84 CLASSIFICATION OF AMPHIBIANS. 



in two elements, but which, during life^ undergo a 

 metamorphosis. Linnaeus indeed placed in his class Am- 

 phibia not only the semi-aquatic reptiles, and the frogs, 

 but the whole of the terrestrial lizards, and serpents, 

 adding thereto the sharks and a large number of true 

 fishes. But modern naturalists have reduced this he- 

 terogeneous assemblage to more order ; and although 

 M. Cuvier^ by a singular oversight, has not retained a 

 name so expressive, our best erpetolcgists now agree 

 in restricting the term to the frogs, the salamanders, 

 and the sirens. These, as it has long ago been proved, 

 form a distinct natural class of vertebrated animals, 

 intervening between fishes and the true reptiles, and to 

 which alone we must be considered to allude in this 

 chapter. 



(89.) '^^The doctrine of continuous affinities," observes 

 Professor Bell, " could scarcely receive a more striking 

 illustration in the animal kingdom, than is afforded by the 

 interesting group constitutmg the Amphibia of modern 

 authors. Intermediate in their structure, and, in many 

 forms, in their habits and modes of life also, between 

 the fishes and the true reptiles, they bear a still more 

 interesting relation to those classes in that remarkable 

 change which many of them undergo at a certain 

 period of life, by which they become transformed from 

 the nature and habits of the former to those of the 

 latter class ; and thus exhibit, in their ov/n indi\ddual 

 life, a beautiful and complete example of transition of 

 organisation, a subject which constitutes one of the 

 most important theories connected with the higher 

 departments of zoological science. To any person ca- 

 pable of appreciating the interest attached to the study 

 of physiological phenomena, the contemplation of an 

 animal, which at one period of its life is endowed 

 exclusively with the organs of aquatic respiration, re- 

 sembling the gills of fishes, with means of locomotion 

 adapted only to a constant residence in water, and with 

 a digestive apparatus fitted exclusively for the assi- 

 milation of vegetable food, assuming \ty degrees the 



