86 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



streams. 48 Ponds and lakes in general have a short life; a number of 

 factors combine to destroy them. Rivers and brooks fill the lakes in 

 mountainous regions with detritus. Lake Constance is constantly being 

 filled by the Rhine, and it is estimated that this process will be com- 

 pleted in 12,500 years; the life of Lake Geneva, Switzerland, into 

 which the Rhone brings an annual sediment of 2,000,000 cu.m., is 

 reckoned at 45,000 years, and that of Lake Lucerne is placed at 

 23,000 years. Shallower ponds and ox-bows, whose banks are covered 

 with vegetation, gradually disappear from the encroachment of the 

 vegetation; plant remains raise the level of the bottom, marsh and 

 bog plants grow out farther and farther toward the middle and limit 

 the open water until it is finally entirely gone. The process of extinc- 

 tion proceeds rapidly in a senescent lake when once a bog margin and 

 false bottom have been established. 49 



Many lakes and rivers fall victims to drought, and as they dry 

 up, their faunae are destroyed. During a part of the Quaternary, the 

 desert valleys of the Great Basin in North America were lake basins 

 of vast extent, known to geologists as Lake Bonneville and Lake La- 

 hontan. The surface of the latter at its maximum was 167 meters 

 above that of the present Pyramid Lake. 50 In recent times, geologi- 

 cally speaking, the greater part of the Waterberg District of the 

 Transvaal was covered by a large lake. 51 Lake Ngami in Bechuana- 

 land was a real lake only 50 years ago; it is now a marsh in process 

 of disappearing. Many Siberian lakes are in process of drying up; 

 the Sary Kupa, for example, at 50° N. latitude, formerly an elliptic 

 basin with a longer diameter of more than 100 km., is now broken 

 up into 20 considerable lakes. 52 With the disappearance of such waters, 

 their faunae must be extinguished, at least such elements as were 

 effectively isolated and hence on the way toward changing to new 

 species. The frequent intercommunications between river systems, in- 

 cluding the well-known phenomenon of stream piracy, have also oper- 

 ated to break down the isolation of their faunae and to further the 

 general phenomenon of wide distribution in fresh-water animals. 



There are, however, a number of large fresh-water basins in which 

 the faunae differ strikingly from the usual fresh-water aspect, and 

 exhibit the results of long-continued isolation. They have numerous 

 endemic species and genera, and their genera are strongly differenti- 

 ated into species adapted to the various habitats available. All these 

 basins are characterized by large size and considerable depth. The 

 principal ones are Lake Baikal (35,000 sq. km., 1706 m. deep) ; 53 the 

 three central lakes of Celebes (Posso, 420 sq. km., 440 m. deep, 

 Towoeti, 160 sq. km., 203 m. deep, and Matano 800 sq. km. and 590 



