I EMIGRATION TO LAND AND FRESH WATER 13 



rest only down to 3 fath. Under stones close to the shore of 

 the Skargard at Stockholm ^ are found young Cardium and 

 Tellina^ and at 3 to 6 fath. Limnaea peregra^ and Physa 

 fontinalis. Near Gothland Lim^iaea is found in the open sea 

 at 8-12 fath., and with it occur Cardium and Tellina. At 

 the Frisches Haff^ Mya areriaria 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 ]\Iollusca 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 Pelecy- 

 poda, 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 relation- 

 ships with such deep-sea genera as the Volutidae, Caneellariidae^ 

 Terehridae, or even with genera trenching on the lowest part 

 of the littoral zone, such as the ffallotidae, Conidae, OUvidae, 

 Capulidae. The signs of connexion are rather with the Neritidae^ 

 Cerithiidae, and above all the Litto7nnidae, which are accustomed 

 to live for hours, and in the case of Litto7'ina for days or even 

 weeks, without being moistened by the tide. Similarly the 



1 Lindstrom, Oef. K. Vet. Fork. Stockh., 1855, p. 49. 



2 Mendthal, Schr. Ges. Konigsh., xxx. p. 27. 



