146 THE EDIBLE FROG. [CH. Ill 



range, which means power of dispersal. The edible frog, 

 Rana esculenta, is a case in point. This species extends 

 from Great Britain, where however it is supposed to have 

 been introduced, in the West to Corea and Japan in the 

 East and to Algeria in the South. Its range in fact is 

 over the entire Palsearctic region ; but individuals from 

 Siam agree with Japanese specimens, so that the frog is 

 Oriental as well as PalaBarctic in habitat. In a paper 

 upon the distribution of this frog Mr Boulenger 1 quotes 

 14 synonyms, generic as well as specific, thus illustrating 

 the difficulties introduced into the study of geographical 

 distribution by imperfect acquaintance with zoological 

 fact. Intermediate forms however prove that there is but 

 one species, divisible into four varieties. It is a question 

 whether certain of these varieties which occur in the 

 same country and do not interbreed are not rather of 

 specific than varietal value ; but in any case they must be 

 derived from the same stock, and the eastern variety does 

 not overlap any of the western. These facts seem to 

 argue powers of extended migration hampered by infre- 

 quent opportunity. A river system appears to be, at least 

 as a rule, inhabited by an identical fauna throughout, 

 though its course may be, as is that of the Nile, from 

 tropical to temperate regions. But river systems are 

 isolated from each other by intervening mountains ; and 

 moreover the rapid streams at the source are not suitable 

 for Amphibian life ; hence a species which inhabited one 

 river system would not in the ordinary course of events 

 get within a measurable distance of the tributaries of 



1 P.Z.S. 1891, p. 374. 



