332 THE STRUCTURE OF EVOLUTIONARY THEORY
that should define the basic attributes of Darwinism. The theory of natural
selection is functionalist, by Darwin's own recognition and definition, and by
routes (and roots) of causality inherent in its proposed mechanics. Intrinsic factors
supply copious, small-scale, isotropic variation—raw material only, and no direct
cause or impetus of change. Evolutionary change occurs by natural selection, as
organisms adapt to modify local environments. The mechanism of evolutionary
change therefore remains functionalist in Darwin's theory; selection powers
change, and organisms adapt as a primary result. Darwin also defined his major
problem squarely within the functionalist theme of adaptation, as he wrote, in the
oft-quoted statement in the Introduction to the Origin (1859, p. 3), that much
evidence, from many sources, could validate the factuality of evolution itself, but
that "nevertheless, such a conclusion, even if well founded, would be
unsatisfactory until it could be shown how the innumerable species inhabiting this
world have been modified, so as to acquire that perfection of structure and
coadaptation which most justly excites our admiration." The argument from
imperfection (pp. 111-116) demonstrates the factuality of evolution, but we need to
explain adaptation if we wish to understand the mechanism.
DARWIN ON CORRELATION OF PARTS
Many evolutionists can cite the specific example given by Darwin in Chapter 6 of
the Origin ("Difficulties on Theory") in allowing that not all useful structures arise
by natural selection for their current role, however essential that function may be to
the life of the organism: "The sutures in the skulls of young mammals have been
advanced as a beautiful adaptation for aiding parturition, and no doubt they
facilitate, or may be indispensable for this act; but as sutures occur in the skulls of
young birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has been taken
advantage of in the parturition of the higher animals" (1859, p. 197).
But I suspect that few of our colleagues know that Darwin took this example
directly from the much longer and more detailed treatment in Richard Owen's
famous essay (1849) On the Nature of Limbs (see quote from Owen on p. 325).
This example not only provides direct evidence for an influence of the leading
British structuralist thinker upon Darwin (a link often denied because we misread
Owen as a creationist, and a leader of the rearguard), but also illustrates a
fascinating change of emphasis between Owen's invocation and Darwin's
borrowing. For Owen, the example lies at the heart of nature's primary causal
structure, and serves as a point of entry to his most trenchant critique of
functionalism in general—particularly his claim that explanations of organic form
in terms of utility match the barrenness of the Vestal Virgins in Bacon's famous
aphorism. For Darwin, on the other hand, the example illustrates an exception to
natural selection included within a chapter entitled "Difficulties on Theory."
But Darwin's serious concern with structural constraint cannot be denied