612 



DR. W. A. CUNXINGTON ON THE 



the conditions in Tanganyika produced a more striking series of 

 such forms than elsewhere. This view is not regarded as accept- 

 able. — It has been suggested that the salinity of the Tanganyika 

 water has produced marine-like forms, but further evidence is 

 needed. — The view that Tanganyika owes its remarkable organ- 

 isms to a prolonged period of isolation is regarded as the most 

 likely suggestion. It doae not run counter to geological 

 conceptions. 



Tliis theory does not account for the medusa. Boulenger 

 suggests that it may have survived from an Eocene sea in 

 Northern Africa. Gravier considers, on the contrary, that it 

 may be a recent migrant from the ocean. 



The faunas of the remaining lakes are of less interest. Victoria 

 Nyanza is next in importance to Tanganyika. The most con- 

 spicuous group is the fishes, with a large proportion of endemic 

 species. The Mollusca also are noteworthy. The fauna of Nyasa 

 is similar in character, but with fewer peculiar types. There are, 

 however, 5 endemic genera of fishes. The smaller lakes contain 

 representatives of fewer animal groups as well as fewer species. 

 Kivu is the extreme case, with only 23 species, of which 4 are 

 endemic. The poverty of its fauna may be associated with its 

 exceptional salinity. Albert Nynnza displays the smallest pro- 

 portion of endemic types, viz. 13 per cent. It is suggested that 

 periods of isolation would account for the peculiarities of Victoria 

 Nyanza and Nyasa. 



Certain animal types are unexpectedly al)sent fi'om tlie African 

 lakes. Such are the fresh-water Crustacea Aselhi.s and Gammarus 

 a,nd the fish parasite Echhiorhyiichus. No explanation of this is 

 forthcoming. 



The number of specific forms in these lakes appears to be 

 proportional to the size. This would seem to hold good also for 

 the number of genera and families and even for tlie number of 

 groups represented. It is possible that this principle is of 

 general application. It is thought to be analogous to the phe- 

 nomenon exhibited by the flora of oceanic islands. 



The aflinity between African and Indian fresh-water types is 

 recognisable in several instances among the inhabitants of the 

 lakes. This affinity is explained by the former existence of a 

 continent which embraced these countries in the Carboniferous 

 and subsequent periods. 



Little is known of the deeper regions of any of the lakes. It 

 remains to be discovered whether associations of abyssal animals 

 exist in them. Neither the vertical distribution of plankton 

 forms, nor their seasonal variations have yet been studied. 

 Further information is needed on the salts dissolved in the water, 

 the depth and nature of the lake basins, water temperatures, etc. 

 It is nevertheless possible from established facts to form a true 

 conception of the na,ture of the lakes and their organisms. 



