SECT. XXVI. BOTANICAL DISTRICTS. 251 



tary, at elevations as high or even higher than Mont Blanc, 

 ahound in Alpine and European genera, the species are univer- 

 sally different, with the single exception of the Rhodiola rosea, 

 which is identical with the species that blooms in Scotland. It 

 is not in this instance alone that similarity of climate obtains 

 without identity of productions ; throughout the whole globe a 

 certain analogy both of structure and appearance is frequently 

 discovered between plants under corresponding circumstances 

 which are yet specifically different. It is even said that a differ- 

 ence of 25 of latitude occasions a total change, not only of 

 vegetable productions, but of organised beings. Certain it is 

 that each separate region both of land and water, from the 

 frozen shores of the polar circles to the burning regions of the 

 torrid zone, possesses a flora peculiarly its own. The whole 

 globe has been divided by physical geographers into various 

 botanical districts, differing almost entirely in their specific vege- 

 table productions, the limits of which are most decided when 

 they are separated by a wide expanse of ocean, mountain chains, 

 sandy deserts, salt plains, or internal seas. A considerable 

 number of plants are common to the northern regions 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 almost complete diversity of species that everywhere prevails. 

 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. Humboldt and Bonpland 

 in Equinoctial America identical with those of the Old World ; 

 and Dr. Robert Brown not only found that a peculiar vegeta- 

 tion exists in Australia between the 33rd and 35th parallels of 

 south latitude, but that at the eastern and western extremities of 

 these parallels not one species is common to both, and that cer- 

 tain 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 conveyed 

 thither by the colonists ; but the greater part of that continent 

 is still unexplored. However, this proportion exceeds what has 



