Seeds of Hierarchy 227
for sorting and selection at the species level—and Darwin, with characteristic
honesty, faced his distress head on.
I am fascinated that the exegetical literature on Darwin's attitude to supra-
organismal selection (Ghiselin, 1974; Ruse, 1980, for example) has focused
entirely on putative cases of group selection (sterility in plant hybrids; neuter
castes in Hymenoptera) and has quite properly concluded that Darwin, with a
possible exception for invoking family or clan selection to explain human moral
traits, doggedly and consistently carried through his program for the exclusivity of
organismic selection (see Chapter 2, pp. 125-137). In so doing, these scholars have
missed the one area—the heart of Darwin's argument about diversity—where his
logic falters because he needs (but hesitates to embrace in his distress) the
apparatus of species selection. I suspect that the internal problems in this
centerpiece of Darwin's thought have not been addressed, or even recognized,
because species selection itself did not become a subject of importance (or even an
acknowledged subject at all) until recently, while debate about conventional group
selection has long raged. Darwin has therefore been well combed for comments
about interdemic selection, while his main engagement with supraorganismal
selection on species went unnoticed.
In any case, whatever our attitude or ignorance today, Darwin clearly
regarded his solution to the problem of divergence as his second great achievement
(after natural selection), and as the capstone to his theory. As Ernst Mayr notes
(1985, pp. 759-760): "He referred to it always with great excitement, as if it had
been a major departure from his previous thinking." On June 8,1858, Darwin wrote
to Hooker after completing his extended discussion of the principle of divergence
for Chapter 6 of Natural Selection (the "big species book" that would never be
completed because Wallace's paper, arriving within 10 days of this letter to
Hooker, derailed his leisurely plans and led him to compose the "abstract" (in his
own description) that we call The Origin of Species). "I am confined to the sofa
with boils," he begins, "so you must let me write in pencil." He then goes on to
describe "the 'Principle of Divergence,' which, with 'Natural Selection,' is the
keystone of my book" (in F. Darwin and Seward, 1903, volume 1, p. 109).
A year before, in September 1857, Darwin wrote his first complete account of
the principle of divergence in a famous letter to Asa Gray at Harvard University.
Gray had explicitly asked Darwin for an epitome of his evolutionary theory
(previously revealed only to Darwin's closest confidants, Hooker and Lyell in
particular): "It is just such sort of people as I that you have to satisfy and convince
and I am a very good subject for you to operate on, as I have no prejudice nor
prepossessions in favor of any theory at all" (quote in Kohn, 1981, p. 1107).
Darwin responded positively with a lucid summary of his theory in six points:
- The power and effect of artificial selection.
- The even greater power of natural selection working on all characters at
once and over vastly longer spans of time.