SOIL SCIENCE 139 



low level for the last forty-five years, and yet the productive capa- 

 city of the land has been systematically destroyed. The western 

 section of the Kamba tribe in Kenya is a case in point, and their 

 outlook to-day is very serious: their country has been well-nigh 

 ruined, they can no longer migrate elsewhere and famines 

 are frequent. The country occupied by the Sukuma south of 

 Mwanza in Tanganyika is also in a bad condition owing to over- 

 stocking, although the population has probably not increased much 

 since European occupation. As a third example the great migra- 

 tion of the Jaluo (Nilotic Kavirondo) southward from the hills east 

 of Gondokoro to their present situation on the Kavirondo Gulf of 

 Lake Victoria, is held by some to have been caused by decreasing 

 fertility of the land they left. Whether or not soil erosion was com- 

 mon before the European occupation of Africa, the coming of the 

 white m.an has probably given it a stimulus by causing increase of 

 population and at the same time encouraging the extension of cul- 

 tivation. 



The human activities which produce erosion are shifting culti- 

 vation combined with land hunger and overgrazing, each of 

 which is considered fully in Chapters XIII and XIV. Some authori- 

 ties would make a third category, namely fire, but burning of 

 forest or grassland is usually a feature of the native agricultural 

 systems, though it is also carried out in some areas to facilitate 

 movements of man for the purpose of collecting wild products such 

 as honey and gum. It is alleged that pastoral tribes associate the 

 dry grass with tick-borne disease such as east coast fever, and the 

 burning does in fact destroy the ticks. In some parts annual burn- 

 ing appears to be a traditional practice for which no economic 

 reason is given. Undoubtedly the burning of vast areas of grass- 

 land and savannah forest is a more potent factor in destroying 

 vegetation in some districts than is either grazing or cultivation, 

 and it lets in the desert on an extensive scale in arid regions. 



It is only during the last fifteen years or so that attention has 

 been focused on the dangers of soil deterioration, but it is impor- 

 tant to realize that the European influences to which soil erosion 

 is partly ascribed have been at work in parts of the continent for 

 some centuries. T. D. Hall (1934) for example, in summarizing the 

 historical evidence for South Africa, points out that changes in 



