276 SCIENCE IN AFRICA 



must resort to the shade of dense vegetation along streams in 

 order to survive during a large part of the dry season. Accordingly 

 conditions are less favourable to the fly in Nigeria than in Tangan- 

 yika, where it need never leave the woodland. 



For these reasons, if the stream-banks were cleared wherever 

 native villages are situated, those villages would become practically 

 free from fly; man-fly contact would be decreased, and a few human 

 carriers would no longer be a menace to the rest of the inhabitants. 

 But far more efficient and easier to control would be the concentra- 

 tion of population in each district into a central area where all 

 streams were cleared. These settled areas would be linked together 

 by trade routes rendered fly-free by clearings made at all places 

 where the path crosses streams clothed with riverine vegetation. 

 The success of such projects will depend to a large extent on im- 

 proved methods of agriculture, which will keep the land in perma- 

 nent cultivation; these methods which include mixed farming and 

 ploughing with oxen are under investigation by the agricultural 

 department in Nigeria; they are considered in Chapter XI. 



In the Southern Provinces of Nigeria, where G. palpalis is the 

 prevalent species, the whole problem is different. Sleeping sickness 

 seems to have been endemic there for a much longer period, so 

 that the population as a whole has acquired a measure of resistance. 

 The possibility of control by modifying vegetation is less in this 

 region; the rate of growth is so great that it is impossible to keep 

 any area in the rain forest belt thoroughly clear of vegetation. Yet 

 the densification of vegetation is also difficult to organize, since 

 wherever man exists he cuts down the forest growth for purposes of 

 shifting cultivation. Here, as in the north, efforts to concentrate 

 the population and to introduce a settled system of agriculture 

 appear to offer the best hope of controlling the fly. 



Another important piece of work was carried out at Gadau by 

 Professor P. A. Buxton and Mr. D. J. Lewis (1934) of the London 

 School of Hygiene and Tropical Medicine, during a few months' 

 visit. By breeding tsetse in the laboratory under controlled con- 

 ditions they confirmed the view that humidity is the principal 

 limiting factor for reproduction. Temperature is also important, 

 and at certain times of the year under natural conditions the sur- 

 face temperature of the soil is so nearly lethal to pupae that partial 



