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 Melania characteristic 
of the fauna of Nyassa cannot have migrated into it, 
for they do not exist outside the lake — they are, in fact, 
