232 THE STRUCTURE OF EVOLUTIONARY THEORY
description of diversity, but emphatically not a "principle of divergence." Darwin's
insight in his carriage did not merely systematize the notions of maximization and
division of labor; Darwin had known and used these concepts for years. The
transforming insight—the argument worthy of being called a "principle of
divergence" and becoming a keystone of his entire evolutionary theory—occurred
when Darwin recognized how he could apply his distinctive style of argument,
based on organismal selection, to the higher-level phenomenology of diversity. In
other words, the "principle of divergence" embodies Darwin's argument for how
and why ordinary natural selection must, as a predictable consequence, yield
divergence of character, leading to multiplication of successful taxa, extinction of
others, ecologic plenitude, maximization of life, and the hierarchical structure of
taxonomy.
THE GENESIS OF DIVERGENCEThis perspective on the jelling of Darwin's principle of divergence resolves a
recent debate among historians about the timing and reason for Darwin's
formulation. Proposals for timing have ranged from the late 1840's to 1858, with
cardinal inspirations from biogeography (Sulloway), systematics (Limoges,
Ospovat), the "botanical arithmetic" of chapter 2 in the Origin, leading Darwin to
defend the greater evolutionary potential for diversification in large genera
(Browne), inspiration from the arguments of political economy (Schweber) and
switch from allopatric to sympatric models of speciation (Kohn). All these
influences surely played their parts, for the principle of divergence calls upon a
wide and complex range of convictions, spanning many years and much turmoil in
Darwin's mind. But Darwin's formulation and formalization of the "principle of
divergence" records his conviction, and his great pleasure, that he could encompass
all these ideas as predictable consequences of natural selection working by
struggle among organisms—that he could, in other words, bring all the higher-
level phenomenology of maximization, division of labor, and so forth, into his own
distinctive explanatory framework.
Schweber dates the first full formulation to 1856 and writes (1988, p. 135):
"That Darwin had the 'keystone' of the argument by January 1855 is probably
correct, but I would also suggest that the argument was still not complete in an
important way—at least insofar as an explicit presentation is concerned. All the
arguments up to that point referred to levels of descriptions above individuals:
varieties, species, and higher taxa. Natural selection operated on individuals, and
the linkage by which diversity is accomplished had to be explicitly stated." Using
this insight, Schweber regards the following note of September 23, 1856, as the
first explicit formulation. * Just
*Since these words directly follow the statement, quoted on p. 230, about division of
labor in political economy, Schweber locates a primary influence in this interdisciplinary
transfer—not only, of course, via the specific linkage in this particular quotation, but pri-
marily because the dominant political economy of individualism, the philosophy of Adam
Smith, Jeremy Bentham and scores of followers, had always been a central inspiration for
Darwin from the Malthusian insight onward—see pp. 121-125 and Schweber, 1977.