Historical Constraints and the Evolution of Development 1037
though the outcome may have been generated in a deterministic manner by a process
that would be called "causal" in any standard scientific usage—then we achieve a
better understanding of how subtle, and how extensive, the clutches of convention
can become, even among people committed to innovation and the value of novelty.
When language unconsciously promotes orthodox mechanisms, setting barriers
against our examination of alternative modes of causality, then we should vigorously
analyze our terminological usages to seek a clarity that might open new possibilities.
When we understand the relative meaning of constraint as a theory-bound term,
expressing the orthodoxy of selection and designating all other causes of change as
limitations upon an expectation*— and when we come to view this relative sense of
"constraint" as a positive definition that urges us to explore alternatives to standard
explanations— then we can stand a terminological bias on its head, for potential use
against the same conceptual lock that engendered such a peculiar terminology in the
first place.
HETEROCHRONY AND ALLOMETRY AS THE LOCUS CLASSICUS
OF THE FIRST POSITIVE (EMPIRICAL) MEANING: CHANNELED
DIRECTIONALITY BY CONSTRAINTI advocate nothing original in asking evolutionists to focus upon the empirically
positive concept of constraint as channels for change, rather than (as in the negative
meaning) limits to natural selection imposed by insufficient raw material in variation.
The "consensus paper" of Maynard Smith et al. (1985), while stressing a minimalist
definition of absent variability for change in certain directions (as a strategy for
achieving a "least common denominator" of agreement among authors of very
disparate opinions), emphasized both the legitimacy and greater interest of the
positive meaning: "Does development merely prevent evolution from following
particular paths or does it also serve as a directing force, accounting in part for
oriented features of various trends and patterns?" (Maynard Smith et al., 1985, p.
281). Alberch (1982, p. 313) also accentuated the positive by stressing the two great
themes—saltations and channels, or speed and directionality—that have always
anchored the formalist or structuralist critique of Darwinian functionalism (see
Chapters 4-5): "Development does not only define the apportionment of phenotypic
*As one more example of how "constraint" terminology can be biased by assumptions
that equate "ordinary" evolution with selection and adaptation, Schwenk (1995, p. 251)
argued that constraints "can have either negative or positive evolutionary effects at the lin-
eage level (i.e., hamper or promote organismal adaptation)." I doubt that Schwenk truly
believes what he literally says—that any "evolutionary effect" hampering organismal adap-
tation must be labeled as "negative"—but his statement illustrates the common conception
that evolution "ought" to build better adapted organisms, and that any other result must be
regarded as disappointing or somehow wrongheaded. But all manner of highly interesting
phenomena beyond (and sometimes opposed to) organismal adaptation pervade such a
richly varied, universal, many-leveled, highly complex process as organic evolution. Do we
really want to label this lifeblood of fascination for our favorite subject as negative?