opment could not
always be relied upon in deciding homologies (p. 89). But he could not
deny that the archetype was better shown in the embryo than in the adult
(_supra_, p. 108).

J. V. Carus[237] likewise stood firm for the older method of determining
homologies by comparison of adult structure. "We can regard as
homologous," he writes, "only those parts which in the fully formed
animal possess a like position and show the same topographical relations
to the neighbouring parts" (p. 389). Parts homologous in this sense
might develop in different ways, but no great importance was to be
attached to such a circumstance. Membrane and cartilage bones developed
in practically the same way, from the same skeleton-forming layer, and
no morphological significance attached to their distinction (pp. 227,
457). Embryology was of considerable value in helping to determine
homologies, but the evidence that it supplied was contributory, not
conclusive. Perhaps the greatest service which the study of development
rendered was to disentangle, by a comparison of the earliest embryos,
the generalised type (p. 389).

We have now traced, by our historical study of the theory of the skull,
the gradual evolution of the tendency to find in development the surest
guide to determining homologies. We have seen how the embryological
"type" came to be substituted, in whole or in part, for the anatomical
"type" derived from the study of adult structure. But we have had to do
only with a modification, not with a transformation, of the criterion of
homology recognised by the anatomists. Homology is still determined by
position, by connections, in the embryo as in the adult. "Similarity of
development" has become the criterion of homology in the eyes of the
embryologist, but "similarity of development" means, not identity of
histological differentiation, but similarity of connections throughout
the course of development. For the purposes of morphology, development
has to be considered as an orderly sequence of successive forms, not in
its real nature as a process essentially continuous. Morphology has to
replace the living continuity by a kinematographic succession of stages.
Since it is the earliest of these stages that manifest the simplest and
most generalised structural relations of the parts, it is in the earlier
stages that homologies can be most easily determined. But these
homologies are still determined solely by the relative positions and
c