CHAPTER XXVI. 



PHYSICAL CHARACTERISTICS OF THE INHABITABLE EARTH.— 

 BIONOMIC CHARACTERS OF THE ENVIRONMENT.* 



A comprehensive survey of the inhabitable world shows its 

 divisibility into three great bionomic realms, each of which has its 

 peculiar characteristics and its specially adapted forms of animal 

 and plant life. The essential character of each is determined by the 

 nature of the medium in which the organisms live, i. e., whether salt 

 water, fresh water, or air. We have, accordingly: 



1. The Marine, or Halo-biotic realm. 



2. The Fresh Water, or Limno-biotic realm. 



3. The Terrestrial, or Atmo-hiotic (also called Geo-hiotic) 



realm. 



While each of these is distinct from the others, they also grade 

 into one another to a greater or less extent along their lines of in- 

 tersection. Thus the marine and fresh-water realms intergrade 

 in the estuaries of rivers, where the water is brackish. The marine 

 and terrestrial realms intergrade at the shore, Avhere organisms 

 are periodically exposed between tides first to the one and then 

 to the other medium. Again, the terrestrial and fresh-Vv^ater flu- 

 vial, lacustrine, etc., realms intergrade along the margins of streams 

 and the shores of lakes. 



Each one of the bionomic realms may be further subdivided 

 into a light, or photic region, and a dark, or aphotic region. Be- 

 tween these two may frequently be determined a dusk, or dysphoHc 

 region. Schimper (6) has defined these regions in the marine 

 realm on the basis of plant life as follows : 



The photic region extends over such depths wherein the in- 

 tensity of sunlight is sufficient for the normal development of 

 macrophytes. The dysphotic region is insufficiently lighted for the 



* This and the succeeding two chapters are in part a reprint, with additions 

 and emendations, of a paper pubHshed by the author in 1899 under the title 

 Relation of Marine Bionomy to Stratigraphy. Bulletin of Buffalo Society Natural 

 Sciences, Vol. VI, pp. 319-365. 



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