174 THE FITNESS OF THE ENVIRONMENT 



of the Atlantic is 3.2 per cent to 3.3 per cent. 

 Areas where the concentration ranges from 

 3.3 per cent to 3.4 per cent are still more 

 extensive. The greater part of the North 

 Atlantic ranges in concentration from 3.5 

 per cent to 3.6 per cent. In general there is a 

 region of maximum concentration between the 

 Equator and the Pole. 



The Mediterranean, the Red Sea, and other 

 similar bodies of water possess a somewhat 

 higher salt concentration, dependent upon 

 excessive evaporation and the absence of 

 great currents ; but only in exceptional cases 

 and small isolated bodies of water does the 

 concentration rise above 4.1 per cent. 



The salinity of ocean water varies also with 

 the depth. In seas where there is a great 

 influx of fresh water the surface is less con- 

 centrated than the depths; in seas where 

 there is much evaporation the surface is more 

 concentrated than the depths. In the latter 

 case the higher temperature of the surface 

 causes expansion of the more concentrated 

 water, and enables it to remain above. When 

 these two influences of dilution and evapora- 

 tion are combined, they may bring about a 

 yearly variation of salinity. Such variations 

 of the environment are important to animal 

 life, slight though they may be. Thus the 



