170 THE DISTRIBUTION OF 



The protozoa are as well represented on the shore as 

 in other parts of the sea, but present no special adapta- 

 tions to life here. 



(2) We come next to the pelagic region, where there 

 is no substratum, but where many at least of the 

 organisms are fuUy exposed to the action of light. 

 With regard to the movements of the water here, we 

 must notice first that, in contrast to the littoral area, 

 tides are of little importance. On the other hand, 

 currents are very important. As is well known, 

 the difference in the amount of heat which streams 

 down from the sun upon the surface of the sea gives 

 rise to differences in temperature and density, which 

 cause the surface waters to move in great whirls, the 

 ocean currents. As the surface in the different latitudes 

 is exposed to different types of winds, and therefore to 

 differential evaporation, we have also differences of 

 salinity. Roughly speaking, the salinity is greatest in 

 warm regions exposed to the drying trades, e. g. in the 

 Red Sea and in the Mediterranean, and least towards 

 the poles. In not a few parts of the globe we find that 

 currents bringing cold water of relatively low salinity 

 meet warm currents of highly saline water. When this 

 occurs the warmer water floats on the surface of the 

 colder, and there is a junction layer where a sudden 

 change in temperatiu"e, sahnity, and specific gravity 

 occurs. Such a meeting of warm and cold currents is 

 found, for instance, in the Norwegian Sea, where the 

 Polar drift meets the warm currents of that sea ; it 

 occTirs also off the eastern coast of North America, 

 where the Labrador current meets the Gulf Stream ; 

 it occurs off the Cape of Good Hope, where the cold 

 Benguela current meets the warm Agulhas one, and 

 so on. Now, as has been already suggested, such regions 



