264 ESS A YS. 



geological period, and bounded on the west by a great moun- 

 tain range mainly clothed with an alpine flora requiring the 

 protection of snow in winter, and on the north by a warm- 

 temperate region whose flora is mainly of modified sub-tropical 

 origin — the only plants that could occupy the newly-formed 

 region were the comparatively few which, though developed 

 under very different conditions, were sufficiently tolerant of 

 change to adapt themselves to the new environment. The 

 flora is poor, not because the land cannot support a richer one, 

 but because the only regions from which a large population 

 could be derived are inhabited by races unfit for emigration." 



Singularly enough, this deficiency of herbaceous plants is 

 being supplied from Europe, and the incomers are spreading 

 with great rapidity ; for lack of other forest material even 

 Apple-trees are running wild and forming extensive groves. 

 Men and cattle are, as usual, the agents of dissemination. 

 But colonizing plants are filling, in this instance, a vacancy 

 which was left by nature, while ours was made by man. We 

 may agree with Mr. Ball in the opinion that the rapidity with 

 which the intrusive plants have spread in this part of South 

 America " is to be accounted for, less by any special fitness 

 of the immigrant species, than by the fact that the ground is 

 to a great extent unoccupied." 



The principle applies here also ; and in general, that it is 

 opportunity rather than specially acquired vigor that has 

 given Old World we%ds an advantage may be inferred from 

 the behavior of our weeds indigenous to the country, the 

 plants of the unwooded districts — prairies or savannas west 

 and south, — which, now that the way is open, are coming in 

 one by one into these eastern parts, extending their area con- 

 tinually, and holding their ground quite as pertinaciously as 

 the immigrant denizens. Almost every year gives new exam- 

 ples of the immigration of campestrine western plants into 

 the eastern States. They are well up to the spirit of the 

 age; they travel by railway. The seeds are transported, 

 some in the coats of cattle and sheep on the way to market, 

 others in the food which supports them on the journey, and 

 many in a way which you might not suspect, until you 



