Chapter VII 



-103- 



Artificial Factors 



Man has played a considerably greater role in the distribution of 

 weeds, which have either accompanied the crops grown by man or 

 directly accompanied man in his migrations. These are plants possess- 

 ing in most cases a wide range of tolerance, so hardy and unfastidious 

 that they can spread rapidly far and wide. 



All lands now have their specific weeds, a study of the origin of 

 which reveals a remarkably close connection with the trade routes and 

 migrations of man. Thus, North America has at present about 500 

 species of weeds brought from Europe. All the American representa- 

 tives of a number of genera — Lamium, Malva, Medicago, Melilotus, 

 etc. — had their origin in Europe and bear an adventive character. 



Fig. 19. — Map showing the gradual migration of the adventive plant, Senecio vernalis, 

 into western Europe from the East beginning with the year 1726. (After Beger, from Hegi). 



Conversely, Europe has a number of species that were brought from 

 America, such as Erigeron canadensis, now common throughout almost 

 all of Europe, and Elodea canadensis, which has become so widely 

 distributed in European waters that it constitutes an obstacle to 

 shipping. 



The large number of weeds in existence was, until recently, taken 

 as proof of the predominant significance of man as a factor in the geo- 

 graphical distribution of plants. Basing themselves on the many, 

 diverse possibilities for the transfer of plants by man and the great 

 abundance of weeds in existence, the extreme advocates of the role of 

 man concluded that all aberrations in plant distribution — discontinu- 

 ous areas, isolated habitats, relics out of harmony with present-day 



