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 {328} applies 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. [39] See on this interesting subject: Martens, “On the occurrence of marine animal forms in fresh water.” Annals of Natural History, 1858: Lovèn. “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. Several species of prawns (_Palæmon_) 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 (_Palæmon_) 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.