12 IOWA STUDIES IN NATURAL HISTORY 
SOIL 
Soil, except for its physical characteristics, is not a primary 
factor in determining animal habitats. It must be admitted, 
however, that such physical characteristics as are represented by 
alluvial loam, gravel bars, or rocky bluffs, limit the components 
of the animal associations to certain forms whose adaptations fit 
them to meet with the physical conditions of the environment. 
Yet animals primarily depend upon the vegetation rather than 
the soil as a distribution control, and as Shimek* has pointed out, 
such vegetational features as prairie and forest are influenced 
little by soil composition. The main types of soil for this county 
are shown on the map comprising Figure I, to which reference 
has already been made. 
RAINFALL AND CLIMATE 
The conditions of rainfall and climate are best and most tersely 
expressed in the table of climatological data which is here given. 
These figures are based upon careful observations, made in con- 
nection with the United States Weather Bureau.t These aver- 
ages may be taken as representative, as they are drawn from 
daily records made during a period of twenty years. The aver- 
age rainfall is computed from this table to be 33.82 inches. The 
minimum annual rainfall for the driest year of the twenty from, 
1896 to 1916 was 22.32 in 1910; the maximum was 47.80 in 1902. 
The most fairly representative variation in temperature 
throughout the year may be deduced by subtracting the lowest 
monthly average absolute minimum temperature from the high- 
est monthly average absolute maximum temperature. This gives 
—15.6° to be subtracted from 97.7°, leaving an annual average 
variation in temperature of 113.3°.. The columns giving the 
average mean maximum and average mean minimum tempera- 
tures may be interpreted as representing the optimum condi- 
tions for the existence of our native animals, or those temper- 
*Shimek, ‘The Prairies,’ Bulletin from the Laboratories of Natural History of the 
State University of Iowa, Volume VI, Number 2, pp. 169-240: 14 plates including 
state map with prairies and forested areas. 
71896 to October 1905, by A. A. Veblen; November 1905 to August 1909, by Karl 
E. Guthe; September 1909 to October 1916, by A. G. Smith; since October 1916 by 
J. F. Reilly. These data were published regularly in Climatological Data, Iowa 
Section, in codperation with the Iowa. Weather and Crop Service, U. S. Department 
of Agriculture, Weather Bureau Office, Des Moines, Iowa. 
