CORRELATION OF DISTRIBUTIONAL FEATURES. 505 



extent of the Deciduous Forest is somewhat less than that of the two 

 vegetations just mentioned. The most significant changes of ampU- 

 tude that may be observed in comparing these three vegetations are 

 found in the normal daily mean for the 14 coldest days and in the 

 normal daily mean for the year. Both of these conditions are much 

 narrower in their amplitude for the Deciduous Forest than they are 

 for the Grassland and the Transition region, indicating that these con- 

 ditions are of increasing importance as we pass from pure grass to pure 

 forest. 



It has already been shown that a comparison of Grassland, Transi- 

 tion, and Deciduous Forest exhibits respective increase in the values 

 for all of the moisture conditions, except, of course, that a reciprocal 

 relation exists with regard to the number of dry days. While this fact 

 might well be anticipated, it is somewhat surprising to find that the 

 amplitude of several of the moisture conditions is greater for the 

 Deciduous Forest than for either of the other two vegetations men- 

 tioned. The amplitude of the normal daily mean precipitation for the 

 frostless season is slightly greater for the Grassland than for either of 

 the other areas, and the number of days in the longest normal dry 

 period for the frostless season is greater for the Grassland than it is 

 for the Deciduous Forest, although it is less for the Transition area 

 than for either of these. The mean total precipitation for the year and 

 the number of days in the longest normal rainy period of the frostless 

 season are both conditions that show the widest amplitude in the 

 Deciduous Forest region. 



The evaporation conditions, which show such wide amplitude in 

 the Grassland and such narrow amplitude in the Transition region, 

 again show a relatively wide amplitude in the Deciduous Forest. This 

 fact determines the great amplitude of the moisture ratios in the 

 Deciduous Forest as compared with the Transition region. While 

 the moisture ratio appears to be of great importance in controlling the 

 limits of the Desert and Grassland, and also those of the Deciduous 

 Forest, the evidence shows it to be of even more critical importance in 

 controlUng the boundaries of the Grassland Deciduous-Forest Transi- 

 tion. 



To continue our comparison of the three vegetations which have 

 already been contrasted, we find that the value for relative humidity 

 becomes progressively greater from Grassland to Deciduous Forest, 

 and that its amplitude is very narrow for the Transition region, while 

 it is relatively broad for the Grassland and Forest, ranging through 

 a third to a fourth of the total amplitude for the United States. The 

 evaporation conditions bear a reciprocal relation to relative humidity, 

 but show the same narrow amplitude for the Transition region and 

 wider amplitudes for the adjacent vegetations. It appears from this 

 circumstance that relative humidity is the strongest determinant of 



