CHAPTER XLVIII. 



LAWS AND LIMITS OF PLANT MIGRATION. 



961. The object of this chapter is to discuss briefly the natural 

 laws of the movement of plants over the earth, not only the move- 

 ment of plants from one part of the earth to another, into terri- 

 tory beyond its "range," but the movement of plants within ter- 

 ritory already occupied by a given species. Manifestly when 

 there are no great climatic or physiographic changes taking place, 

 the laws of movement in general would be the same whether the 

 species was moving into new territory or moving about within 

 territory already occupied by it. The word distribution is some- 

 times employed instead of migration, and it is also used to indi- 

 cate the "range" or the territory already occupied by a species, 

 or refers in general to the location of the different elements of the 

 earth's floral covering. Migration has also a twofold signifi- 

 cance. It may be used to indicate great movements of plants 

 from one region to another during prolonged climatic changes; 

 or movement of species into new territory where conditions are 

 congenial and climate is stable, or back and forth over territory 

 already occupied. The terms are to some extent interchange- 

 able and will not be used here in any strict sense, since the con- 

 nection in the text will usually make the sense clear. It is not 

 proposed to discuss the geographic distribution of plants in the 

 sense of the "static" distribution. We are not dealing here with 

 the floral elements of the earth, but with the -vegetation elements.* 



* Flora, or floral elements, refers to species. Vegetation elements refers 

 to the character of the vegetation without regard to species. Thus several 

 different species or floral elements may show but one vegetative type. 



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