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480
TYCHO NOR LINDH
Osteospermum muricatiun, in contrast to the preceding species,
is a xerophyte. It lias a very extensive distribution in semi-arid areas
(map fig. 4). A glance at the map will show that its distribution is
very discontinuous and that it occurs most frequently in the steppe
and semi-deserl areas of South Africa. In a north-weslerly direction
from this cent re it extends right up into Angola. The large gap between
its ränge in S.W. Africa and Angola (c. 700 km) should be regärded
as due to lack of investigation. It will very likely be discovered in
several places in this area. Compared with the enormous gap exhibited
by Ihe species in its distribution in East Africa, ils disjunctions in
South Africa are relatively insignificant. They may be designated as of
second order, the disjunction in East Africa being of first order. That
is the reason whv I have characterized O. muricatum as bicentric. The
gap between the least distant localilies situated in S. Rhodesia and
British Somaliland, is c. 3600 km.
Ön a previous occasion I have already attempted to explain this
interesting disjunction in O. muricatum |l p. 190). The most plausible
interpretation appears lo me to be that O. muricatum, which is a
xero-philous species with a wealth of biotypes, at one lime had a fairly
continuous distribution from South Africa up to the region bordering
upon Ihe Gulf of’ Aden and Ihe Red Sea. I consider that Ihe species
attained this unbroken range during a comparatively long semi-arid
or ariel period in tropical East Africa. Geological investigations
(Nilsson 1932, 1940, Leakey 1936) in East Africa have proved that arid
and pluvial periods have alternated with eacli other. Also in South
Africa arid and pluvial periods have alternated (Sühnge, Visser and van
Riet Lowe 1937). That part of Ihe species population which extended
farthest to the north during an semi-arid period will, during a
sub-sequent pluvial linie, have succeeded in retaining a foothold in a
number of localities favourable to the species. In the northern hemispliere
the species now survives as a relict ön the mountains of British
Somaliland and Ihe Yemen. In Ihe area between Somaliland and Rhodesia
the species population will probably have become enlirely extinct owing
lo changes in climate and competition with other plants.
The above-mentioned five species occur in Ihe mountains and
high-lands hut are wanting in the lowlands of tropical East Africa. This
|ribe can be designated as bipolar (Du Rietz 1940), since its area is
discontinuous between the northern and southern hemispheres hut
abseilt from the tropical lowlands. Osteospermum Vaillantii, O. mono-
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