of phyletic change, implied by Kelvin's dates, were entirely acceptable under their
reading of evolution.
Chapter 7: The modern synthesis as a limited consensus- From the anarchic situation that prevailed at the Darwinian centennial
celebrations of 1909 (confidence in the factuality of evolution, linked with
agnosticism about theories and mechanics, as the first fruits of Mendelism seemed,
initially, to refute the gradualism and incrementalism of natural selection), the
Modern Synthesis eventually emerged in two stages (following the union of
Darwinian and Mendelian perspectives in the work of Fisher and others): first, by a
welcome restriction that eliminated Kellogg's three alternatives in oppositional
modes that would have destroyed Darwinism (Lamarckism as a substitute
functionalism, and saltationism and orthogenesis as formalist alternatives), and
reasserted, now in a context of Mendelian particulate inheritance, the adequacy of
natural selection as a creative force; and second, by an increasingly dubious
hardening, culminating in centennial celebrations for the Origin in 1959, that
substituted an increasingly rigid adaptationism for an earlier pluralism that
embraced all mechanisms (including genetic drift) consistent with known genetic
principles, while favoring selection as a primary force. - In his founding book of 1930, The Genetical Theory of Natural Selection,
R. A. Fisher showed how slow, gradualist evolution in large, panmictic populations
(treated almost as an ahistorical system, analogous to effectively infinite
populations of identical gas molecules free to move and diffuse by physical
principles) could validate strict Darwinism under Mendelian particulate inheritance
(with Darwin's own acceptance of blending inheritance exposed as a more serious
impediment than Darwin himself had realized), and disprove saltational
alternatives by the inverse correlation of frequency and magnitude in variation. To
these mathematical and general chapters, Fisher appended a long closing section
devoted to his eugenical theory that Western society had begun to degenerate
seriously as a consequence of the social promotion of infertility (the rise in class
level of "good" genetic stock, largely by their correlated tendency to have fewer
children, thereby husbanding their economic resources to potentiate their social
elevation). Fisher conceived this eugenical "blight" as entirely Darwinian in
character—invisible in its gradual expression generation by generation, but
ultimately more deadly than the explicit saltational degenerations stressed by most
eugenicists. - In contrast with the initial pluralism of Haldane and Huxley (in the book
that coined the Modern Synthesis), and of the first editions of founding documents
for the second phase of the Synthesis (Dobzhansky's 1937 Genetics and the Origin
of Species, Mayr's 1942 Systematics and the Origin of Species, and Simpson's 1944
Tempo and Mode in Evolution), later editions of these three documents
encapsulated the hardening of this second phase, as initial pluralism yielded to an
increasingly firm and exclusive commitment to adaptationist scenarios, and to
natural selection as a virtually exclusive mechanism of change. Even Sewall
Wright's views on genetic drift and shifting bal-
Defining and Revising the Structure of Evolutionary Theory 71