234 ANIMALS OF THE SEA FLOOR 



in temperature are rather seasonal and gradual than 

 diurnal or rapid, and are not dependent immediately 

 on the alterations of the temperature of the air. The 

 mean temperatures in various regions show, of course, 

 very great differences. 



The salinity, like the temperature, is locally fairly 

 constant, the changes being slow, and the range com- 

 paratively small, though great differences may be 

 noticed between different seas. The light is very 

 much diminished, passing from a dim twilight in the 

 more shallow parts to complete darkness in the deeper. 

 The actual depth to which light can penetrate depends 

 very much on the clearness and freedom from sediment 

 of the water ; but it may be taken that at depths 

 exceeding 100 fathoms practically no light can be re- 

 ceived from above, while at depths considerably less 

 the amount of light is so small as to have little influence 

 on life. 



The absence of light entails the absence of plant life 

 and consequently of vegetable feeders, with the excep- 

 tion of such forms as subsist on microscopic floating 

 vegetable matter. The greater part of the area is 

 too deep to be troubled by wave action, and the 

 force of tides and currents is much diminished. The 

 bottom is fairly uniform over large areas, though the 

 actual types of bottom do not differ materially from 

 those found immediately adjoining the shore, the 

 materials of the sea floor in both cases being derived 

 from the debris of the adjacent land. It is usually the 

 case that the composition of the bottom becomes finer 

 as the depth and the distance from shore increase, but 

 this is by no means a universal rule, gravel, rocks, and 

 stones being often met with even beyond the edge of the 

 Continental Shelf. The water overlying this zone 



