on adversity, Darwin
added a passage to later editions of the Origin. He now acknowledged the vector of
diminishing intensity in change, admitted higher rates of evolution on an early
earth, and made peace with Kelvin's temporal restrictions by awarding evolution an
ontogeny with a speedy childhood: "It is, however, probable, as Sir William
Thompson [sic] insists, that the world at a very early period was subjected to more
rapid and violent changes in its physical conditions than those now occurring; and
such changes would have tended to induce changes at a corresponding rate in the
organisms which then existed" (Origin, 1872, 6th edition).
In a reciprocal move, Darwin also quietly dropped the following passage of
his first edition, with its opposing claim for slower initial rates of evolution— no
doubt with regret, for this original argument had embodied his favored theme of
biotic control: "During early periods of the earth's history, when the forms of life
were probably fewer and simpler, the rate of change was probably slower; and at
the first dawn of life, when very few forms of the simplest structure existed, the
rate of change may have been slow in an extreme degree" (1859, p. 488).
If all evolutionists had reacted to Kelvin in the same way, we might have
learned something about conflicts across scientific disciplines, but little about
Darwin's distinctive view. However, in a fascinating and little known aspect of this
familiar story, Darwin stood virtually alone in loathing Kelvin's restriction—and
his reaction therefore reveals some important implications of his own strict
selectionist logic applied with firm gradualist commitments.