ANIMAL KINGDOM. , 275 



leton which they possess in the adult state, and finally, by 

 that remarkable character of Reptiles, namely, that the 

 heart sends to the lung only a part of the blood which it 

 receives from the veins, and that the remainder of this fluid 

 returns into the general circulation without having come in 

 contact with the air. From Reptiles they are even still 

 further distant ; for besides the branchiae of their larva state, 

 which are always more or less analogous to those of fish, 

 and are supported in much the same manner, the circula- 

 tion is then totally different, as is also the system of gene- 

 ration in the perfect state. The passage however to the 

 Ophidians, by means of the genus Cecilia, is so direct 

 and manifest as scarcely to require notice. 



Thus we have seen five distinct classes of animals to 

 compose the great group of VERTEBRATA, viz. 



1. Reptilia, 



2. AVES, 



3. Mammalia, 



4. Pisces, 



5. Amphibia. 



There is nothing new in this arrangement, unless it be 

 that the Cetacea are not separated from the Fishes by the 

 intervention of Birds, and I hardly imagine that such a 

 novelty is likely to be objected to by those who have more 

 regard for truth of Nature than the authority of an artifi- 

 cial system. But to those who may be inclined to dis- 

 pute the accuracy of this innovation I will mention, among 

 a thousand which it is possible to produce, one anatomi- 

 cal argument which may be considered as conclusive. 

 On comparing the general structure of a bird, a quadru- 

 ped and a fish, naturalists find three very distinct essen- 

 tial plans of organization which influence, as Geoffrey St. 

 T 2 



