1180 THE STRUCTURE OF EVOLUTIONARY THEORY
secure better fit to their environments. We prefer Darwin and reject Lamarck because
nature's mechanisms of heredity and variation validate the efficacy of natural
selection and disprove the existence of soft inheritance, not because we can specify
any basic difference in their shared commitment to a functionalist account of
evolutionary mechanics.
Structuralist or formalist theories, on the other hand, generally seek to explain
the origin of adaptive design in terms of such internal forces as constraint and
directed variability. In the strictest versions of these theories, external causes can only
act as editors to distill the most workable phenotypes from the full range of potential
shapes that structural rules engender. Function may therefore determine what lives
and what dies, but not what can (and does) originate.
Chapters 4 and 5 discussed the two major structuralist theories that we now
reject for their operation as strict alternatives and denials of Darwinian functionalism:
(1) orthogenesis, with its central claim that evolutionary trends follow internal drives
in variation, and that selection can only accelerate or retard these inherent and
inevitable pathways; and (2) saltation, with its premise that occasional fortuitous
discontinuities in variation create new species all at once, and that selection can only
intensify the process by preserving lucky sports and eliminating old, superseded
designs.
Chapters 4 and 5, and much of Chapter 10 as well, also discuss more acceptable
forms of structuralism that do not attempt to replace natural selection, but rather work
in concert with known Darwinian mechanisms to channel possible directions of
evolutionary change "from the inside" along pathways of variation that record
constraints of history or principles of physical construction. I shall return to this
theme of internal constraints in the last two sections of this chapter, where I discuss
the important structural principle of non-adaptive origin followed by cooptation for a
descendant's utility.
This initial section of Chapter 11, however, represents an interlude between
historical constraints (Chapter 10) and structural constraints based on mechanically
forced (or, at the opposite end of this spectrum, simply inherited) correlations with
actively selected features (Sections II and III of this chapter). This interlude also
discusses a form of structural constraint—but of a markedly different nature: direct
molding by physical laws and forces acting upon the developing organism. This
"maverick" theme has played only a small role in the history of evolutionary thought
(a fact that should elicit no judgment about actual importance, for we all recognize
that today's ignored or ridiculed theme can become the centerpiece of tomorrow's
revolutionary theory).
If adaptive phenotypes originate directly and immediately from the imposition
of physical forces upon the "yielding putty" (if you will) of organic material, then we
need no functionalist account of "that perfection of structure and coadaptation"—for
good form emerges automatically from the nature of physical reality (by external
forces imposed upon the organism, or internal forces exerted from within as the
organism grows). We get, in Kauffman's memorable phrase (1993) "order for free,"
and need not posit any explicit organismal mechanisms, as functionalist theories
propose (Darwinian selection