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 interésting essay, ‘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 Professor 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 Ku- 
rope renders the ripening of some seed precarious, etc. He 
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 “ 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,” yet “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 
