The Structure of Evolutionary Theory

(Michael S) #1

446 THE STRUCTURE OF EVOLUTIONARY THEORY


(for the two themes fuse well into a coherent anti-Darwinian philosophy, as
Bateson recognized and articulated).
When such basic themes commingle, our experimental traditions lead us to
search for "pure" end-member cases—examples of one item without the other, so
that we may assess the unadulterated contribution of each theme treated separately.
We can identify several pure channeling theorists—biologists, who extolled
directional variation, but supported gradualism and rejected saltation (several
orthogeneticists fall into this category). But we rarely encounter a "pure"
saltationist who accepts isotropy for large-scale changes and rejects all notions of
preferred directionality. (Since saltation also implies an internal control upon
change, the allied theme of preferred directions usually gains assent as well—as in
Bateson's arguments on homeotic variation.) But de Vries represents our
instructive "pure" case of one without the other— a saltationist who accepted the
isotropy of these immediate and substantial changes. Moreover, de Vries
explicated the logic of his unusual commitment to recognize the interesting role
that such a "nonstandard combination" must imply for Darwinism. (Saltationism
precluded any role for selection in the origin of species; but mutational isotropy
resurrected the Darwinian apparatus at the higher level of evolutionary trends.) As
a "test case," therefore, de Vries—all by himself—balances the rest of this chapter,
with its primary emphasis on internally channeled variation. De Vries's brilliance,
particularly his clear attention to Darwinian logic, makes him a particularly
attractive and instructive figure in our quest to understand the components of
internalist thinking.


De Vries on macroevolution
De Vries never placed primary emphasis upon macroevolutionary themes in his
books, but these issues do receive more than passing attention. By his own intent
and reckoning, de Vries' main contribution to macroevolution lay in his resolution
of Kelvin's paradox—an earth too young to permit evolution by Darwinian
gradualism (see Chapter 6). Obviously, if new species arise per saltum in a single
generation, limits on the earth's age would not preclude the evolutionary work
actually accomplished. "The demands of the biologists and the results of the
physicists are harmonized on the ground of the theory of mutation" (1905, p. 712).
"One of the greatest objections to the Darwinian theory of descent arose from the
length of time it would require if all evolution was to be explained on the ground
of slow and nearly invisible changes. This difficulty is at once met, and fully
surmounted by the hypothesis of periodical but sudden and quite noticeable steps.
This assumption requires only a limited number of mutative periods, which might
occur within the time allowed by physicists and geologists for the existence of
animal and vegetable life on the earth" (1905, p. 29).
De Vries even made a semi-quantitative assessment, based again on
Oenothera. Given Kelvin's 24 million year estimate for a habitable earth, even one
mutation (adding a character) every 4,000 years would provide 6,000 new features
for any extant lineage. Since neither Oenothera, nor any

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