12 THE ALLEGED DESICCATION OF EAST AFEICA 
state that the marks of the old water-levels on Lake Nj^asa 
are now many feet above the present water-level. This 
may, of course, be due to the wearing away of the sill where 
the Shire river leaves the lake. To go further afield, we know 
that in Livingstone’s day Lake Ngami was an open sheet of 
water, and is now nothing more than a swamp ; but these 
examples may be considered too remote from the area under 
consideration to be of any weight. According to the Duke 
of Mecklenburg, the salt lake of Katwe in South Toro is drying 
up, and the salt deposits may be seen several yards above the 
present lake-level. This is probably due to desiccation in 
historic times, as sodium chloride is such a soluble salt. 
One must exercise great caution in an inquiry of this kind 
to separate evidence of a geological character from evidence of 
what may be termed an historical character, because in dealing 
with the two classes of evidence one is thinking in different 
terms. It must be borne in mind that our accurate 
meteorological records in these parts of Africa only go back 
some twenty-five years, and in that period no great decrease 
of rainfall is marked. The evidence of the coast settlements 
above quoted goes back probably some six hundred years, 
and may go back much farther, for, as has been mentioned, 
it is possible that they date back to the zenith of the Himyaritic 
civilisation in South Arabia ; the fact that there are ruined 
mosques may not prove that these erections were coincident 
with the foundation of the settlements. For all this, however, 
there is a great jump from historic times to the later Tertiary 
age, when the Eift valley was in an unstable condition owing 
to volcanic disturbance, but there is nothing to show that the 
conditions which set in then have not continued up to the 
present era. 
After having stated a problem it is always considered 
advisable to propound a plausible solution, and it is here that 
I fear I shall fail to satisfy the critic. There are, however, a 
few points in this connection which should be set forth. One 
is the great decrease of the area under forest in the higher parts 
of East Africa, particularly in the Kikuyu region, i.e., the 
south-south-west slopes of Kenya and on the Aberdare ridge, 
during historic times. Owing to the increase of population 
