476 Evolution and Distribution of Fishes 



with other animals and with environal lake surroundings, 

 there would be a natural tendency for these animals to 

 segregate in waters that were of varying but specially 

 considerable depth; that were at the same time in contact 

 with inflowing streams, or with swamp areas, which were 

 in turn connected wtih wide stretches of country during flood 

 period; and that in time became permanently cut off from 

 each other owing to denudation, or to more sudden changes, 

 with upheavals or depressions of the earth's crust. Such 

 environal alterations would start and more or less accentu- 

 ate localized variation-changes in the organisms there, from 

 steady action of the great law of proenvironment (p. 15). 

 Such seem to have existed as incipient conditions, toward 

 close of the Oligocene, or in the early Miocene, and in the 

 six or eight principal lakes of the central African region. 



If then there started an extensive but steady change 

 in the earth's crust over the above area, as is demonstrated 

 to have taken place through the observations of Scott- 

 Elliott, Gregory, Moore, and others since, shrinkage, fault- 

 ing, downthrow and upthrow of strata must have occurred. 

 The heat generated by such extensive stratigraphic changes 

 would start volcanic action, and originate mountains such 

 as are now seen in the Fort Hill, Mfumbiro, and Ruwen- 

 zori mountains. Great stretches of country would thus 

 be elevated or depressed. And while at times and in some 

 places cataclysmic destruction of life would follow, incipient 

 variations in species that were saved alive would often 

 tend to be preserved, and even in time further accentuated. 

 The very inequalities in elevation and depression shown by 

 Lakes Nyasa, Rikwa, Tanganyika, Kivu, Victoria and 

 Rudolf, would involve differences in many environal fact- 

 ors, as travellers have noted. Such in turn would by pro- 

 environal reaction, affect and alter the organisms involved. 



But insufficient knowledge of biological relations, or of 

 the changes proceeding, may cause one to assume mistaken 

 positions. Thus Moore (op. cit. p. 128) in comparing 

 Lakes Tanganyika and Kivu, concluded that "their faunas 

 are entirely distinct," and then recorded five species of fish 

 from Kivu. But further on he remarks (p. 136) : "Tilapia 

 hurtoni is found outside the confines of Tanganyika, this 



