988 PRINCIPLES OF STRATIGRAPHY 



penetrate to the bottom and in the subterranean streams and lakes. 

 The fauna of the Hmno-Httoral district is much less diverse than 

 that of the corresponding district in the sea. Whole classes of ani- 

 mals, like those of the Echinodermata, the Anthozoa, the Brachio- 

 poda, Cephalopoda, Pteropoda, Scaphopoda, etc., are normally ab- 

 sent from fresh water, while most of the remaining ones are poorly 

 represented by genera and species, though often prolific in indi- 

 viduals. Plant life, on the contrary, is abundantly represented, not 

 only by desmids, diatoms, and fresh-water algiie, but also by the 

 large number of swamp plants which grow partly submerged and 

 represent the transition zone between the terrestrial and limno- 

 littoral districts. 



C. Terrestrial. The terrestrio-littoral district is coextensive 

 with the surface of the land. No other life district comprises such 

 a range of physical characteristics, and no other district is inhabi- 

 ted by such a variety of highly specialized types of animal and 

 plant life. Here we pass from the cold of the arctic snow-fields 

 to the burning sands of the tropics, from the land of nearly continu- 

 ous rains to the rainless desert regions, parched by the continued 

 drought of years, and from the region of plentiful food supply to 

 the stony, arid wastes, where the nature of the soil is hostile to 

 practically all forms of plant life. It is to be expected that under 

 such widely varying conditions a fauna and flora should develop 

 which in its variety outruns that of any other life district, and which 

 in its own extremes reflects the range of its environment. 



II. The Pelagic Districts. 



A, Marine. The marine pelagic, or halo-pelagic district, is 

 the common meeting-ground of most of the life districts. It 

 touches all shores and communicates with the corresponding dis- 

 tricts of both the terrestrial and fluvial realms. It has direct com- 

 munication with the littoral district, many inhabitants of which 

 leave the bottom at times to lead a temporary existence in the pela- 

 gic district; while many pelagic animals, in turn, visit the bottom 

 or shores for 'food. Occasionally inhabitants of the pelagic dis- 

 trict enter for a time the corresponding district of the terrestrial 

 realms, i. e., the aerial ; as, for example, the so-called flying-fish ; 

 and, in turn, as already noted, many aerial animals spend a part 

 of their lives in the marme pelagic district, or at least show a de- 

 cided preference for a pelagic life. The passage of land animals 

 to the halo-pelagic district has already been noted. Similar in- 



