1084 THE STRUCTURE OF EVOLUTIONARY THEORY
nonoperational nature—about the most frustrating situation one can face in science—
of the distinction originally made in defining parallelism and convergence. Both Scott
and Osborn grasped the importance of separating homoplasy due to underlying
homology of generators ("latent or potential homology" in Osborn's apt phrase in the
title of his 1902 paper) from homoplasy rooted exclusively in a similar external
context of adaptation. But the biology of their time provided no way to specify or
identify these generators. Scott and Osborn therefore had to invoke the entirely
unsatisfactory, indirect and vague surrogate of "degree" of taxonomic resemblance—
arguing (quite properly of course, however nonoperationally) that the closer the
relationship between two separated lineages, the more likely that any homoplastic
characters will arise by parallelism. Scott expressed his frustration at this
unsurmountable situation on the page following his initial definitions (1891, p. 363):
"The distinction between the two classes of phenomena [parallelism and
convergence] is obviously one of degree rather than of kind, and it will therefore be
convenient to consider them together."
THE GREATER SALIENCE OF PARALLELISM FOR NON-DARWINIAN FORMALISTS, AND
FOR ANTI-DARWINIAN THEORISTS OF VARIOUS STRIPES, IN LATE 19 TH AND EARLY 20 TH
CENTURY EVOLUTIONARY DEBATES. We understand why parallelism faded from
general consideration when the strict adaptationism of later and hardened versions of
the Modern Synthesis pushed the general subject of internal constraint to a periphery
of intellectual concern and presumed relevance. Similarly, we should easily
comprehend why the same phenomenon—and the importance of distinguishing its
component of constraint from the purely adaptational basis of convergence—would
have generated more interest and greater clarity of definition during the period of its
initial formulation (1890's to 1920's), when non-Darwinian formalist, and more
overtly anti-Darwinian orthogenetic and saltational, theories enjoyed considerable
vogue as adjuncts or alternatives to natural selection.
Two linguistically and geographically defined traditions of argument reinforce
my contention that parallelism has always been understood and debated as a theory of
constraint based on homologous generators for the origin of homoplastic similarities.
First, the American paleontologists who initially codified the concept of parallelism
did so in the context of pluralistic support for non-Darwinian internal mechanisms of
evolutionary change (working in conjunction, or potentially in opposition, to natural
selection, which they also accepted as a valid mechanism). We find parallelism
sufficiently interesting today as an indicator of preferred internal channels that
selection can exploit in coordinated evolutionary change. Imagine the even greater
theoretical interest of parallelism for evolutionists who hoped to discover, in its
workings, new principles and mechanisms of change that might fundamentally enrich
or alter the basis of evolutionary theory.
In the article that first defined parallelism, for example, Scott (1891, pp. 370-
371) argued that the orthogenetic linearity of parallel series implied a primary
nonselectionist cause for phylogenetic transformation, since lineages