parsimony/biology

[Kirk Fitzhugh]

At 10:30 AM 3/1/01 -0500, you wrote:
>What I dont understand about your reading of Owen is that you seem to claim
>that the term "homologues" refers only to similarities, and that only after
>calling some property observed in different objects "homologues", do we then
>investigate the "why" question. To me this begs the question of why
>similarities are not simply called similarities, until the "why" question is
>answered. To refer to them as "homologues" seems to me to imply that one has
>accepted homology as the answer to the "why" question.

Tom,

I will have to reply to your last message in a couple of parts - too busy
today to write everything out. Your above points on homologue and homology
are very important, so I want to quote here what Owen said as to the
distinction between homologue and homology. I'm only invoking his authority
from a historical perspective, but it can be argued that his understanding
of the progression from perception to naming (homologue) to causal
hypothesis (homology) is correct. This progression is reflected in his
writings.

In his "Report on the Archetype and Homologies of the Vertebrate Skeleton,
" Owen (1847: 173; see also Owen, 1849: 16-17, footnote) defines homologue as:

"The corresponding parts in different animals being thus made namesakes,
are called technically 'homologues.'  The term is used by logicians as
synonymous with 'homonyms,' and by geometricians as signifying 'the sides
of similar figures which are opposite to equal and corresponding angles,'
or to parts having the same proportions...."

Subsequent to the perception of similarity of properties, one applies the
same name to the things with those properties. At the point one applies the
same name to objects with the same properties, one has denoted homologues.
It is after giving this definition that Owen then states that "homology"
can be recognized as having three forms: special, general, and serial. For
our interests, special and general homology are of greatest interest.
Unfortunately, Owen never formally defines the term homology in the way he
did homologue, but rather characterized homology as relations between
homologues and an archetype. On special homology, Owen (1847: 175) says,

"Relations of homology are of three kinds: the first is that above defined
[for homologue], viz. the correspondence of a part or organ, determined by
its relative position and connections, with a part or organ in a different
animal; the determination of which homology indicates that such animals are
constructed on a common type: when for example, the correspondence of the
basilar process of the human occipital bone with the distinct bone called
'basi-occipital' in a fish or crocodile is shown, the special homology of
that process is determined."

Here, Owen does not say that homologue is equivalent to homology, but that
subsequent to "the determination of" homologues, i.e., the naming of shared
similariteis, "homology indicates" that perceived correspondency is due to
a "common type." In today's terminology, the "relations" Owen referred to
are causal relations. Special homology denotes that homologues have a
common causal basis, the archetype.

In his "On the Nature of Limbs," Owen (1849: 2-3) specifically
characterizes homology as the study of the causal basis of shared similarities:

"'Homology' seems now to be accepted as the name of that study or doctrine
the subject of which is the relations of the parts of animal bodies
understood by the German word 'Bedeutung.'... I have used therefore the
word 'Nature' in the sense of the German 'Bedeutung,' as signifying that
essential character of a part which belongs to it in its relation to a
predetermined pattern, answering to the 'idea' of the Archetypal World in
the Platonic cosmogony, which archetype or primal pattern is the basis
supporting all the modifications of such part for specific powers and
actions in all animals possessing it, and to which archetypal form we come,
in the course of our comparison of those modifications, finally to reduce
their subject."

Owen (1847: 175) then recognizes another type of relation, called general
homology:

"A higher relation of homology is that in which a part 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.  Thus when the basilar process of
the human occipital bone is determined to be the 'centrum' or 'body of the
last cranial vertebra,' its general homology is enunciated."

"General" homology only differs from "special" in that one has accounted
for homologues by specifically recognizing a given archetype in "general
homology," whereas a statement of "special" homology simply recognizes that
homologues are due to an unspecified archetype.

Owen (1847: 311) readily pointed out the intimate dependencies between
special and general homologies:

"If then the special homology be admitted on the ground of the constancy of
the connections of the part, with what show of reason can its general
homology be rejected which forms the very basis or condition of the
characters determinative of such admitted special homology?"

A very accurate summary of Owen's special and general homology are provided
by Panchen (1994: 35-36), for which we can obviously see that homology is
the causal accounting, be by archetype or common ancestor, of homologues:
Special homology: "...equivalent parts of the skeleton in two different
vertebrates are homologues because each is equivalent to the same structure
in the archetype."

General homology: "...the comparison is made directly between organism and
archetype."

Serial homology: "...different structures are equivalent because the
archetypal equivalents have the same basic structure."

It should also be noted that Owen was very cognizant of his derivation of
terms, especially since his "Report" was intended to formalize the use of
terminology throughout vertebrate skeletal structure. I think he was very
aware of how he was using "homologue" and "homology," especially when we
look at the derivation of the terms:

homo-logue, homos, "same," "equal," or "similar," and logos, "word";
homo-logy, logy, "knowledge," "science," or "the study of."

Hope this helps,

Kirk