FAUNA OF THE AFRICAN LAKES. 



597 



relatioiisliips — wliicli are quite independent of habitat— can be 

 obtained. 



The 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 conchision that life had its origin 

 in the ocean. Thus the forms now found in fresh-waters must 

 liave attained their present distribution in one of three wa.ys : — 

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

 becoming terrestrial or swampdoving in nature, and secondaiily 

 adapting themselves to life in fresh water ; (3) as a residt of tlie 

 isolation and subsequent freshening of some portion of the sea, 

 due to movements of the earths crust. Without speculating as 

 to which of these methods has played the most imjiortant part, it 

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

 constant throughout the ages, but is doubtless greater now than 

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

 recently migrated from the sea, it is not hai-d to believe that many 

 forms may have achieved the change during former epochs when 

 the obstacles to be surmounted wei-e somewhat less. It is hardly 

 necessary to repeat that the view advocated by Moore assumes 

 that the remarkable oiganisms found in Tanganyika have attained 

 their present distribution by the third means, and have been 

 modified from marine types in a basin cut off from the sea. 



Since the flora of a lake perforce 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 ceitainly 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 

 Algae of Tanganyika differ markedly from those found in the 

 other big lals:es, 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 Algje may perhaps be 

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

 lishment of an outlet. (For further particulars consult the 

 detailed Report on the Fresh-watei' Algje of the Third Tanganyika 

 Expedition, 2 CO). 



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 



