The Essence of Darwinism and the Basis of Modern Orthodoxy 153
Some fidei defensores of the Darwinian citadel have sensed the weakness of
this third version of gradualism, and have either pointed out that the creativity of
natural selection cannot be compromised thereby (quite correct, but then no one
ever raised such a challenge, at least within the legitimate debate on punctuated
equilibrium); or have argued either that Darwin meant no such thing, or that, if he
really did, the claim has no importance (see Dawkins, 1986). This last effort in
apologetics provides a striking illustration of the retrospective fallacy in
historiography. Whatever the current status of this third formulation within modern
Darwinism, this broadest style of gradualism was vitally important to Darwin; for
belief in slow change in geological perspective lies at the heart of his more
inclusive view about nature and science, an issue even larger than the mechanics of
natural selection.
Darwin often states his convictions about extreme slowness and continuous
flux in geological time—as something quite apart from gradualism's second
meaning of insensible intermediacy in microevolutionary perspective.
Evolutionary change, Darwin asserts, usually occurs so slowly that even the
immense length of an average geological formation may not reach the mean time
of transformation between species. Thus, apparent stasis may actually represent
change at average rates, but to an imperceptible degree even through such an
extensive stretch of geological time! "Although each formation may mark a very
long lapse of years, each perhaps is short compared with the period requisite to
change one species into another" (p. 293).
Change not only occurs with geological slowness on this largest scale; but
most transformations also proceed in sufficient continuity and limited variation in
rate that elapsed time may be roughly measured by degree of accumulated
difference: "The amount of organic change in the fossils of consecutive formations
probably serves as a fair measure of the lapse of actual time" (p. 488).
Darwin presents his credo in crisp epitome: "Nature acts uniformly and slowly
during vast periods of time on the whole organization, in any way which may be
for each creature's own good" (p. 269). Note how Darwin concentrates so many of
his central beliefs into so few words: gradualism, adaptationism, locus of selection
on organisms.
But the most striking testimony to Darwin's conviction about gradualism in
this third sense of slow and continuous flux lies in several errors prominently
highlighted in the Origin—all based on convictions about steady rate (gradualism
in the third sense), not on the insensible intermediacy genuinely demanded by
natural selection (gradualism in the second sense), or on the simple continuity of
historical information required to validate the factuality of evolution itself
(gradualism in the first sense). For example, Darwin makes a famous calculation
(dropped from later editions) on the "denudation of the Weald"—the erosion of the
anticlinal valley located between the North and South Chalk Downs of southern
England (pp. 285-287). He tries to determine an average value for yearly erosion of
seacliffs today, and then extrapolates his figure as a constant rate into the past. His
date of some 300 million years for the denudation of the Weald overestimated the
true duration by five