468 Evolution and Distribution of Fishes 



fishes spread from eastern S. America to West Africa, and 

 thence across the Continent. 



So it seems not unlikely that a tendency to incipient 

 parasitism, on the part of some Cyclopidae, started in the 

 extensive swamp and river system of eastern S. America, 

 and thence spread eastward with the fishes that acted as 

 hosts. For these and other reasons then, the writer would 

 consider that the apparent derivative marine forms of Sars 

 \ are species that have persisted amid freshwater, while most 

 of their near allies have become by slow degree, and by in- 

 creasing selection of a marine host, in the manner experi- 

 mentally demonstrated by Wilson, dwellers in the sea. 



What has been said above would apply equally to the 

 Ostracoda, described by Sars (Proc. Zool. Soc. (1910) 

 732). The group however is well-known to be an ancient 

 one, and may well have passed through varying vicissi- 

 tudes of geographic and geologic kinds, before taking on 

 the relations shown by those of the Central African region. 



It is not intended to treat here of the Molluscs. The 

 writer would venture the prediction however, that not only 

 the typical freshwater species given by Moore (op. cit. p. 

 217), but such as he also discusses on pp. 218-265, will all 

 prove, when critically studied, to have a freshwater an- 

 cestry. 



The group of fishes has received careful attention by 

 many investigators of the Tanganyika fauna. As a result 

 upwards of 120 species are now known as composing it, 

 and this mainly through the skilled taxonomic efforts of G. 

 A. Boulenger. Two of his latest papers (joo: 17,399) 

 give recent additions. But to appreciate past geographic 

 and geologic changes, a correlation must be effected between 

 the freshwater fish fauna of S. America as synopsized main- 

 ly by Eigenmann {301) that of the West Niger and Congo 

 areas as given by Boulenger (227), and that of the Nile 

 basin in the widest sense, as treated by the same author 

 {302). 



When survey is made of the above wide area, many 

 striking features are revealed, some of which have already 

 been slightly referred to. Thus the large freshwater fami-( 

 ly Siluridae shows fully 300 species in S. America, about 



