THE TANGANYIKA PROBLEM. 
147 
compared with the poorness of the fauna of the Albert 
Nyanza. The water of the Albert Nyanza is quite as full, 
if not fuller, of animals than the water of Nyassa ; but the 
number of species which the lakes contain is, as we have 
seen, widely different. In this way it would appear to be a 
fact that the number of animals living in any particular lake 
may be, and probably is, related to the food supply which 
the lake presents, but the number of species is for some 
reason a function of the area of the lake and not of the food 
which it contains. This is a very remarkable circumstance, 
and has, in all probability, a wide significance ; the only 
analogous phenomena with which I am acquainted being the 
contrast which has been found to subsist between conti- 
nental and island florae. It has been shown by botanical 
enquirers that the number of species of plants which flourish 
upon an island is less than the number of species which 
flourish upon a similar continental land area, and that, 
roughly speaking, the number of species which constitutes 
an island flora is proportional to the size of the island ; the 
less the island, the fewer the species of plants which it sup- 
ports, although the island may be just as thickly covered 
with vegetation as a similar continental area. From these 
considerations it would appear probable that in the case of 
the different sized lakes of Africa and in the case of island 
floras we are dealing with analogous phenomena. For in 
the one case we are dealing with sheets of water of different 
sizes, which are, so to speak, detached from the sea, and 
in the other, with different sized pieces of land occurring in 
the ocean which are detached from the surface of the 
continents. 
In the case of island floras it has been found, however, 
that there is a distinction to be drawn between what have 
been called oceanic islands and continental islands, or in 
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