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.

Supposing, however, that the ancestral forms of the Potamobiidae obtained access to the river basins in which they are now found, from the north, the hypothesis that a mass of fresh water once occupied a great part of the region which is now Siberia and the Arctic Ocean, would be hardly tenable, and it is, in fact, wholly unnecessary for our present purpose.

But

The vast majority of the stalk-eyed crustaceans are, and always have been, exclusively marine animals; the crayfishes, the Atyide, and the fluviatile crabs (Thelphusida), being the only considerable groups among them which habitually confine themselves to fresh waters. even in such a genus as Penæus, most of the species of which are exclusively marine, some, such as Penaus brasiliensis, ascend rivers for long distances. Moreover, there are cases in which it cannot be doubted that the descendants of marine Crustacea have gradually accustomed themselves to fresh water conditions, and have, at the same time, become more or less modified,

CONVERSION OF MARINE INTO FRESHWATER ANIMALS. 327

so that they are no longer absolutely identical with those descendants of their ancestors which have continued to live in the sea.*

In several of the lakes of Norway, Sweden and Finland, and in Lake Ladoga, in Northern Europe; in Lake Superior and Lake Michigan, in North America; a small crustacean, Mysis relicta, occurs in such abundance as to furnish a great part of the supply of food to the fresh water fishes which inhabit these lakes. Now, this Mysis relicta is hardly distinguishable from the Mysis oculata which inhabits the Arctic seas, and is certainly nothing but a slight variety of that species.

In the case of the lakes of Norway and Sweden, there is independent evidence that they formerly communicated with the Baltic, and were, in fact, fiords or arms of the sea. The communication of these fiords with the sea having been gradually cut off, the marine animals they contained have been imprisoned; and as the water has been slowly changed from salt to fresh by the drainage of the surrounding land, only those which were able to withstand the altered conditions have survived. Among these is the Mysis oculata, which has in the meanwhile undergone the slight variation which has converted it into Mysis relicta. Whether the same explanation ap

* See on this interesting subject: Martens, "On the occurrence of marine animal forms in fresh water." Annals of Natural History, 1858: Loven. "Ueber einige im Wetter und Wener See gefundene Crustaceen." Halle Zeitschrift für die Gesammten Wissenschaften, xix., 1862: G. O. Sars, "Histoire Naturelle des Crustacés d'eau douce de Norvège," 1867.

plies to Lakes Superior and Michigan, or whether the Mysis oculata has not passed into these masses of fresh water by channels of communication with the Arctic Ocean which no longer exist, is a secondary question. The fact remains that Mysis relicta is a primitively marine animal which has become completely adapted to fresh-water life.

Several species of prawns (Palamon) abound in our own seas. Other marine prawns are found on the coasts of North America, in the Mediterranean, in the South Atlantic and Indian Oceans, and in the Pacific as far south as New Zealand. But species of the same genus (Palamon) are met with, living altogether in fresh water, in Lake Erie, in the rivers of Florida, in the Ohio, in the rivers of the Gulf of Mexico, of the West India Islands and of eastern South America, as far as southern Brazil, if not further; in those of Chili and those of Costa Rica in western South America; in the Upper Nile, in West Africa, in Natal, in the Islands of Johanna, Mauritius, and Bourbon, in the Ganges, in the Molucca and Philippine Islands, and probably elsewhere.

Many of these fluviatile prawns differ from the marine species not only in their great size (some attaining a foot or more in length), but still more remarkably in the vast development of the fifth pair of thoracic appendages. These are always larger than the slender fourth pair (which answer to the forceps of the crayfishes); and, in the miles especially, they are very long and strong, and

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are terminated by great chelæ, not unlike those of the crayfishes. Hence these fluviatile prawns (known in many places by the name of "Cammarons ") are not unfrequently confounded with true crayfishes; though

the fact that there are only three pair of ordinary legs behind the largest, forceps-like pair, is sufficient at once to distinguish them from any of the Astacide.

Species of these large clawed prawns live in the

brackish water lagoons of the Gulf of Mexico, but I am not aware that any of them have yet been met with in the sea itself. The Palamon lacustris (Anchistia migratoria, Heller) abounds in fresh-water ditches and canals between Padua and Venice, and in the Lago di Garda, as well as in the brooks of Dalmatia; but its occurrence in the Adriatic or the Mediterranean, which has been asserted, appears to be doubtful. So the Nile prawn, though very similar to some Mediterranean prawns, does not seem to be identical with any at present known.*

In all these cases, it appears reasonable to apply the analogy of the Mysis relicta, and to suppose that the fluviatile prawns are simply the result of the adaptive modification of species which, like their congeners, were primitively marine.

But if the existing sea prawns were to die out, or to be beaten in the struggle for existence, we should have, scattered over the world in isolated river basins, more or less distinct species of freshwater prawns,† the areas inhabited by which might hereafter be indefinitely enlarged or diminished, by alteration in the elevation of the

* Heller, "Die Crustaceen des südlichen Europas," p. 259. Klunzinger, "Ueber eine Süsswasser-crustacee im Nil," with the notes by von Martens and von Siebold: Zeitschrift für Wissenschaftliche Zoologie, 1866.

This seems actually to have happened in the case of the widely. spread allies and companions of the fluviatile prawns, Atya and Caridina. I am not aware that truly marine species of these genera are known.