The Modern Synthesis as a Limited Consensus 515
corners of the new world of fusion between Darwin and Mendel, for the two
internalist theories in Kellogg's triad of alternatives—saltation and orthogenesis.
(In fact, Haldane even repeats the "standard" anti-Darwinian claim for selection's
merely subsidiary and negative role in enhancing and stabilizing a saltational
change arising by other means—even though Haldane regards this alternative
mechanism as rare in nature.) Galton's polyhedron cannot be fully rounded by the
emerging Darwinian consensus:
But if we come to the conclusion that natural selection is probably the main
cause of change in a population, we certainly need not go back completely
to Darwin's point of view. In the first place, we have every reason to
believe that new species may arise quite suddenly, sometimes by
hybridization, sometimes perhaps by other means. Such species do not
arise, as Darwin thought, by natural selection. When they have arisen they
must justify their existence before the tribunal of natural selection, but that
is a different matter... Secondly, natural selection can only act on the
variations available, and these are not, as Darwin thought, in every
direction. In the first place, most mutations lead to a loss of complexity
(e.g. substitution of leaves for tendrils in the pea and sweet pea) or
reduction in size of some organ (e.g. wings in Drosophila)... Mutations
only seem to occur along certain lines (1932, pp. 138— 139).Two modes of non-Darwinian change especially intrigued Haldane. First,
though he tried to reinterpret as many cases as possible in a Darwinian manner,
Haldane accepted some paleontological claims for supposedly orthogenetic trends,
and he admitted that the developing Darwinian synthesis could find no place for
such phenomena: "Many such cases—for example the development of large size or
large horns—can, I think, be put down to the ill effects of competition between
members of the same species. Others, such as the exaggerated coiling of Gryphaea
cannot at present be explained with any strong degree of likelihood" (1932, p.
141). (This example seems especially ironic in retrospect, because Gryphaea's
supposed overcoiling to necessary extinction never occurred, and the claim rested
upon misreported and misinterpreted data—see Chapter 10, pp. 1040-1045 and
Gould, 1972.)
As his favorite general argument for awarding a small space to orthogenesis,
Haldane cited the putatively higher frequency of degenerational over progressive
evolution, arguing that such a tendency probably required an internalist
explanation rooted in a bias for deletional mutations: "Degeneration is a far
commoner phenomenon than progress. It is less striking because a progressive
type, such as the first bird, has left many different species as progeny, while
degeneration often leads to extinction, and rarely to a widespread production of
new forms ... But if we consider any given evolutionary level we generally find
one or two lines leading up to it, and dozens leading down" (1932, pp. 152-153).
Second, Haldane accepted the common wisdom of taxonomists in his
generation that most differentia of species expressed no adaptive significance. He