104 PLANT-LIFE ON LAND [CH. 



their habitat : or perhaps rather the negative side of 

 the question might force itself more clearly upon the 

 mind, viz. the failure of germs to establish themselves 

 where the conditions are more severe than their 

 constitution will stand. This study would require to 

 be carried out with separate reference to each of the 

 species represented on the list observed, since the 

 reasons for the presence of each may have been 

 distinct and separate from those of others : indeed it 

 is probable that they were so. Such questions may 

 be allowed to stand aside for the present, and the 

 first factor will remain as the subject for discussion 

 here, viz. how the given area of ground acquired the 

 germs of the plants now established there. The ques- 

 tion divides itself naturally again into two branches, 

 first the origination of the germs, and secondly, the 

 means of transfer of the germs themselves, or those 

 of their ultimate parents, to the spot where they have 

 developed into the plants which we see. 



It will be obvious to any one who recognises fully 

 the ordinary forces of Nature, and especially the 

 movements of air and of water, and their secondary 

 effects upon the surface of the soil, and upon the 

 organisms growing upon it, that chance will enter 

 largely into the determination whether any given 

 germ, when separated from its parent plant and 

 exposed to their action, shall lodge finally in one spot 

 or another. The limited area within reach as we sit 



