The Tanganyika Problem Reviewed 471 



The rather abrupt manner in which the great groups 

 of fishes above-named disappear when the northern part of 

 the Soudan is passed, suggests that from Cretaceous times 

 onward either a Cretaceous, and later in the northern and 

 N. E. Sahara an Eocene sea covered most of that region; 

 or when such seas were withdrawn physical conditions due 

 to hot suns, intense desiccation, sand storms etc., prevented 

 northward migration except at a late period, and indirectly 

 down the Nile Valley, then thence along the south Mediter- 

 ranean coast. Similar obstacles long acted to prevent ex- 

 tension into S. Africa until comparatively recently, and then 

 only to a small degree. 



Though some writers have tried to minimize the re- 

 semblances — one might truly say the abruptly close affini- 

 ties — traceable between many genera of the Siluridae, 

 ICharacinidae, Cyprinodontidae, and Cichlidae of Africa 

 and eastern S. America, such must be accepted as existing, 

 and the writer adduced reasons (p. 400) for considering 

 that all primarily evolved in America, and became estab- 

 lished as increasingly dominant groups during late Cre- 

 taceous time. How then, it may be asked, did they pass 

 to Africa? Before giving answer, it should be pointed out 

 that the greatest number of African species in these four 

 families occur in western and central Africa, while the 

 number markedly decreases toward the Nile, and becomes 

 still smaller when one traces them eastward into Asia. 



Any attempted explanation therefore must involve the 

 former existence of a wide tract of land that was of such 

 elevation, and was so connected in its marshes, lakes or 

 river systems, as to permit the gradual migration from the 

 Guiana-Brazilian or Archenchelis continent to what is now 

 the great Nile system, of at least some representative 

 genera in each of the four families above-named. 



The chart on next page (Fig. 72) gives in outline what 

 the writer would regard as the condition of affairs, toward 

 the close of the cretaceous period. At that time the great 

 Archenchelis massif seems to have been of considerable 

 elevation, but was being drained and denuded by rivers such 

 as are now represented by the Amazon, the Araguay, the 

 Parnahyba, and the San Francisco. Extending from the 



