1028 THE STRUCTURE OF EVOLUTIONARY THEORY
gains free rein (and reign) as the cause of change—in sharp contrast to nearly all
competing 19th century evolutionary theories, with their stress on internally
generated directionality.
To epitomize the theoretical importance of constraint in a single sentence, the
concept of non-isotropy in variation may be roughly synonymized with notions of
"constraint"—that is, with claims that internal factors restrict the freedom of natural
selection to establish and control the direction of evolutionary change.
The crucial importance of constraint in evolutionary theory therefore centers
upon this potential challenge, particularly upon the nature and scope of the
restrictions. In Chapters 4 and 5—the longest section of this book's first half, because
the subject consistently commanded the principal attention of Darwin's most cogent
critics—I considered both pre and post-Darwinian theories of evolutionary
internalism, as rooted in the common claim that any significant non-isotropy of
variation would reduce, or even cancel, the creative role of natural selection by
identifying potent internal forces of evolutionary change in either directed or
saltatory variation. These two persistent bugbears of "quick" and "channeled"
received their most memorable (and influential) joint expression in the challenging
metaphor of Galton's polyhedron (see pages 342-351 of Chapter 5).
Of course, no sophisticated Darwinian ever denied some limited domain of
validity to the concept of internally generated "constraints." (As I have emphasized
throughout this book, validation in natural history rarely follows the criterion of
"never in principle for this would violate nature's laws," as favored in some
constructions of the so-called exact sciences, but rather the standard of "conceivable
in principle, but not occurring often enough to matter," as followed in historical
sciences that formulate most basic judgments by analysis of relative frequencies.)
Rather, Darwinian functionalists have tended to admit certain kinds of constraints,
and have then tried to limit their modes of occurrence and domains of action in such a
manner that the central principle of Darwinian theory—the control of evolutionary
change by natural selection—will not be threatened.
In short, and to summarize these few pages of argument in a paragraph,
orthodox Darwinians have not balked at negative constructions of constraint as limits
and impediments to the power of natural selection in certain definable situations. But
they have been far less willing to embrace positive meanings of constraint as
promoters, suppliers, and causes of evolutionary direction and change. This
distinction follows logically from the basic premises of Darwinian functionalism,
because the admission of a potent and positive version of constraint would
compromise the fundamental principle that variation (the structuralist and internalist
component of evolution) only proposes, while selection (the functionalist and
externalist force) disposes as the only effective cause of change.
In considering how structural constraints might limit the power of natural
selection to adapt each feature of an organism to each local environment, we
recognize that some modes will rank as "benign" for Darwinian