useful analysis of the
concept of homology, and established certain classes and degrees of it.
He distinguished first between general and special homology, in quite a
different sense from Owen.

General homology, in Gegenbaur's sense, relates to resemblances of
organs within the organism, and includes four kinds of resemblance,
homotypy, homodynamy, homonomy and homonymy. Right and left organs are
homotypic, metameric organs are homodynamic; homonomy is the relation
exemplified by fin-rays or fingers, which are arranged with reference to
a transverse axis of the body; homonymy is a sort of metamerism in
secondary parts (not the main axis) of the body, and is shown by the
various divisions of the appendages (_Grundzuege_, p. 80).

Special homology, on the other hand, relates to resemblances between
organs in different animals. The interesting thing is that Gegenbaur
defines it genetically. Special homology is the name we give "to the
relations which obtain between two organs which have had a common
origin, and which have also a common embryonic history" (_Elements_, p.
64). This is his definition; but, in practice, Gegenbaur establishes
homologies by comparison just as the older anatomists did, and infers
common descent from homology, not homology from common descent.

"Special homology," he continues, "must be again separated into
sub-divisions, according as the organs dealt with are essentially
unchanged in their morphological characters, or are altered by the
addition or removal of parts" (p. 65). In the former case the homology
is said to be "complete," in the latter "incomplete." Thus the bones of
the upper arm are completely homologous throughout all vertebrate
classes from Amphibia upwards, while the heart of a fish is incompletely
homologous with the heart of a mammal.

Independently of Gegenbaur, Sir E. Ray Lankester proposed in 1870 a
genetic definition of homology.[385] He proposed, indeed, to do away with
the term homology altogether, on the ground that it included many
resemblances which were obviously not due to common descent--as, for
instance, the resemblance of metameres. So, too, organs which were
homologous in the ordinary sense, as the heart of birds and mammals,
might have arisen separately in evolution. He proposed, therefore, that
"structures which are genetically related, in so far as they have a
single representative in a common ancestor," should be called
_homogenous_(p. 36). All other resem