3 o 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 

 Jialolimnic series of the world. 



