1382 THE STORY OF THE UNIVERSE 



shrunk, and rapidly disappear, their function being 

 superseded by the lungs, which have been called into 

 play; and the animal now emerges from the water 

 and begins a new mode of existence, having become 

 a perfect frog. It still, however, retains its aquatic 

 habits, and swims with great ease in the water by 

 means of its hindfeet, which are very long and mus- 

 cular, and of which the toes are furnished with a 

 broad web, derived from a thin extension of the in- 

 teguments. 



No less curious are the changes which take place in 

 all the other organs for the purpose of effecting the 

 transformations rendered necessary by this entire 

 alteration in all the external circumstances of that 

 animal this total reversal of its wants, of its habits, 

 of its functions, and of its very constitution. 



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 progression in the water and on land is probably 

 the cause which prevents their having the form best 

 adapted for either function. The hindfeet of the 

 frog, being well constructed for striking the water 

 backward in swimming, are, in consequence, less 

 capable of exerting a force sufficient to raise and sup- 

 port 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 pro- 

 ceed only by leaps; an action which the length and 

 great muscularity of the hindlegs particularly fit 

 them for performing. The toad, on the other hand, 



