in] THE FACTORS OF DISTRIBUTION 59 



regular decrease of temperature is modified by the 

 influence of the land and by ocean currents : thus the 

 temperature of the Red Sea, and of the Atlantic Ocean 

 to the west of Ireland are higher than they ought to 

 be if the latitude were the only factor, because of 

 the influence of the land in the first case, and that of 

 the Gulf Stream in the second. The temperature 

 of the Atlantic off the coast of North America is lower 

 than it ought to be because of the influence of the 

 Labrador Current. The temperature of the sea is 

 very generally highest at the surface and then falls 

 towards the bottom, quickly at first and then more 

 slowly. Towards the bottom in very deep seas it may 

 rise a very little, probably because of the generation 

 of heat from radio-active substances in the bottom 

 deposits, but it is a general rule that almost every- 

 where the temperature at the ocean bottom lies 

 between 2 C. and zero, and in places it may fall 

 below the freezing point of fresh water. Occasionally 

 the deeper or the intermediate layers may be warmer 

 than those of the surface, but these conditions are 

 anomalous and are to be traced to the action of 

 currents. 



The salinity of the sea is rather low along a belt 

 of ocean between the equator and 5 N., that is in the 

 region of tropical calms and torrential rainfalls. It 

 is highest along two belts of ocean situated between 

 25 N. and 30 N.; and between 20 S. and 25 S., 



