ENVIRONMENTAL CLASSIFICATION 37 



ment. The relations are still more evident among the decapods. In the 

 islands of the Indian Ocean 12 only four out of twenty genera are con- 

 fined to fresh water; Caridina and Palaemon live principally in fresh 

 water, but have species which occur in the sea and in brackish water, 

 Palaemon carcinus even in all three. Finally, many marine genera* 

 have single species in fresh water. Palaemonetes vulgaris is the com- 

 mon prawn of the Atlantic coast from Massachusetts to Florida, 13 

 while P. paludosa occurs in fresh-water streams and lakes as far west 

 as the Chicago area. 14 Among fishes in the Indian Ocean, of twenty- 

 two species of selachians occurring in fresh water, only seven are 

 confined to it; the rest live in the ocean as well. 15 A species of bass, 

 Lates calcarifer, in southeast Asia, lives in fresh, brackish, and salt 

 water. 16 A variety of the shad, Alosa finta, ascending the European 

 streams to spawn, has established itself in the Lake of Lugano. Of 

 the marine genera Syngnathus, Blennius, Belone, and others, single 

 species are confined to fresh water. 



The immigration of marine animals into fresh water continues in 

 recent time primarily in the tropics. Of the fresh-water selachians, for 

 example, no species is found beyond latitude 35°N. or S., and only a 

 few beyond 30°. 15 The fresh waters about the Gulf of Bengal, the 

 Islands of the Malay Archipelago, Madagascar, and tropical America 

 are rich in new immigrants from the surrounding seas. The small 

 variation in temperature of tropical streams probably facilitates the 

 entrance of marine forms. It is also possible that the tremendous 

 rainstorms, which are frequent at certain places and seasons in the 

 tropics, by reducing the salt content of the surface waters, help to 

 accustom marine animals to a lower salinity. Temperature may have 

 something to do with the varying behavior of Alosa finta. In Scandi- 

 navia this species spawns among reefs in the sea ; from the North Sea 

 on it occasionally ascends streams to spawn; and about the Adriatic 

 it has become in part permanently at home in fresh waters, on ac- 

 count of its spawning migrations. The presence of large amounts of 

 calcium, characteristic of so-called hard waters, makes the transition 

 to fresh water less difficult. 17, 18 



Another process which leads to the production of fresh-water forms 

 is the freshening of arms of the sea which become cut off from the 

 ocean. This may occur through changes in the strand line, as in the 

 lakes of Finland and south Sweden, which have been separated from 

 the sea by the rise of the land, or in the lakes of the south Russian 

 steppe, which were left behind by the lowering of the surface of the 

 former Sarmatian Sea. Separation of arms of the sea by tongues of 



* Pseudograpsus, Leander, Penaeus. 



