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 



FIG. 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" (xoth 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 



1 A glance at the map shows us that our area of study is in the center of the 

 Forest-Border Region. 



