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Ch. XL.] 



MIGEATION OE PLA.::^TS. 



385 



There are however, not a few species whicli are common 

 to two or more than two of these provinces, and often repre- 



geographical varieties 



me naturahsts wonld class as mere 

 The six ornithological divisions of 



the slobe before aimaed to (p. 335), four of them 



World and two in the New 



com 



view of the leading features in their geographical distribution, 

 especially as regards genera and families. 



This holds true, particularly of the Neoarctic and ISTeo- 

 tropical regions, each of which contains a distinct assem- 

 blage of peculiar vegetable forms. Those of the table-land 

 of Brazil, which has an elevation of from 2,000 to 4,000 feet, 

 are described by Sir Charles Bunbury, after he had explored 

 the district, as belonging for the most part to generic types, 

 little known except to botanists, for they have not been 

 cultivated in Europe. But when he descended from the 

 Brazilian uplands towards the south, or to the grassy plains 

 of Uruguay and La Plata, he found plants still belonging to 



■American tyi: 



by diiferent and local species. 



affinity 



between the 



specific forms proper to the more elevated and to the lower 

 stations agrees well with the idea of certain original types 

 having been gradually adapted by variation and natural 

 selection to all the diversified conditions of the surface of the 



land. 



The Pampas and banks of the Plata are also remarkable 

 for the extraordinary manner in which some foreign European 

 plants, especially the thistles and trefoils, have overpowered 

 the indigenous vegetation."^ The intruders have been intro- 

 duced by man sometimes unintentionally, and, having na- 

 turalised themselves, have become more conspicuous than any 

 of the native products of the soil. They illustrate a principle 

 before laid down, that the organic beings of each great region 

 which man finds in possession of w^ide areas are not those 

 which are most fitted of all contemporary species to flourish 

 there to the exclusion of all others. They appear to be 



* Sir C. BunLury, ' Characters of S. American Vegetation,' Eraser's Magazine, 

 July, 1867. 



VOL. II. C C 



