226 THE STRUCTURE OF EVOLUTIONARY THEORY
taxa. So much of what Darwin needed to explain—plenitude in ecology, branching
models in phylogeny, the hierarchical structure of taxonomy, to name just a few
items of obvious centrality—rested upon the fact of diversification, not adaptation
(see Mayr, 1992, on Darwin's several theories of evolution).
One might say—indeed many of us do say, thus leading us to downgrade and
misinterpret Darwin's explanation of diversity—that "divergence of character"
requires no separate principle beyond adaptation, natural selection, and historical
contingency. After all, the earthly stage of evolution provides ecological and
biogeographical prerequisites for diversification. Climates alter; topography
changes; populations become isolated, and some, adapting to modified
environments, form new species. What more do we need? Insofar as Darwin
considered the issue at all between 1838 and the early 1850's, his thinking followed
this general line (Sulloway, 1979; Ospovat, 1981). But Darwin grew dissatisfied
with a theory that featured a general principle to explain adaptation, but then relied
upon historical accidents of changing environments to resolve diversity. He
decided that a fully adequate theory of evolution required an equally strong
principle of diversity, one that acted intrinsically and predictably. If adaptation and
diversification specify the central phenomena of evolution, each must have its
principle, and their union would then define his complete theory.
(In modern evolutionary parlance, we may relate the growing intensity of
Darwin's search for a general, "law-of-nature" explanation of divergence to his
changing views about allopatric and sympatric speciation. During the 1840's, when
diversity did not greatly trouble him as a theoretical issue, Darwin tended to view
speciation as allopatric, and therefore as a consequence of historical accidents in
geography and ecology. When a population becomes spatially isolated, he
reasoned, natural selection can act independently upon it, and eventually
accumulate enough divergence from the ancestral form to establish a new species.
But Darwin's preferences then shifted to sympatric views of speciation—and he
therefore developed a conviction that some general law, and not just historical
accidents of isolation, must promote the multiplication of species. A complete
theory of natural selection required that this elusive "law" of speciation or
divergence also be based on the predictable operation of organismic selection. In
the light of our current preferences for allopatric speciation, Darwin's shift may
seem ironic, but our opinions and certainties, as presently defined, must be deemed
irrelevant to such historical analysis.)
In the context of this book and its principal theme of hierarchical selection, I
stress the centrality of Darwin's changing views on divergence because I think that
I have made a small discovery about the structure of his argument. I shall try to
show that this most brilliant of all theorists, this rigorously honest thinker who
worked so diligently to explain all evolution as a consequence of organismic
struggle, tried mightily to render his second touchstone, his "principle of
divergence," by ordinary natural selection—and he failed. He could not succeed
because the logic of his argument demands a major role