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 between _A. nobilis_ and _A. leptodactylus_, than between _A. nobilis_ and _A. torrentium_; it seems not improbable that Astacus torrentium was the original tenant of the whole western European area outside the Ponto-Caspian watershed; 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 {322} has led Sartorius von Waltershausen[38] 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 westward 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. [38] “Untersuchungen ueber die Klimate der Gegenwart und der Vorwelt.” Natuurkundige Verhandelingen van de Hollandsche Maatschappij der Wetenschappen te Haarlem, 1865. The extremely severe climatal conditions which obtain in northern Siberia may sufficiently account for the {323} absence of crayfishes (if they are really absent) in the rivers Obi, Yenisei, and Lena, and in the great lake Baikal, which lies more than 1,300 feet above the sea, and is frozen over from November to May. Moreover, there can be no doubt that, at a comparatively recent period, the whole of this region, from the Baltic to the mouth of the Lena, was submerged beneath a southward extension of the waters of the Arctic ocean to the Aralo-Caspian Sea and Lake Baikal, and a westward extension to the Gulf of Finland.