8 SCIENCE IN AFRICA 



every cultivator bases his selection of plots for different crops on 

 recognized associations between certain plants and certain types 

 of soil. The deterioration and erosion of soil is perhaps the best 

 example of any to show the rapidity of environmental change. This 

 likewise has been recognized by African farmers for centuries, and 

 on it have been built up the complicated systems of shifting culti- 

 vation which so admirably suit the soils and climate, provided the 

 area of land is sufficiently large. There are many examples of 

 individual tribes which have apparently evolved methods of avoid- 

 ing soil erosion which could hardly be bettered by science. Thus 

 the primitive pagan peoples of the Bauchi plateau in Nigeria pick 

 the stones from their millet fields and arrange them in rows 

 along the contours, with the result that in a few seasons the annual 

 soil-wash leads to a definite terracing of the land. Furrowing along 

 the contours and mound-cultivation in wet climates are likewise 

 admirably suited to special conditions. 



Before enlarging on agriculture the vegetation must be con- 

 sidered. This is directly dependent on the factors already dis- 

 cussed. The changes which constantly take place are dependent 

 on, and themselves cause changes in, rainfall and water-supply. 

 Perhaps man himself is the principal agent in these changes; for 

 example, he fells an area of evergreen forest to cultivate a garden, 

 to establish a cocoa plantation or to obtain timber. He burns 

 hundreds of square miles of savannah for the sake of hunting honey, 

 or of grassland to stimulate the young nutritious shoots and to kill 

 the carriers of animal disease; and he may ruin pasture by too 

 concentrated grazing on the part of his stock. After any of these 

 drastic actions the natural balance is, of course, upset, and a series 

 of changes in vegetation and soil set in which, if left to go their own 

 way, would eventually lead on to the original type of vegetation 

 by a succession of plant associations leading to the climax. But 

 more usually a continual or intermittent interference on man's 

 part prevents the natural succession, so that an entirely new type 

 of vegetation becomes established; for example, in Uganda the 

 huge areas of elephant-grass were once closed forest. 



The effect of natural vegetation and particularly forests on 

 water-supply is an aspect of the changing environment about 

 which very little is yet known. It is recognized, though it is diffi^ 



