SOME PROBLEMS OF RESEARCH 1 3 



anxiety to those who are responsible for conserving the pastures as 

 well as destroying the ticks. 



Another important source of animal disease is the tsetse fly. It 

 is well known that the so-called tsetse belt covers the greater part 

 of tropical Africa between a line from the Senegal River to Italian 

 Somaliland and another in the south from the coast of Angola to 

 Mozambique. A large part of this area is dominated by one or other 

 of the twenty species of fly rather than by man. The numerous 

 fly-fronts are continually subject to oscillations, many of them 

 recurring annually when in the dry season high temperatures 

 drive the fly to take refuge in the narrow bands of vegetation 

 bordering the watercourses. But the great changes in fly distribu- 

 tion which are now hoped for depend primarily on human en- 

 deavour. In Tanganyika, for example, the clearing by organized 

 native labour of the bush which harbours the fly has enabled 

 5,000 natives to return to land in one chieftainship from which it 

 had previously driven them. Again, in Southern Rhodesia, the 

 destruction of the wild animals on which the fly feeds has resulted 

 in freeing considerable areas from trypanosomiasis. 



Periodic locust invasions injure man through the destruction of 

 his crops. Here too, advances in studies which are slowly leading to 

 an understanding of locust ecology may be expected before many 

 years pass, to prevent or at least reduce these visitations, and there- 

 by render the African environment far more favourable to man. 



In connection with the numerous other pests of agriculture, it is 

 noteworthy that the change from extensive to intensive land utiliza- 

 tion tends to make the environment more favourable to pests. The 

 distribution of an insect or fungal parasite must clearly be easier 

 where the plant on which it lives is cultivated as a single crop over 

 a large area, than where a number of plants are grown together, 

 as in the native system, in small fields which may be separated by 

 forest areas. A parasite, which in these circumstances causes prac- 

 tically no damage, may become a pest of first-rate importance 

 where modern methods favour its rapid spread. Another factor 

 has been the introduction of domesticated plants from one territory 

 to another, and from other continents. These often bring their own 

 parasites with them, which find suitable hosts in indigenous crops. 

 Thus the mosaic disease of cassava, which is growing in importance 



