148 THE TANGANYIKA PROBLEM. 



other words islands which have long been remotely isolated 

 in the midst of the ocean, and islands which are more or less 

 narrowly-detached annexes of a continental land-mass ; the 

 floras being different in character on these two types of 

 islands. On comparing the floras of continental with oceanic 

 islands, it is found that on continental islands there are more 

 genera and fewer species as compared with the fewer genera 

 and more species found on oceanic islands. The continental 

 islands are dominated by migrations of certain genera from 

 the continent ; whereas the floras of oceanic islands are less 

 exposed to the effect of the incursion of vigorous continental 

 forms ; and the floras of such islands are able to gradually 

 adapt themselves to the varying conditions which the island 

 presents in various parts, the flora of an oceanic island be- 

 coming affected by the natural restrictions which deliminate 

 different portions of the island from one another ; and in 

 different areas individual varieties gradually attain specific 

 rank. It is apparently the oceanic island which is most 

 nearly comparable to the remotely inland lake, for it is ob- 

 vious that when the depressions holding lakes upon a con- 

 tinental area have become established, the migrations of new 

 forms into these depressions from the ocean will have been 

 rendered difficult or impossible, as the case may be. From 

 what we saw in Chapter VII. it would appear that to a large 

 extent the fresh-water fauna of a continent is something 

 which in its origin is bound up with the origin of the con- 

 tinent itself, and consequently the production of species in 

 the fresh waters of a continent is, in a sense the measure of 

 the age of the fresh waters to which these species may 

 belong. From general considerations it would appear that 

 the specific varieties of the genus Melanin characteristic 

 of the fauna of Nyassa cannot have migrated into it, 

 for they do not exist outside the lake — they are, in fact, 



