3 ° 
THE TANGANYIKA PROBLEM. 
Once established in the fresh-waters of the globe, it is 
probable that the fishes and the more active of the inver- 
tebrates migrated slowly from point to point, but as we 
have seen, there is much evidence to show that in the 
great fresh-water areas of the different continents the 
invertebrate components of the primary fresh-water series 
have migrated very little from the point at which they 
first originated. 
In many fresh-water areas however, as in the Lago di 
Garda in Italy, there are often to be found organisms 
which have not the ancient attributes of the constituents 
of the primary fresh-water series, and these organisms, like 
the Lago di Garda prawns, are to be regarded as the com- 
paratively rare and conspicuous examples of voluntary 
colonisation from the sea. Still further, in a few places, as 
in the Caspian Sea, and, as we shall see later, in the case 
of Lake Tanganyika, there are to be found whole batches 
of animals which have become independently detached 
from the sea and which bear no proximate relationship 
either to the modern marine fauna, or to the primary fresh- 
water types. Thus we find in the fresh-waters of to-day 
animals which can naturally be arranged under three dis- 
tinct heads. We have in the first place the primary fresh- 
water series , in the second the sporadic and voluntary 
colonists of fresh-water which we may call the secondary 
fresh-water series , and thirdly the relics of entire marine 
faunas which in a few places are found to persist and which, 
like those of the Caspian and Tanganyika, we may call the 
halolimnic series of the world. 
