436 THE MECHANICAL FUNCTIONS. 



to form an intermediate link in the chain con- 

 necting the strictly aquatic, with the terrestrial 

 vertebrated animals : then, taking up this latter 

 series, I shall consider the more simple confor- 

 mation, 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 terminates this extensive 

 series. 



2. J3atrachia. 



THE order of Batrachia, or Amphibious Rep- 

 tiles, constitutes the first step in the transition 

 from aquatic to terrestrial vertebrata. It is more 

 particularly the function of respiration that re- 

 quires to be modified in consequence of the 

 change of element in which the animal is to reside; 

 and as if it had been necessary, conformably to 

 the laws of animal creation, that this change 

 should not be abruptly made, we find that Ba- 

 trachian reptiles, with which this series com- 

 mences, are constructed at first on the model of 

 fishes ; breathing the atmospheric air contained 

 in the water by means of gills, and moving 

 through the fluid by the same instruments of 

 progression as fishes, which indeed they exactly 



