238 ESS A YS. 



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 had 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. 



