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18 THE WORLD OF LIFE CHAP.
rotundi folia), which are found in all Northern Europe, Asia,
and America ; while our common Sowthistle (Sonchus
oleraceus) is found scattered over the whole globe, tropical
as well as temperate, and is perhaps the nearest of any
known plant to being truly cosmopolitan.
By a laborious comparison the author arrives at the
conclusion that the average area occupied by the species of
flowering plants is yj^th part
the globe.
of the whole land surface of
But the area varies enormously in different parts
of the world. Thus, in the whole Russian Empire, species
have a mean area of
-^jth
the land surface, owing to the fact
that so many range east and west over a large part of
Europe and North Asia ; while in South Africa the mean
range is only 20 1 oo t^1 ^ ^at surface, which expresses the
fact of the extreme richness of the latter flora, many of the
species composing which have extremely restricted ranges.
He also reaches the conclusion that in passing from the pole
to the equator the mean areas of the species become smaller.
A few examples of very limited areas are the following :
Several species of heaths are found only on Table Mountain,
Cape of Good Hope ; Campanula isophylla grows only on
one promontory of the coast of Genoa ;
Gromwell (Lithospermum Gasfoni),
the beautiful Alpine
on one cliff in the
Pyrenees ; Wulfenia Carinthiaca, on one mountain slope in
Carinthia ; Primula imperialis, on the summit of Mount
Pangerago in Java, and many others.
In order to compare the plants of different parts of the
world in their various relations, De Candolle divides the
whole land surface into fifty botanical regions, each dis-
tinguished by the possession of a considerable proportion of
peculiar species of plants. These regions are of greatly
varying extent, from No. 18, comprising the whole of
Northern Asia, to No. 10, limited to the small island of
Tristan d'Acunha in the South Atlantic.
The list is as follows :
1. Arctic zone.
2. Europe, temperate.
3. Mediterranean.
A. DE CANDOLLE'S BOTANICAL REGIONS
4. Azores, Madeira, Canaries.
5. Sahara, Cape Verde Islands.
6. Guinea N., Soudan.
7. S., Congo, Benguela.
8. Island of St. Helena.