Plant Communities of the World 265 



morph in continental Algiers. Temperature modifies the mois- 

 ture factor. Factors not only modify others; they may replace 

 them. Replaceability of factors is very important. It makes it 

 possible for a vegetation to thrive in localities where various 

 factors have changed, and this allows the spreading of plant 

 communities. For the investigator, it makes the characterizing 

 of vegetation difficult. Language employs factors for character- 

 izing. A locality is judged differently, according as we empha- 

 size the changed or the unchanged factor. Formerly, vegetation 

 was grouped climatically and the change of situation by edaphic 

 factors was neglected. Edaphically conditioned vegetation did 

 not fit. These difficulties are solved if the replaceability of fac- 

 tors is duly taken into consideration. 



The replaceabilities modify thoroughly the law of minimum. 

 A replacing factor may raise the minimum of another factor or 

 may lower it — Roosevelt has annihilated the hampering factor 

 of lameness by an extraordinary factor of will. A species or a 

 community may be well adapted to a climate and thrive if un- 

 limited soil is at its disposition. But it may lack strength to battle 

 against the competition of invaders. Good, thriving, natural 

 vegetation in New Zealand has been dispossessed by invaders 

 accidently brought into the country by man — invaders which 

 had a stronger power of competition and therefore raised higher 

 than the existing conditions the needed minimum for mainte- 

 nance of the old vegetation. Dispossession was the result. Taking 

 replaceability into account, edaphic factors find their modifying 

 value in climatic bases for classifying vegetation. The entire 

 ecology may be more or less grasped, and the system becomes 

 more and more natural.* 



* More explicitly than can be done here, but still concisely, all these questions 

 of general sociology are treated in my Soziologie ("Geographie der Pflanzen" 

 Handworterbuch derNaturwissenschaften, Bd. IV, p, 1044; Gustav Fischer, Jena.) 



