Historical Constraints and the Evolution of Development 1071
more or less symmetrical classes and groups in the organic world, established
by some inherent law of Nature which limited her productive powers to
arbitrary special plans or types of structure, and has taught us to see, in the
variously isolated and variously connected kinds of animals and plants, simply
the parts of one great genealogical tree, which have become detached and
separated from one another in a thousand different degrees, through the
operation of the great destroyer Time, yet certain terms and ideas are still in
use which belonged to the old Platonic school, and have not been defined
afresh in accordance with the doctrine of descent.In particular, Owen had specified three categories of homology: special, general,
and serial. (His classical and definitive 1848 treatise, "On the archetype and
homologies of the vertebrate skeleton," comprises three chapters, titled "special
homology," "general homology," and "serial homology" respectively.)
Owen's famously vague and broad definition of "homologue" as "the same organ
in different animals under every variety of form and function" (1848, p. 7, repeated
from 1843, p. 374) invokes a Platonic notion of sameness as "proceeding from a
common archetype." Lankester had the good sense and vision to recognize (and we
continue to assent today) that this concept did enjoy philosophical coherence, and
could be translated into evolutionary terms—but that the Darwinian version implied
different distinctions, requiring a subdivision of meanings and significances within a
general notion that remained usefully unitary.
Owen's three categories share tighter bonding in the idea that parts can be called
homologous so long as they can be construed as expressions or embodiments of the
same idealized archetype (a key pre-evolutionary notion of formalist, as opposed to
functionalist, thinking, and therefore particularly difficult to translate into a
functionalist theory of evolution like natural selection). Obviously, as a first pass for
evolutionary translation, we should redefine the Owenian archetype as the Darwinian
common ancestor—thus substituting the real flesh and blood of physical continuity
for a Platonic notion of formal identity. We can then proceed, as Lankester notes,
with evolutionary versions of Owen's three categories.
For Owen (1848, p. 7), special homology refers to "the correspondence of a part
or organ, determined by its relative position and connections, with a part or organ in a
different animal." In evolutionary terms, we regard these two parts (in two different
organisms) as homologous because they descend from the same feature in a common
ancestor. Lankester (1870, p. 36, first paragraph) recognized this criterion of common
ancestry as paramount—the definition that "without doubt the majority of
evolutionists" would assign to the concept of homology. Lankester proposed—
although his name never took hold—that this aspect of Owen's broader concept
("special homology") be called homogeny (or homology sensu stricto.) What then
becomes of Owen's other two categories?
Owen defined general homology as "a higher relation ... in which a part