1074 THE STRUCTURE OF EVOLUTIONARY THEORY
pressures (the pool cue of natural selection, in Darwinian terms), but he stresses the
internal basis of inherited common building patterns and materials (1870, p. 42):
"Under the term 'homology,' belonging to another philosophy, evolutionists have
described and do describe two kinds of agreement—the one, now proposed to be
called 'homogeny,' depending simply on the inheritance of a common part, the other,
proposed to be called 'homoplasy,' depending on a common action of evoking causes
or moulding environment on such homogenous parts, or on parts which for other
reasons offer a likeness of material to begin with."
The foregoing exegesis raises an obvious question: if Lankester restricted
homoplasy to independent origin of similar features based on common and
phyletically distinctive internal constraints (though not common ancestral structures)
in two or more lineages—thus drawing the phenomenon close enough to the essential
and defining theme of homology (the "hold of history") to rank, in Lankester's
system, as a subcategory of homology (broadly defined)—then how did the term
migrate to the opposite meaning now universally and unambiguously understood
today? In other words, how did homoplasy move from a subcategory of homology to
become the diametric opposite of homology, with the domain of homology then
shrinking to encompass only Lankester's narrower category of homogeny, and the
domain of homoplasy expanding to include all similarities evolved independently and
not directly inherited from a common ancestral structure?
What looks like an enormous difference—the expulsion of homoplasy as a
subcategory of homology (sensu lato), and its establishment as a phenomenon
directly contrary to homology (sensu stricto)—actually rests upon a small point: the
migration of convergence into the category of homoplasy as now defined. If we
decide that the crucial distinction between homology and homoplasy should rest upon
common ancestry vs. independent origin, then one important phenomenon,
necessarily included within homoplasy by the defining criterion of independent origin
for similar structures, shares too much conceptual overlap with homology to permit a
clear and comfortable theoretical separation (however firm the descriptive division):
independent origin channeled by common internal constraints of homologous genes
or developmental pathways—in other words, the phenomenon known as parallelism.
But Lankester originally defined homoplasy exclusively on the basis of
phenomena that we would attribute to parallelism (Owen's general and serial
homologies). Therefore, for him, homoplasy could legitimately count as a
subcategory of homology (sensu lato), even though he recognized that he had to
separate homoplasy from homogeny (homology sensu stricto) by the genealogical
criterion of common ancestry vs. independent origin. But, if the scope of homoplasy
ever expanded to embrace convergences as well—a defendable move because
convergences also record an independent origin of similarities—then the combination
of parallelisms plus convergences into one category would destroy the conceptual
linkage of homoplasy with homology. With the addition of convergence (based on
explicit denial of common