238 ESSAYS. 
indisposition to change ;” that “the European is thus better 
able to adapt itself to the strange climate and conditions — 
that is to emigrate —than the American: and thus, being 
more plastic or adaptable, it succeeds in the New World, 
while the less adaptable American flora fails in the Old 
World.” | 
So far as we know, the greater plasticity of European 
as compared with American plants is purely hypothetical. 
“More plastic” would mean of greater variability, which, if 
true, might be determined by observation. Because Europe 
once al more species or types in common with North Amer- 
ica than it now has, it does not seem to follow that the former 
has “a younger plant-life,” or that its existing plants are more 
recent than those of the American flora. And as already in- 
timated, so refined an hypothesis is hardly necessary for the 
probable explanation of the predominance of Old World weeds 
in the Atlantic United States. 
Mr. Henslow, in his remarkable memoir, “ On the Self- 
Fertilization of Plants,” derives from different but equally 
theoretical premises an opposite conclusion, — namely, that 
weeds or intrusive and dominant plants in general, and of 
great emigrating capabilities, have ‘‘a longer ancestral life- 
history than their less aggressive relatives.” He also main- 
tains that weeds, and plants best fitted for domination in the 
manner of weeds, possess a common characteristic to which 
this dominance may be attributed, namely, that they are in 
general self-fertilized plants. A rapid generalizer might find 
confirmation of this in the converse, which is obviously true, 
that plants with blossoms very specially adapted for cross-fer- 
tilization by particular insects, and therefore dependent on 
such special aid, are comparatively local and unaggressive ; 
yet some of these are widely distributed. It will also be 
understood that self-fertilization may give advantage to an 
intruding plant at the outset by enabling an exceptionally 
well-fitted individual to initiate a favored race. And self- 
fertilization, with its sureness, would always be most advan- 
tageous unless cross-fertilization brings some compensatory 
advantage greater on the whole than that of immediate sure- 
ness to fertilize. 
