Human Factor in Plant Evolution 295 



greatly increased, and a wider range of variation 

 may be thus developed, of which the experimenter 

 takes advantage. It is in this way that man has 

 most conspicuously acted as a real creator of new 

 plant forms, many of which, as we have said, are 

 very different from any natural species. How far 

 plants naturalized for a long time in a new country, 

 either intentionally or otherwise through human 

 agency, have become permanently altered, has been 

 but little investigated ; but it would certainly be very 

 interesting to know whether or not weeds, for ex- 

 ample, after two or three centuries, have diverged 

 perceptibly from the type of the same species grow- 

 ing in the original habitat, and whether such differ- 

 ences would be lost if these plants were grown for 

 a series of years in their old home. 



Changes in European Plants Introduced into 

 America. It is well known that the standard fruits 

 introduced into America from Europe have varied 

 extremely, without any conscious selection on the 

 part of man. This has been very carefully studied 

 with reference to apples, especially, many varieties 

 of which were introduced originally from Europe. 

 By 1817, according to Professor Bailey, over 60 

 per cent of the best varieties in the United States 

 were of American origin, and at present probably 

 over 80 per cent are native varieties. Most of these 

 American types do not succeed in England, owing 

 to the different climatic conditions. It is quite 

 probable that a similar study of the accidentally in- 



