PERTINACITY AND PREDOMINANCE OF WEEDS. 237 



Professor Claypole, of Antioch College, Ohio, has recently 

 contributed to the " Third Report of the Montreal Horticul- 

 tural Society " (1877-8) an interesting- essay, u On the Migra- 

 tion of Plants from Europe to America, with an Attempt to 

 explain Certain Phenomena connected therewith." The phe- 

 nomena which he would explain are the abundant migration 

 of numerous weeds from Europe to the shore of North Amer- 

 ica, while others fail to come, and the general failure of North 

 American weeds to invade Europe. We have offered a fairly 

 good explanation of the first. And Prof essor Claypole goes 

 far toward explaining the second when he notes that seed is 

 (or formerly was) mainly brought from the Old World to the 

 New, and the same may be said of cattle and other emigra- 

 tion ; that the cooler and shorter summer of the north of Eu- 

 rope renders the ripening of some seed precarious, etc. lie 

 does not mention the fact that American plants by chance 

 reaching Europe have to compete with a vegetable world in 

 comparatively stable equilibrium of its species, while Euro- 

 pean weeds coming — or which formerly came — to the 

 United States found the course of nature disturbed by man 

 and new-made fields for which they could compete with ad- 

 vantage. But this ingenious hypothesis is that weeds have a 

 peculiarly " plastic nature, one capable of being moulded by 

 and to the new surroundings," by which the plant w * ere long 

 adapts itself, if the change is not too great or sudden, to its 

 new situation, takes out a new lease of life, and continues in 

 the strictest sense a weed ; that the plants of the European 

 flora possess more of this plasticity, are less unyielding in 

 their constitution, can adapt themselves more readily to new 

 surroundings," and that it is "the lack of this plasticity in 

 the American flora which incapacitates it from securing a foot- 

 hold and obtaining a living in the different conditions of the 

 New World ; " that although "in the Miocene era the Euro- 

 pean and American floras were very much alike."* yel " since 

 that era the European flora has been vastly altered, while the 

 American flora still retains a Miocene aspect, and is therefore 

 the elder of the two ; that this long persistence of type in 

 the American flora may have induced, by habit, a rigidity or 



