ssages
show that Aristotle had some conception of homology as distinct from
analogy. He did not, however, develop the idea. What Aristotle sought
in the variety of animal structure, and what he found, were not
homologies, but rather communities of function, parts with the same
attributes. His interest was all in _organs_, in functioning parts,
not in the mere spatial relationship of parts.

This comes out clearly in his treatise _On the Parts of Animals_,
which is subsequent to, and the complement of, his _History of
Animals_. The latter is a description of the variety of animal form,
the former is a treatise on the functions of the parts. He describes
the plan of the _De Partibus Animalium_ as follows:--"We have, then,
first to describe the common functions, common, that is, to the whole
animal kingdom, or to certain large groups, or to members of a
species. In other words, we have to describe the attributes common to
all animals, or to assemblages, like the class of Birds, of closely
allied groups differentiated by gradation, or to groups like Man not
differentiated into subordinate groups. In the first case the common
attributes may be called analogous, in the second generic, in the
third specific" (i, 5, 645^b, trans. Ogle). The alimentary canal is a
good example of a part which is "analogous" throughout the animal
kingdom, for "all animals possess in common those parts by which they
take in food, and into which they receive it" (Cresswell, _loc. cit._,
p. 6).

The _De Partibus Animalium_ becomes in form a comparative
organography, but the emphasis is always on function and community of
function. Thus he treats of bone, "fish-spine," and cartilage together
(_De Partibus_, ii., 9, 655^a), because they have the same function,
though he says elsewhere that they are only analogous structures (ii.,
8, 653^b). In the same connection he describes also the supporting
tissues of Invertebrates--the hard exoskeleton of Crustacea and
Insects, the shell of Testacea, the "bone" of _Sepia_ (ii., 8,
654^a). Aristotle took much more interest in analogies, in organs of
similar function, than in homologies. He did recognise the existence
of homologies, but rather _malgre lui_, because the facts forced it
upon him.

His only excursion into the realm of "transcendental anatomy" is his
comparison of a Cephalopod to a doubled-up Vertebrate whose legs have
become adherent to its head, whose alimentary canal has doubled upon
itself in