ea of that theory was correct;
that the skull does in fact correspond to a series of coalescent
vertebrae, but that the separate bones of the skull are not to be
regarded as representing parts of such modified vertebrae. The
skull-bones of all recent vertebrate animals are rather, for the most
part, dermal bones, which have come into closer connection as
supplementary to the cartilaginous primitive skull. We can even now
trace the number and position of the original vertebrae, from which
this primitive skull originated, by the number of the vertebral arches
(gill-arches) which are attached to it, as well as by the number and
position of those vertebrae, from nine to ten. Of all the recent
vertebrata, the cartilaginous fishes, or Selachians, have most nearly
preserved the form and structure of this primordial skull. These
Selachians, the Rays and Sharks, are on the whole the creatures which
throw the clearest light on the history of the lineage of the
vertebrata and on the organisation of our primeval fish-natured
ancestors. It is one of the particular merits of Gegenbaur that he
clearly and firmly established the place in nature of the Selachians
as the common ancestors of all vertebrate animals from fish up to man.

None but those who have thoroughly studied the comparative morphology
of the vertebrata, who have sought the genetic issue from that
labyrinth of intricate morphological problems at the hands of the
theory of descent, can duly value the immeasurable service which
Gegenbaur has done by this and other "Investigations into the
Comparative Anatomy of the Vertebrata." These investigations are as
much distinguished by a profound knowledge and careful working out of
the wonderfully-extensive empirical materials for the subject, as by
their critical acumen and philosophic grasp. At the same time they set
in the clearest light the immeasurable value of the theory of descent
in the causal explanation of the most difficult morphological
problems. Gegenbaur might, therefore, with perfect right, enunciate
this axiom in the Introduction to his "Comparative Anatomy." "The
theory of descent will at once find a touchstone of proof in
comparative anatomy. Up to this time no experience in comparative
anatomy has transpired which contradicts that theory; on the
contrary, they all lead up to it. Thus it will receive back from
science that which it has given to scientific method: clearness and
certainty." In point of fact we ca