CHAPTER VI 



DISTRIBUTION AND ADJUSTMENT 



]HE purpose of this and the succeeding 

 chapters is to show how the physio- 

 graphic features of the Chicago region, 

 the origin of which has been outlined in 

 the preceding pages, determine the dis- 

 tribution of plants and animals, not 

 directly but by shaping in large meas- 

 ure the interplay of those factors that 

 do condition the plant and animal life. The chief factors for 

 plants are available moisture and light; for animals, the oxy- 

 gen supply, food, nest-forming materials, light, heat, currents, 

 foes, etc. Furthermore, plants and animals are nicely adjusted 

 in structural peculiarities and habits to the complex of these 

 limiting factors, and it will be the aim of succeeding pages to 

 point out also some of these adjustments. 



It is a matter of common knowledge that, the world over, 

 there is a zonation of life. The vegetation of the tropics is 

 totally unlike that of the temperate regions, and the life of the 

 latter zones is quite different from that of the arctic. The ascent 

 of a mountain carries the traveler through a succession of life- 

 zones, from the luxuriant growth about the base through scant 

 vegetation and disappearing animals to a bleak and almost 

 uninhabited peak. 



Temperature is on the whole a, very great factor in the deter- 

 mination of the abundance and character of both plant and 

 animal forms. Locally its effect is to settle the distribution in 

 time rather than fix the place in which animal or plant shall 

 grow, for naturally there is no very great difference in tempera- 

 ture in the various parts of the Chicago area unless it be a 



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