THE TANGANYIKA PROBLEM. 
149 
characteristic of the fauna of Nyassa, and consequently they 
would appear to have originated in the lake itself. In a great 
lake like Nyassa the origin of new forms peculiar to the 
region appears to be related to the existence of conditions 
there which have already been observed to be conducive to 
the separation of definite variations among animal forms. As 
Darwin pointed out, it is definitely known that the fencing- 
off of cattle in a park tends towards the formation of stock 
which differs on different sides of the fences, and in a great 
lake like Nyassa we find that the fauna is often actually very 
definitely fenced off into isolated areas by the natural condi- 
tions which exist in such a lake. Thus the molluscs in 
Nyassa live chiefly in the sheltered bays and creeks which 
communicate with the open body of the lake, each bay 
forming in itself a colony wherein a number of molluscs live 
practically isolated from all the other colonies which are dis- 
tributed round the shore. Storms and floods effect the dis- 
persion of members of these colonies at different times, and 
in this way every colonisable area of the lake becomes filled 
with molluscs, but in general the individual population of 
each colonisable area remains cut off and isolated from all 
the rest. The actual illustrations of these phenomena 
are often very interesting and instructive. Thus the 
Vivipara unicolor , which is generally found in Nyassa, 
and frequents more diversely conditioned parts of the 
lake than any other form, is replaced in Monkey Bay by 
a type nearly three times as large, and, so far as I 
know, this variety occurs in no other part of the lake. 
So again in Tanganyika, there are at least three very 
well marked varieties of the ally of the genus Vivipara 
Neothauma. One of these, with the type of shell repre- 
sented on page 261 ( a ), occurs exclusively at the south end of 
the lake, swarming in the broad and more or less sheltered 
