GEOGllArilY OF PLANTS. 



267 



though, as we have said, these are rather famUies, as Pahnse, 

 Scitamineae, Museas, Myrteas, Sapindeae, and Anoneas ; or 

 genera, as Epidendrum, Santalum^ Olax, Cymhidiumy and 

 so forth : yet there are particular species, which grow in all 

 parts of the world only between the tropics, as, for instance, 

 Heliotropium Jndicum, Ageratum coiiyzoides^ Pistia stra- 

 tiotes, Scoparia dulcis, Guilandina Bonduc, Sphenoclea zey- 

 lanica, Abrus precatorius, Boerhama mutabilis, and so forth. 



But most commonly there are other species, which, under 

 the same degree of latitude, supply in the new world the 

 place of related species in the old. Dryas octopetala, in- 

 deed, grows equally upon the mountains of Canada, and in 

 Europe ; but Dryas tenella of Pursh, which is very like the 

 former, grows only in Greenland and Labrador. Instead 

 of the Platanus orientalis^ there grows in North America the 

 Platanus occidentalis ; instead of Thuia orientalis, Thuia 

 occiden talis ; instead of Pinus Cembra, in Europe and Asia, 

 there grows in North America, Pinus Strobus ; instead of 

 Prunus Laurocerasus^ in Asia Minor, there grows under the 

 same latitude in North America, the Prunus Caroliniana. 



394. 



There are many exceptions to this rule, however, depending 

 on circumstances that have been already noticed. In the first 

 place, countries are wont to share their Floras with neighbour- 

 ing regions, especially islands lying under the same latitude, 

 as the Azores possess the Floras of Europe and of Northern 

 Africa, rather than those of America, because they are scarce- 

 ly ten degrees of longitude from the coast of Portugal. Si- 

 cily, and still more Malta, possesses a Flora made up of those 

 of the south of Europe and the north of Africa. The Aleu- 

 tian Islands share their Flora with the north-west coast of 

 America and the north-east of Asia. But the most distant 

 countries, lying under the same latitude, may have the 

 same, or a similar vegetation, while countries, or islands 

 which lie between them, have not the least share in this parti- 

 cular Flora. The island of St Helena, whicli is scarcely 

 eighteen degrees of longitude from the west coast of Africaj 



