THE EVOLUTION OF ANIMAL COMMUNITIES 81 



The oaks succeed each other, first black, then 

 white and finally the red oaks. Each has its 

 characteristic set of accompanying plants and to 

 some extent, a characteristic set of animals, but 

 the larger animals and particularly the birds, tend 

 to range throughout. As the sand becomes more 

 firmly bound and more thoroughly mixed with 

 humus, the plant growth becomes thicker, and 

 the resulting leaf mulch tends to hold the soil 

 moisture while the growing density of the tree 

 crowns lessens light and so lessens the amount of 

 evaporation. These processes go on, with each 

 community gradually making conditions that it 

 can no longer tolerate and so preparing the way 

 for the coming of another; at last the climax forest 

 of this region, the beech and maple, begins to grow 

 on a sandy sub-soil that has accumulated humus 

 from the numberless plants and animals that have 

 lived on and over it since it was originally cast 

 up along the bare lake shore. The beeches and 

 maples and their associates gradually replace the 

 majority of the oaks and the type of commun- 

 ity develops which once dominated the eastern 

 section of the United States. Unlike its prede- 

 cessors in this succession which we have been 

 tracing, this community is immune to the effects 

 which it produces and so becomes virtually per- 

 manent, changing only with a change in climate. 



