The Essence of Darwinism and the Basis of Modern Orthodoxy 151
formulation: "Natural selection can only act by the preservation and accumulation
of infinitesimally small inherited modifications" (p. 95). At some level of
discontinuity, of course, Darwin's strong statement must prevail. If the altered
morphology of new species often arose in single steps by fortuitous
macromutation, then selection would lose its creative role and could act only as a
secondary and auxiliary force to spread the sudden blessing through a population.
But can we justify Darwin's application of the same claim to single organs?
Suppose (as must often happen) that developmental heterochrony produces a major
shift in form and function by two or three steps without intermediary stages. The
size of these steps may lie outside the "normal" variation of most populations at
most moments, but not beyond the potential of an inherited developmental
program. (Incidentally, these types of changes represent the concept that
Goldschmidt embodied in the legitimate meaning of "hopeful monster," before he
made his unfortunate decision to tie this interesting concept to his fallacious
genetics of "systemic mutation"—see Chapter 5 and Gould, 1982a.)
Would natural selection perish if change in this mode were common? I don't
think so. Darwinian theory would require some adjustments and compromises—
particularly a toning down of assertions about the isotropy of variation, and a more
vigorous study of internal constraint in genetics and development (see Chapter 10
for advocacy of this theoretical shift)—but natural selection would still enjoy a
status far higher than that of a mere executioner. A new organ does not make a new
species; and a new morphology must be brought into functional integration—a
process that requires secondary adaptation and fine tuning, presumably by natural
selection, whatever the extent of the initial step.
I believe, therefore, that Darwin's strong, even pugnacious, defense of strict
gradualism reflects a much more pervasive commitment, extending far beyond the
simple recognition of a logical entailment implied by natural selection—and that
this stronger conviction must record such general influences as Darwin's attraction
to Lyell's conflation of gradualism with rationality itself, and the cultural appeal of
gradualism during Britain's greatest age of industrial expansion and imperial
conquest (Gould, 1984a). Huxley's savvy assessment of the Origin still rings true,
for while he offered, in his famous letter to Darwin, written just as the Origin
rolled off the presses, to "go to the stake" for Darwin's view, he also stated his
major criticism: "You have loaded yourself with an unnecessary difficulty in
adopting Natura non facit saltum so unreservedly" (in L. Huxley, 1901, p. 189).
Darwin persevered nonetheless. We often fail to recognize how much of the
Origin presents an exposition of gradualism, rather than a defense of natural
selection. As a striking example, the famous (and virtually only) statement about
human evolution asserts the pedagogical value of gradualism—not natural
selection—in our Socratic quest to know ourselves: "Psychology will be based on a
new foundation, that of the necessary acquirement of each mental power and
capacity by gradation. Light will be thrown on the origin of man and his history"
(p. 488).