50° ANIMAL ENVIRONMENT 
and elm, occur in groves and along streams. It has the general form of a 
bow, with its central and most eastern point at Chicago (Fig. 8). To the 
east of Valparaiso, Ind., the forest is chiefly beech and maple (see 
frontispiece). The types are believed to stand in close relation to 
climate, especially to ratio of rainfall to evaporation (Fig. 7). 
The vegetation of local conditions, as indicated on p. 42, is different 
from that of the region as a whole and we are concerned in part with 
0% 130% 
100% 
Fic. 7.—Map showing ratio of rainfall to evaporation in percentages, with area 
of special study inclosed in rectangle (after Transeau). Compare with Sargent’s 
map of the ‘‘ Forests of North America” (roth Census Report and, Fig. 8 below). 
the relations of the animal communities of local conditions to animal 
communities of the climatic vegetation. 
VI. LOCALITIES OF STUDY 
In beginning the investigation of any biological subject from the 
point of view of general principles, the most important step is the selec- 
tion of the material (animals to be studied). In ecological work we 
tA glance at the map shows us that our area of study is in the center of the 
Forest-Border Region. 
