293 
Plant Distribution in South Africa. 
development is forest. If now we suppose that two separate groups of 
species were produced by mutation at the same time, one inside the forest, 
the other outside, but, as might easily happen, very near to each other 
geographically, the former would only spread up to the limits of the forest 
climatic area. In South Africa at the present time such areas are com- 
paratively limited in extent, and are separated by drier areas, which any 
forest species has difficulty in crossing. The species produced outside the 
forest are of necessity of a more xerophytic light-demanding type, and are 
able to spread over all the drier areas, but cannot invade the forest. They 
can, however, extend into regions, such as all the dry western parts of South 
Africa, where the climatic conditions are such that no forest development is 
possible. In course of time they come to have a much wider distribution 
than the species which we have postulated to be of the same age. 
It may be well to point out that it is not necessary to take a large area 
like the whole of South Africa to find an application for the above principle. 
Even in smaller areas, such as Natal, it applies equally well. The early stages 
of succession contain the same species on the coast-belt, in the mid- 
lands, and in the mountain regions, but later stages differ much more in 
the different parts. Grassland is far more uniform over Natal than the 
scrub and forest which invade it. The widespread species over any area, 
large or small, belong to early stages of the succession. It must be re- 
membered that in Natal climatic variations are great, even in small areas, 
the valley climates differing from those of the surrounding hills. The 
climax type in the former is dry thorny scrub, but that of the latter meso- 
phytic forest. Nevertheless the differences in the two types of grassland 
which precede (the low veld in the case of thorny scrub and the high 
veld in the case of forest) are only slight. The same grassland species 
range over both (2, 5). 
Though it appears to be true (for South Africa at least) that wide- 
spread species appear early in the succession, it does not of course follow 
that all species which appear early in the succession are widespread. 
Many pioneer species, e. g. numerous species of Crassula , which colonize 
rocks, are, so far as is known, not particularly widespread. Such species 
have not reached the limits of their climatic areas, and, if we apply the 
age and area law to them, may be ranked as relatively young ; but it 
should further be remembered that pioneer species are readily killed 
out by species which appear later in the succession, and for strict pioneers 
any close relatively stable plant community is itself a barrier. Most 
species of Crassula cannot invade grassland, much less forest, and they may 
find it difficult to migrate from one rocky region to another across great 
stretches of grassland. 
