470 



NATURALIST'S GUIDE TO THE AMERICAS 



north latitudes, and its position in regard 

 to moisture bearing winds. 



Generally speaking, Illinois is a 

 state with warm summers and cold 

 winters. The average temperature for 

 January, the coldest month, is 26.9°F. 

 and that of July, the warmest month, 

 is 75.9°F. Changeableness of weather 

 is a common phenomenon and the 

 changes are often rapid and extreme. 

 Increases and decreases in the tem- 

 perature of 30°F. within twenty-four 

 hours are not uncommon, and the tem- 

 perature has been known to rise 48°F. 

 within twenty-four hours and to fall 

 65°F. within eighteen hours. The aver- 

 age temperature for the state is approxi- 

 mately 52°F. and the extreme range in 

 temperatures is 147°F. The difference 

 in annual temperatures between those 

 places having the lowest (Northern 

 Illinois) and highest (Southern Illinois) 

 annual temperatures is a matter of about 

 10.5°F. The growing season varies 

 from five months in the northern to 

 seven months in the southern part of 

 the state. 



In Illinois the rainfall is well cor- 

 related with suitable conditions of 

 temperature for an abundant growth of 

 plant life. The normal precipitation 

 for the state is 36.54 in. This precipita- 

 tion is distributed throughout winter, 

 spring, summer, and fall, respectively, 

 as follows: 6.98, 10.33, 10.76, and 8.47 

 in. The month of May with 4.07 in. 

 of rainfall has the largest average 

 precipitation of the months of the j'ear, 

 and December the smallest with a 

 precipitation of 2.16 in. Nearly 60% 

 of the normal rainfall therefore falls 

 during the time of year when conditions 

 of temperature are favorable for plant 

 growth. 



Though the averages of rainfall are 

 well balanced for the four seasons of 

 the year, there occur many wide de- 

 partures in local areas from the aver- 

 age. Variations in the amount of 

 monthly precipitation from to 20 

 in. have been recorded. Such deviations 

 from the average monthly rainfall, 

 together with the distribution of 'the 



rainfall within the month or season, 

 are responsible for occasional local or 

 general drouths and flooded conditions 

 in the river and stream valleys. 



S. Original biota 



Illinois is unique among the states 

 in that it contains plant and animal 

 forms representative of the eastern 

 forests, of the western prairies, of the 

 northern bogs, of the southern moun- 

 tains and the Lake Michigan coastal 

 plain. 



a. Deciduous forest 



About 44% of the state was originally 

 deciduous forest. The northeastern cor- 

 ner and the southern third of the state 

 were almost continuous forest. In the 

 other parts of the state, the forest was 

 confined to the stream margins and to 

 occasional groves away from streams. 

 The principal trees were white oak 

 (Quercus alba), red oak {Quercus rubra), 

 black oak (Quercus velutina), elm {Ulmus 

 americana), basswood {Tilia americana) , 

 soft maple {Acer saccharinum) , and 

 especially in the southeastern part 

 some beech (Fagus grandifolia), and 

 sugar maple (Acer saccharum). The 

 larger animals of the deciduous forest 

 included the Virginia white-tailed deer 

 (Odocoileus virginianus), black bear 

 (Ursus americanus), timber wolf {Canis 

 lycaon), red fox (Vulpes fulva), gray fox 

 {Urocyon cinereoargenteus) , opossum 

 (Didelphis virginiana), cotton-tail rab- 

 bit (Sylvilagus floridanus inearnsii), 

 gray squirrel (Sciurus carolinensis leuco- 

 tis), deer-mouse (Peromyscus leucopus 

 noveboracensis), timber rattlesnake (Cro- 

 lalus horridus), ruffed grouse, wild 

 turkey, and the tufted titmouse. 

 Among the smaller animals such forms 

 as the polyphemus moth (Telea poly- 

 phemus), the wood cockroach {Parco- 

 blatta pennsylvanica), the katydid {Mi- 

 crocentrum laurifolium) , the oak-pruner 

 (Elaphidion villosum), the two-spotted 

 bumblebee (Bremus bimaciilatus), the 

 white-lipped wood snail {Polygyra albo- 

 labris), the striped wood snail {Pyrami- 

 dula alternata), and a couple of small 



