THE TANGANYIKA PROBLEM. 
117 
Once started, the groups of trees and patches of 
bush which marked the graves of their former bene- 
factors, the euphorbias, spread gradually under the pro- 
tection of their own shadows, until finally the patches 
ran together and more or less coalesced into the ragged 
forest which covers the higher portions of these long- 
alluvial slopes. 
It will be obvious that we have thus, in the simple 
natural dispersion and growth of euphorbias over desert 
steppes and in their mode of growth, a completely satisfac- 
tory explanation of the formation of park-lands, and their 
relation to steppes and forest ; the process of their forma- 
tion being a natural sequence of events following upon 
the scattering-, through the agency of the wind or birds, 
of the seeds of a single tree. But at the same time the 
appearance of a park-land is seen also to be one 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. 
And further, it appears to be as true of the natural park as 
of the artificial one, that unless it is kept up, it must, in the 
course of time, disappear and become converted into more 
or less thick forest. Its appearance is simply the ex- 
pression of progressive physical change. There appears to 
be no agency, except a park-keeper which is capable of 
maintaining a park, any more in Africa than in England ; 
and perhaps the most singular, or at any rate the most 
interesting thing, which the foregoing observations teach us, 
is that the African parks are absolutely impermanent, and 
are, in reality, direct and incontestable evidence in them- 
selves of wide-spread physical changes in the lands on 
which they exist. 
But not only is the existence of a park-land evidence of 
recent change in any district in which it may occur, it is 
