XX 
THORNS 
26 
Moore, in liis interesting journeys in and around 
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 Eift 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 
