3i]H THE MECHANICAL FUNCTIONS. 



vancing; others outstrip the winds in fleetness. 

 Some families of reptiles are entirely destitute of 

 any external organs of motion, the whole trunk of 

 the body resting on the ground ; while Man occu- 

 pies a place where he stands alone, being distin- 

 guished by the exclusive faculty of permanently 

 sustaining himself on the lower extremities. 



In reviewing the developements and the mecha- 

 nical functions exhibited by so great a diversity of 

 structures, I shall commence with an examination 

 of those amphibious reptiles which appear to form 

 an intermediate link in the chain connecting the 

 strictly aquatic, with the terrestrial vertebrated ani- 

 mals: then, taking up this latter series, I shall 

 consider the more simple conformation and less 

 perfect motions of terrestrial animals destitute of 

 limbs ; and gradually ascend to those in which the 

 support and progression of the body is effected by 

 extremities, more and more artificially formed : 

 concluding with the human structure, which termi- 

 nates this extensive series. 



§ 2. Batrachia. 



The order of Batrachia, or Amphibious Reptiles, 

 constitutes the first step in the transition from 

 aquatic to terrestrial vertebrata. It is more par- 

 ticularly the function of respiration that requires to 

 be modified in consequence of the change of ele- 

 ment in which the animal is to reside ; and as if it 

 had been necessary, conformably with the laws of 

 animal creation, that this change should not be 



