432 THE MECHANICAL FUNCTIONS. 



depressed, they perform the same office as the tail 

 of the cetacea, and propel the aiihual forwards : 

 but when employed separately, they are more 

 qualified to act as oars. The Walrus has feet still 

 more developed, and distinctly divided into toes, 

 which are disposed so as to strike backwards 

 against the water. 



'^ 4. Mammiferoiis Quadrupeds in generuL 



From the imperfectly developed aquatic and am- 

 phibious 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 developement, 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 concen- 

 trating their energies, so as ultimately to attain the 

 extent of power and harmony of action, which are 

 displayed in the higher orders of warm-blooded 

 fjuadrupeds. 



It is to these favoured tribes that we must look 

 for examples of the most complete developement 

 of the skeleton, and the most advantageous dispo- 

 sition of mechanic force. We have seen that rep- 

 tiles, from the comparative shortness of their limbs, 

 and the torpidity of their muscular powers, are but 

 ill adapted for rapid progression. All the more 

 perfectly formed quadrupeds of the class mam- 

 malia, having the trunk of the body raised high 



