AMPHIBIA. 487 



§ 3. Amphibia. 



In the small tribe denominated by Cuvier 

 Amphibia, and consisting of the Phoca, or Seal, 

 and the Trichecus, or Walrus, we perceive that 

 an advance is made towards a fuller develope- 

 ment of the limbs : these animals having a 

 distinct neck and pelvis, and both hind and 

 fore extremities. In the seal the hind legs are 

 drawn out posteriorly to a considerable length, 

 and placed parallel to each other : w^hen united 

 and alternately raised and depressed, they per- 

 form the same office as the tail of the cetacea, 

 and propel the animal forwards : but when em- 

 ployed separately, they are more qualified to act 

 as oars. The walrus has feet still more deve- 

 loped, and distinctly divided into toes, which are 

 disposed so as to strike backwards against the 

 water. 



§ 4. Mammi/erous Quadrupeds in general. 



From the imperfectly developed aquatic and 

 amphibious tribes we gradually ascend to the 

 more finished structures of mammiferous quad- 

 rupeds, which are expressly fitted for progression 

 on land. In these the powers of dev elopement. 



