. "Homology ...
corresponds to the hypothetical genetic relationship. In the more or the
less clear homology, we have the expression of the more or less intimate
degree of relationship. Blood-relationship becomes dubious exactly in
proportion as the proof of homologies is uncertain" (_Elements_, p. 63).

It is worth noting that while Gegenbaur agrees with Haeckel generally
that morphological relationships are really genealogical, that, for
instance, each phylum has its ancestral form, he enters a caution
against too hastily assuming the existence of a genetic relation between
two forms on the basis of the comparison of one or two organs. "In
treating comparative anatomy from the genealogical standpoint required
by the evolution-theory," he writes, "we have to take into consideration
the fact that the connections can almost never be discovered in the real
genealogically related objects, for we have almost always to do with the
divergent members of an evolutionary series. We derive, for instance,
the circulatory system of insects from that of Crustacea ... but there
exists neither a form that leads directly from Crustacea to insects nor
any organisatory state (_Organisationszustand_), which as such shows the
transition. Even when one point of organisation can be denoted as
transitional, numerous other points prevent us from regarding the whole
organism strictly in the same light" (_Grundzuege_, p. 75). The real
ancestral forms cannot, as a rule, be discovered among living species,
nor often as extinct. "When we arrange allied forms in series by means
of comparison, and seek to derive the more complex from the simpler, we
recognise in the lower and simpler forms only similarities with the
ancestral form, which remains essentially hypothetical" (p. 75).

The facts of development, Gegenbaur goes on to say, help us out greatly
in our search for ancestral forms, for the early stages in the ontogeny
of a highly organised animal give us some idea of the organisation of
its original ancestor. Characters common to the early ontogeny of all
the members of a large group are particularly important in this respect
(_cf._ von Baer's law).

Gegenbaur distinguishes homologous or morphologically equivalent
structures from such as are analogous or physiologically equivalent,
just as did Owen and the older anatomists. Like von Baer he recognises
homologies, as a rule, only within the type.

He contributed, however, to the common stock a