DISCUSSION AND PRELIMINARY INTERPRETATIONS OF 



THE CORRELATION DATA. 



I. CORRELATIONS AS INDICATING CONTROLLING CONDITIONS. 



An adequate discussion of the extreme values of each of the climatic 

 conditions which we have used, and of the probable importance of each 

 of them in controlling the distribution of the various vegetations and 

 species, would lead beyond the bounds of practicality if extended to 

 all of the 3,906 readings comprised in the tables just given. It has 

 seemed desirable, therefore, to discuss the controls for the vegetational 

 areas and groups of growth-forms, and for only a selected series of the 

 individual species. Such a discussion involves repeated reference to 

 the tables of climatic extremes and to the graphs in which selected 

 extremes are shown, as well as frequent direct comparison of the 

 plates showing the isoclimatic lines and those showing the distribu- 

 tional limits in question. 



We are far from taking the position that it is possible to point out a 

 single climatic condition which may be regarded as acting alone in the 

 control of any distributional feature. Enough has been said in pre- 

 ceding sections to indicate the importance that we attribute to the 

 combined operation of the entire constellation of climatic conditions 

 in determining the distribution of species and vegetations, as well as 

 in controlling their physiological processes. Our desire to consider the 

 climatic conditions collectively, so far as possible, has been responsible 

 for the elaboration of the data of the moisture ratio and of the moisture- 

 temperature indices. The evaporation data may also be regarded as 

 a collective expression of separate climatic elements, although the 

 plant is influenced by these elements collectively just as the atmometer 

 is, in addition to whatever other separate effects they may have. 



The tables of climatic extremes show a very large number of cases 

 in which species or vegetations are subjected to at least one- third of 

 the entire gamut of difference exhibited by the climatic conditions, 

 at least in the United States, and there is a large number of cases in 

 which they are subjected to more than half of it. The endurance of 

 such widely differing conditions is partly real and partly such as to 

 require qualification. Widely dissimilar temperature conditions are 

 encountered by those grasses of the Great Plains region which extend 

 from the Canadian boundary to the Rio Grande, and widely unlike 

 conditions of evaporation and the moisture ratio are encountered by 

 plants of southern transcontinental distribution, such as Daucus 

 pusillus or Cephalanthus occidentalis. In the case of Daucus, however, 

 the seasonal habits of the plant are entirely different in the three sec- 

 tions of its range, so that a comparison of the moisture conditions of 



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