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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