Historical Constraints and the Evolution of Development 1057
theme of constraints as limitation, while parallelism features the "positive" empirical
theme of constraints as enabling channels. Both themes, however, have forced
evolutionary biologists to reassess the importance of constraints at the historical
vertex for explaining the actual distribution of adaptive form within potential organic
morphospace. In this sense, both themes count as "positive" in my second (or
conceptual) sense that any powerful argument challenging a stale and limiting
consensus must be treasured in science.
No good or experienced naturalist could ever fully espouse the reductionistic
belief that all problems of organic form might be answered by dissolving organisms
into separate features, each with a specified function, and each optimized
independently by natural selection. But theories do drive, or at least nudge, adherents
towards their extreme formulations—and even such sophisticated versions of
Darwinism as the Modern Synthesis (see Chapter 7) biased the perspectives of
biologists in this direction by advocating natural selection as, effectively, the sole
cause of evolutionary change. Various pleas, heard with increasing frequency during
the past generation (Goodwin, 1994, for example), to "put the organism back into
evolution," or to "reestablish a meaningful science of morphology," should be
understood as expressions of a growing conviction that theories of part-by-part
functionalism cannot explain the major patterns of life's history and current
morphological distribution.
We do need to reformulate, in modern and operational ways, the old notions of
organic integrity, and structural determination from the "inside" of genetics and
development, thus balancing our former functionalist faith in the full efficacy of
adaptationism with positive concepts of internal and structural constraint. Only in this
way can we forge a unified science of form to integrate the architecture and history of
organisms with their daily struggles to survive, prosper, and propagate in a complex
ecological surround—a world that Western culture once perceived as "the face of
nature bright with gladness" (Darwin, 1859, p. 62), but that we now recognize as the
material domain of natural selection, a process carrying no moral implications for
human life (thus permitting us to throw aside the crutch of comforting imagery that
Darwin so rightly rejected), but operating with relentless (though not exclusive) force
throughout living nature.
An epitome for the theory-bound nature of constraint terminology
We may use the model of the aptive triangle to illustrate how my second, or
conceptually "positive," meaning of constraint (including both the "positive" and
"negative" empirical modes of channels and limits) rests upon the theory-bound
nature of all scientific terms and definitions. As I argued previously (see pp. 1032-
1037), if we designate a set of causes as canonical within an orthodox theory, then
vernacular usage designates other causes lying outside the theory, but nonetheless
influencing phenomena that should fall under the aegis of orthodoxy, as "constraints"
upon the power or validity of the standard theory. (My designation of such
constraints as "positive" then follows