MAMMALS OF AXGLO-EGYPTIAN SUDAX — SETZER 451 



companion ranges, the Dongotonas and the Didingas) has a close 

 affinity with the East African fauna but shows a high degree of in 

 situ development. These mountain ranges of the southeastern Sudan 

 need much further field work in order to evaluate properly their 

 zoogeographic status and the degree of isolation of their faunas from 

 those of East Africa and from each other. Of the three, only the 

 Imatong Range is adequately represented by specimens. It is in- 

 teresting that the mammals showing the greatest differentiation are 

 those whose normal habitat is in a forest envh-onment. In these 

 mountains, the forest has become isolated from adjacent forested areas 

 by large regions of arid savanna. This isolation is apparently the 

 result of the dessication of central Africa after the last Pleistocene 

 Pluvial period. Worthington (1937, p. 316) with regard to the great 

 lakes of Africa and theu- fish faunas, saj^s: 



It is yet premature to date the earlier changes involved — the time at which 

 the main rivers of Africa became set in their courses, the formation of the rift 

 valle3's, the depression of the Lake Victoria basin, and the reversal of many of 

 the rivers of Uganda. But the later changes, including the dessication of Lake 

 Edward and probably of Lake Victoria can now be dated within reasonably cer- 

 tain limits by correlation with climatic change during the Pleistocene in other 

 parts of Africa and with the glacial and inter-glacial periods in the northern hemi- 

 sphere. The exact correlation of the individual pluvial and interpluvial periods 

 is not yet fixed, but in general terms the climatic changes which were taking place 

 soon after the beginning of the Pleistocene were responsible both for the glacia- 

 tions in the north and the pluvials on the equator. 



In Europe well-known studies have led to the enumeration or estimation of the 

 number of years since the ice receded from such localities as Stockholm or from 

 certain lakes in Switzerland, and figures of the order of 9,000 to 14,000 years have 

 resulted. If we take the last pluvial of Africa to have finished at about the same 

 time as the last glacial in Europe and apply these figures, we conclude that Lake 

 Rudolf was cut off from its connection with the Nile, say 12,000 years ago, and 

 in that comparatively short time the fish isolated in that lake have changed into 

 the endemic species and subspecies referred to above. Somewhat before this, 

 say between 15,000 and 20,000 years ago, the plateau lakes at the main source of 

 the Nile were dried up, and since they were refilled, adaptive radiation up to the 

 present day has given rise in Lakes Edward and George to eighteen endemic 

 species of Cichlid fish and four of non-Cichlids, and in Lakes Victoria and Kioga, 

 with their more diverse environments and partial isolation from each other, to 

 fifty-eight endemic Cichlids and twenty-seven non-Cichlids. The vast assort- 

 ment of unique forms in Lake Nyasa and Lake Tanganyika has naturally taken 

 much longer, and to date and understand these we must await the result of future 

 geological and biological studies. 



In substantiation of the dating of the end of the Pluvial in central 

 Africa, a corollary may be drawn from the pocket gophers (Thomomys) 

 of the Salt Lake Valley and envu'ons in North America. In attempt- 

 ing to determine the time level at which habitat became available 

 for pocket gophers, Durrant (1952, p. 497) says: "The Postpluvial, 

 the last period of Lake Bonneville, has endured from the second 



