The Modern Synthesis as a Limited Consensus 509
and uncompromising strict Darwinian among early synthesists. As a great logician
and statistician, Fisher explored the Darwinian consequences of small changes in
large populations with a consistency and completeness never achieved before. We
may choose, in retrospect, to regard the views of other evolutionists as more subtle,
or more attuned to nature's diversity and ambiguity—but no one can match Fisher
as an advocate of the main line in its pure form. We must therefore treat him as
inaugural, in both senses of priority and centrality.
Fisher begins The Genetical Theory of Natural Selection by explaining how
the logic of Darwinism requires a particulate theory of inheritance in order to
achieve self-sustainability as a mechanics of change without special pleading and
additional forces. He states in his introduction (1930, pp. vii-viii): "That an
independent study of Natural Selection is now possible is principally due to the
great advance which our generation has seen in the science of genetics."
As an opening gambit, Fisher develops the intriguing argument that Darwin's
forced allegiance to blending inheritance entailed far more than mere
inconvenience and surmountable difficulty, thus confuting the usual claim,
advanced by scientists who have traditionally viewed the Origin as a complete and
fully workable theory of natural selection. Rather, Fisher maintains, blending
inheritance represented a central impediment, debarring natural selection as the
chief agent of evolution. By constantly degrading favorable genotypes, blending
requires an enormous input of new mutation to fuel any process of change. But
mutation rates this high could easily overwhelm any reasonable force of natural
selection. Evolution would then be propelled largely from within, and natural
selection, as a theory of "trial and error externalism," could not operate as the
major cause of change. But a particulate theory provides raw material for favorable
change without degradation—and a vastly lowered rate of new input now suffices.
Evolution need not be driven from within, and may now be pulled by the external
force of natural selection.
Fisher conceived his argument as a proof by elimination. Once we dispense
with a need for such lavish internal stirring, no credible force but natural selection
remains. Mendelism therefore validates Darwinism: "The whole group of theories
which ascribe to hypothetical physiological mechanisms, controlling the
occurrence of mutations, a power of directing the course of evolution, must be set
aside, once the blending theory of inheritance is abandoned. The sole surviving
theory is that of Natural Selection" (1930, p. 20).
Fisher illustrates the restrictive character of this first synthetic phase by
invoking the fusion of Darwin and Mendel to discard each alternative of Kellogg's
triad. Lamarckism (as noted before) goes quickly and gently into that good night—
for soft inheritance cannot exist under the newly validated mechanics of
Mendelism. Saltationism surrenders when small-scale continuous variability gains
a Mendelian basis and wins evolutionary effectiveness thereby. This small-scale
Mendelian component provides superior evolutionary raw material for two
reasons: (1) overwhelming predominance in sheer