9 
illustrated by the South African Flora . 
where tropical species are able to cross the continent without encountering 
any barriers. This explains the connexions between the subtropical areas 
in the west and those in the east, but not the connexion between the Cape 
Peninsula and Natal. 
The Origin of New Species and Varieties. 
Species which occupy any of these primitive or relatively primitive 
habitats listed above naturally have no difficulty in spreading all over 
South Africa, and a large number of them are thus widespread. Now, if 
a species which is adapted to extreme conditions in the course of its spread 
comes into contact with more mesophytic conditions it is, as a rule, rapidly 
ousted in the course of plant succession by more vigorous growing, more 
mesophytic species. But if the primitive species is capable of varying, it 
may give rise to a new variety or it may produce a new species capable of 
holding its own with other competitors. Many examples of this will be 
given later. The genus Rhus , e. g., has many species which have developed 
in this way. 
It is a well-known fact among gardeners that the cultivation of 
a species often leads to variation or mutations. The same thing would 
appear to happen in nature when a species in the course of its migration 
comes into environmental conditions which differ from those which pro- 
duced it. In South Africa this, as a rule, implies a change from xerophytic 
or hydrophytic to mesophytic conditions, since the former are widespread, 
the latter more local. That is why widespread species belong to early 
stages of the succession. Succession is usually towards the mesophytic. 
If, however, mesophytic conditions were widespread and xerophytic condi- 
tions local, the reverse might happen. A widespread mesophytic species 
in the course of its wanderings might give rise to one more xerophytic. 
This is of much less importance in South Africa at the present time, what- 
ever it may have been in the past. However, another type is important, 
namely, when a tropical species which cannot withstand frosts gives rise to 
a temperate species which can. It has already been pointed out that to 
the north of South Africa tropical species are able to spread across the 
continent. Derived subtropical or warm temperate species invade South 
Africa at different points. 
Various Methods of Origin. 
When we seek to apply the process just outlined to concrete examples, 
we find that these tend to fall into several different categories, and the 
process itself requires further analysis. If for ‘species’ we read ‘ varieties’ 
in each case, the number of possible examples is, of course, enormously 
increased. 
i. A species A, which is widely distributed, may give rise to a closely 
allied but much rarer, more mesophytic species B in Natal, another closely 
