Human Factor in Plant Evolution 289 



In the case of primitive man, especially in the 

 tropics, the effects of cultivation were probably in- 

 significant. A few acres of forest burned off, or 

 the trees girdled and left to die, furnished the crude 

 field where he planted his crop of yams and corn, 

 and after a few crops had been taken off, the plot 

 was left to revert to forest. Very different has 

 been the case in modern times, where man has 

 spread over the whole world, and profoundly 

 changed the character of the vegetation of vast 

 areas of the earth's surface. These changes have 

 been going on steadily in many parts of the Old 

 World for many centuries, but it is in the more re- 

 cently settled regions, like the United States and 

 Australia, that the great changes in the vegetation 

 due to the invasion of man can be best appreciated. 

 Perhaps the United States, more than any other 

 country, will illustrate this most vividly, owing to 

 the great rapidity with which it has been settled 

 during the past century. 



Changes in America Due to Cultivation. A hun- 

 dred years ago, all but a small part of the Atlantic 

 third of the United States was an almost unbroken 

 forest, with very little open land except marshes 

 and swamps, and in the western parts, the small 

 prairies, which were the outposts of the great plains 

 of the trans-Mississippi region. Aside from the 

 trees, the undergrowth consisted of shrubs and 

 herbs fitted for the most part to growth in the 

 dense shade of the forest, and quite unadapted to 



