THE DISTRIBUTION OF THE CRAYFISHES. 321 



water, still less of crossing the many miles of sea which 

 separate England from even the nearest point of the 

 Continent. In fact, the existence of the same kind of 

 crayfish on both sides of the Channel appears to be 

 only a case of the general truth, that the Fauna of the 

 British Islands is identical with a part of that of the 

 Continent; and as our foxes, badgers, and moles cer- 

 tainly have neither swum across, nor been transj)orted 

 by man, but existed in Britain while it was still con- 

 tinuous with western Europe, and have been isolated 

 by the subsequent intervention of the sea, so we may 

 confidently explain the jDresence of Astacus torrentium 

 hj reference to the same operation. 



If we take into account the occurrence of Astacua 

 nobilis over so large a j)art of the area occupied by 

 Astacus torrentium; its absence in the British Islands, 

 and in Greece ; and the closer affinity which exists be- 

 tween A. nobilis and A, leptodactylus, than between A. 

 nobilis and ^. torrentium; it seems not improbable that 

 Astacus torrentium was the original tenant of the whole 

 western European area outside the Ponto- Caspian water- 

 shed ; and that A, nobilis is an invading offshoot of the 

 Ponto- Caspian or leptodactylus form which has made its 

 way into the western rivers in the course of the many 

 changes of level which central Europe has undergone ; 

 in the same way as A. leptodacti/lus is now passing into 

 the rivers of the Baltic provinces of Russia. 



The study of the glacial phenomena of central Europe 

 15 



