2 12 
Bews . — The South-east African Flora : 
The vegetation of the Tugela and its tributary, the Lower Mooi River, 
though, as the above list shows, it is rather distinct from that of the rest of 
Natal, has certain fairly close connexions with that of the dry kopjes in the 
northern Transvaal, e. g. Heeria , Vitex , Croton , Pappea , Ficus . For 
a time this appeared somewhat of a distributional puzzle. The two areas 
are separated by a great stretch of high veld grassland, many hundreds 
of miles wide, where these species do not occur. The explanation is 
suggested when we observe that the Northern Transvaal bears the same 
relationship to the great valley of the Limpopo as the Lower Mooi River 
area does to the Tugela. The species in question spread along both of these 
main river valleys. It is clear that this river migration must be connected 
up with the general coast-belt invasion of tropical and subtropical species 
from the North. 
Coast-belt Migration. 
As we have already pointed out, the maximum distribution of the 
coast-belt species is in a line parallel to the coast. The whole flora has 
close tropical affinities, but the actual tropical species diminish in numbers : 
towards the south. 
At the outset it is necessary to distinguish clearly between seashore 
migration and that of the rest of the coast-belt. Strand plants and seashore 
sand-dune species, as well as mangroves and other plants of the mud 
lagoons, whose seeds are capable of withstanding submergence in sea water, 
are rapidly and widely distributed, as Guppy has fully demonstrated ( 5 ). In 
South Africa the southern limit of the mangroves and other tropical forms 
in Tembuland is probably determined by the increasing influence of the cold 
shoreward current from the south, which flows northwards as a counter- j 
current to the warm Mozambique current from the north. The latter 
is, of course, so much larger that it warms the whole eastern side of South 
Africa and raises the average temperatures much above those of correspond- 
ing latitudes on the western side. The cold counter-current varies in 
strength at different times and in a way not thoroughly understood. 
Occasionally its influence is very marked, as when it kills the fish of the 
tropical waters with which it mixes, and these float ashore in shoals. It is 
just such extreme and exceptional occurrences that are of the greatest 
importance in their effect on the vegetation. 
While the actual coast-line is a uniform easy pathway leading to rapid 
migration for its own characteristic flora, farther inland on the coast-belt 
migration is slower, and rather pronounced changes in topography and soil 
conditions have tended to impose a check on the invasion of many species. 
Reference to any physical map of Africa will show that the 1,000 foot coast- 
belt, which is several hundreds of miles broad in Portuguese East Africa, 
becomes narrowed like the neck of a bottle at Port Durnford in Zululand. 
