322 



Bionomic Districts. 



For bionomic 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: the character of 

 the medium in which the organism lives, the degree of illumi- 

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

 medium may be cither 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 from 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 Hmnobiotic ; and the 

 terrestrial, or geohiotio. 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 

 occupying such regions are dependent for food on organic 

 matter brought to them from the light 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 absenceof the 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. 



*Schimper 1 '98) makes a more precise division with reference to the distribution of plant 

 life in the sea. He distinguishes : first, a light or photic region, where the intensity of the 

 light insufficient for tin- normal development of macrophytes; secondly, a dusk or dys- 

 photic region, where macrophytes exist hut 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 re jions piss into one an »ther vary with the locality and the purity of the water. 





