DEVELOPEMENT OF BATRACIIIA. 



629 



at that date, was reduced to a stump, fig. 441, on the 18th, and was 

 removed by May 20 ; the metamorphosis behig fully completed, as 

 in fig. 442, in ail the tadpoles by May 22. 



4.3S 



440 



=^^ 



Aii,,dalT;uliiolc. 



(^luadruiiodni Tadpule. 



44 1 



442 



Bipedal Tadiiule. 



Young Frogs. 



Itinui tr-nii'ornna. 



The figures 438 to 441 illustrate the chief outward changes which 

 accompany the batrachian metamorphosis, as exemplified in Runa. 



In Bufo the tadpole is smaller and blacker in all the stages of 

 growth and metamorphosis. In both genera of Anourans the 

 growth is greatest at the phase figured in 439 ; with the subse- 

 quent phases the bulk of the l>ody is diminished : and this is 

 remarkably the case in the Rana paradoxu. 



In the Newts {Triton) the gills are in three pairs, larger and 

 more complex than in the Frog : the fore-limbs are the first to 

 emerge, and the gills persist long after the hind-limbs are deve- 

 loped. If late hatched and in a cold season, the gills may be re- 

 tained through the ensuing winter : they are absorbed before the 

 next breeding season comes on. 



Much ingenious conjecture has been expended on the influence 

 of external circumstances and internal volitions and eftbrts during 

 the struggles for existence in the origin of species by progressive 

 transmutation ; and their succession on this planet has been 

 speculatively assigned to such causes. In the metamorphoses of 

 the Batrachia we seem to have such process carried on before our 

 eyes to its extremest extent. Xot merely is one specific form 

 changed to another of the same genus ; not merely is one generic 

 modification of an order substituted for another; the transmu- 

 tation is not even limited by passing from one order ( Urodela) to 

 another (Anoura): it affects a transition from class to class. The 

 Fish becomes the Frog ; the aquatic animal changes to the terres- 

 trial one ; the water-breather becomes the air-breather ; an insect 

 diet is substituted for a vegetable one. And these changes, more- 

 over, proceed gradually, continuously, and without any interruption 



