CORRELATION OF DISTRIBUTIONAL FEATURES. 393 



conditions of the plant in question, in addition to its limiting ones. 

 We publish our climatic data in detail partly with the hope that it may- 

 be possible for future workers in this field to do more than we have 

 been able to do with this other and equally important half of the field 

 of distributional etiology, and that our assemblage of figures may be 

 of use in that connection. 



It is in default of full data as to the location of the ecological centers 

 of distribution that we have used the maps showing the cumulative 

 occurrence of trees and grasses and the maps showing the relative 

 abundance of Pinus tceda, Liriodendron tulipifera, and Bulhilis dacty- 

 loides, in the different portions of their areas. These maps probably 

 approach the appearance that might characterize maps drawn by 

 combining all of the criteria mentioned by Adams if such charts were 

 possible. 



With respect to both the climatological and the vegetational data 

 that we have been able to secure, the limitations of our material have 

 at all times been more serious than the limitations of our methods, and 

 the methods might readily have been much improved in many respects 

 if the available observational data had seemed to warrant this. 



PRESENTATION OF THE CORRELATION DATA. 

 I. METHODS OF CORRELATION. 



The method here adopted for determining the extreme climatic 

 values for each botanical area is a simple cartographic one. After 

 assembling all of the values for a given element or condition of the 

 climate, these were placed on a base-map of the United States (U. S. 

 Geological Survey sheets, 17 by 28 inches), with a heavy dot locating 

 each station used. From each of the 39 chmatic maps (see Part II) it 

 was then possible to learn at a glance the climatic index value for any 

 given station and to follow the regions of similar readings by aid of the 

 isoclimatic lines. The data on plant and vegetational distribution 

 were placed on maps of the same size, and from them overlays were 

 made, on thin tracing-paper, for each of the 33 botanical maps. It was 

 then possible to take each distributional tracing in turn and to lay it 

 successively on each of the climatological maps, reading the figures for 

 the stations which showed the highest and lowest values within each of 

 the distributional areas. In this manner 126 botanical areas w^ere 

 compared with 31 of the climatological maps and 3,906 readings of 

 climatic extremes were secured. These readings are presented in 

 tables 23 to 151. 



Our method of determining optimum conditions, in the few cases in 

 which we obtained centers of distribution by the method of cumulative 

 occurrences, was to secure the readings of highest and lowest climatic 



