246 THE STRUCTURE OF EVOLUTIONARY THEORY
drought, but not at the same time enduring an equal amount of moisture with the
parent M, both parent and modified offspring might co-exist: the parent (with
perhaps a more restricted range) in the dryer stations, and m^1 -^10 in the very driest
stations" (ibid., p. 240).
Darwin then rejects this reasonable but debilitating scenario with his ad hoc
assumption, though he senses the weakness of his proposal and salvages his
argument almost apologetically, especially at the end:
In the imaginary case of the varieties m^1 -^10 which are supposed to inherit all
the characters of M, with the addition of enduring more drought; these
varieties would inhabit stations, where M could not exist, but in the less dry
stations m^1 -^10 would have very little power of supplanting their parent M;
nevertheless during unusually dry seasons m^1 -^10 would have a great
advantage over M and would spread; but in damper seasons M would not
have a corresponding advantage over m^1 -^10 for these latter varieties are
supposed to inherit all the characters of their parent. So there would be a
tendency in m^1 -^10 to supplant M, but at an excessively slow rate. It would be
easy to show that the same thing might occur in the case of many other new
characters thus acquired; but the subject is far too doubtful and speculative
to be worth pursuing (ibid., pp. 241-242).Species selection based on propensity for extinction
Relentlessly probing as usual, Darwin now identifies another weak point in his
argument. If divergence follows this predictable and necessary pattern, given the
propensity of natural selection to favor extreme variants in all directions, then what
prevents this inexorable process from reaching absurdity in a final state of such
precise and extended diversification that each species contains but a single
individual? (This issue became important for Darwin when he moved from his
earlier allopatric view of speciation to embrace a largely sympatric model with an
intrinsic and predictable "motor" for the generation of species by selection of
extreme variants): "But if the time has not yet arrived, may it not at some epoch
come, when there will be almost as many specific forms as individuals? I think we
can clearly see that this would never be the case" (in Stauffer, ed., 1975, p. 247).
Darwin proposes three reasons for nature's avoidance of such an absurd
outcome, the first conventional, but the second and third invoking species selection
on population properties of size and variability. For the first reason, Darwin cites
ecological notions that have since become standard—limiting similarity and a
restricted number of "addresses" in the economy of nature. Diversity does beget
more diversity, and the physical environment sets no strict a priori upper bound.
But limits imposed by "inorganic conditions" will eventually cause selection to
rein in the intrinsic process of ever finer diversification:
Firstly, there would be no apparent benefit in a greater amount of
modification than would adapt organic beings to different places in the
polity