1070 THE STRUCTURE OF EVOLUTIONARY THEORY
by inheritance from a common ancestor. Homoplastic structures are similar by
independent evolution, for we can infer that the common ancestor did not possess the
structure. In other words, the dichotomy of these two terms captures the essential
difference between common ancestry and independent origin. At a first level of
interpretation (but here we immediately plunge a toe into troubled waters, as we shall
soon see), the dichotomy also marks a conceptual distinction between the hold of
history and the power of adaptation.
So far so good—and I will not challenge the accepted and codified current
definition of these two terms for describing an important logical distinction in
evolutionary biology. But we often gain better understanding—and do not merely
indulge an antiquarian passion for trivial and superannuated detail— when we
explore the historical origin of a word, and then discover a marked discrepancy
between initial and current usage. I intend no criticism of current usage in making
such an observation. Words change their meanings, just as organisms evolve. We
would impose an enormous burden upon our economy if we insisted upon payment in
cattle every time we identified a bonus as a pecuniary advantage (from the Latin
pecus, or cattle, a verbal fossil from a former commercial reality).
E. Ray Lankester, T. H. Huxley's protege and the finest evolutionary
morphologist in the generation just after Darwin, proposed the concept of homoplasy
in 1870 (see Lester and Bowler, 1995, and Gould, 1999a, on Lankester's life and
general views). I suspect that most evolutionary biologists could cite Lankester as a
source, but I will wager a substantial sum that very few colleagues could identify (to
their pecuniary benefit) a supreme irony in Lankester's original paper, entitled "On
the use of the term homology in modern zoology, and the distinction between
homogenetic and homoplastic agreements"—namely, that he defined homoplasy as a
subcategory of homology, in apparent defiance of current usage (which, I repeat, I do
not challenge) of homology and homoplasy as dichotomous opposites. The reasons
for his distinctions, and for subsequent changes and refinements of meaning, tell an
interesting story that can unlock the essential distinction between parallelism and
convergence, and also explain the significance of evo-devo in unleashing the capacity
of parallelism to rebalance formalist and functionalist causes within evolutionary
theory.
Richard Owen enjoyed the height of his influence when Lankester wrote his
paper, and the younger morphologist properly went to the source of all later usage
(Owen, 1848 and 1849) in defining his terms. Lankester, as a Darwinian acolyte, also
correctly noted the philosophical difficulty facing anyone who sought to translate
such vital terms as homology into the new evolutionary context. Owen, as Lankester
notes, defined homology within a Platonic theory of archetypal form (see discussion
of Owen's concepts on pp. 312-329). How can the term be carried over into Darwin's
world? Lankester (1870, p. 34) began his paper by stating this problem:
Whilst the adoption of the theory of evolution has broken down the notions at
one time held by zoologists and botanists as to the existence of