Historical Constraints and the Evolution of Development 1085
under the control of natural selection should exhibit more temporal fluctuation: "So
far as the series of fossil mammals which we have been considering are concerned,
the developmental history appears to be very direct, and subject to comparatively
little fluctuation, advancing steadily in a definite direction, though with slight
deviations."
In his 1902 article, Osborn invoked parallelism more explicitly as a central
argument for internal control of phylogenetic directionality, and against natural
selection as a primary cause of change. In fact, following a standard tradition of
continental non-Darwinian argument, Osborn demoted natural selection to a mere
"exciting cause" ("exciting," that is, in the literal sense of "initiating," not in the
modern meaning of "thrilling") that can arouse the inherent channels of necessary
change, and provoke homoplastic evolution along parallel paths. In his typically regal
way, Osborn begins his paper by quoting his own prophetic words of 1897: "My
study of teeth in a great many phyla of Mammalia in past times has convinced me
that there are fundamental predispositions to vary in certain directions; that the
evolution of teeth is marked out beforehand by hereditary influences which extend
back hundreds of thousands of years. These predispositions are aroused under certain
exciting causes [note his verbal demotion of natural selection] and the progress of
tooth development takes a certain form converting into actuality what has hitherto
been potentiality."
Osborn then ends his paper (1902, p. 270) by explicitly citing the "latent or
potential" homology of parallelism as an alternative to natural selection among causes
of evolutionary change:
These homoplastic cusps [of teeth in independent lineages of mammalian
evolution] do not arise from selection out of fortuitous variations, because
they develop directly and are not picked from a number of alternates... We
are forced to the conclusion that in the original tritubercular constitution of the
teeth there is some principle which unifies the subsequent variation and
evolution up to a certain point. Herein lies the appropriateness of Lankester's
phrases, "a likeness of material to begin with." Philosophically,
predeterminate variation and evolution brings us upon dangerous ground. If
all that is involved in the Tertiary molar tooth is included in a latent or
potential form in the Cretaceous molar tooth we are nearing the emboitement
hypothesis of Bonnet or the archetype of Oken and Owen.Second, continental European theorists in the formalist tradition (see Chapters 4
and 5) had always emphasized constraint channeled by laws of form as a primary
alternative to functionalist theories like natural selection. These scientists should
therefore have taken a particular interest in parallelism, especially in its distinction
from convergence for the origin of homoplastic similarity—for convergence exalts
natural selection, while parallelism stresses internal channeling and supports the
standard continental view of selection as a mere potentiator, or at most a minor
diverter, of predictable and