AMPHIBIA. 337 



the same office as the tail of the cetacea, and propel the ani- 

 mal forwards: but when employed scparatcl}', they are more 

 qualified to act as oars. The walrus has feet still more de- 

 veloped, and distinctly divided into toes, which arc disposed 

 so as to strike backwards against the water. 



§ 4. Ma 7717711 fcr oil s Quachnipeds in general. 



From the imperfectly developed a'quatic and amphibious 

 tribes, we gradually ascend to the more finished structures of 

 mammiferous quadrupeds, which are expressly fitted for 

 progression on land. In these the powers of development, 

 not being expended in the mere effort of giving expansion 

 to the several textures, and of swelling the bulk of the frame, 

 sometimes to inordinate dimensions, are employed rather in 

 reducing the elements of the organization into compact forms, 

 and in concentrating their energies, so as ultimately to at- 

 tain the extent of power and harmony of action, which are 

 displayed in the higher orders of warm-blooded quadrupeds. 



It is to these favoured tribes that we must look for exam- 

 ples of the most complete development of the skeleton, and 

 the most advantageous disposition of mechanic force. We 

 have seen that reptiles, from the comparative shortness of 

 their limbs, and the torpidity of their muscular powers, are 

 but ill adapted for rapid progression. In all the more per- 

 fectly formed quadrupeds of the class mammalia, the trunk 

 of the body, being raised high upon the limbs, possesses great 

 range of motion, and can traverse with fewer steps a given 

 space. 



The office of the limbs, as far as they are concerned in 

 progressive motion, is two-fold. They have, first, to sus- 

 tain the weight of the body, which they must do by acting 

 in opposition to the force of gravity; and they must, second- 

 ly, give the body an impulse forwards. Let us consider 

 more particularly the relations which the structures bear to 

 each of these two functions. 



Vol. I. 43 



