630 SCIENCE PROGRESS. 



latter is only i 500 feet above the sea. This shut-in character 

 of Tanganyika, and its consequent high temperature, is 

 witnessed in a still higher degree in the valley of Lake 

 Rukwa, whose lofty coasts rise almost perpendicularly from 

 the water's edge from 5000 to 7000 feet. Owing to this 

 cause the Rukwa region is extremely hot, and as the lake 

 is without outlet the water is extremely salt. Again the 

 Victoria Nyanza forms a vast expanse of shallow water 

 like an enlarged edition of Bangweolo. But the characters 

 of Tanganyika are repeated to some extent in the Albert 

 Edward, the Albert Nyanza and Lake Rudolph to the 

 north and east. 



It is thus seen that the conditions under which the 

 faunas of the lakes exist vary a good deal from lake to lake. 

 The lakes themselves moreover vary considerably in the 

 directness or the indirectness of their connections with each 

 other and the sea. Thus Nyassa flows directly south and 

 east to the Zambesi and the east coast, there being only 

 one great series of rapids, at the Murchison Falls, throughout 

 the whole of the effluent river's course. The effluents of 

 Lakes Bangweolo, Mwero and Tanganyika, on the other 

 hand, all flow together down the Lualabwa to the Congo 

 and the Atlantic Ocean. This series of lakes is now, and 

 must have been for a prolonged period of time, entirely cut 

 off from all contamination throuQ^h incursions of all but the 

 most active marine animals coming up the rivers from the 

 sea, for the courses of both the Lualabwa and the Congo ^ 

 are intersected by formidable rapids and falls. 



The effluents of the Victoria Nyanza, the Albert Edward 

 and the Albert Nyanza go north and eventually reach the 

 Nile, and in this way it will be seen that some of the great 

 equatorial lakes have water communications with the seas 

 and oceans north, east and west. The upper reaches of 

 these rivers are, however, effectually blocked, so far as 

 migration from without inward is concerned, by successions 

 of falls and rapids where the rivers pass downwards from 

 the high interior plateau, and of these several river courses, 

 that from the west, stretching from the Lualabwa and 

 ^ See Stanley's description of this region in his Darkest Africa. 



