INTRODUCTORY OBSERVATIONS. 73 



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 developement through the 

 whole of the animal kingdom. That such phenomena are 

 exhibited by the typical forms of this class will be sufficiently 

 established by the slight sketch of their structure, habits, 

 and developement, which will presently be offered. 



The AMPHIBIA have by many zoologists been considered 

 in the light of an order of the class Reptilia ; but the charac- 

 ters by which they are distinguished appear to me to be suf- 

 ficiently marked and important to justify their separation as 

 a distinct class. They may be characterised as " vertebrated 

 animals with cold blood, naked skin, oviparous reproduction, 

 and most of them undergoing a metamorphosis, having refer- 

 ence to a change of condition, from an aquatic to an atmo- 

 spheric medium of respiration." " The class has been va- 

 riously divided into groups, according to the different views 

 of the naturalists by whom they have been arranged. The 

 division adopted by many zoologists of the present day, ac- 

 cording to the mere presence or absence of the tail in the 

 perfect state, is not only liable to the objections which be- 

 long to all merely dichotomous arrangements, but appears to 

 be far less natural arid 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 posses- 

 sion of branchiae, which are subsequently lost and replaced 

 by true lungs, and in undergoing consequently a total change 

 in the medium of their respiration ; whilst the Proteus and 

 the Siren retain their branchiae with lungs, (rudimentary, at 

 least,) and probably throughout life possess synchronously the 

 two-fold function of aquatic and atmospheric respiration. 



