56 ANIMAL LIFE AND SOCIAL GROWTH 



students present at different seasons of the year. 

 It must be remembered, however, that even with 

 college communities such data are considered 

 to be sufficiently important to be published an- 

 nually and frequently are broadcast widely in an 

 attempt to give some definite information con- 

 cerning the structure of such a human community. 



The same sort of picture but with a different 

 background and with some interesting implica- 

 tions has been revealed by recent studies in the 

 aspen parkland of southern Canada. This aspen 

 forest extends in a great crescent a thousand miles 

 long, from northern Minnesota northward through 

 Manitoba and Saskatchewan, west through Al- 

 berta to the Rocky Mts. and then south along 

 their eastern foothills to western Montana. At 

 its widest, this crescent extends north and south 

 for 150 miles. To the north of this parkland lies 

 the great transcontinental evergreen forest and 

 to the south lies the great plains; it is the con- 

 necting link, the zone of transition, between the 

 two. 



The aspen is the dominant tree of this woodland. 

 In places the box elder, the American elm, the 

 burr oak and the balsam poplar also occur. The 

 aspen trees are advancing southward invading the 

 prairie all along their southern front, while to the 

 north the evergreens are filtering in, replacing 

 the aspens locally. 



