This passage from a marine to a non-marine life—in other words, this direct derivation of non-marine from marine genera—is illustrated by the faunal phenomena of an inland brackish-water sea like the Caspian, which is known to have been originally in connexion with the Mediterranean, and therefore originally supported a marine fauna. The Mollusca of the Caspian, although without exception brackish- or fresh-water species, are in their general facies distinctly marine. Of the 26 univalve species which inhabit it 19 belong to 4 peculiar genera (Micromelania, Caspia, Clessinia, Nematurella), all of which are modified forms of the marine Rissoidae. The characteristic bivalves belong to the genera Adacna, Didacna, and Monodacna, all of which can be shown to be derived from the common Cardium edule. We have here a case where complete isolation from the sea, combined no doubt with a gradual freshening of the water, has resulted in the development of a number of new genera. The singularly marine facies of several of the fresh-water genera now inhabiting Lake Tanganyika, has given rise to the belief, among some authorities, that that lake was at one time an inlet of the Indian Ocean. In the upper waters of the Baltic, marine and fresh-water Mollusca flourish side by side. So complete is the intermixture, that an observer who had lived on no other shores would probably be unable to separate the one set of species from the other.[18] Thus between Dagö and Papen-Wiek[19] Mytilus edulis, Cardium edule, Tellina balthica, Mya arenaria, Littorina rudis, and Hydrobia balthica are the only true marine species; with these live Unio, Cyclas, Neritina, Limnaea, and Bithynia. The marine species and Neritina live down to 15–20 fath., the rest only down to 3 fath. Under stones close to the shore of the Skärgård at Stockholm[20] are found young Cardium and Tellina, and at 3 to 6 fath. Limnaea peregra, and Physa fontinalis. Near Gothland Limnaea is found in the open sea at 8–12 fath., and with it occur Cardium and Tellina. At the Frisches Haff[21] Mya arenaria is the only marine species, and lives in company with 6 sp. Limnaea, 1 Physa, 9 Planorbis, 1 Ancylus, 4 Valvata, 2 Sphaerium. Were the Sound to become closed, and the waters of the Baltic perfectly fresh, it would be inevitable that Mya arenaria, and such other marine species as continued to live under their changed conditions, should in course of time submit to modifications similar in kind to those experienced by the quondam marine species of the Caspian.
It seems probable, however, that the origin, at least in a great part, of the land and fresh-water Mollusca need not be accounted for by such involuntary changes of environment as the enclosure of arms of the sea, or the possible drying up of inland lakes. These cases may be taken as illustrations of the much more gradual processes of nature by which the land and fresh-water fauna must have been developed. The ancestry of that fauna must be looked for, as far as the Gasteropoda are concerned, in the littoral and estuarine species; for the Pelecypoda, in the estuarine alone. The effect of the recess of the tide, in the one case, and the effect of the reduced percentage of salt, in the other, has tended to produce a gradual adaptation to new surroundings, an adaptation which becomes more and more perfect. It may be safely asserted that no marine species could pass into a land or fresh-water species except after a period, more or less prolonged, of littoral or estuarine existence. Thus we find no land or fresh-water species exhibiting relationships with such deep-sea genera as the Volutidae, Cancellariidae, Terebridae, or even with genera trenching on the lowest part of the littoral zone, such as the Haliotidae, Conidae, Olividae, Capulidae. The signs of connexion are rather with the Neritidae, Cerithiidae, and above all the Littorinidae, which are accustomed to live for hours, and in the case of Littorina for days or even weeks, without being moistened by the tide. Similarly the fresh-water Pelecypoda exhibit relationships, not with genera exclusively marine, but with genera known to inhabit estuaries, such as the Mytilidae, Corbulidae, Cardiidae.
It would be natural to expect that we should find this process of conversion still going on, and that we should be able to detect particular species or groups of species in process of emigration from sea to land, or from sea to fresh water. Such species will be intermediate between a marine and a land or fresh-water species, and difficult to classify distinctly as one or the other. Cases of Mollusca occupying this intermediate position occur all over the world. They inhabit brackish swamps, damp places at high-water mark, and rocks only at intervals visited by the tide. Such are Potamides, Assiminea, Siphonaria, Melampus, Hydrobia, Truncatella, among the univalves, and many species of Cyrena and Arca among the bivalves.
Origin of the Fresh-water Fauna
(a) Pelecypoda.—Estuarine species, which have become accustomed to a certain admixture of fresh water, have gradually ascended the streams or been cut off from the sea, and have at last become habituated to water which is perfectly fresh.
Fig. 8.—A, The common Mytilus edulis L., a marine genus and species. B, Dreissensia, a fresh-water genus, closely allied to Mytilus.
Fig. 9.—A, Arca navicella Reeve, Philippines, a marine species. B, Arca (Scaphula) pinna Bens., R. Tenasserim, a fresh-water species which lives many miles above the tide-way.
Thus Dreissensia (rivers and canals throughout N. Europe and N. America) and Mytilopsis (rivers of America) are scarcely modified Mytili (Fig. [8]); Scaphula is a modified Arca, and lives in the Ganges, the Jumna, and the Tenasserim at a distance of 1600 miles from the sea (Fig. [9]). Pholas rivicola is found imbedded in floating wood on the R. Pantai many miles from its mouth. Cyrena, Corbicula, and probably Sphaerium and Pisidium are derived, in different degrees of removal, from the exclusively marine Veneridae; Potamomya (rivers of S. America), and Himella (R. Amazon) are forms of Corbula. The Caspian genera derived from Cardium (Adacna, Didacna, Monodacna), have already been referred to. Nausitora is a form of Teredo, which lives in fresh water in Bengal. Rangia, Fischeria, and Galatea probably share the derivation of the Cyrenidae, while in Iphigenia we have one of the Donacidae which has not yet mounted rivers, but is confined to a strictly estuarine life. The familiar Scrobicularia piperata of our own estuaries is a Tellina, which lives by preference in brackish water.