128 THE STRUCTURE OF EVOLUTIONARY THEORY
selection works "for the good of individuals" recur, almost in catechistic form,
throughout the Origin: "Natural selection will never produce in a being anything
injurious to itself, for natural selection acts solely by and for the good of each (p.
201)... Natural selection acts only by the accumulation of slight modifications of
structure or instinct, each profitable to the individual under its conditions of life"
(p. 233). Even if higher-level order arises as a result, the causal locus must be
recognized as individual benefit: "In social animals [natural selection] will adapt
the structure of each individual for the benefit of the community; if each in
consequence profits by the selected change" (p. 87).
Several other statements illustrate Darwin's emphasis on struggle among
organisms, and his desire to avoid all implication that members of a species might
amalgamate to collectivities functioning as units of selection in themselves. He
continually stresses, for example, that competition tends to be more intense among
members of a single species than between individuals of different species—thus
emphasizing the difficulty of forming such collectivities. Moreover, Darwin's
development of the theory of sexual selection, and his increasing reliance on this
mechanism as his views matured, also forestalls any temptation to advocate group
selection—as no form of intraspecific competition can be more intense than
struggle among similar individuals for personal success in mating.
RESPONSE TO CHALLENGES IN THE ORIGIN. The primary commitments of a
theory lie best revealed, not so much in the initial exposition of their logic, but in
their later employment to resolve difficulties and paradoxes. Darwin devotes much
more of the Origin than most readers have generally realized to defending his
single-level theory of selection on organisms.
Darwin structured the Origin as a trilogy—a first part (4 chapters) on the
exposition of natural selection, a last section (5 chapters) on the evidence for
evolution, and a middle series of 5 chapters on difficulties and responses. Two
chapters of this middle section treat a broad range of potential challenges to the
creativity of selection and its sequelae—chapter 9 on the geological record (to
defend gradualism in the face of apparently contradictory evidence), and chapter 5
on laws of variation (to assert the isotropy of variation—see pp. 144-146). A third
(chapter 6) treats general "Difficulties on Theory," mostly centered on gradualism.
Darwin therefore devotes only two of these five chapters, 7 on "Instinct" and
8 on "Hybridism," to specific difficulties—that is, to issues of sufficient import in
his mind to merit such extensive and exclusive treatment. Readers have not always
discerned the common thread between these two chapters— Darwin's defense of
struggle among organisms as the locus of selection. The chapter on hybridism
presents, as its central theme, an argument against species selection as the cause of
sterility in interspecific crosses. The chapter on instinct treats the more general
subject of selection's application to behavior as well as to form, but Darwin
devotes more than half of this chapter to social insects, and he presents his primary
examples of differentiation among castes and sterility of workers as threats to the
principle of selection on organisms.