THE EVOLUTION OF ANIMAL COMMUNITIES 75 



usually spoken of as forming an ecological suc- 

 cession. There are many other small habitats in 

 which succession of communities can readily be 

 traced within a few months or years. One good 

 example is the animal community inhabiting a 

 fallen tree. Animals such as the wood boring 

 beetles attack and feed upon the solid wood. 

 They open tunnels through its structure by means 

 of which other animals or water or fungus spores 

 can enter. The feeding of these animals, the 

 freezing of the water and the growth of the fungus 

 spores all help to disrupt the woody structure 

 until as it softens, the first pioneers can no longer 

 remain. The wood is too soft and decayed for 

 their food; they must either die or migrate and 

 their place in the decaying log is taken by other 

 forms that do feed upon the softened wood. 

 Finally through the activities of these animals, of 

 bacteria and other plants, and of weathering 

 processes, the once solid log is reduced to a mere 

 mound of much decayed humus occupied by a 

 few ants and earthworms, a totally different 

 community from that of the newly fallen log. 



The same sort of evolution of plant and animal 

 communities can be observed in nature on a much 

 grander scale than in a protozoa infusion or in the 

 decay of a log on the forest floor. The forest 

 communities themselves show such successions. 



