THE TANGANYIKA PROBLEM. 151 



of species in a great lake as compared to a small one is really 

 clue to the existence of a greater number of more or less 

 definitely isolated aquatic areas in the greater lake as com- 

 pared to the less. The greater lake corresponds to a park 

 in which the cattle have been fenced off into a greater 

 number of pens, and thereby a greater number of varieties 

 have been produced, while the smaller lake corresponds 

 to one of these areas, and contains only a single variety. 



The phenomena presented by lacustrine faunas would thus 

 seem to be very similar to the phenomena presented by 

 island floras, for, as we have seen, the number of species en- 

 countered in the different African lakes vary directly with 

 the size of the lake, but it should at the same time be 

 clearly apprehended that the specific varieties characteristic 

 of individual great African lakes are usually only so many 

 structural changes which have been rung by time and cir- 

 cumstance upon the types of fresh-water organisms which 

 are universally distributed throughout the earth. These 

 varieties which appear in the greater African lakes have 

 nothing to do with the halolimnic forms peculiar to Tangan- 

 yika, although these forms may and have been, as we have 

 just seen, affected in a similar way. It is, moreover, not 

 always the same forms which tend to vary in the different 

 lakes. In Nyassa there are numerous endemic varieties 

 of Melania, Ampidaria, and Vivipara ; in the Victoria 

 Nyanza numerous varieties of the Meiania, to which there 

 are however added noticeable modifications of normal fresh- 

 water Lamellibranchs, as in the case of sEtheria. 



