Seeds of Hierarchy 235
So Darwin recognized in early 1855 that maximization would have to be
explained by natural selection (a "mere result from struggle"); he also stated that
development of such an argument would be complex and difficult ("I must think
out last proposition"). The "principle of divergence of character," or more
succinctly the "principle of divergence," emerged as the result of this intellectual
labor. How, then, did Darwin finally render maximization of life as a consequence
of struggle, or ordinary natural selection?
Darwin's solution, embedded as Kohn notes (1985) in his increasing will-
ingness to accept sympatric speciation, holds that natural selection will generally
favor the most extreme, the most different, the most divergent forms in a spectrum
of variation emanating from any common parental stock. Thus, each vigorous and
successful stock produces a cone of varying forms about its own modal design (see
Fig. 3-5 on p. 242). If natural selection generally favors extreme variants in such
arrays—the core claim of the "principle of divergence of character"—then
vigorous ancestors will generate two or more descendant taxa fanning out towards
maximally different form and adaptation. Two sequelae now complete the
argument by drawing both ecological plenitude and taxonomic structure from the
principle of divergence: First, the process of divergence must continue (see Fig. 3-
5), impelling each vigorous descendant to produce still more advantageous
extremes—thereby entraining phyletic trends of constantly increasing
specialization. (The full extension elevates subspecies to species, species to genera,
etc.—as extreme variants proliferate and diversify. The taxonomic tree of life
emerges as an ultimate result.) Second, descendants will, in general, be
competitively superior to parents, and must therefore tend to exterminate them in
competition—for the number of species cannot increase indefinitely, and some
ecological mechanism for replacement of ancestors must exist.
In the Origin, Darwin begins, in his characteristic fashion, by analogy to
artificial selection. Breeders, he argues, tend to favor extreme variants when trying
to improve a stock; nature must follow suit (1859, p. 112). For the breeder's
conscious aim, Darwin substitutes the natural advantages of extreme variants: "The
more diversified the descendants from any one species become in structure,
constitution, and habits, by so much will they be better enabled to seize on many
and widely diversified places in the polity of nature, and so be enabled to increase
in numbers" (1859, p. 112). With a botanical example, Darwin then strongly argues
that divergence occurs because natural selection tends to favor extreme variants:
We well know that each species and each variety of grass is annually
sowing almost countless seeds; and thus, as it may be said, is striving its
utmost to increase its numbers. Consequently, I cannot doubt that in the
course of many thousands of generations, the most distinct varieties of any
one species of grass would always have the best chance of succeeding and
of increasing in numbers, and thus of supplanting the less distinct varieties;
and varieties, when rendered very distinct from each other, take the rank of
species (1859, pp. 113-114).