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500
TYCHO NOR LINDH
number of much advanced types in tbat area. All Ihe same, I will nol
from that draw Ihe cpnclusion that the entire |ribe Calenduleae derives
ils origin from the South African Senecioninae, as Small assumes.
It is hardly likely that the ancestral forms of the genus Calendula
have migrated from South Africa to the Mediterranean. My map of the
distribution of the tribe Calenduleae |map p. 474) indicales rather that
Ihe phylogenetic slock ont of which the Calenduleae developed has had
a continuous extension from South Africa to the Mediterranean.
Pre-sumably the Calenduleae has during the Tertiarv become differentiated
möre or less contemporaneously in northern, middle and southern
Africa from a series of very closely allied populations belonging to
Senecioninae’’s plexus.
Al ihe present time the tribe mainly comprises such species as are
adapted for subtropieal and warm temperate climates, hut earlier il
has possibly also embraced some purely tropical species.
How is the present, pronouncedly bicentric or bipolar distribution
of Calenduleae sens. str. best to be explained? No doubt the changes
in climate have played a most significant part in the formation of
these two centra. Uf still greater importance, however, have been the
mountain areas in North and South Africa, for in them a rich
diffe-rentiation of taxonomical units has undoubtedly taken place. These
mountain areas have, besides, constituted important refugia for the
species during periods of unfavourable climate or of keen competition
in the lowlands.
As already mentioned above, 1 consider that the pre-Calenduleae
ancestors during some period of Ihe Tertiary have had cpiite a wide,
continuous distribution from North Africa to South Africa. During Ihe
alternations between arid or semi-arid and pluvial periods in tropical
Africa the species populations of the tribe were very greatlv decimated.
The extremely keen competition between the plants in the tropical belt
has certainly also been a cause of this decimation of Ihe populations.
Only insignificant relics of this earlier connection between Calendtdeac
of Ihe northern and southern hemispheres are now lef t in tropical
Africa. As they have been discussed in detail in previous chapters, 1
shall here give only a brief survev of them.
Near the equator grows Osteospermum Volkensii, which must be
regarded as a fragment of a species population that has no doubt had
a wide distribution in East Africa during an earlier period. This species,
which is a large shrub with opposite leaves, has hitherto only been
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