323 



The marine realm may, then, be divided into the following 

 life-districts, the medium in all cases being salt water:* 



1. Littoral district : Light; substratum present. 



2. Pelagic district : Light; substratum absent. 



3. Abyssal district : Dark ; substratum present. 



4. Abysso-pelagic district : Dark; substratum absent. 



The littoral district extends from the shore at high-water 

 mark to the edge of the continental shelf, where it quickly 

 merges into the abyssal district. Around oceanic islands 

 and young continents the littoral district is very narrow, the 

 ocean floor soon falling off to deep water. The origin of 

 the littoral district is to be sought in the activities of various 

 geologic agencies, chief among which are the inland cutting 

 of waves and currents, thus extending the sea landward, 

 and the deposition of the land-derived detritus on the edge 

 of the continental shelf, thus pushing this edge seaward and 

 widening the submarine platform. Subsidence of the land 

 permits the advance of the sea over the low country, and 

 broad Epicontinental seas, as Chamberlin has called them, 

 are formed by the "creeping out upon the low parts of the 

 land 'of a film of the sea, as it were. " f 



The conditions become more favorable for the develop- 

 ment of littoral life as the continent grows older, provided, 

 of course, no important oscillatory movements occur. As 

 time progresses, the breadth of the submerged continental 

 shelf increases, — both by landward cutting and by seaward 

 building, — and the surface of the land becomes more and 

 more reduced, thus bringing about a decrease in the amount 

 of detrital material, which is carried into the sea, and a con- 

 comitant increase in the purity of the water. When peneple- 

 nation, or the reduction of the land to base level, has been 

 accomplished, the amount of detritus carried into the sea 

 is practically nil, and organisms, like corals, adapted only 



* Ortmann ('95) makes the terrestrial and fluvial realms each coordinate in rank with 

 the littoral, pelagic, and abyssal districts of the marine realm, treating them as districts. 

 It seems more natural, however, to divide his terrestrial and fluvial " districts "' into districts 

 comparable to the three marine districts, though, of course, the resulting divisions will not 

 always be of uniform quantitative value. 



f Chamberlin, '98, p. 603. 



