Agassiz— Paleontological and Embryological Development. 295 
on the other. It is to embryology that we owe the explana- 
tion of the affinities of the old Fishes in which Agassiz first 
recognized the similarity to the embryo of Fishes now living, 
and by its aid we may hope to understand the relationship of 
the oldest represeutatives of the class. It has given us the 
only explanation of the early appearance of the Cartilaginous 
Fishes, and of the probable formation of the earliest vertebrate 
limb from the lateral embryonic fold, still to be traced in the 
young of the Osseous Fishes of to-day. 
Embryology has helped us to understand the changes ve ap 
) 
animals must gradually undergo in order to become capa 
Polyps, in fact of every single class of Invertebrates, and per- 
aps in none more than in the Brachiopods, to show how far-’ 
reaching has been the influence of embryology in guiding us to 
a correct reading of the relations between the fossils of succes- 
sive formations. There is scarcely an embryological mono- 
graph now published dealing with any of the later stages of 
growth which does not speak of their resemblance to some type 
of the group long ago extinct. It has therefore been most 
natural to combine with the attempts constantly made to estab- 
lish the genetic sequence between the genera of successive 
formations an effort to establish also a correspondence between 
their paleontological sequence and that of the embryonic stages 
of development of the same, thus extending the mere simi- 
larity first observed between certain stages to a far broader 
generalization. 
