1082 THE STRUCTURE OF EVOLUTIONARY THEORY
within recapitulatory theory (see Gould, 1977b, for relevant sources and quotations).
"The relation of genera," Cope writes (1887, p. 45), "which are simply steps in
one and the same line of development, may be called exact parallelism." In other
words, different genera belonging to the same parallel series will run, during their full
ontogenies, down varying lengths of a common developmental (and phyletic)
trackway. In this sense, the adult of one genus may be virtually identical (exactly
parallel in Cope's terms) with the juvenile form of another genus that runs further
along the common track during its own ontogeny. Obviously, these common
trackways, regulating both the ontogeny and phylogeny of entire series of related
genera, invoke a concept of internal constraint with a vengeance. Cope, in this early
version of his developing ideas, placed far more stress on internal channeling to
explain taxonomic relationships than his later attraction for the functionalist theory of
Neo-Lamarckism would allow (see Gould, 1977b and 1981b, for an analysis of
Cope's changing views on the relative importance of constraint and function).
The first use of parallelism in its modern meaning, including its dichotomous
pairing with convergence, can also be traced to two of the greatest American
vertebrate paleontologists of the late 19th century: W. B. Scott and H. F. Osborn. If
the concept can claim a "founding" quotation at all, Scott (1891, p. 362), in a long
and famous article on the osteology of early perissodactyls and artiodactyls, invoked
degree of taxonomic relationship to distinguish parallelism from convergence, while
emphasizing their common attributes as homoplastic confounders of phylogeny: "But
if the various species of the ancestral genus may acquire the new character
independently of each other (parallelism), or if the species of widely different genera
may gradually assume a common likeness (convergence), then it is plain that such a
genus is an artificial assemblage of forms of polyphyletic origin."
In his 1895 summer lecture at the Marine Biological Laboratory in Woods Hole,
Scott (1896, p. 56) provided a more formal definition: "By parallelism is meant the
independent acquisition of similar structure in forms which are themselves nearly
related, and by convergence such acquisition in forms which are not closely related,
and thus in one or more respects come to be more nearly alike than were their
ancestors."
More importantly, Scott then explicitly argued that he needed to distinguish
these two categories of homoplasy because parallelism, based on constraints of
inherited channels for preferred change, will generally confound phylogeny less than
convergences that arise by similar functional impact upon truly different starting
points (1896, p. 58): "It seems the most obvious of commonplaces to say that
numerous and close resemblances of structure are prima facie evidences of
relationship. Yet the statement is true, even though the resemblances have been
independently acquired, because parallelism is a more frequently observed
phenomenon than convergence, and because the more nearly related any two
organisms are, the more likely are they to undergo similar modifications."