Vegetation Problems Involving Climate. 103 



of geographic divisions upon either of these bases. When we 

 recall that for North America alone not less than sixty different 

 proposals of geographic zones and regions have been published 

 during the last century, the futility of the point of view which 

 disregards all but one or two climatic factors and emphasizes 

 boundary lines, must be apparent. But we shall be still more 

 impressed with the inadequacy of these proposals if we attempt 

 to relate the actual distribution of plants or plant formations 

 to these "regions." 



Recently there has been a rapid increase of local ecological 

 studies in which the successional processes of vegetation have 

 been emphasized. These studies have apprehended to a greater 

 or less extent the dynamics of the habitat and the plant forma 

 tion. The separation of the local vegetation into stages has 

 assumed the dominance in each of a distinct complex of environ- 

 mental factors. The occurrence of distinct boundaries has 

 neither been assumed nor insisted upon. 



Local studies, however, can not lead to general conceptions 

 of vegetation unless compared and united into larger units. This 

 brings us to the fact that the larger units generally recognized 

 are transcontinental zones and regions. But zones and regions 

 are static entities. They are developed upon assumptions 

 wholly different from those upon which the local studies have 

 been founded. Usually, in zonal classification, temperature is 

 recognized as all-important, and rainfall an unfortunate dis- 

 turber of symmetry. Not a few are based upon phenological 

 assumptions long since proved untenable or still awaiting exper- 

 imental evidence. In other words, the succession of local plant 

 formations which has consciously depended upon changes in 

 the concomitant action of many soil, climatic, and historical 

 factors, is made to fit into a larger unit whose fundamental basis 

 is a single, or at most two, climatic factors. It is to be noted 

 further that the problem most debated in connection with zonal 

 arrangements is the boundary; that the term "zone" implies 

 uniformity of structure and homogeneity of composition. But 

 the most striking fact about the geographic distribution of 

 individual species is their dominance in some region and their 

 decline in importance and frequency as we depart thence in any 

 direction. Plant formations in their distribution show the 



