322 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES. 



has led Sartorius von Waltershausen * to the conchision 

 that at the time when the glaciers of the Alps had a 

 much greater extension than at present, a vast mass of 

 freshwater extended from the valley of the Danube to 

 that of the Ehone, around the northern escarpment of the 

 Alpine chain, and connected the head-waters of the 

 Danube with those of the Ehine, the Rhone, and the 

 northern Italian rivers. As the Danube debouches into 

 the Black Sea, and this was formerly connected with 

 the Aralo-Caspian Sea, an easy passage w^ould thus be 

 opened up hy which crayfishes might pass from the Aralo- 

 Caspian area to western Europe. If they spread by this 

 road, the Astacus torrentium may represent the first wave 

 of migration westward, while A. nohilis answers to a 

 second, and A. leptodactylus, with its varieties, remains 

 as the representative of the old Aralo-Caspian crayfishes. 

 And thus the crayfishes would present a curious parallel 

 with the Iberian, Ar3^an, and Mongoloid streams of west- 

 ward movement among mankind. 



If we thus suppose the western Eurasiatic craj^fishes 

 to be simply varieties of a primitive Aralo-Caspian stock, 

 their limitation to the south by the Mediterranean and by 

 the great Asiatic highlands becomes easily intelHgible. 



The extremely severe chmatal conditions which obtain 

 in northern Siberia may sufficiently account for the 



* " Untersuchungen ueber dieKlimate derGegenwartundderVorwelt." 

 Natuurkimdige Verhandelingen van de Hollandsche Maatschappij der 

 Wetenschappen te Haarlem, l8Go. 



