1032 THE STRUCTURE OF EVOLUTIONARY THEORY
selection can dispose. Internal forces supply the possibilities; but natural selection
builds the pattern (because the possibilities nearly always exist in sufficient
abundance to fuel the changes that natural selection might favor). Constraints impede,
but do not direct.
Thus, a standard Darwinian "truce" accepts the notion, even the potential
importance, of constraint as a negative force that can impede rates and amounts of
change (and therefore cannot be dismissed as irrelevant to the generation of
evolutionary pattern in this limited sense). But strict Darwinian functionalists
generally try to hold the line by denying importance, or even legitimacy, to positive
definitions of constraint as causes of directionality in evolutionary change.
In one of the best examples I have ever encountered of the vital (almost morally
enjoined) principle that the forebears of our current struggles demand our continuing
study and respect—and that we often gain, as recompense for this fealty, substantial
practical benefit in getting our own thoughts straight, and channeled in useful
directions—the rich history of debate about Darwinian theory has brought the theme
of positive constraints into sharp focus (see Chapters 4-5). This clarity emerges from
the common emphasis placed by all major structuralist critics upon (1) the difference
between positive and negative meanings of constraint (accompanied by specifications
that only the positive meanings could pose serious difficulties for Darwinism), and
(2) the parsing of positivity into two essential themes of speed, or enhancement of
rates beyond the power of natural selection to instigate, and channeling, or the
preferential (perhaps even requisite) flow of change in particular directions set by
internal possibilities, even if natural selection must supply an initial impetus. We
should also note the ironic sense in which this argument inverts the canonical roles of
the two central components in Darwinian theory. In natural selection, an internal
source of variation provides the impetus, whereas selection determines direction. In
channeled change by constraint, natural selection supplies the impetus by "getting the
ball rolling" (to use Galton's metaphor of the pool table), but the directionality of
evolutionary change, or "where the ball rolls," emerges from internal channels that,
so to speak, "use" natural selection as their convenient source of power. In short,
variation as raw material and selection as the shaper of change in Darwinism; vs.
selection as raw power, and channeled variation for shaping in theories of positive
constraint. Such an epitome, needless to say, remains far too simple to resolve
nature's ways—but this formulation does embody a clear and useful conceptual
dichotomy for clarifying our thoughts.
The second (definitional) positive meaning of causes outside
accepted mechanisms
A second, and conceptually quite distinct, sense of positivity for the concept of
constraint also arises from a vernacular meaning of the word, but embodies a
philosophical position about the general nature of theories and arguments in science,
rather than a specific empirical claim about the nature of evolution.