INTRODUCTORY OBSERVATIONS. 77 



carnivorous appetite, will not only excite feelings of the 

 deepest admiration, but necessarily lead to the investiga- 

 tion of the laws by which such extraordinary changes are 

 governed, and of the relations which they bear to the 

 theory of continuous affinity before alluded to, and to that 

 of progressive development through the whole of the 

 animal kingdom. That such phenomena are exhibited by 

 the typical forms of this class will be sufficiently esta- 

 blished by the slight sketch of their structure, habits, and 

 development, which will presently be offered. 



The Amphibia have by many zoologists been considered 

 in the light of an order of the class Beptilia ; but the cha- 

 racters by which they are distinguished appear to me to 

 be sufficiently marked and important to justify their sepa- 

 ration as a distinct class. They may be characterised as 

 " vertebrated animals with cold blood, naked skin, and 

 oviparous reproduction, and most of them undergoing a 

 metamorphosis, having reference to a change of condition, 

 from an aquatic to an atmospheric medium of respiration." 

 " The class has been variously divided into groups, accord- 

 ing to the different views of the naturalists by whom they 

 have been arranged. The division adopted by many zoolo- 

 gists of the present day, according to the mere presence or 

 absence of the tail in the perfect state, is not only liable 

 to the objections which belong to all merely dichotonous 

 arrangements, but appears to be far less natural and less 

 consistent with the physiological characters of the groups 

 than that which is derived from the absence or presence 

 and the duration of the branchiae. Thus the Frogs and 

 Toads, which in the adult state have not the vestige of a 

 tail, and the Salamanders and Tritons, which retain that 

 organ through life, all agree in the early possession of 

 branchiee, which are subsequently lost and replaced by true 



