Seeds of Hierarchy 229
Darwin did not draw this unique diagram simply to illustrate the generality of
evolutionary branching, but primarily to explicate the principle of divergence.
After more than a century in limbo, Darwin's principle of divergence has been
exhumed and subjected to careful scrutiny by historians of science. No subject in
Darwinian studies has been more actively pursued during the past 25 years, and
many excellent analyses have been published on the genesis and utility of the
principle of divergence (Limoges, 1968; Sulloway, 1979; Browne, 1980;
Schweber, 1980, 1985, 1988; Ospovat, 1981; Kohn, 1981, 1985). Therefore, the
importance of this principle has finally been recognized. Ospovat, for example,
writes (1981, pp. 170-171): "Darwin's 'principle of divergence' [is] the most
important addition to his theory between 1838 and 1959 [sic, for 1859] and the one
most intimately associated with the transformation of his theory after 1844." In all
this literature, however, only Schweber has grasped Darwin's difficulties with
divergence as an unresolved struggle between levels of explanation. Yet this
theme, particularly Darwin's inability to "cash out" his usual argument about
organismal struggle at the level of species birth and death, holds, I believe, the key
to Darwin's treatment.
Darwin's argument about divergence begins with an unquestioned premise
that strikes us as curious today (for we are immediately tempted to mount a
challenge), but resonates with a central theme of Darwin's century—the clear and
inherent "good" of maximizing the amount of life in any given region, and the
consequent necessity for a cause to insure this natural goal. Maximization, Darwin
argues, arises by diversification: the more taxa in a given area (and the more
different), the greater the total quantity of life. This theme can be traced to
Darwin's earliest "transmutation notebooks" of the 1830's, the primary documents
of his quest to formulate evolution: "The end [that is, goal] of formation of species
and genera is probably to add to quantum of life possible with certain pre-existing
laws—if only one kind of plant not so many" (C Notebook, p. 146—in Barrett et
al., 1987).
In the fullest discussion within Natural Selection (written in early 1858),
Darwin firmly links maximization of life to diversification of taxa: "I consider it as
of the utmost importance fully to recognize that the amount of life in any country,
and still more that the number of modified descendants from a common parent,
will in chief part depend on the amount of diversification which they have
undergone, so as best to fill as many and as widely different places as possible in
the great scheme of nature" (p. 234 of Stauffer edition, 1975).
Darwin proposes that the vague concept of "amount" or "maximization" of
life be quantified chemically as total metabolic flow through a given area in a
given time—and he illustrates the primary dependence of this quantity on
diversification:
The fairest measure of the amount of life [is] probably the amount of
chemical composition and decomposition within a given period. Imagine
the case of an island, peopled with only three or four plants of the same
order all well adapted to their conditions of life, and by three or four insects