44 Routes from the Sea 



been brought about by the lake drying out and becoming more 

 sahne; then with more rains again developing fresh-water forms 

 that come in from rivers and ponds. Certain blind cave fishes have 

 affinities with marine groups and have apparently rem.ained in cave 

 waters as they freshened (Eigenman, 1909; Pearse, 1938) . On the 

 northern border of the Sahara, Gauthier (1927) found a species of 

 shrimp in an isolated spring-fed desert reservoir. He believed this 

 crustacean had been marooned when the course of a river changed. 

 In many small bodies of water which are cut off from the ocean, 

 salinity increases much above that of sea water. In isolated inland 

 pools also evaporation may cause salinity to become very high. In 

 such situations the salts present may differ in character and relative 

 amount from those in the sea. When water evaporates from a salt- 

 containing basin, salts will precipitate out in a certain order; double 

 salts may be formed and separate again when rains dilute the water 

 (Richardson, 1928) . In so-called magnesium sulphate lakes in the 

 Caucasus, Wornichin (1926) describes three characteristic stages: 

 (1) a period when there is little salt and Ruppia is the dominant 

 plant; (2) a freshening of the water and an increase in Vaucheria 

 and other plants; and (3) an increase in salinity and the formation 

 of felt-like growths of plants. In Devil's Lake, North Dakota, live 

 several species of fresh-water rotifers which have become adapted 

 to life in brackish water (about 1% salt) which is unlike the sea in 

 salt content (Bryce, 1925) . Perhaps fresh-water animals are less 

 likely to become adjusted to salt water than are salt-water animals 

 to become adjusted to various dilutions. For example, in the Kaiser 

 Wilhelm Canal, which runs from the North Sea to the East Sea 

 and connects with the River Eider, Brandt (1896) found that 

 animals had entered from either end and become established in 

 waters which varied in salt content from 4.7 to 19.0 parts pro milk, 

 but only a few stonefly and beetle larvae had come in from fresh 

 water. However, paleontological evidence indicates that in the past 



