328 GEOLOGY. 



part, absent, though certain islands, hke Cuba and some of its associates, 

 may be regarded as the crests of systems which are chiefly submerged. 

 If the water were drawn off from the ocean's bed so that it could be seen 

 as the land is, its most impressive feature would be its monotony. The 

 famihar hills and valleys which, in all their multitudinous forms, give 

 the land surface its most characteristic features are essentially absent. 

 A large part of its surface would be found to be so nearly flat that the 

 eye would not detect its departure from planeness. 



The reason for this profound difference is readily found. On the 

 land, the dominant processes which shape the details of the surface 

 are degradational, and though the final result of degradation is flatness 

 (base-level), the immediate result is relief, and, most commonly, relief 

 of the hill-and- valley type. In the sea, the dominant processes are 

 aggradational, and tend to monotonous planeness. 



Distribution of marine life. — Marine hfe has been of such impor- 

 tance in the history of the earth that the elem.entary facts concerning its 

 distribution and the principles which control it are here recalled. The 

 distribution of marine hfe is influenced by many factors, chief among 

 which are temperature and depth of water. Not only is hfe more abun- 

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

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

 range through great variations. Many forms of life are restricted to 

 shallow water. Many more, especially those which do not live on the 

 bottom, swim about freely without reference to the depth of the water 

 beneath them, while relatively few are restricted to great depths. 

 Many species are also influenced by the salinity of the water, which varies 

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

 charged; by the character of the sediment at the bottom, some species 

 preferring mud, others sand, and others gravel; by the movement of the 

 waters, some species preferring still waters and others rough; and some 

 species by the abundance and nature of the food-supply, and by rival and 

 hostile species. 



Subject to the exceptions determined by temperature, etc., plant 

 life abounds in shallow water out to depths of 100 fathoms or so, and 

 is found in abundance at the surface where the depth is much greater. 

 Animal life abounds in shallow water, both at the bottom and above it, 

 out to depths of 200 or 300 fathoms, and occurs in great profusion in the 

 surface-waters of temperate and tropical regions without regard to the 



