530 THE STRUCTURE OF EVOLUTIONARY THEORY
single process. Although he favored selection leading to adaptation as a primary
theme, he explicitly denied that all evolution could be adaptive and under selective
control. He concluded: "The aspects of tempo and mode that have now been
discussed give little support to the extreme dictum that all evolution is primarily
adaptive. Selection is a truly creative force and not solely negative in action. It is
one of the crucial determinants of evolution, although under special circumstances
it may be ineffective, and the rise of characters indifferent or even opposed to
selection is explicable and does not contradict this usually decisive influence"
(1944, p. 180).
When pressured for a new edition of Tempo and Mode, Simpson realized that
evolutionary theory had developed too much in the intervening ten years to permit
a reissue or even a simple revision. The field that he pioneered had stabilized and
flourished: "It was [in the late 1930s] to me a new and exciting idea to try to apply
population genetics to interpretation of the fossil record and conversely to check
the broader validity of genetical theory and to extend its field by means of the
fossil record. That idea is now a commonplace" (1953, p. ix). Thus, Simpson
followed the outline of Tempo and Mode, but wrote a new book more than double
the length of its ancestor—The Major Features of Evolution, published in 1953.
The two books differ in many ways (see p. 522 for my personal and
professional introduction to the distinctions), most notably in Simpson's increasing
confidence that selection within phyletic lineages must represent the only
important cause of substantial change. Consider the following addition to the 1953
book, a speculative comment on trends in titanothere horns, with its prompt
dismissal—tinged with impatience, if not incipient dogmatism—of the venerable
argument that no evident function can be ascribed to the incipient stages of useful
structures: "This long seemed an extremely forceful argument, but now it can be
dismissed with little serious discussion. If a trend is advantageous at any point,
even its earliest stages have some advantage. Thus if an animal butts others with its
head, as titanotheres surely did, the slightest thickening as presage of later horns
already reduced danger of fractures by however small an amount" (p. 270).
But the most dramatic difference between the two books lies in Simpson's
demotion to insignificance of the concept that had formerly been, by his own
reckoning and explicit announcement, his delight and greatest pride—quantum
evolution. This hypothesis embodied the pluralism of his original approach—a
reliance on a range of genetical models. For he had advocated genetic drift to
propel small populations off adaptive peaks into an ultimately untenable inadaptive
phase. And he had explicitly christened quantum evolution as a mode different in
kind, not only in rate, from phyletic transformation within lineages. But now, as the
adaptationist program of the Synthesis hardened, Simpson decided that genetic
drift could not trigger any major evolutionary event: "Genetic drift is certainly not
involved in all or in most origins of higher categories, even of very high categories
such as classes or phyla" (p. 355).
In an "intermediate stage" of his personal ontogeny—his presentation to