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 transported 
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 presence of Astacus torrentium 
by reference to the same operation. 
If we take into account the occurrence of Astacus 
nobilis over so large a part 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 A. torrentitum; 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. leptodactylus is now passing into 
the rivers of the Baltic provinces of Russia. 
The study of the glacial phenomena of central Europe 
v 
