Historical Constraints and the Evolution of Development 1073
owe their structural similarity, in large part, to construction by homologous genes and
developmental pathways. After all, if no common genetics or development influenced
the ontogeny of the aardvark's arm and leg, we would view the two limbs as purely
convergent and unaffected by internal constraint, while Owen, in such circumstances,
would not have defined them as serially homologous).
We may now return to Lankester's problem: Owen's special homology translates
easily into evolutionary language as descent from common ancestry. This
phenomenon presents no conceptual problem, and Lankester therefore chose to
separate this subcategory as the unambiguous "best case" of homogeny. But how shall
general and special homology be represented in evolutionary language? On the one
hand, Owen applied these terms to separate structures that do not descend from the
same structure in a common ancestor. They should therefore be distinguished from
special homology (Lankester's homogeny) because we must be able to identify true
and unbroken continuity in physical descent as a basis for phylogenetic
reconstruction.
On the other hand, general and serial homologies do record a hold of history
over descent within clades. Some common property, present in a clade as a
consequence of phylogenetic history, does generate the similarities in these two other
Owenian categories. But this property can only be identified as a common generating
pattern, a common constraint, a common pathway of development, or a common set
of hereditary tendencies—and explicitly not as an overt common ancestral structure
retained by descent in subsequent branches of the clade.
What then shall we call these products of common phylogenetic patterns in
organic architecture—these separately evolved results of common developmental
constraints, we would say today—but not of overt and expressed common ancestral
phenotypes? Lankester proposed that we call them homoplasts in contrast with the
homogens of common structural origin, and that we designate the process of their
production as homoplasy, as distinguished from the homogeny of strict descent from
common ancestral structures. But he considered both processes as subdivisions of a
larger and coherent concept of homology—homogeny for Owen's special homology,
and homoplasy for Owen's general and serial homology.
In defending his placement of homoplasy within a broad but coherent concept of
homology, Lankester asks (1870, p. 38): "What is the other quantity covered by the
term homology over and above homogeny?" Lankester answers that many similarities
not due to inheritance of common ancestral structures nonetheless arise as
consequences of the inheritance of unique phylogenetically constrained building
patterns—and therefore deserve inclusion within a broader category of similarity
based upon descent (as opposed to similarity derived purely by independent
adaptation, with no contribution by constraint from an organism's past history). These
independently evolved, but historically constrained, similarities—we would now call
them parallelisms—define Lankester's original concept of homoplasy. Lankester does
acknowledge that homoplastic similarities must be evoked by similar environmental