The Essence of Darwinism and the Basis of Modern Orthodoxy 141
material; as long as change accretes in an insensibly gradual manner; and as long
as the reproductive advantages of certain individuals provide the statistical source
of change; then natural selection must be construed as the directional cause of
evolutionary modification.
These conditions are stringent; and they cannot be construed as vague,
unconstraining, or too far in the distance to matter. In fact, I would argue that the
single most brilliant (and daring) stroke in Darwin's entire theory lay in his
willingness to assert a set of precise and stringent requirements for variation—all
in complete ignorance of the actual mechanics of heredity. Darwin understood that
if any of these claims failed, natural selection could not be a creative force, and the
theory of natural selection would collapse. We pay our highest tribute to the power
of natural selection in recognizing how Darwin used the theory to deduce a set of
necessary properties for variation, well before science understood the mechanism
of heredity—and in noting that these properties then turned out to be both basically
correct and also entailed by the causes later discovered!
The requirements for variation
In order to act as raw material only, variation must walk a tightrope between two
unacceptable alternatives. First and foremost, variation must exist in sufficient
amounts, for natural selection can make nothing, and must rely upon the bounty
thus provided; but variation must not be too florid or showy either, lest it become
the creative agent of change all by itself. Variation, in short, must be copious,
small in extent, and undirected. A full taxonomy of non-Darwinian evolutionary
theories may be elaborated by their denials of one or more of these central
assumptions.
COPIOUS. Since natural selection makes nothing and can only work with raw
material presented to its stringent review, variation must be generated in copious
and dependable amounts (especially given the hecatomb of selective deaths
accompanying the establishment of each favorable feature). Darwin's scenario for
selective modification always includes the postulate, usually stated explicitly, that
all structures vary, and can therefore evolve. He argues, for example, that if a short
beak were favored on a full-grown pigeon "for the bird's own advantage" (p. 87),
then selection would also work within the egg for sufficient beak strength to break
the shell despite diminution in overall size of the beak—unless evolution followed
an alternate route of selection for thinner shells, "the thickness of the shell being
known to vary like any other structure" (p. 87).
Darwin's faith in the copiousness of variation can be gauged most clearly by
his response to the two most serious potential challenges of his time. First, he
acknowledges the folk wisdom that some domestic species (dogs, for example)
have developed great variety, while others (cats, for example) differ far less among
populations. If these universally recognized distinctions arise as consequences of
differences in the intrinsic capacity of species to vary, then Darwin's key postulate
of copiousness would be compromised—for failure of