1072 THE STRUCTURE OF EVOLUTIONARY THEORY
or series of parts stands to the fundamental or general type, and its enunciation
involves and implies a knowledge of the type on which a natural group of animals,
the vertebrate for example, is constructed" (1848, p. 7). This idea that we can assert a
form of homology between two parts (in two organisms) because both express the
same general archetype, rather than because one part can be designated as the "same"
as the other part (as in special homology), does not translate by a similar criterion of
descent from common ancestry because, as Owen noted, general homology records "a
higher relation." For example, Owen regarded arms, legs, and heads for that matter,
as derivations from a common vertebral archetype. Thus, the forearm of an aardvark
is a general homolog of my leg (not to mention the shrew's head and the whale's
flipper).
Obviously, these pairings do not represent homology by direct descent from
common ancestry. And yet, we would not deny that some legitimate evolutionary
commonality links my leg and aardvark's forearm (forget the shrew's head, although I
would not be shocked if the old vertebral theory for the origin of the skull reemerges
some day in a renewed form of validity). Moreover, we can be quite confident that
the similarity marks a genuine historical hold, not a fortuity, or a convergence
separately evolved from different archetypal bases. Still, the hold cannot be equated
with true common ancestry, and must arise instead as a constraint based on common
genesis from a source that imposes limitations or sets preferred channels of change
from within. In his Platonic perspective, Owen called this common source an ar-
chetype. We would identify such a generating source as a developmental constraint
from the historical vertex of the aptive triangle—perhaps arising from homologous
genes or homologous developmental pathways in the two separated lineages.
Finally, Owen defined serial homology as the iteration of an archetypal form
within the same organism in a set of repeated parts, perhaps each specialized for a
particular function, but still bearing signs of the common architectural plan—as in the
biramous appendages of arthropods, whether specialized as antennae, mouth parts,
walking legs or genital claspers; and in the arms and legs of tetrapods.
When asked how he could square serial homology with his basic definition of
"the same organ in different animals..." Owen would reply that the criterion of
sameness trumped the requirement for different organisms, and that different places
within the same organism would suffice. And we do not judge this response today as
flippant or invalid because we share Owen's feeling that some common principle—
although not common ancestry—validates a legitimate comparison of my leg and the
aardvark's forearm (general homology) and the aardvark's forearm with the aardvark's
own leg (serial homology). We would also identify this common principle as
developmental constraint based on homologous genes and embryological pathways
(whether expressed as arms and legs in the same animal, or as similar structures in
two animals—although we would call such structures nonhomologous because they
do not descend from a common ancestor, even though they