664 GEOGEAPHICAL AND GEOLOGICAL BOTANY. 



mopolitan, some water- weeds (Potamoyeton) are nearly so; Shepherd's 

 purse (Capsella bursa pastoris) is another instance. Many of the macro- 

 therms are common to the tropics of both hemispheres, the meiothenns 

 and mesothernis are less cosmopolitan. 



Instances, however, of such cosmopolitan plants are exceptions 

 to the rule, and the majority of the plants occurring over wide 

 extents of the, globe present characters which facilitate their dif- 

 fusion by natural or artificial influences. 



The plants (Phanerogamia) occupying even one third of the earth's 

 surface are but a small fraction of known plants. Many of these are 

 aquatic or subaquatic plants ; and a considerable number belong to the 

 list of weeds which accompany man, growing in cultivated land or 

 rubbish &c. 5 few or no woody plants occur in the lists hitherto pub- 

 lished. 



It is observable that those cosmopolitan species which occur 

 widely spread over two continents are found also in the adjacent 

 islands. 



The Arctic plants occurring in the continents of the Old and New 

 World are found also in the Faroes, Iceland, and the Aleutian Islands. 



Plants of Restricted Distribution. On the other hand, certain 

 plants occur only within very narrow limits ; this is the case with 

 many continental species, but more particularly with insular plants, 

 as mentioned in a subsequent paragraph under the head of insular 

 floras. 



Area of Distribution. Generally speaking, the species of plants 

 of a continent are found most abundantly over a particular more 

 or less extensive tract, growing scarcer, more or less suddenly, at 

 the margins of this space. Such a space, called the area of distri- 

 bution of the species, in these cases exhibits a centre or point of 

 greatest intensity of occurrence. 



The areas of many plants extend not only over continents, but 

 over detached islands, and even to other continents. In many 

 instances species are spread interruptedly over their area, as is the 

 case with the alpine plants common to Norway, Scotland, and the 

 Alps. &c. 



Representative Species. It is usual to find the maximum of a 

 species in one country, forming a single centre ; and this same 

 species does not recur again with a centre in a distant spot with a 

 similar climate and soil. But representative spedes of the same 

 genus occur not unfrequently under such circumstances ; and this 

 is still more the case with genera of particular Orders, sometimes 

 also with the Orders of like habit. 



Thus, the Violets of Europe and those of North America are distinct ; 

 the dwarf Palm, Cliamcerops humilis, of Europe, is represented by C. 



