288 R. T. GUNTHER. 
When such an isolated volume of water has become completely 
separated from the sea at all stages of the tide, it is further 
necessary that the rainfall received in its basin should be in 
excess of the quantity of water lost by evaporation. Given 
all these conditions, the salt will gradually be washed out and 
the water will become fresher and fresher, and those of the 
original inhabitants of the lagoon or bay which could not 
accommodate themselves to the changed environment would 
die, leaving the rest to survive as a fresh-water fauna. 
Such are briefly the changes which have probably occurred 
in the case of Halmonises lacustris, which is the inhabitant 
of a small fresh-water lagoon removed but a few yards from 
the seashore itself. Its ancestors probably wandered into a 
small bay or estuary situated where the fresh-water lagoon is 
now, but in direct connection with the sea. By the upheaval 
of the land or by some other cause the estuary became shut off 
from the sea, and the salt water was gradually flushed out by 
fresh. In the present case the change must have occurred 
with sufficient slowness to allow not only Halmonises, but 
also numerous other marine animals, such as several genera of 
Polycheta, to become acclimatised to a life in fresh water. 
Owing to the proximity of the pool inhabited by Halmo- 
nises to the sea, the change is easy to understand. In the 
case of Limnocnida it is far otherwise. 
At the present day Lake Tanganyika, according to the 
description of Mr. E. C. Hore (8), is situated about twice as 
far from the west coast of Africa as from the east coast. The 
lake lies at an altitude of about 2700 feet above the sea level, 
within a mountainous ring fence encompassing a space of, 
roughly, 600 miles by 300. The lake itself is situated close 
under the mountains on the western side of this ring, and some 
2000 to 3000 feet below the higher parts of the range. All 
along this western side is an enormous chasm 400 miles long, 
with an average width of 20 miles and of great depth, varying 
from 500 to 1000 feet in the middle. The superfluous waters 
escape by the Lukuga, which runs through a great break in 
the mountainous rampart on the west side, and is a tributary 
