ENVIRONMENTAL FACTORS 291 



larly Tubella pennsylvanica shows decided variations correlated with 

 the degree of mineralization of the water; in some cases these abrogate 

 accepted generic criteria. 6 



Other inorganic compounds are also dissolved in great quantities 

 in many inland waters. Borax, for instance, is present in solution in 

 the Puga creek in Ladakh (Kashmir). Crustacea and fishes are never- 

 theless found there. 7 Sarasin found a lake in New Caledonia whose 

 waters are very rich in iron in which the animal life was inhibited by 

 this factor. Crustaceans, snails, and fishes were diminutive. Sulphur- 

 etted hydrogen, which is also occasionally found in inland waters, is 

 just as deadly to animal life there as it is in landlocked arms of the 

 ocean; some Swiss lakes are unfit for animal life at a depth of 13 m. 

 or more because of their hydrogen-sulphuretted water. The surface 

 animals which descend into the depths periodically, or whose winter 

 eggs sink to the bottom, are at a disadvantage. 



Organic content. — Still other inland waters, rivers as well as 

 lakes, are characterized by their content of organic matter. Moor and 

 bog waters and the lakes fed by streams from such sources, and the 

 waters in rivers which flow through extensive swampy virgin forests, 

 are especially rich in humus, which gives them a dark, blackish brown 

 color. The waters of the Dismal Swamp in eastern North America 

 afford an example, and the multiplication of the name "Rio Negro" 

 on the map of South America is due to this phenomenon. This humus 

 content renders such waters uninhabitable for many animals. The 

 water of the Obi River as it flows through the plains takes up so 

 much dirty steppe and morass water that it becomes stagnant and 

 foul, especially when frozen over, so that certain migrating salmon 

 which are found east and west of the Obi do not enter this river, and 

 the regional salmon (Salmo coregonoidcs) is not found in the main 

 stream but only in the mountain tributaries. 



A contamination of inland waters, especially through organic mat- 

 ter, often occurs near the settlements of man. The degree of contamina- 

 tion can well be estimated by the composition of the fauna. Through 

 the disintegration of highly complex organic compounds, the oxygen 

 content is reduced and the sulphuretted hydrogen, iron sulphide, 

 and carbonic acid content increased. In such waters only a few species 

 of animals thrive. These are the saprocolic organisms, mostly proto- 

 zoans, especially flagellates; of Metazoa only the slimeworm Tubifex 

 and the larvae of the drone fly Eristalis are generally present, both in 

 great numbers. Farther down from the origin of contamination, where 

 the albuminous substances have begun to be transformed into amino 



