264 Eduard Riibel 



posed by C. E. Moss in 1910). The following unity is the associa- 

 tion "order" ("subformation" DuRietz) with the ending -etalia 

 (neutrum plurale) introduced by Koch. The recognition of the 

 community orders of the world is not yet far advanced; very 

 much remains to be done. Gapless further unions are not yet 

 possible. With the highest unities of communities the situation 

 is different; they come from another line of research — ^from the 

 attempt at classification. This is an old task. Here, as everywhere 

 in every science, works the fundamental need of the human 

 mind for order, for grouping, so to be able to apprehend the 

 infinity of things existing side by side. Only in orderly fashion 

 can mind render account of things. An investigator needs a 

 synthesis of his field of research, a system. We have systems in 

 philosophy, we have systems of the plants of the world; we need 

 a system of the plant communities of the world. 



A plant community is something eminendy ecologic; to ecol- 

 ogy, then, must be paid the first attention in arranging a system. 

 A very essential point is connected with the law of minimum 

 and the replaceability of factors. The law of minimum, founded 

 by Liebig for cultures, is also valid for natural plant communi- 

 ties — and for one's own self-discernment. Any factor can only 

 be effective when other factors also act sufficiently. It decides the 

 factor which is present in a minimum. A soil may be highly 

 nutrient, yet the nutrient substances do not work if moisture is 

 lacking. Water and sun accomplish nothing if there is no nutri- 

 ment. The minimum factor becomes the deciding one. Which 

 it is, varies. Therefore, a classification may not be founded on a 

 single factor, nor on a few factors. The single factor, moreover, 

 acts difFerendy according to other factors accompanying it. Abso- 

 lute numbers in a factor say very little; all is relative. Fifty centi- 

 meters of rainfall acts hygromorph in oceanic Ireland, xero- 



