The Modern Synthesis as a Limited Consensus 517
with providing the physical explanation for what Darwin could only deduce from
first principles of natural selection, while hoping for later confirmation from
discoveries about the basis of heredity.
In an interesting discussion on the nature of theories and their central logic,
Huxley disputes Lancelot Hogben's claim that the Mendelian fusion had so altered
Darwin's own notion of mechanics, that the reformulation of Fisher, Haldane, and
Wright should neither bear Darwin's name nor even retain the term "natural
selection" for its central mechanism. Huxley replies that all theories must change
by growth, but that the proper standard for maintenance of a name must be defined
by continuity in key precepts in a central logic:
Hogben is perfectly right in stressing the fact of the important differences in
content and implication between the Darwinism of Darwin or Weismann
and that of Fisher or Haldane. We may, however, reflect that the term atom
is still in current use and the atomic theory not yet rejected by physicists, in
spite of the supposedly indivisible units having been divided. This is
because modern physicists still find that the particles called atoms by their
predecessors do play an important role, even if they are compound and do
occasionally lose or gain particles and even change their nature. If this is
so, biologists may with a good heart continue to be Darwinians and to
employ the term Natural Selection, even if Darwin knew nothing of
mendelizing mutation (1942, p. 28).Huxley also follows the English tradition (see pp. 1 16 - 119) for central
emphasis upon adaptation in the definition of evolutionary mechanisms. He speaks
of "a functionally-guided course of evolution" (p. 39), and almost claims an a
priori status for panadaptationism: "Our enumeration will also serve as a reminder
of the omnipresence of adaptation. Adaptation cannot but be universal among
organisms, and every organism cannot be other than a bundle of adaptations, more
or less detailed and efficient, coordinated in greater or lesser degree" (1942, p.
420).
But as further evidence for pluralism in the early synthesis, and despite this
emphasis upon the ubiquity of adaptation, Huxley then speaks favorably of the
same challenges and exceptions that intrigued Haldane—orthogenesis and
nonadaptation. Whereas he does claim (correctly) that most cases of supposed
orthogenesis only represent instances of phyletic constraint, he also provides an
interesting taxonomy of genuine examples. Mirroring our modern distinction
between positive and negative meanings of constraint (see Chapter 10, pp. 1025-
1061, and Gould, 1989a), Huxley speaks of dominant and subsidiary orthogenetic
restriction:
True orthogenetic restriction depends on a restriction of the type and
quantity of genetic variation. When dominant it prescribes the direction of
evolution: when subsidiary it merely limits its possibilities... Dominant
orthogenetic restriction [is] very rare, if indeed it exists at all...