PROGRESSIVE MOTION IN BATRACHIA. 445 



is large, and considerably developed ; making 

 some slight approach to the expansion it receives 

 in the Chelonia. The radius and ulna are united 

 into one bone : the bones of the arm and leg in 

 general resemble in their figure and connexions 

 those of the higher orders of Mammalia, to the 

 type of which this order of reptiles is evidently 

 making an approximation. There are five toes 

 in the foot, with sometimes the rudiment of a 

 sixth : the anterior extremity has only four toes, 

 which are without claws. 



The necessity of employing the same instru- 

 ments for progression in the water and on land, is 

 probably the cause which prevents their having 

 the form best adapted for either function. The 

 hind feet of the frog, being well constructed for 

 striking the water backwards in swimming, are, 

 in consequence, less capable of exerting a force 

 sufficient to raise and support the weight of the 

 body in walking : and this animal accordingly 

 is exceedingly awkward in its attempt to walk. 

 On a short level plane it can proceed only by 

 leaps ; an action which the length and great 

 muscularity of the hind legs particularly fit 

 them for performing. The toad, on the other 

 hand, whose hind legs are short and feeble, walks 

 better, but does not jump or swim so well as the 

 frog.* The Hyla, or tree-frog, has the extremities 



* It is singular that the frog, though so low in the scale of 

 vertebrated animals, should bear a striking resemblance to the 



