1 EMIGRATION TO LAND AND FRESH WATER I 3 



down to 15-20 fatli., the rest only down to 3 fath. Under stones 

 close to the shore of the Skiirgard at Stockholm ^ are found young 

 Cardium and Tellina, and at 3 to 6 fath. Limnaea ^pcregra, and 

 Physa fontincdis. Near Gothland Limnaea is found in the open 

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

 the Frisches Haff" My a arenaria is the only marine species, 

 and lives in company with 6 sp. Limnaea, 1 Fhysa, 9 Flanorhis, 

 1 Ancylus, 4 Valvata, 2 Spliaerium. 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, Terehridae, or even with genera 

 trenching on the lowest part of the littoral zone, such as the 

 Haliotidae, Conidae; Olividae, Ca'pulidae. 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 re- 



^ LiudstiiJm, Oef. K. Vet. Fork. H^itockh., 1855, p. 49, 

 - Mcudthal, Schr. Ges. Konigsh., xxx. p. 27. 



