The Structure of Evolutionary Theory

CHAPTER FOUR

Internalism and Laws of Form:

Pre-Darwinian Alternatives

to Functionalism

Prologue: Darwin's Fateful Decision

Thinking in dichotomies may be the most venerable (and ineluctable) of all human
mental habits. In his Lives and Opinions of Eminent Philosophers (circa A.D. 200),
Diogenes Laertius wrote: "Protagoras asserted that there were two sides to every
question, exactly opposite to each other."
Darwin follows this tradition of dichotomy in a passage that he earmarked for
special impact as the concluding paragraph of his crucial Chapter 6, "Difficulties
on Theory." I regard this passage as among the most important and portentous in
the entire Origin, for these words embody Darwin's ultimate decision to construct a
functionalist theory based on adaptation as primary, and to relegate the effects of
constraint (a subject that also commanded his considerable interest—see Section
IV of this chapter) to a periphery of low relative frequency and subsidiary
importance. Yet this passage, which should be emblazoned into the consciousness
of all evolutionary biologists, has rarely been acknowledged or quoted. Darwin
begins (1859, p. 206), expressing his alternatives in upper case (and using the
categories of the great debate between Cuvier and Geoffroy—see Section III of
this chapter): "It is generally acknowledged that all organic beings have been
formed on two great laws—Unity of Type, and the Conditions of Existence."
Conditions of Existence, of course, express the principle of adaptation— final
cause or teleology to pre-evolutionists. Organisms are well designed for their
immediate modes of life—and intricate adaptation implies an agent of design,
either an intelligent creator who made organisms by fiat as an expression of his
wisdom and benevolence, or a natural principle of evolution that yields such
adjustment between organism and environment as a primary result of its operation.
(Both Darwinian natural selection and Lamarckian response to perceived needs, for
example, build adaptation as the most general consequence of their basic mode of
action.)
Darwin then continues by defining the other side of the classical dichotomy:
Unity of Type (1859, p. 206): "By unity of type is meant that fundamental

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