244 THE STRUCTURE OF EVOLUTIONARY THEORY
(the lower half of Fig. 3-6), where variation to the left and right of A represents
greater or lesser adaptation to drought in plants: "Everything is the same as in
diagram I... except that it is left to mere chance in each stage of descent, whether
the more or less moisture loving varieties are preserved; and the result is, as
graphically shown, that a^10 and l^10 [sic, he has no a^10 in the drawing, but represents
the leftward extreme as f^10 ] differ in this respect; and so in other respects, hardly
more than did the first varieties (a^1 l^1 ) which were produced" (in Stauffer, ed., 1975,
p. 244).
The argument for a trend that can be reduced to natural selection therefore
hinges upon reasons for differential survival of extremes within the fans of varying
species; for the trend cannot emerge simply from the greater evolutionary vigor of
the ancestral extremes themselves. (Interestingly, Darwin never considers the
alternative, more congenial to species selection, of greater production of variants at
the extremes, with random survival within fans.) Darwin now makes his crucial
move for ordinary natural selection, using his principle of divergence: extremes
enjoy differential survival within the fans of variants, because natural selection
favors adaptation to peripheral, over adaptation to central, "stations" in any region.
(We must remember that all members of the fan are well adapted to their own local
bits of the environment). Now, at the crux of his development, Darwin tries to
defend his position on the differential value of extreme stations, and his argument
falls apart—to be rescued only with a forced and self-contradictory ad hoc
hypothesis (explicitly stated in Natural Selection, but wisely omitted from the
Origin).
Darwin provides two potential reasons for differential success of organisms
adapted to extreme environmental stations. The first remains perfectly acceptable,
and would pass muster today as a standard ecological argument featured in all
textbooks—reduced competition in less "crowded" extreme environments: "From
our principle of divergence, the extreme varieties of any of the species, and more
especially of those species which are now extreme in some characters, will have
the best chance, after a vast lapse of time, of surviving; for they will tend to occupy
new places in the economy of our imaginary country" (in Stauffer, ed., 1975, p.
239).
If Darwin had stopped here, his argument would have remained consistent, if
dangerously weak. But his relentless probing would not permit such a course—for
he knew that a key problem remained unsolved: * extreme variants may be favored
in their own extreme environments, but why should the y
*If historians and historically-minded scientists, myself included, often develop an
admiration bordering on reverence for Darwin, our judgment arises from his persistent
thoroughness, his insistence on following a train of complex thought into all ramifications
and difficulties, and his internal need to resolve each and every little puzzle before achieving
satisfaction. Darwin therefore, over and over again, provides resolutions to puzzles that none
of his contemporaries even considered or conceptualized. In this sense, no other evolutionist
of his generation came close to rivaling Darwin in sophistication—and extensive logical
sloppiness permeates the work of many other thinkers. Darwin never resolved several
difficult issues (progress, divergence), but he thought about them with almost chilling clarity
and integrity.