48 THE COMMON FROG. [chap. 



abundant evidence how obscure and recondite may be 

 the conditions which determine the transformations of 

 specific genesis, and how utterly futile are observations 

 as to an apparent homogeneity of readily appreciable 

 conditions. They are so since it seems to be just such 

 recondite ones which really determine the changes 

 just referred to, and probably therefore, other changes 

 analogous to them. 



It may be a question whether the gtnus Mejwbi'anc/ms 

 may not also be a persistent larval ^ form, and one 

 which now never attains its adult form. If so, it is 

 most probable that its lost state was similar to that of 

 the exclusively American genus Spclerpes^ the larva of 

 which MenobrancJms much resembles. With respect 

 to Proteus and Siren no conjecture of the kind can 

 yet be made. 



Individuals belonging to the common English species 

 [Triton cristatiis) occasionally retain some of the 

 external characters of immaturity, in spite of having 

 attained reproductive capability ; and a European 

 species {Triton alpestris) often matures the generative 

 elements while still, as to external appearance, more 

 or less in its tadpole stage of existence. The adult 

 condition, however, is normally and generally attained 

 by it. 



The geographical distribution of the Urodcla is very 

 remarkable. North America is the head-quarters of 

 the order, and, with rare and trifling exceptions, the 

 whole are confined to the Northern hemisphere. The 

 exceptions are certain forms which extend down the 

 Andes into South America, and one or two species of 



^ The young of the Frog or Eft is called a larva. 



