516 CORRELATION OF DISTRIBUTIONAL FEATURES. 



a correlation points to this condition as playing an important part in 

 the existence and distributional limits of these vegetations. This 

 should be true even if there is some overlapping of the values of the 

 condition between the adjacent vegetations, for there is always such 

 overlapping of the vegetations themselves. A comparison of the 

 humidity conditions for Grassland, Grassland Deciduous-Forest 

 Transition, and Deciduous Forest, or a comparison of the moisture 

 ratios for these vegetations and for Desert and Semidesert, shows a 

 progressive change of position of the climatic extremes with respect 

 to the extremes for the entire country (see fig. 25). 



A comparison of the amplitudes and extremes of all conditions for a 

 single vegetational area makes it possible to discover which conditions 

 tend to exhibit great differences in the various parts of the area and 

 which ones tend to show a relative uniformity throughout the area. 

 We have already seen that this method of evaluating the conditions 

 makes it possible to use their relative amplitude as a measure of their 

 comparative importance in establishing the hmiting conditions for 

 the vegetation in question. 



A cartographic representation of the distribution of vegetation and 

 of the distribution of the various intensities of the cHmatic conditions 

 makes it possible to compare distributional limits and isoclimatic lines, 

 and to search for correspondence between the two. But such a search 

 is best carried out by the aid of suggestions from the graphs showing 

 the climatic extremes. 



The use of the three methods of correlation has shown them to be 

 consistent in their indications. If a condition shows a shding scale of 

 values for a given series of vegetations, it is also found to show ampli- 

 tudes, in each of the vegetations, which are narrow as compared with 

 those of other conditions, and the isoclimatic lines showing the dis- 

 tribution of the intensities of this condition are found to approximate 

 the distributional lines between the series of vegetations. The first 

 two methods serve for the discovery of the relative importance of 

 various conditions in determining distribution, and the third method 

 serves to show the critical intensities of the condition which appear to 

 be important. 



In a general review of our examination into the correlations between 

 climatic conditions and the general vegetational areas we find the 

 most salient fact to be the great controlling importance of moisture 

 conditions, embracing precipitation, evaporation, relative humidity, 

 and the moisture ratio, as compared with the small controlling impor- 

 tance of temperature conditions, embracing length of frostless season, 

 number of hot and of cold days, and the temperature summations. 

 The moisture-temperature index partakes strongly of the character of 

 a temperature condition when it is brought into this comparison. 



In the vegetations of group A (including our series from Desert to 

 Deciduous Forest; see figs. 21 to 26) the temperature conditions show 



