314 GEORGE E. NICHOLS 



then, the climate to be a regional constant, it follows that the 

 ecological aspect of an association is the direct combined func- 

 tion of the habitat factors. Carrying the process a step further, 

 it seems obvious that, with certain exceptions like those noted in 

 a preceding paragraph, all of the habitat factors are relatively 

 constant except those which are due to plant and animal agen- 

 cies. It would follow, therefore, that the ecological aspect of 

 an association is the direct combined function of these biotic 

 factors alone. Now, with these deductions in mind, it might 

 perhaps be expected to follow that through the gradual amelio- 

 ration of the habitat, brought about by the progressive reaction 

 of the biotic factors, the culminating member of every succes- 

 sional series throughout a given climatic region would be the 

 same, the only difference between them being in point of the 

 time when this ultimate condition is reached. But in this 

 connection must be taken into account Liebig's Law of the Mini- 

 mum, which states in effect that: if any reaction or process is 

 the combined function of several factors' (or variables), the ex- 

 tent to which the reaction or process may be carried is limited 

 by the effect of that factor which possesses a minimum value, 

 or which is present in relatively minimal amount. The appli- 

 cation of this law to ecological problems has already been sug- 

 gested by Adams (1) and Hooker (15); and herein lies the ex- 

 planation of what the writer proposes to term the edaphic 

 climax association. 



The edaphic climax association. Disregarding for the moment 

 the influence of biotic factors, it may be stated, on the basis of 

 Liebig's Law, that in every habitat the degree of mesophytism 

 which it is possible for the vegetation to attain is conditioned 

 by the limiting edaphic factor. Where all the necessary edaphic 

 factors are sufficiently represented, the highest degree of meso- 

 phytism permitted by the climate is capable of attainment. 

 But the limiting influence of any one factor may prevent suc- 

 cession from proceeding beyond a certain stage. 



The exact limiting factors of course vary with the habitat 

 and are not easy of analysis, except in a very superficial way. 

 A few illustrations, however, may be suggestive. In the case 



