1894.] ON ZOOLOGICAL DISTRIBUTION IN AFRICA. 165 



Dr. J. "W. Gregory, E.Z.S., made some remarks on the factors 

 that appear to have influenced zoological distribution in Africa, 

 commencing with the following observations : — 



"It has long been known that the phenomena of distribution in 

 Equatorial Africa present a series of glaring contradictions and 

 anomalies. Thus many groups extend across Africa east and west, 

 others run north and south, while a third group occurs only as 

 isolated patches on the summits of the highest mountains. Simi- 

 larly the fishes of many rivers and lakes belonging to different 

 basins have identical or nearly allied species. These are absent in 

 North-east Africa and reappear in the lakes and rivers of Syria. 

 As it was believed that the geology of Central Africa was very 

 simple and that the country had been for ages remarkably stable, 

 it has appeared very difficult to explain these facts of distribution. 

 The results of more recent work, however, show that the lake- 

 region of Africa is a district of great instability." 



Dr. Gregory then gave a brief sketch of the probable changes that 

 had occurred in the level of the country: — " Originally the Victoria 

 Nyanza district was probably a high plateau on which rose rivers 

 that flowed on one side into the Congo and on the other into 

 the Indian Ocean and the Bed Sea. The centre subsided and the 

 drainage formed a great lake. This was subsequently further 

 isolated from the Congo and the East Coast river-systems by two 

 long cracks forming rift valleys. Subsequently the Nile cut 

 through the mountains to the north of the Nyanza, and the waters 

 of that lake became discharged into the Nile. The Jordan Valley 

 was connected to the East- African river-system at a time when 

 much of the Eastern Levant was dry land and Palestine was 

 covered by a freshwater lake. The surplus waters of this lake 

 discharged to the south and flowed along the valley that by later 

 subsidence was formed into the basin of the Bed Sea. The living 

 land- and river-mollusks of Abyssinia and the fossil species to the 

 south also show that the connexion between Syria and the Central 

 African lakes was established by a river that flowed across Baringo 

 and Basso Narok and thus into the south end of the Bed Sea. 



" The key to the distribution of the land animals and plants lies 

 in the discovery of the former extension of the glaciers of Mount 

 Kenia. The climate must then have been very different from the 

 present one. The results of the former greater height of the land 

 would have been a depression of the isobaric surfaces and the 

 formation of a high-pressure area over the central plateau. The 

 winds would have been different and far less regular, and the 

 rainfall would have been greater and more evenly distributed. The 

 surface of maximum l-ainfall would have been lower and more 

 extensive. Hence the present alpine flora would have descended 

 from the mountains to the plateaus, and the low-level flora have 

 been luxuriant and better adapted for food than the existing scrub. 

 There would therefore have been no such barriers to the migration 

 of small mammals and many of the invertebrates as exist at 

 present." 



Proc. Zool. Soc— 1894, No. XII. 12 





