The Tanganyika Problem Reviewed 481 



Such however permitted the gradual eastward migra- 

 tion of species of the above famihes of fishes; as well as of 

 various invertebrate groups, that gradually mingled with 

 the older African types. It permitted also the westward 

 passage of Protopterus, and its gradual condensing modi- 

 cation into Lepidosiren, that passed into the Amazon and 

 into tributary as well as more southern rivers. 



The lower Atlantic-African drainage area probably did 

 not exceed 500 feet to 1500 feet, and was continued east- 

 ward to the Upper waters of the White Nile, as well as in 

 bow-like fashion, southeastward to the Nyanzas, Tangan- 

 yika, Rudolf, and Nyasa. Of these lakes Tanganyika was 

 probably the oldest, and even in early Eocene time was 

 probably already a deep "rift" lake-ravine, that for long 

 periods before had accumulated numerous freshwater in- 

 vertebrate types from earlier epochs. At this time the 

 Congo basin seems to have been a more elevated plateau, 

 into which the eastwardly migrant fishes only later pene- 

 trated, when by combined denudation action and depression 

 by faulting its level was considerably reduced. 



By late Oligocene or by Miocene times the Atlantis 

 bridge was largely broken up, by extensive faulting and de- 

 pression, and a segregated evolution of species and of gen- 

 era belonging to the above four families of fishes, proceeded, 

 though in some cases — as with Fiindulus — generic contin- 

 uity can still be traced over the two sundered continents. But 

 further segregation resulted when — from late Oligocene 

 to early Pliocene times, and even on till now — steady but 

 local changes took place, through combined faulting, ele- 

 vation, depression and volcanic outbursts, that made them- 

 selves more or less felt over the entire mid-African area, 

 and which probably were coeval with those depressions, 

 faultings or foldings that almost obliterated the Southern 

 Continent, treated of in last chapter. 



Then started specific and generic variations in the five 

 or six main lake centres, as well as in Chad to the far north. 

 During this process one or two of the four groups of fishes 

 named, became dominant in each lake-centre. Thus the 

 Cichlidae far outreach the other families in Tanganyika, 

 and form a good second in Victoria. The species and even 



