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 T Jxr tn P art °f tne whole land surface of 

 the globe. But the area varies enormously in different parts 

 of the world. Thus, in the whole Russian Empire, species 

 have a mean area of ^th 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 ^0o tn °^ tnat sur f ace > 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 ; the beautiful Alpine 

 Gromwell {Lithospermum Gastoni), 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 : — 



A. De Candolle's Botanical Regions 



i. Arctic zone. 



2. Europe, temperate. 



3. Mediterranean. 



4. Azores, Madeira, Canaries. 



5. Sahara, Cape Verde Islands. 



6. Guinea N., Soudan. 



7. ,, S., Congo, Benguela. 



8. Island of St. Helena. 



