XX THORNS 261 



Moore, in his interesting journeys in and uroiuid 

 Lake Tanganyika, points out that park-lands of this 

 character cover immense areas in the interior of Africa. 

 He also makes the following shrewd observation : — In 

 England the existence of a park implies the operations of 

 park-keepers, or gardeners, to keep the trees free 

 from brambles, briars, and similar bushes. In the 

 natural African parks there are no keepers, but the park- 

 like character of these districts is well maintained. From 

 a careful review of the subject Moore shows that these 

 park-lands do not occur on hill-sides or upon rocky 

 ground : They are invariably found on alluvial plains, 

 or upon old lake deposits ; that is, on flats made up of 

 blown sand, or ground of aqueous origin. An African 

 park-land is a phase in a series of changes which follow 

 the retreat and drying up, or the change in position of 

 water on the face of the land. The production of an 

 African park marks a phase in a gradual physical 

 change. When a lake contracts within its own bed, or 

 the positions of its shores are changed by other means, 

 the exposed floors of mud and alluvium become first 

 desert steppes, then steppes covered with grass and 

 young euphorbias, then plains covered with euphorbias 

 and bushy patches of trees, then park-land, and finally 

 complete forest, in which the euphorbias become buried 

 in the bushes which they originally sheltered. 



It is a remarkable fact that all the lakes in the 

 western as well as the eastern arm of the Rift Valley 

 exhibit clear signs of shrinking. So far as the eastern 

 arm is concerned it is a simple observation to determine 

 that each lake has been far more extensive than to-day. 

 Probably it is due to some slow physical alterations in 

 the land, and the forces producing them are acting to-day. 

 Moore suggests that the relations of euphorbias to park- 

 lands could be made to throw light on the matter. 

 Euphorbias have a definite average rate of growth, and 

 if this rate could be determined, it would be possible to 

 speak with some certainty about the time occupied by 



