466 Evolution and Distribution of Fishes 



that started In freshwater. The prawns of Tanganyika 

 are now known to include twelve species. Of these Caridina 

 nilotica was first reported from freshwaters of the Nile, 

 but later was discovered in Algiers and in Oran. It is now 

 known to be represented in Tanganyika by a variety C. n. 

 gracilipes. The remaining eleven species all belong, like 

 the last, to the purely freshwater family Atyidae. 



In his revision of this family Ortmann (29^: 397) pro- 

 ceeds throughout on the assumption that it is an ancient 

 freshwater derivative from an older marine tribe, and as to 

 the genera of it, that "their several characters are connected 

 with a change of habits, and with immigration to fresh- 

 waters." The writer hopes in time to show that the reverse 

 is true. But Ortmann fully accepts it that the family Atyi- 

 dae as such "contains only freshwater forms." Caiman 

 writes (295: 187) "every one of the twelve species found 

 in Tanganyika is, so far as we yet know, peculiar to that 

 lake." Such might suggest that the lake has been an iso- 

 lated centre for evolution of new species, through a con- 

 siderable period of time; and such is the view accepted by 

 not a few. 



Two species of crab, belonging to the freshwater family 

 Thelphusidae have been secured at depths of 100-600 feet. 

 One genus of the family, Thelphusa, includes about a hund- 

 red species scattered over the tropics and subtropics. We 

 need not now discuss the views expressed by Moore (op. 

 cit. pp. 284-86) as to the possible phylogeny of these. 



In the report of G. O. Sars on the Copepoda and 

 Ostracoda he describes 29 species from the Tanganyika 

 area ; 1 1 from Nyasa, and 7 from Lake Victoria (296 : 3 1 ) , 

 belonging to the genera Cyclops, Diaptomus, Ergasiloides, 

 Schizopera, and Ilyophilus. While accepting the first three 

 as typically freshwater, he says regarding the two last: 

 "both of these genera must evidently be regarded as of 

 marine origin, and the question thus arises, how we shall 

 explain the occurrence of species of these genera in the 

 purely freshwater lakes of Central Africa." His conclusion 

 is, not that they were inland "relict" forms of primitively 

 marine origin, but that they were carried inland by birds 

 from the seashore. This means that they were suddenly 



