ENVIRONMENTAL SELECTION 25 



The reduction in total number of species with depth is shown as 

 follows in the Challenger collection: 5 



A similar impoverishment is caused by any sort of deviation from 

 the optimum. Variation in the salt content of sea water has an espe- 

 cially important influence on the life of marine animals. If this varia- 

 tion is large, great demands are made on the adaptability of the 

 organisms concerned. They must be protected from osmotic changes 

 in the salinity of their body fluids, either by an impenetrable surface 

 or by the ability to eliminate excessive amounts of salt or water 

 through their excretory organs. This form of selection is nowhere 

 better illustrated than in salt pits, in which sea water, evaporated in 

 successive shallow basins, is concentrated for the production of salt. 

 In the salt pits at Bourg de Batz (Loire Inferieur), the channels 

 which conduct the salt water to the pits contain 2 nemertines, 4 

 lamellibranchs, 9 snails, 6 annelids, 1 crab, and 9 amphipods and 

 isopods; the first evaporation basins contain 3 turbellarians, 3 lamelli- 

 branchs, 7 snails, 6 annelids, 1 crab, 1 shrimp, and 7 amphipods and 

 isopods; the second series of tanks (7-8° Baume sp. gr.) contain 1 

 turbellarian, 2 annelids, 1 crab, 1 isopod, and 1 amphipod; in the next 

 series of evaporation basins (17-18°B.), only the turbellarian and one 

 annelid remain; finally, in the salt beds (up to 27°B.) there is only 

 the salt-tolerant crustacean, Artemia salina. The number of species 

 in the fauna is regularly decreased with increasing concentration by 

 the continued selection of euryhaline forms. 6 



The complete elimination of a species is frequently preceded by 

 the production of stunted forms, as in Cardium edule, Macrostoma 

 hystrix, and Nereis diversicolor. Even Artemia, which is especially 

 adapted to waters of high salinity, becomes stunted or defective as the 

 salt content approaches limiting values. Similar phenomena may be 

 observed wherever the salinity of waters increases, as in salt marshes 

 and in regions without outlet to the sea. 



Parallel phenomena appear with decrease of the salinity of sea 

 water. The Baltic offers an excellent example. The salinity decreases 

 from 3% at the Kattegat to 0.8% in the Belt, and sinks to 0.4 and 

 0.3% in the Gulf of Bothnia (cf. map at the close of Chapter XV). 

 This change has a selective and transforming effect upon the fauna. 



