BRITISH FROGS 



restriction of watery environment will often make 

 AmUystomas out of axolotls. The case of the axolotl 

 is not so unique as was once thought. " Neoteny," as 

 this extension of childhood is scientifically called, both 

 occurs normally, and can be produced in various am- 

 phibia. Even in frogs, the tadpoles can in the case of 

 some species be compelled to remain tadpoles for a long 

 if not indefinite period of time, behaving, however, pre- 

 cisely as tadpoles, though swollen in bulk ; the two 

 frogs of this country, viz. Rana temporaria and R. escu- 

 lenta, are instances to the point. But these tadpoles, if 

 they do not ultimately become frogs, die as tadpoles 

 without begetting fresh tadpoles. The axolotl and the 

 common newt of this country, as well as other newts in 

 the strict sense, that is to say, of the genus Triton, not 

 only retain their larval characters, but are able, as we 

 have seen, to breed as such. These facts lead us to 

 some highly interesting conclusions. It used to be a 

 common idea, which had the advantage of seeming 

 quite philosophical, that the existence of external 

 arborescent breathing organs in such amphibians as 

 the axolotl and the Japanese salamander, were proofs 

 of the antiquity of these forms. The retention of the 

 fish-like mode of breathing by gills seemed to argue a 

 basal position in the amphibian world. Not so, how- 

 ever, do we now think. For it is clear that some at 

 least of the gilled forms are merely precociously de- 

 veloped young, which, taking time by the forelock, 

 commence breeding before structural maturity, and 

 that others, like the Japanese salamander, may be 

 (here we can only guess for the present) entirely similar 

 in origin. Hence it is clearly a recent modification 

 which we are dealing with ; and therefore the types 

 which, like the frog, have not given rise to a persistent 

 and sexual larval form are the older. 



Cortez, when he descended from his " peak in Darien," 

 296 



