GENERAL FEATURES 169 



on land. In addition there are great depressions comparable- to the 

 great basins of the land. 



Apart from these general features, there is little in common be- 

 tween the topography of the sea bottom and that of the land. If 

 the ocean's bed could be seen as the land is, its most impressive 

 feature would be its monotony. The familiar hills and valleys 

 which give the land its most familiar features are essentially absent. 

 A large part of the ocean bottom is so nearly flat that the eye would 

 not detect its departure from planeness. 



The reason for this difference is readily found. The dominant 

 processes which shape the details of the surface of the land are 

 degradational, and though the final result of degradation is flatness 

 (base-level), the earlier result is roughness. In the sea, the domi- 

 nant processes are aggradational, and tend to planeness. 



Distribution of marine life. Marine life has been of such im- 

 portance in the history of the earth that the elementary facts 

 concerning its distribution and the principles which control it are 

 here recalled. Its distribution is influenced by many factors, chief 

 among which are temperature and depth of water. It is more abun- 

 .dant in the warmer parts of the ocean than in the colder, the species 

 inhabiting cold waters are different from those in warm, and few 

 species range through great variations of temperature. Many 

 forms are restricted to shallow water; many others, especially those 

 living near the surface, swim about freely without reference to 

 depth; while a few are restricted to great depths. Some species 

 are influenced by (i) the salinity of the water, which varies con- 

 siderably along coasts where the fresh waters from the land are dis- 

 charged; (2) the character of the sediment at the bottom, some species 

 preferring mud, others sand, etc.; (3) the movement of the water, 

 some species preferring quiet water and others rough water; (4) 

 'he abundance and nature of the food-supply; and (5) the presence 

 or absence of rival and hostile species. 



Subject to exceptions determined by temperature, etc., plant 

 life abounds in the superficial parts of the ocean, and down to the 

 bottom where the depth does not exceed 100 fathoms. Animal 

 life is abundant in shallow water at all depths down to 200 or 300 

 fathoms, and in the surface-waters of temperate and tropical regions 

 regardless of depth. The great body of the ocean-water lying below 

 the depth of a few hundred fathoms has but little life, though 

 animals exists sparingly at the bottom, even where the depth is great, 



