PROGRESSIVE MOTION IN BATRACIIIA. 309 



general, resemble, in their figure and connexions, those of 

 the higher orders oi 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 instruments for pro- 

 gression in the water and on land, is probably the cause 

 which prevents their having the form best adapted for ei- 

 ther function. The hind feet of the frog, being well con- 

 structed 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 at- 

 tempt 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^hort and fee- 

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

 frog."^ The Hyla, or tree-frog, has the extremities of each 

 of its toes expanded into a fleshy tubercle, approaching in 

 the form of its concave surface to that of a sucker, and by 

 the aid of which it fastens itself readily to the branches of 

 trees, which it chiefly inhabits, and along which it runs with 

 great agility. * 



The Salamander is an animal of the same class as the 

 frog, undergoing the same metamorphoses from the tadpole 

 state. It differs much, however, in respect to the develop- 

 ment of particular parts of the skeleton. The anterior ex- 



• It is singular that the frog", though so low in the scale of vertebratcd ani- 

 mals, should bear a striking resemblance to the human conformation in its or- 

 gans of progressive motion. This ai-ises from the exertions which it makes 

 in swimming being similar to those of man in walking, in as far as they both 

 result from the strong action of the extensors of the feet. Hence, we find a 

 distinct calf in the legs of both, produced by the swelling of similar muscles. 

 The muscles of the thigh present, also, many analogies with those of man; 

 particularly in the presence of the long muscle called the sariorius, the use 

 of which is to turn the foot outwards, both in stepping and in swimming. 



