io Bews. — Some General Principles of Plant Distribution as 
allied species C in the northern Transvaal, and still another D, more 
xerophytic perhaps, in Namaqualand, &c. This is a very common type of 
occurrence, and in some of the larger genera numerous rare endemics can be 
grouped around a single widespread species in this way. Even in smaller 
and more uniform areas, such as the south-western region or Natal, the same 
thing can be shown to take place. In other cases, only two species are 
involved, the one widespread, the other usually more mesophytic and always 
more restricted in its range. 
2 . A species A is produced in or brought into a certain suitable 
climatic area and it spreads until it reaches the limits of that area. Then 
it gives rise to another closely allied species B, which spreads over the next 
climatic area, and the process may even be repeated with the production 
of a species C. Examples of this can be given, but it is hardly worth 
while speculating whether B could again produce A on the other side of B’s 
own area. As long as one is only dealing with habitat varieties, no doubt 
this could happen, but with differences of specific rank the matter would be 
considered highly controversial and it would be impossible to prove in any 
particular case that A had not been distributed to the other side of B’sarea 
by ordinary methods of migration. 
3. Several species A, B, C, &c., which are closely allied are often found 
to have more or less the same distribution. Different explanations of their 
origin are possible. Species A may have given rise to B and to C ; or A 
may have given rise to B and then B to C, or, on the other hand, an 
ancestral species may have broken up into A, B, and C. 
4. The same kind of thing may happen in the case of various species 
A, B, C, &c., which do not have the same distribution but whose areas 
overlap, though this category is hardly worth distinguishing. 
5. Polygenesis . There are several examples to be given later of 
a widespread species A, and another closely allied species B, the latter 
having widely discontinuous distribution. B is often a subtropical species 
recorded only for the Natal coast-belt and the northern Transvaal, or for 
the north-western and again for the north-eastern side, while A is a tropical 
species widespread to the north. Or A may be a widespread species in 
one of the primitive habitats from Natal across the Karroo to the Cape, 
while B occurs only in Natal and at the Cape in mesophytic forest habitats. 
Now the usual assumption has been that B has migrated from one meso- 
phytic habitat to the other, or from one subtropical habitat to the other, 
an assumption which of course it is difficult to disprove and probably 
is correct in many or indeed in the majority of the cases. Ordinary 
migration, of course, must explain things when B, as in the case of 
many forest trees, is an isolated species without any closely connected 
species A growing in between its mesophytic habitats. It is worth 
noting that, in such cases, the fruits or seeds are usually distributed 
