6 SCIENCE IN AFRICA 



or husbandry: for example, in the more arid regions pastoralists 

 migrate with their cattle back and forth to the wet-season pastures, 

 resorting during the dry season to country where a permanent 

 water-supply remains. What happens then when this annual regi- 

 men of rainfall and water-supply changes, and in fact, does it 

 change? 



In answer to this question it is only possible to mention the sup- 

 posed progressive desiccation of Africa: to point out that the con- 

 tinent is still emerging from the last pluvial period; that river- 

 capture has caused many internal drainage basins to run out to 

 sea; that some believe the Saharan sands to be shifting slowly 

 southwards; that the filtering action of swamps tends to the pro- 

 duction of dry land; and finally, but more important still, to point 

 to the effects of human activity in accelerating run-off by the 

 destruction of forests, overcultivation and overgrazing. 



Apart from this general change to drier conditions, if such 

 exists, there are undoubtedly minor changes in rainfall of a secular 

 or perhaps cyclical nature. The period of records is too small to 

 show whether the Bruckner cycles with a periodicity of about 

 thirty-five years, which have been traced back in Europe through 

 several centuries, also exist in Africa. It is generally supposed that 

 the Bruckner cycles are less well marked in tropical than in tem- 

 perate regions; but the eleven-year cycle of solar activity, repre- 

 sented by a periodic increase and decrease in the number of sun- 

 spots, is said to show up better in the tropics, where the annual 

 cycle is more constant. The level of Lake Victoria and of some 

 other lakes in East Africa, rises and falls with the increase and 

 decrease of sunspots, and there is evidence of similar correlations 

 with records of rainfall in West Africa, but this is not so conclusive. 

 The eleven-year cycle is believed by some to account for the apparent 

 southward migration of peoples along the southern borders of the 

 Sahara. During the dry years of the cycle they are driven south to 

 maintain connection with permanent water, and then, taking to a 

 more fixed state of agriculture, they do not move north again dur- 

 ing the wet years. If this cycle is eventually proved to be estab- 

 lished over wide areas, it may well explain a number of variations 

 in the biological environment; for example, there is already reason 

 to suppose that the periodic outbreaks of locusts, which have caused 



