8 Illinois Natural History Survey Circular 49 



about change in the physical form of the surface of the earth, pro- 

 cesses that extend over tens of thousands and milHons of years. To 

 understand how the Dunesland area came into existence, we must 

 therefore pry into the history of earth changes for many millions of 

 years. When we do this, we discover that not only has the earth been 



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Fig. 7. — Marshy swales in the hollows and oak forest along the ridges in the mature part of 

 the Dunesland. One line of trees at the left and another in the center of the picture represent thin 

 strips of black oak; trees in the extreme background are the deciduous forest on a higher and 

 older lake shore. 



different in the past from what it is now, but that the kinds of life 

 also were different. The present Dunesland plants and animals and 

 the landscape on which they occur are thus the products of a long 

 history of earth and life changes. 



LEARNING ABOUT THE PAST 



Civilized man was not present at the beginning of Dunesland 

 history and therefore could not collect field notes and write books 

 about it. For this reason we have to rely on special methods and 

 techniques to unravel the past of the Dunesland. The special methods 

 consist of (1) deducing the phylogenetic or family relationships of 

 different kinds of organisms and (2) accumulating and analyzing 

 data concerning the distribution of these organisms in relation to 

 time and geographic area. 



Evidence for unraveling this past falls into two categories. First 

 is fossil evidence, comprising the field of investigation called paleon- 



