Distribution of Tropical Diseases in Africa. 421 
an intervening belt of calms at the equator, and it is to these 
winds that the unequal distribution of the rainfall is to be 
attributed. 
The heat, rainfall, and winds naturally affect the fertility 
of the country, and the whole area of Africa may be divided 
up as follows :—36'4 per cent. is occupied by deserts, 14°6 by 
steppes, 5°3 by serub, 21°3 by savannahs, 21°8 by forests and 
cultivated land, and 6 per cent. by the large lakes. Therefore 
half the continent is occupied by deserts and steppes. 
The typical zones of vegetation are—(1) the Mediterranean 
zone, having a vegetation similar to that of southern Europe, 
if somewhat more tropical; (2) the Sahara Desert zone, of 
which much might be cultivated, numerous oases certainly 
existing there; (3) the zone of tropical vegetation, more or 
less coincident with the areas of the highest mean annual 
temperature and heaviest rainfall; and lastly, the south- 
central and South African savannah zone. In general terms, 
the vegetation is richer as we proceed from the south to 
the north, and also from the west to the east. 
It is unnecessary to refer in detail to the fauna of Africa. 
The mammalian fauna is exceptionally varied, the bird fauna 
meagre, the reptile fauna largely developed. Insect life is 
very abundant, and the tetze fly infests large parts of the 
country which are uncleared. 
The population of Africa may be estimated at about 20 to 
a square mile. Boehm and Wagner estimate the population 
at 205 millions, Mr Ravenstein at 127 millions, with a rate ° 
of increase of 10 per cent. per decade. 
The people of Northern Africa were probably in pre- 
historic times of the same ethnical stock as that of the people 
inhabiting Southern Europe. The Arabs of the Soudan are 
probably all descendants from the proto-Semitic stock. For 
the rest we have Negroes of various kinds, but alldistinguished 
by projecting jaws, flat features, broad noses, woolly hair, 
shining skin, and pouting lips, and it is probable that they 
are all of one origin. We find also in Africa the Bushman 
and Tikki-Tikki or Akka dwarfs, all of whom I believe to be 
among the oldest primitive inhabitants of the continent. 
In classifying the ethnographic distribution of a population, 
