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Show 1894.] ON ZOOLOGICAL DISTRIBUTION IN AFRICA. 165 Dr. J. W. Gregory, F.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. Similarly 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 rainfall 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 |