THE TANGANYIKA PROBLEM. 15 



life — (1) a universal series of types, and (2) another com- 

 posed of organisms which only occasionally appear in fresh- 

 water, and are related to a particular district or to a lake. If 

 now we examine the character of the animals that form the 

 types of the first series, we are confronted with this very 

 curious and remarkable feature characteristic of them all. 

 They have no counterparts or close allies in the ocean at the 

 present day; and on anatomical examination they are found to 

 stand in the relationship of ancestors to numerous well-known 

 marine forms. If, on the other hand, we consider the forms 

 which are peculiar to the two lakes in question, and more 

 especially if we consider the forms similarly peculiar to a 

 number of other lakes and distinct fresh-water areas, we 

 find that these forms have almost invariably, like the 

 prawns in the Lago di Garcia, in Italy, close allies in the 

 existing marine fauna, and generally exhibit closest affinities 

 with some of the organisms which inhabit the sea nearest to 

 the particular fresh-water which may be studied. We find, 

 in fact, usually in the fresh-waters of the globe a fauna 

 which is composed of an ancient and almost universal stock to 

 which it appears that there may be added in any particular 

 fresh- water a more or less large and variable stock of animals 

 which have migrated into this area at one time or other from 

 the nearest seas. The origin and nature of this latter series 

 is obvious and plain, but the origin and relationships of the 

 first — i.e., the ancient and universal series — is not so clear. It 

 will, in the first place, therefore, be well to distinguish this 

 universal series by a special name, and to speak of it as the 

 primary fresh-water faima, and, dealing with the second 

 series in the same way, to call it the secondary fresh-ioater 

 series. That the types representing the groups consti- 

 tuting the primary fresh-water fauna of the world should 

 generally exhibit ancestral characters in their organisation is 



