444 THE STRUCTURE OF EVOLUTIONARY THEORY
upon the validity of the remaining two criteria at this higher level: copiousness and
isotropy. The criterion of copiousness will surely be fulfilled in lineages
undergoing a de Vriesian period of mutability. The full operation of Darwinian
logic at this larger scale will then stand or fall upon the remaining criterion of
isotropy. De Vries analyzed the issue in these appropriate terms and, by stating
strong support for isotropy. He therefore identified a proper fealty with Darwinian
logic (in a domain different from Darwin's own application, after denying the
efficacy of the same general logic in Darwin's own favored realm).
De Vries often states the principle of isotropy as one of his central
conclusions: all characters may mutate in all directions. "Single variations
[mutations] seem to be presented by all characters, to proceed in every direction
and to be apparently without limit" (1909a, volume 1, p. 33). Moreover, de Vries
expresses this view not only as an empirical conclusion but as a logical
consequence of his theory: "The mutation theory demands that organisms should
exhibit mutability in almost all directions" (1909a, volume 1, p. 204).
Even more significantly, de Vries recognizes that isotropy must be asserted to
validate Darwinian selection at the higher level of evolutionary trends. Tying
isotropy directly to the efficacy of selection, de Vries writes (1905, p. 574):
"Nearly all qualities vary in opposite directions and our group of mutants affords
wide material for the sifting process of natural selection." Selection can only
operate as a sieve, but if variations are copious, isotropic and small (a good
description for the status of de Vriesian mutations relative to the full extent of a
geological trend), then these species-forming mutations can forge no large-scale
cladal trend by themselves—and even a sieve, by extended directional
accumulation, becomes a creative mechanism. (This metaphor beautifully restates
Darwin's basic argument about the creativity of selection—see Chapter 2. But de
Vries denies selection at Darwin's own favored level of organisms in populations,
and grants power to Darwin's mechanism only at the higher level of sustained
trends among species in clades.)
According to Darwin, changes occur in all directions, quite independently
of the prevailing circumstances. Some may be favorable, others
detrimental, many of them without significance, neither useful nor
injurious. Some of them will sooner or later be destroyed, while others will
survive, and which of them will survive is obviously incumbent on the
question, whether their particular changes agree with the existing
conditions or not. This is what Darwin has called the struggle for life. It is a
large sieve, and it only acts as such. Some fall through and are annihilated,
others remain above and are selected, as the phrase goes. Many are
selected, but more are destroyed: daily observation does not leave any
doubt upon this point. How the differences originate is quite another
question. It has nothing to do with the theory of natural selection, nor with
the struggle for life (1905, p. 571).De Vries strongly rejected any notion of directed variability (nonisotropy) in
the production of trends—and therefore maintained no sympathy at all for