CLASSIFICATION OF LIFE DISTRICTS 983 



normal development of macrophytes, which exist there but scantily 

 or not at all, while certain moderately assimilating microphytes, 

 especially diatoms, still exist. The aphotic region comprises that 

 part of the sea where only the non-assimilating vegetable organisms 

 can exist. The depth at which these regions pass into one another 

 varies with the locality and the purity of the water. In the open 

 sea the dark region begins at a depth of about 200 fathoms. In 

 the fresh water realm the aphotic region is not often found, though 

 deep lakes, like Lake Superior and Lake Geneva, extend to depths 

 not penetrated by sunlight. Such cases are, however, comparatively 

 uncommon. A limno-aphotic region is further found in the streams 

 and lakes of caves where the sunlight is shut out by the roof 

 of the cavern. Caverns, too (natural and artificial), constitute the 

 only aphotic region of the terrestrial realm, and they are often 

 peopled with a well-adapted fauna and flora. 



The final division into life districts depends on the absence or 

 presence of a substratum, and this division can be equally well 

 carried out in the marine, fresh water, and terrestrial realms. The 

 absence of a substratum compels the organism to float or swim in 

 the medium, and for this purpose special organs and a specially 

 modified body- form commonly exist. The substratum may be 

 visited for food or other purposes, but the organism is perfectly 

 at home in the medium. 



Each realm may, then, be divided into the following life-districts, 

 the medium being salt water, fresh water, or air, according as the 

 marine, fluvio-Iacustrine, or terrestrial realms are considered : 



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, 



I. The Littoral Districts. 



A. Marine. The marine 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. The depth to 

 which the littoral district extends is approximately 200 meters, 

 though the hundred-fathom line is commonly taken as the seaward 

 boundary of this district. Around oceanic islands and young con- 

 tinents the littoral district is very narrow, the ocean floor soon 

 falling ofif to deep water. The origin of this district is to be 

 sought in the activities of various geologic agencies, chief among 



