238 THE STRUCTURE OF EVOLUTIONARY THEORY
levels, though he does not provide the technical arguments detailed below. In
Schweber's view, Darwin was driven to formulate an argument that "does not
cohere" (Schweber, personal communication) because his century's ignorance of
hereditary mechanisms drove him to describe variation within species and varieties
treated as units, while the causal structure of natural selection rested upon
individual organisms. Arguments about organisms and species are not comfortably
intertwined or mutually supporting within Darwin's conceptual structure: rather,
the two levels remain discordant and inadequately (if not illogically) bridged. "This
difference in the 'units' used is important. It accounts for the fact that at times
levels of description were interchanged and some confusion necessarily crept in"
(Schweber, 1980, p. 240). "There was no link between adaptation and speciation,
except whatever could be supplied by a quasi-historical developmental idea of
optimizing the amount of life" (ibid., pp. 287-288). "The problem of the different
levels of descriptions was confined to how the properties of variations in
individuals ... were responsible for the assumed variability characteristic of
varieties and species. This problem Darwin never solved" (ibid., p. 288).
We can exemplify Schweber's perceptions about Darwin's incoherence of
argument by dissecting the logic of Darwin's attempt to use ordinary natural
selection as the basis of divergence. For three basic reasons, his attempt to invoke
selection among organisms as an explanation for patterns in speciation and
extinction—the heart of the "principle of divergence," and the primum desideratum
for a complete theory of natural selection—fails because the level of species must
be addressed both directly and causally, while Darwin's rationale for explanation
from below includes gaps and fatal weaknesses.
The calculus of individual success
Darwin treats the principle of divergence in two extensive discussions—the long
and even labored account of chapter 4 in the Origin of Species (1859, pp. 111-126),
and the even more detailed exposition intended for the "big species book" that
Wallace interrupted and Darwin never published because he rushed to compose the
Origin instead. The manuscript for most of this larger project survives, including
the full discussion of divergence intended for chapter 6 "On Natural Selection."
This text, published under R. C. Stauffer's editorship in 1975, treats the principle of
divergence on pages 227-251.
When, in the 1970's, I first read the Origin with the notion of hierarchical
selection in mind, I was fascinated by Darwin's struggle to bridge the levels, and
his ultimate lack of success. Schweber speaks of "incoherence"; I would rather
describe Darwin's "moves" of argument as an oscillation between one mode and
the other. In some passages (including those cited above), Darwin speaks of
ordinary natural selection and the advantages enjoyed by extreme variants. In
others, he judges the success of a parental form not by the vigor or competitive
prowess of offspring, but by the number of descendant species emanating from a
rootstock.
These themes could, of course, be complementary. One perspective might