430 THE STRUCTURE OF EVOLUTIONARY THEORY
The theory of mutations is a starting point for direct investigation while the general
belief in slow changes has held back science from such investigations during half a
century" (1905, p. 30).
As an experimental reductionist, committed to finding the mechanism for
large-scale evolution of phenotypes in the smallest cellular parts, de Vries sought
the causal basis for his mutation theory in the character of variation and its putative
causes. As a foundation for all his theorizing, de Vries proposed a strict separation
between two distinct types of variation: fluctuating and mutational. (This division,
of course, establishes the same false dichotomy that prompted the famous
"biometrician" vs. "Mendelian" debate— a struggle that de Vries' context did much
to promote). De Vries stated that, in the early 1870's, he had read Quetelet's work
on normal curves and Galtonian regression to the mean—and had determined
thereby that the omnipresent, small scale or, in his favored term, "fluctuating"
variation could not be parlayed into directional evolutionary change, as Darwin's
theory required (Fig. 5-10). Evolution must therefore require a conceptually and
causally distinct mode of sudden, larger-scale, true breeding and non-regressing
variation—a necessary source eventually found in the "mutations" that yielded
distinct new phenotypes in Oenothera.
De Vries acknowledged that selection of fluctuating variation could produce
new agricultural races and stocks of domesticated animals. But this Darwinian
alteration can only yield a minor change from the mean of a parental stock: "It is
responsible only for the smallest lateral branches of the pedigree, but has nothing
in common with the evolution of the main stems. It is of very subordinate
importance" (1905, p. 801). These new races, if not constantly superintended, will
rapidly revert towards parental characters by regression to the mean (1909a,
volume 1, pp. 88-89). De Vries, who understood the logic of Darwinism so keenly
(see pp. 446-451), knew that the most promising
5 - 10. De Vries' illustration of continuous "fluctuating" variation, which he regarded as
ineffective as a source for evolutionary change. From volume 1 of De Vries' Mutation Theory.