19 
illustrated by the South African Flora . 
When we look at the present-day distribution of the species of Rhus 
in South Africa, we find certain of them (e. g. Rh. den tat a, Rh. villosa) 
very widespread, as pioneer species in the xerosere. Others (e. g. Rhus 
obovata ) occur along the streams and are important ip the hydrosere, or 
(e. g. Rh. crenata) occur on the coast sand-dunes in the psammosere. As 
succession advances, we find species of Rhus (Rh. dregeana , Rh. erosa ) 
dominant in climax vegetation on the dry doleritic Karroo kopjes. These 
two species are closely allied and may have been derived one from the other 
or by the splitting of an ancestral type. In more mesophytic forest areas 
we find various steps in the adaptation to more favourable conditions in 
a large series of forms till we reach such large forest-trees as the Red 
Currant (Rh. laevigata ), which grows fifty to eighty feet high, and two to 
four feet in stem diameter. Rhus also shows adaptation to purely grassland 
conditions in the species Rh. discolor , which takes its place with numerous 
other associated plants of the grassveld. There are few examples of 
adaptation to grass-land conditions from the genera of trees and shrubs, the 
evolution of species being, as far as South Africa is concerned, usually in 
the other direction, widespread xerophytic types producing more meso- 
phytic forms. 
Leguminosae. 
It will be necessary now to pass over certain large and important 
families in the briefest possible way. The present-day development of 
the Leguminosae could be made the subject of a monograph by itself. 
If we take any two allied species or any group of allied species, again 
and again we find that species A with a wide distribution has apparently 
produced one or more, B, C, D, &c., each with restricted and in some 
cases with discontinuous distribution, or there has been an apparent 
breaking up. Examples are seen in the genera Podalyria , Crotalaria , 
Rafnia , Lotononis , Argyrolobium , Lebeckia , Buchenroedera , Aspalathus , 
Psoralea , Indigofer a, Tephrosia , Lesser tia^ Vigna , Dolichos , Eriosema , and 
Acacia. At the same time examples of isolated species with no obvious 
connexions are to be found, e. g. Sutherlandia frutescens , but they are not 
numerous. The suborders should, of course, be studied separately. The 
Caesalpineae invade South Africa from the tropics only along the eastern 
side as far as Uitenhage, and in the case of Schotia speciosa as far as Mossel 
Bay. Several of these tropical outliers in South Africa are interesting, 
e. g. Bauhinia tomentosa extends from Mozambique to Natal and is 
recorded from Angola. In South Africa, B. garipensis in Namaqualand, 
B. Bowkeri ‘along the Bashe River, Kaffraria ’, are both closely allied. 
The Mimoseae has the great genus Acacia , which is all over Africa and 
is even more characteristic of Australia. Acacia arabica extends from India 
through the whole of Africa to Natal, though the Natal variety Kraus- 
siana is now separated as a distinct but allied species, A. Benthami. 
C 3 
