322 DISTRIBUTION AND ETIOLOGY OF THE CEATFISHES. 



has led Sartorius von Waltershauseu * to the conclusion 

 that at the time when the glaciei's 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 Rhone, around the northern escarpment of the 

 Alpine chain, and connected the head-waters of the 

 Danube with those of the Rhine, 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 would thus be 

 opened up by which crayfishes might pass from the Aralo- 

 Caspian area to western Europe. If they spread by this 

 road, the Astacus torrentium may represent the fi.rst wave 

 of migration westward, while A. nobilis 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, Aryan, and Mongoloid streams of west- 

 ward movement among mankind. 



If we thus suppose the western Eurasiatic crayfishes 

 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 intelligible. 



The extremely severe climatal conditions which obtain 

 in northern Siberia may sufiiciently account for the 



* ' ' TJntersuchung-en ueber die Klimate der Gegenwart und der Vorwelt." 

 Xatuurkundige Verhandelingen van de HoUandsche Maatscliappij d<a 

 Wetenschappen te Haarlem, l6G5. 



