60 LIFE IN THE SEA [CH. 



that is, in the regions of high evaporation due to the 

 trade winds. On each side of the trade-wind belts 

 the salinity falls rapidly towards the north, and 

 less rapidly towards the south. In the Arctic and 

 Antarctic seas the salinity is low because of the 

 melting of ice which contains less salt than sea- water. 

 Here and there we find marked deviations from this 

 general distribution of salinity the Red Sea, for 

 instance is salter than it ought to be because of the 

 great evaporation due to the high temperature ; and 

 the sea off the coasts of Denmark at some parts of 

 the year is fresher than it ought to be because of the 

 influence of the outflowing Baltic fresh current. 



The intensity of sunlight is greatest on the surface 

 of the tropical seas, and least on that of the circum- 

 polar seas. But the amount of solar radiation falling 

 on the surface of the sea is greatly affected by the 

 amount of fog and cloud, and variations due to these 

 causes affect the abundance of plant life at the surface 

 of the sea. 



The physical conditions over the oceans are far 

 more uniform than over the land. Thus the extremes 

 of temperature on the land are 90 C. and 65 C., a 

 range of 155C., while the extremes in the sea are 

 only -2'8 C C. and 31 C., a range of 33'8C. There 

 are no changes in the sea corresponding to changes 

 of humidity, or to rain, hail, snow, etc., on the land. 

 As we ascend the high mountains the temperature 



