52 ANIMAL LIFE AND SOCIAL GROWTH 



dominant living things. Subject to the greater 

 power of climate and soil, both of which they 

 modify, they control the habitat in that they 

 meet the full impact of the physical environment, 

 wind, rain and sun and so modify the effects of 

 these that the reactions of the plants and animals 

 associated with the trees are profoundly affected; 

 frequently these associates could not tolerate 

 conditions within the environment were it not for 

 the modifications produced by the dominant 

 trees. Under the shelter of the forest trees there 

 has been built a community which not only shows 

 marked interactions between the different sub- 

 sidiary constituents, but some of these attack 

 the trees at all times and in periods of unusual 

 stress may help destroy these dominant elements 

 that make the community possible. 



Some of the relationships among the animals 

 living in these bits of Illinois woodland are illus- 

 trated in Fig. 1. The most important animals 

 of the permanent nucleus of strongly influential 

 animals are shown in the center. The symbols 

 placed around these give at a glance the relative 

 numbers of the three invertebrate animals present 

 in greatest numbers at different seasons of the 

 year. These symbols are placed end to end 

 with the least abundant next to the base line so 

 that in determining the relative numbers present 



