574. 
This summary would not be complete without a word on the causes 
of the prairie. No other question of the prairie has been discussed so 
frequently or widely, and the answers to the question are as widely 
variable as their authors. This is a result, largely, of the various authors 
emphasizing local factors which would not apply to the prairies as a 
whole, or emphasizing only one of several factors which operating to- 
gether may explain the prairies, but when considered separately fall 
far short of an adequate explanation. For instance, prairie fires, grazing, 
and the character of the soil are factors that may affect the local dis- 
tribution of prairie and forest at their borders, but become insignificant 
in any attempt to explain the prairies as a whole. In some cases they 
have been reported as favorable to forest, in others as favorable to 
prairie. As a matter of fact, prairie fires and grazing are an effect 
rather than a cause of the prairies. As shown above, this may be true 
also of the prairie soils in so far as they differ from forest soils. On the 
other hand, temperature, wind, humidity, rainfall, and topography were 
all important factors in causing the treelessness of the prairies, but no 
one of these five factors considered alone will explain the prairies. 
Omitting topography for the moment, it is possible to analyze the 
effect of the remaining four factors in terms of only two factors. Tem- 
perature, wind, and humidity all affect the rate of evaporation of water, 
and it is in this way that they operate in affecting the development of 
forest and prairie. They may be dealt with as one factor, namely, 
evaporation. We have to consider, therefore, only the factors, evapora- 
tion and rainfall. Both rainfall and evaporation are measured in inches 
per year by the Weather Bureaus. The amount of evaporation is ob- 
tained by exposing standard open vessels of water and keeping the record 
of water loss each day. At the end of the year this may be calculated 
to terms of inches of water lost by evaporation. 
If the number of inches of annual rainfall (precipitation in general) 
is divided by the number of inches of annual evaporation, these two 
factors may be expressed as a single factor, namely, the per cent. or ratio 
of rainfall to evaporation. When the rainfall-evaporation ratios obtained 
at various places in the United States are plotted on a map, we are able 
to determine the combined effects of the four climatic factors—tempera- 
ture, wind, humidity, and rainfall—upon the distribution of the natural 
vegetation. (See map on page 524.) The region of the United 
States in which the ratio of rainfall to evaporation is between 60 
and 80 per cent. coincides very closely with the prairie region. In gen- 
eral, where the ratio is less than 60 per cent. plains or deserts result; 
where it is more than 80 per cent. forests are found. Since the annual 
distribution of rainfall is also important, these figures are not absolute 
for all localities: Hence, the chief cause of the prairie lies in the 
combined action of the four climatic factors named above through their 
effect upon the water relations of the plants concerned. 
Topography may effect the distribution of prairies through its 
effect on some one or more of the climatic factors, as in the exposure 
