THE TANGANYIKA PROBLEM. 
15 1 
of species in a great lake as compared to a small one is really 
due 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 , Ampul-aria , and Vivipara ; in the Victoria 
Nyanza numerous varieties of the Melania , to which there 
are however added noticeable modifications of normal fresh- 
water Lamellibranchs, as in the case of AEtheria. 
