40 PLANT SUCCESSION AND CROP PRODUCTION 



actual conditions, the influence of soils and climate. The factors 

 are like the legs of a three-legged stool. 



The economist can never avoid the fact that it is sometimes 

 the edaphic group and sometimes the climatic group of factors 

 which stand back of this very profit he is studying. Climatic 



Fig. 6. Zones of production on a purely hypothetical economic basis. 



conditions determine the physiological limits of the cotton crop. 

 (Fig. 6). It is the climatic conditions and not the economic only 

 that determines the dominance of cotton. Corn is not and never 

 can be a serious competitor of cotton. In the case of spring wheat, 

 edaphic factors limit its distribution to the Dakotas and to Western 

 Minnesota. Here is a level tract of land with rich productive soil, 

 a bequest of the long-deceased Glacial Lake Agassiz. 



Finally it would be well to point out an economic factor, not 

 a part of the theory of rent, which is intimately related to the pro- 



