266 DISTRIBUTION OF PLANTS. SECT. XXVII. 



sandy deserts, salt plains, or internal seas. A consider- 

 able number of plants are common to the northern re- 

 gions of Asia, Europe, and America, where the continents 

 almost unite ; but in approaching the south, the Floras 

 of these three great divisions of the globe differ more 

 and more even in the same parallels of latitude, which 

 shows that temperature alone is not the cause of the al- 

 most complete diversity of species that everywhere pre- 

 vails. The Floras of China, Siberia, Tartary, of the 

 European district including Central Europe, and the 

 coast of the Mediterranean, and the Oriental region, 

 comprising the countries round the Black and Caspian 

 Seas, all differ in specific character. Only twenty -four 

 species were found by MM. Bonpland and Humboldtin 

 Equinoctial America identical with those of the old 

 world: and Mr. Brown not only found that a peculiar 

 vegetation exists in New Holland, between the 33d and 

 35th parallels of south latitude, but that, at the eastern 

 and western extremities of these parallels, not one spe- 

 cies is common to both, and that certain genera also are 

 almost entirely confined to these spots. The number of 

 species common to Australia and Europe are only 166 

 out of,4100, and probably some of these have been con- 

 veyed thither by the colonists. This proportion exceeds 

 what is observed in Southern Africa, and from what has 

 been already stated, the proportion of European species 

 in Equinoctial America is still less. 



Islands partake of the vegetation of the nearest con- 

 tinents, but when very remote from land their Floras 

 are altogether peculiar. The Aleutian Islands, extend- 

 ing between Asia and America, partake of the vegeta- 

 tion of the northern parts of both these continents, and 

 may have served as a channel of communication. In 

 Madeira and Teneriffe, the plants of Portugal, Spain, 

 the Azores, and of the north coast of Africa are found ; 

 and the Canaries contain a great number of plants be- 

 longing to the African coast. But each of these islands 

 possesses a Flora that exists nowhere else ; and St. 

 Helena, standing alone in the midst of the Atlantic 

 Ocean, out of sixty-one indigenous species, produces 

 only two or three recognized as belonging to any other 

 part of the world. 



