AREA OF STUDY 



49 



chiefly between 581 and 600 feet, and presents very little relief. The 

 lowest point of land on our map is in the valley of the Illinois River 

 below the entrance of the Kankakee. This is 480 feet above tide, or 

 101 feet below the level of Lake Michigan. In passing from the lowest 

 point in the lake shown on our map to the vicinity of Lake Zurich, which 

 is the location of one of the high points on the moraine, one would 

 travel 64 miles and make an ascent of only 12 feet per mile on the 

 average. Indeed, if Lake Michigan were to become dry and its bottom 

 a prairie, it would appear an undulating plain. 



V. CLIMATE AND VEGETATION or THE AREA 



I. METEOROLOGICAL CONDITIONS AFFECTING ANIMALS (68) 



The table (I) illustrates the fact that there are some notable differ- 

 ences between the different parts of our area. Extreme points would 



TABLE I 



show greater differences. The evaporating power of the air is probably 

 one of the best indices of conditions which affect animals. The ratio of 

 rainfall to evaporation is the only expression of the evaporating power 

 of the air which has been mapped. Fig. 7 shows this phenomenon in 

 Central North America, with our area indicated. 



2. VEGETATION (69, 70) 



Those features of the vegetation which are called climatic must be 

 discussed first. The two main climatic divisions of vegetation represented 

 in the Chicago area are savanna including the prairie vegetation, and 

 deciduous forest. The prairie, or savanna, as distinguished from steppe, 

 is a strip of country (the forest-border area) a few hundred miles wide, 

 from Athabaska to Texas, where trees, chiefly oak, hickory, basswood, 



