Human Factor in Plant Evolution 287 



It still exists in the wild state in Chili and possibly 

 in the mountainous regions to the north. 



Man's Spread over the Earth Due to Agriculture. 

 With the adoption of agricultural habits, the pos- 

 sibilities of man's expansion over very wide areas 

 of the earth's surface became possible, and as a re- 

 sult of his migrations the vegetation of the invaded 

 regions has been very greatly altered. The clearing 

 of large tracts for agricultural purposes, and the 

 replacement of native plants by cultivated ones, has 

 very much changed the aspect of great areas of the 

 earth's surface all over the world. Little of Europe 

 is in a state of nature, and the same is true of south- 

 ern and eastern Asia and northern Africa. In the 

 old settled countries, like most of Europe and much 

 of Asia, all traces of the original forest have long 

 ago disappeared, and one must go to the most re- 

 mote mountain regions to find any remains of it. 



In the tropical regions the jungle quickly grows up 

 again when cultivation is neglected, but this is not 

 the case in the colder and drier climates of most 

 parts of Europe. In the United States, the greater 

 part of the dense forest of the Atlantic region has 

 given way to great cities, or to fields, meadows, and 

 orchards largely occupied by alien plants wheat, 

 corn, clover, and fruit trees, of various kinds, and all 

 the familiar garden flowers and vegetables. These 

 are, with very few exceptions, foreigners, which 

 have replaced the native forest trees and under- 



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