XXIX 



in external conditions which could have occurred within 

 the areas over which plants of limited distribution have 

 been disseminated, are capable of bringing about those 

 diversities of form which we have here ventured to attribute 

 to them, how is it that we find others spreading, as it 

 would seem, over the entire globe, and yet presenting a 

 remarkable uniformity of character ? There are plants, for 

 instance, like the Sonchus oleraceus, which, without the aid 

 of man, have been dispersed through all latitudes from 

 England to New Zealand; others, like the Primula farinosa, 

 which, although everywhere rather rare, start up now and 

 then in the most distant countries, without any apparent 

 reason; and a still larger number, which seem to follow 

 everywhere the migrations of man, such as the common 

 Shepherd's Purse, the Clover, and the like. Now, in these 

 instances, no change of importance has been induced by 

 climate, but each plant continues to preserve the impress of 

 its kind. 



The answer to this perhaps may be, that the very reason 

 why such plants have been able to diffuse themselves over 

 such distant regions, is their comparative unsusceptibility 

 of change, their power of resistance, that is, to external 

 agents, in short, their relative hardihood of structure. 



