2o THE TANGANYIKA PROBLEM. 



on any supposition of their having originated as a relic 

 fauna in some one arm of the sea which became cut off 

 from the ocean, and ultimately fresh ; for there is no 

 evidence that there has been any connection between the re- 

 mote state land masses which these fishes now inhabit, since 

 the origin of types now common to them both. It is the 

 same with the Cichlidae and many other forms of fish. The 

 idea which Sollas advocates, that the fresh-water faunas 

 of the world are relic faunas emanating from the sea, may 

 be true, but that these faunas have intercommunicated one 

 with the other, and thus produced the universal distribu- 

 tion of the primary fresh-water types, does not appear to 

 be supported by any capacity for dispersion exhibited by 

 their present constituents. We seem, in fact, from the 

 evidence which is now available, to be driven either to 

 the supposition of such a complete parallelism in the 

 evolution of forms of different origin in different areas, 

 that it makes generic distinction impossible, and conse- 

 quently upsets our fundamental conceptions in a manner 

 for which there is no warrant ; or we must find some other 

 cause for the universal distribution of numerous fresh- 

 water types. 



It has already been emphasised that the members of 

 the universal fresh-water series possess the characters of 

 forms which there is every reason to believe were once 

 widely spread in the sea, and, this being so, it also becomes 

 clear that if we can discover some cause which would 

 have made their migration from the sea to fresh-waters a 

 necessity, we shall have found an explanation of some of 

 the most remarkable features of the fresh-water faunas of 

 to-day ; for through such a cause all the members of the 

 primary fresh-water series might have been produced about 

 the same time, and would consequently present all over 



