1142 THE STRUCTURE OF EVOLUTIONARY THEORY
outgrowths. Once established, this system provided the genetic and developmental
foundation for the evolution of structures as diverse as wings, fins, antennae and
lobopodia."
Movement three, Scherzo: Does evolutionary change often proceed
by saltation down channels of historical constraint?
As I documented in Chapters 4 and 5, internally channeled evolution (orthogenesis)
has been intimately linked with discontinuous change (saltationism) in the history of
structuralist thought (with the model of "Galton's polyhedron" serving as the classical
image for the connection). The linkage isn't physically necessary or logically
impelled, for some orthogeneticists have favored gradualism (C. O. Whitman, pp.
383 - 395), whereas some saltationists have rejected internal directionality (Hugo de
Vries, pp. 415-451). But in expanding the causes of evolutionary change beyond the
incremental gradualism of externally directed Darwinian selection, and in regarding
internal channels of developmental constraint as important mediators of phyletic
trending, most advocates of formalist or structuralist explanation (Bateson, D'Arcy
Thompson, and Goldschmidt, for example) have supported some linkage of
channeled directionality with at least the possibility of saltational movement down
the channels—if only because the potential phyletic analog of such ontogenetic
phenomena as metamorphosis seems intriguing and worth exploration.
Thus, with the reintroduction of internal channeling by historical constraint
(based on genetic homology) into our explanatory schemes, we must ask whether
saltational themes (that had been even more firmly rejected by the Darwinian
Synthesis) can also advance a strong case for a rehearing. My own conclusions are
primarily negative (hence my parsing of this theme as a scherzo, and as the shortest
movement of my analysis), but the subject clearly merits some airing (and
undoubtedly holds limited validity), if only as a sign of respect for the intuition of so
many fine evolutionists, throughout the history of our subject, that structural
channeling—now clearly affirmed as a theme of central importance—implies a
serious consideration of saltational mechanics.
As discussed extensively in Chapter 9, in the context of debate over punctuated
equilibrium, notions of "rapidity" depend strongly upon the time scales of their
context. Invocations of suddenness raise quite different evolutionary issues at each
level of consideration. In this section, I shall discuss true saltation (discontinuous
changes, potentially across a single generation, and usually mediated by small genetic
alterations with major developmental effects), and not punctuational patterns at larger
scales of time (continuous changes that would be regarded as slow and gradual across
human lifetimes, but that appear instantaneous when scaled against the millions of
years in stasis for a resulting species or developmental Bauplan).
Nonetheless, I note in passing the relevance of developmental themes to
punctuational patterns at these larger, and very different, scales of explanation. For
example, several authors have argued that our emerging concepts of deep homology
might help to elucidate such macroevolutionary "classics" of