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 fully exposed to the action of lght. 
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 temperature, salinity, 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 
occurs 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 
soon. Now, as has been already suggested, such regions 
