322 DISTRIBUTION AND ATIOLOGY OF THE CRAYFISHES. 
has led Sartorius von Waltershausen* to the conclusion 
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 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 first 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 sufficiently account for the 
*«¢Untersuchungen ueber die Klimate der Gegenwart und der Vorwelt.” 
Natuurkundige Verhandelingen van de Hollandeshe: Maatschappij der 
Wetenschappen te Haarlem, 1865. 
