1086 THE STRUCTURE OF EVOLUTIONARY THEORY
law like changes that must follow internally specified rules of morphogenetic
transformation.
Haas and Simpson (1946) cite all the major evolutionary theorists among
continental paleontologists of the early to mid 20th century—particularly Abel,
Dacque and Schindewolf—in support of these weights and definitions. In 1921, for
example, Dacque compared parallelism with Eimer's anti-Darwinian concept of
orthogenesis (see pp. 355-365 for full discussion on Eimer's views), while stressing
the distinction of parallelism and convergence by the predominant causality of
constraint vs. adaptation (Haas and Simpson, 1946, p. 335).
G. G. SIMPSON AND THE CAUSAL VS. GEOMETRIC DEFINITION OF PARALLELISM.
With parallelism thus falsely depicted as somehow contrary to selection, one can
hardly blame the resurgent Darwinians of the Modern Synthesis for their diminished
attention to a phenomenon that had been unfairly cited against the cause of change
that they now wished to reassert as primary, if not virtually exclusive. (This history
provides another concrete illustration of a general argument about older vs. modern
versions of constraint that I advance throughout this book. The older versions
interpreted constraint as contrary to selection, thus earning the indifference or enmity
of Darwinian theorists when they regained ascendancy during the 1930's and af-
terwards. This unfortunate historical situation clouded the utility of constraint within
Darwinian theory as an adjunct, a potentiator, or (at most distinction) an orthogonal
source of evolutionary change. Modern versions of constraint can overcome this
unfortunate division and reunite these two vital sources, formalist and functionalist,
into an expanded and more general theory of Darwinian evolution.)
But the most perceptive of Darwinian theorists would not let such a contingent
historical happenstance extinguish an important concept and distinction within the
scope of evolutionary causality. In particular, G. G. Simpson—indisputably the most
brilliant and biologically sophisticated of 20th century evolutionary paleontologists—
continually emphasized the significance of a causal concept of parallelism based
upon constraint, and the importance of distinguishing this mode of homoplasy from
the opposite style of convergence based entirely upon shared adaptive contexts rather
than shared homologous generators.
In his epochal 1945 treatise on principles of taxonomy and classification of
mammals, Simpson drew a sharply dichotomous distinction between homology and
convergence (1945, p. 9): "Animals may resemble one another because they have
inherited like characters, homology, or because they have independently acquired like
characters, convergence." Simpson then spoke of parallelism as "a third sort of
process [that] also produces similarities" (p. 9)—for he recognized the "hybrid"
nature of a concept that required independent episodes of similar selection, but
nonetheless constructed homo-plastic likenesses from homologous generators in two
separate lines. With his usual insight, Simpson made the proper theoretical
separation, but then ran