FAUNA OF THE AFRICAN LAKES. 597 



veliitioiialiips — which aie (|uit.H iiulepeiident of lialntat — can be 

 obtained. 



Tlie undoubted affinities existing between marine and fresh- 

 water organisms are the direct result of a community of descent, 

 for there is no escape from the conclusion that life had its origin 

 in the ocean. Thus tlie forms now found in fresh-Avaters must 

 have attained their present distribution in one of three ways :■ — 

 (1) by a. direct, active or passive migration from the sea ; (2) by 

 becoming terrestrial or swamp-loving in nature, a.nd secondarily 

 adapting themselves to life in fresh water ; (3) a,s a result of the 

 isolation and subsequent freshening of some portion of the sea, 

 due to movements of the earth's crust. Without speculating as 

 to which of these methods has played the most imj)ortant part, it 

 may be pointed out that the salinity of the ocean has not been 

 constant throughout the ages, but is doubtless gre;iter now than 

 in past geological times. Since certain types are known to have 

 receritly migrated from the sea, it is not hard to believe that many 

 forms may have achieved the change during former epochs when 

 the obstacles to be surmounted were somewhat less. It is hardly 

 necessaiy to repeat that the view advocated by Moore assumes 

 that the remarkable organisms found in Tanganyika have attained 

 their present distribution liy the third means, and have been 

 modified from marine types in a basin cut oft' from the sea. 



Since the flora of a lake perfoice exists under the same con- 

 ditions as the fauna, it will be well, before proceeding, to make 

 further reference to the plants of Tanganyika. The higher plants 

 show no outstanding peculiarities. There are certainly 8 species 

 of true aquatics which have been collected in Tanganyika alone 

 among the lakes, but all these are well-known African — or even 

 cosmopolitan- forms {cf. Rendle, 147). On the other hand, the 

 Algfe of Tanganyika difier markedly from those found in the 

 other big lakes, a number being endemic, while a few are usually 

 marine or brackish in habit. In all, some 21 species and 5 

 varieties are described as peculiar to Tanganyika. It is, however, 

 the phytoplankton of the lakes which afibrds the most interesting 

 comparisons. The plankton of Tanganyika is much richer in 

 species than that of either Nyasa or Victoria Nyanza. Out of a 

 total of 85 species, more than 70 per cent, do not occur in the 

 other two lakes, so that in this instance once more the features 

 characteristic of Tanganyika are exhibited. The presence in the 

 lake of brackish-water and quasi-marine Algse may perhaps be 

 accounted for by a period of growing salinity prior to the estab- 

 lishment of an outlet. (For fui^ther particulars consult the 

 detailed Report on the Fresh-water Algpe of the Third Tanganyika 

 Expedition, 200). 



It is natural that a good deal of attention has been attracted 

 to Tanganyika by the singular nature of its fauna, and it is 

 not surprising that various suggestions have been offered in 

 explanation of the facts. While it will be necessary to recount 

 the several views which have been advanced, it is appropriate to 



