884 THE AMERICAN NATURALIST. | [Vor. XXXIX. 
represent the possibilities of evaporation from a free water sur- 
face inside the instrument shelters. Just as the figures for the 
rainfall do not represent the amount of water actually available 
for plants (since it includes the run-off, the part that evaporates, 
and that which sinks into the ground) so the figures for evapora- 
tion do not correspond to the water vapor actually given off by 
plants, because this is determined largely by the ecological adap- 
tations of the individual plants. But the figures have a compara- 
tive value in both cases and when combined probably give a fairly 
correct idea of the distribution of these climatic factors in the 
eastern United States. The map was not extended to the west- 
ern states owing to the paucity of data and the disturbing influ- 
ence of the mountains. However, sufficient evidence is at hand 
to show that a desert center is clearly indicated in southern 
Arizona and California, and a forest center in the Puget Sound 
region. A comparison of the data for Colorado Springs and 
Pike's Peak indicates that the ratio increases from about 20 per- 
cent to 100 percent as one goes from the base to the summit of 
the Peak. This suggests the importance of taking other factors 
than temperature into account when explaining the distribution 
of the forests on mountains. 
Turning now to the map (Fig. 6) showing the distribution of 
the rainfall-evaporation ratios in the eastern states, it will be 
noted that the Great Plains are marked by a rainfall equal to 
from 20 to 60 percent of the evaporation called for. The 
prairie region where forests are confined to the low grounds, is 
indicated by a ratio of from 60 to 80 percent. Its limits as in- 
dicated show a remarkable agreement with the actual distribu- 
tion of the prairie. The region indicated by ratios between 80 
and 100 percent is more or less coincident with the occurrence 
of “oak openings," “open forests," and “groves” on the up- 
lands, and dense forests on the low grounds. 
The southeastern area where the rainfall is from 100 to 110 
percent of the evaporation, corresponds to the region of the 
Deciduous forest center. The distribution of the ratios above 
IIO percent in the region of the coastal plain is remarkably 
similar to the position of the Southeastern Conifer forest center. 
