which it is soon to be raised; occasionally taking in a mouthful of air, which is received into its newly developed lungs, and afterward discharging it in the form of a small bubble. When the necessary internal changes are at length completed, preparations are made for getting rid of the tail, which is now a useless member, and which, ceasing to be nourished, diminishes by degrees, leaving only a short stump, which is soon removed. The gills are by this time shrunk, and rapidly disappear, their function being superseded by the lungs, which have been called into play; and the animal now emerges from the water and begins a new mode of existence, having become a perfect frog.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 muscular, and of which the toes are furnished with a broad web, derived from a thin extension of the integuments. 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 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 hindlegs particularly fit them for performing. The toad, on the other hand, whose hindlegs are short and feeble, walks better, but does not jump or swim so well as the frog.[5] 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.