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 



