322 



BioxoMic Districts. 



For bionouiic purposes, the inhabitable portion of the 

 earth may be divided into a number of life districts; the 

 character and limitation of each being determined by the 

 interrelation of three primary factors, viz : tlie character of 

 the medium in whicli the oi-ganism lives, the degree of illumi- 

 nation, and the absence or presence of a substratum. The 

 medium may be either air or water, and determines the 

 method of respiration of the organism. The medium water 

 is naturally divided into salt water and fresh water, each 

 distinct fi-om the other and containing its own types of 

 organisms, though an intermingling occurs near the junction 

 of the two. 



Thus three great organic realms may be recognized : the 

 marine, or halobiotic ; the fluvial, or limnohiotic ; and the 

 terrestrial, or geohiotie. Each of these realms is again divis- 

 ible into a light and a dark region, the latter being of chief 

 moment in the marine realm, where it is characteristic of the 

 abyssal districts. The deeper parts of great lakes, and per- 

 petually lightless cave districts, represent the dark regions of 

 the fluvial and terrestrial realms respectively. Where light is 

 absent, assimilating plant life is absent; and hence, animals 

 occupving such regions are dependent for food on organic 

 matter brought to them from the liglit regions, where food 

 material is produced through the influence of the sun's rays.* 



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, fluvial, and terrestrial realms. 

 The absence of the substratum compels the organism to float 

 or swim in the medium, and for this purpose special organs 

 and a specially modified body -form commonh' exist. The 

 substratum may be visited for food or other purposes, l)ut 

 the organism is perfectly at home in the medium. 



* Schimper ("98) makes a more precise division with reference to tliedistrilmtiuM of plant 

 life in tlie sea. He distinguishes : first, a light or photic region, where thi' intt^iisitr of the 

 light is sufficient for the normal development of macrophytes ; secondly, a dusk or dus- 

 photic region, where macrophytes exist but scantily or not at all, while certain moderately 

 assimilating microphytes, especially diatoms, still exist ; and, thirdly, a dark or aphotic 

 region, where only the non-assimilating vegetable organisms can exist. The depths at which 

 these regions pass into one another vary with the locality and the purity of the water. 



