Pattern and Progress on the Geological Stage 501
false impression from our recent (and unusual) climatic stability that evolution
must always proceed with imperceptible slowness. But the general pace of natural
selection can be much faster, matching the usual rate of climatic fluctuation. When
we recognize this "slower change of species since the glacial epoch than at any
former period" (1870, p. 455), we can correct our false allegiance to evolution's
perpetual sloth, "thus allowing us to suppose change of form in the organic world
to go on more rapidly than we had before thought possible" (1870, p. 455).
Wallace then proposes a correction to Lyell’s estimates of time based on
turnover in molluscan species. He obtains an age of 24 million years for the base of
the Cambrian, proclaims himself in agreement with Kelvin's 100 million years
since the solidification of the earth's surface (leaving some 75 million years for the
Precambrian interval that Darwin had proclaimed longer than all subsequent time),
and urges Darwin to accept this framework with good grace and confidence in
natural selection:
These figures will seem very small to some geologists who have been
accustomed to speak of "millions" as small matters; but I hope I have
shown that, so far as we have any means at present of measuring geological
time, they may be amply sufficient. Taking Sir William Thomson's
allowance of a hundred million years for the time during which the earth
can have been fit for life, it yet allows Mr. Darwin, for the process of
development from the primordial germ, three times as many years anterior
to the Cambrian epoch as have elapsed since that date, an amount of time
which, I believe, will fully satisfy him (1870, pp. 454-455).I find an ironic similarity between this tale of Darwin and his intellectual
brethren, and the story of Solomon and the baby claimed by two women. The true
parent loves her child so fiercely that she would rather give her offspring to an
imposter than to see the infant dismembered in mock compromise. The false
mother feels no such protective love, thus permitting Solomon's wise
identification. I do not wish to push the analogy too far. Huxley and Wallace
cannot be called false parents, but the child of natural selection had not originated
in their womb (or, in Wallace's case, at least not for so long a gestation, or so slow
and prideful a growth). They had not nurtured this idea through so many years of
thought, through such struggles of upbringing, to build such an extensive and
coherent edifice of logic and implication. They could therefore compromise,
sacrifice a bit here, and trim a bit there, to preserve the entity, however tarnished.
But, for Darwin, any departure from full integrity became unthinkable. Darwin
also understood, however, that intransigence compromises the spirit of science, and
that all good thinkers must make and admit mistakes. For this reason, Darwin
separated his core commitments (the "essences" of my treatment) from postulates
more easily compromised. To abandon, or even seriously to mitigate, any of the
core commitments might be equated with the unacceptable solution of dividing the
child. A conviction about the generally slow and steady character of geological
change, a prerequisite