20 
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 
