438 THE MECHANICAL FUNCTIONS. 



ception of atmospheric air into new cavities, for 

 the acquisition of limbs suited to new modes of 

 progression ; in a word, for a terrestrial life, and 

 for all the attributes and powers which belong to 

 quadrupeds. The succession of forms, which 

 these metamorphoses present, are in themselves 

 exceedingly curious, and bear a remarkable ana- 

 logy with the progress of the transformations of 

 those insects, which in the first stages of their 

 existence are aquatic. To the philosophic in- 

 quirer into the marvellous plans of creation, the 

 series of changes which mark these singular tran- 

 sitions cannot fail to be deeply interesting ; and 

 occurring, as we here find them, among a tribe 

 of animals allied to the more perfect forms of 

 organization, they afford us a better opportunity 

 of exploring the secrets of their developement 

 by tracing them from the earlier stages of this 

 complicated process so full of mystery and of 

 wonder. 



The egg of the frog (Fig. 196) is a round 

 mass of transparent nutritive gelly, in the centre 

 of which appears a small black globule. By 

 degrees this shapeless globule exhibits the ap- 

 pearance of a head and tail, and in this form it 

 emerges from its prison, and moves briskly in 

 the water. From the sides of the neck there 

 grow out feathery tufts (Fig. 198, B, B), which 

 float loosely, and without protection, in the sur- 

 rounding fluid. These, however, are mere tern- 



