302 EMBRYOLOGY AND CLASSIFICATION. 
finement. In this stage, they are, moreover, 
generally too small to be readily seen in their 
natural element. Nevertheless, this is the most 
important period of their growth, with reference 
to their natural affinities, and I shall take an 
early opportunity to show how our young fishes, 
aping the Gadoid or Blennioid type in their tran- 
sition period, pass gradually into that of Labroids 
and Lophioids; how fish embryos, resembling 
the tadpoles of frogs and toads, gradually as- 
sume the form of Cyprinodonts; how Apods are 
transformed into Jugulars and Abdominals, and 
Malacopterygians into Acanthopterygians; and, 
finally, how a natural classification of the fishes 
may be founded upon the correspondence which 
exists between their embryonic development and 
their structural gradation. | 
In order to show further how much we may ex- 
pect from such investigations, I will allude briefly 
to some of the facts with which my own studies 
have thus far made me acquainted. One impor- 
tant truth already assumes egrcat significance in 
the history of the growth of animals; namely, that 
whatever the changes may be through which an 
animal passes, and however different the aspect 
of these phases at successive periods may appear, 
they are always limited by the character of the 
type to which the animal belongs, anil never 
pass that boundary. Thus, the Radiate begins 
