Dilution and Salation 43 



the ocean and persisted in bodies of water that gradually became 

 fresher, so that they finally became fresh water animals. In the 

 deep lakes in the northern United States (Hoy, 1873), Canada 

 (Adamstone, 1924), and Scandinavia (Ekman, 1920, 1930) there 

 are shrimps (Mysis) , amphipods (Pontoporeia) , and other animals 

 which belong to genera and families which still exist in the ocean 

 and are looked upon as representative marine animals. Ponds and 

 pools in sandy beaches which have been cut off from the sea remain 

 fresher than the neighboring marine areas, even though they are 

 subject to tidal influences by percolation of water through the sand. 

 Such ponds are inhabited by marine burrowing animals (Calian- 

 assa) , and such marine fishes as mullets and needle-fishes may 

 persist, but, as the ponds grow fresher, they are soon invaded by 

 progressive and aggressive types of insects, such as various Diptera, 

 Coleoptera, Hemiptera, and Odonata (Pearse, 1932) . On the 

 beaches in Puget Sound, Miles (1920) observed three species of 

 blennies that lived near low tide mark. He believed they chose 

 this habitat because oxygen was more abundant there. Mytilus 

 eggs, sperms, and larvae do not survive in salinities that are slightly 

 below those of sea water, and these clams are absent from quiet 

 waters (Young, 1941). A beach snail in Florida (Thais floridana) 

 became immobile and died in low salinities (Schechter, 1943). 



It has been claimed that some lakes, like Tanganyika, have a 

 "marine" fauna (Moore, 1903; Germain, 1913), but "recent dis- 

 coveries do not favor Moore's hypothesis of a marine Jurassic origin 

 for Tanganyika . . . there is no support for the view that the ocean 

 at one time extended over the Congo basin. . . . The view that 

 Tanganyika owes its remarkable organisms to a prolonged period 

 of isolation is regarded as the most likely suggestion" (Cunning- 

 ton, 1920). 



The moUusks in the lake, however, show both marine and fresh- 

 water affinities. Schweitz dC Darteville (1948) believe this may have 



