FAUNA OF THE AFRICAN LAKES. 



515 



which bears on the matter, ()iit the coiichisions nrrived at may 

 be summarised as follows : — Since the area drained is not very 

 great, and the lainf'all is relatively low, the amount of water 

 which finds its way into the lake is nearly balanced by the evapor- 

 ation (cf. 190, ^ol. ii. p. (58 ct seq.). It follows that under 

 avei-a.ge conditions there will be little change in the smface- 

 level and no occasion for an outlet for the suri)lus water. There 

 are reasons for believing that at some former period the waters 

 of Tanganyika did not flow out at all, and were consequently 

 more saline than at present. As a result of the formation of 

 great volcanic cones north of Lake Kivu, that portion of the 

 Nile drainage system which once drained north into Edward 

 Nyanza l)ecame cut off, and its waters linally di\erted south- 

 wards into Tanganyika {cf. 137, p. 89 et seq.). 



Slowly raised in level by this additional supply, the water 

 eventually found an outlet to the west, along the course of the 

 Lukuga,, a former alHuent, and through a, sandstone ridge 

 denuded in part by the action of a, tributary of the Congo. The 

 lake having risen abnormally before the establishment of an 

 outlet, sank at first rapidly, remaining at a, lowei' level, subject 

 to fluctuations with the time of year and the amount of rainfall. 

 A few consecutive seasons of unusual drought would suffice to 

 reduce the level of the lake to a point at which all outHow woidd 

 cease. It seems likely that under such circumstances a barrier 

 might be formed in the bed of the effluent, due to the combined 

 effects of wind-blown sand and ti'opical swamp vegetation, and 

 va,rying in nature with the length of time taken to produce it. 

 If a tolerably firm dam was formed, a considerable rise in the 

 level of the lake would be necessary in order to develop enough 

 power to sweep awa v the obstruction. There is reason to believe 

 that the high water level which Cameron and Stanley observed, 

 and the subse(pient i-e-establishment of an outflow, were phases in 

 a cycle of this nature. If a periodicity of this kind really exists, 

 it must of necessity be extremely variable in nature, and due to 

 irregidarities in the rainfall. 



The important bearing of these inferences on the pioblem of 

 the fauna of Tanganyika it is hardly necessary to emphasise. 

 If the Nile has lost a not inconsideiable draiiiage aiea, which 

 has ])een added to that of Tanganyika, it seems very reasonable 

 to suppose that this accession of water produced, for the first 

 time, the overflow of the lake. If this supposition be cori'ect, 

 Tanganyika, must have been completely isolated and without an 

 outlet, until the formation of the great volcanic dam in geolo- 

 gically recent times. 



The lists of animals from the various lakes given in the body 

 of this work are, as regards extent, partly dependent on the 

 amount of biological exploration which each lake has leceived. 

 That is to say, a comparatively small fauna may be merely due 

 to less complete examination. Whether the lakes considered 

 here can be regaided as having been at all equally investigated, 



