>" 



£74 ON THE CLASSES OF THE 



scribed groups this type of form, being a real existence, is 

 not very difficult to be ascertained ; but in the larger assem- 

 blages, such as those into which the Vertehrata seem to have 

 been distributed by Nature, it is almost impossible to ascer- 

 tain the type from its being a sort of ideal being, uniting 

 in itself all the various perfections of organization which may 

 in reality be dispersed throughout the group. The phy-' 

 siologist however will be always interested with the study 

 of the Amphibia, because it leads him by the most accu- 

 rate process of ratiocination to establish some fixed points 

 in the group of Vertehrata, which he may make the foun- 

 dation of his more minute investigations. Thus, if that 

 state in which an animal is fitted to continue the species 

 be more perfect than that of infancy, it follows that the 

 tadpole form and structure, which approaches to that of a 

 Fish, is not so perfect as the adult form, which approaches 

 to that of a Reptile. This reptile form is therefore the 

 type or perfection of the group ; and as the Salamander is 

 more nearly allied in shape to the true reptiles than any 

 others of the Amphibia, we may safely conclude it to be 

 the type to which the Proteus and Siren tended in the de- 

 velopement of their form, but which they are never able 

 to arrive at from some limits which have been imposed on 

 their metamorphosis by Nature. This metamorphosis 

 then, whether perfect, as in the case of the Salamander 

 and perhaps of the Frog, or imperfect, as in Proteus and 

 Siren, is the distinguishing character of the group of Am' 

 phibia, though evidently like most other natural characters 

 not that which may easily be made use of for purposes of 

 arrangement. The Amphibia are then clearly situated be- 

 tween Fishes and Reptiles ; from the former of which ani- 

 mals they are separated by the lungs and form of the ske^ 



