CH. vn] PLANT POPULATION 103 



limited circle to have been duly recognised; any 

 active mind will ask, Why are these plants here? 

 How did they get here? Why these instead of a 

 hundred others? You may perhaps be able, where 

 the ground is open and not under cultivation, to rule 

 out the direct hand of Man, so often a disturber of 

 the order of Nature. With some certainty it may 

 then be concluded that it is really Nature's own 

 achievement which has been observed and analysed. 

 How then does Nature bring it about that certain 

 species are present, and others absent from this 

 particular area of the earth's surface ? 



There are two main factors in this problem. It is 

 quite clear that as all living plants arise from their 

 like, in some way the germs of the plants recognised, 

 or of their ultimate parents, must have arrived within 

 the area observed. The first factor is then how this 

 area acquired the germs of the plants living there. 

 But further the growing plant developes from the 

 germ only when the conditions are favourable. From 

 the presence of any such growing plant it may then 

 be inferred that the conditions have been at least 

 tolerable.^ That they must necessarily be so is the 

 second factor in our problem. It is not proposed 

 here to consider the effect of external conditions 

 upon the growth and establishment of each germ as 

 an independent plant. That would involve some 

 review of the means of accommodation of plants to 



