The Crayfish: An Introduction to the Study of Zoology.

by Thomas Henry Huxley

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.

The great lakes and inland seas which stretch, at intervals, from Baikal, on the east, to Wenner in Sweden, on the west, are simply pools, isolated partly by the rising of the ancient sea-bottom and partly by evaporation; and often completely converted into fresh water by the inflow of the surrounding land-drainage. But the population of these pools was originally the same as that of the Northern Ocean, and a few species of marine crustaceans, mollusks, and fish, besides seals, remain in them as living evidences of the great change which has taken place. The same process which, as we shall see, has isolated the Mysis of the Arctic seas in the lakes of Sweden and Finland, has shut up with it other arctic marine crustacea, such as species of Gammarus and Idothea. And the very same species of Gammarus is imprisoned, along with arctic seals, in the waters of Lake Baikal.

The distribution of the American crayfishes agrees equally well with the hypothesis of the northern origin of {324} the stock from which they have been evolved. Even under existing geographical conditions, an affluent of the Mississippi, the St. Peter’s river, communicates directly, in rainy weather, with the Red river, which flows into Lake Winnipeg, the southernmost of the long series of intercommunicating lakes and streams, which occupy the low and flat water-parting between the southern and the northern watersheds of the North American Continent. But the northernmost of these, the Great Slave Lake, empties itself by the Mackenzie river into the Arctic Ocean, and thus provides a route by which crayfishes might spread from the north over all parts of North America east of the Rocky Mountains.

The so-called Rocky Mountain range is, in reality, an immense table-land, the edges of which are fringed by two principal lines of mountainous elevations. The table-land itself occupies the place of a great north and south depression which, in the cretaceous epoch, was occupied by the sea and probably communicated with the ocean at its northern, as well as at its southern end. During and since this epoch it became gradually filled up, and it now contains an immense thickness of deposits of all ages from the cretaceous to the pliocene—the earlier marine, the later more and more completely freshwater. During the tertiary epoch, various portions of this area have been occupied by vast lakes, the more northern of which doubtless had outlets into the Northern sea. That crayfish existed in the vicinity of the Rocky Mountains {325} in the latter part of the tertiary epoch is testified by the Idaho fossils. And there is thus no difficulty in understanding their presence in the rivers which have now cut their way to the Pacific coast.

The similarity of the crayfish of the Amurland and of Japan is a fact of the same order as the identity of the English crayfish with the Astacus torrentium of the European Continent, and is to be explained in an analogous fashion. For there can be no doubt that the Asiatic continent formerly extended much further to the eastward than it does at present, and included what are now the islands of Japan. Even with this alteration of the geographical conditions, however, it is not easy to see how crayfishes can have got into the Amur-Japanese fresh waters. For a north-eastern prolongation of the Asiatic highlands, which ends to the north in the Stanovoi range, shuts in the Amur basin on the west; while the Amur debouches into the sea of Okhotsk, and the Pacific ocean washes the shores of the Japanese islands.

But there are many grounds for the conclusion that, in the latter half of the tertiary epoch, eastern Asia and North America were connected, and that the chain of the Kurile and Aleutian islands may indicate the position of a great extent of submerged land. In that case, the sea of Okhotsk and Behring’s sea may occupy the site of inland waters which formerly placed the mouth of the Amur in direct communication with the Northern Ocean, just as the Black Sea, at present, brings the basin of the {326} Danube into connection, first with the Mediterranean and then with the western Atlantic; and, as in former times, it gave access from the south to the vast area now drained by the Volga. When the Black Sea communicated with the Aralo-Caspian sea, and this opened to the north into the Arctic sea, a chain of great inland waters must have skirted the eastern frontier of Europe, just such as would now lie on the eastern frontier of Asia if the present coast underwent elevation.