154 Changing Plant and Animal Nature 



ward which it strikes. A perfectly adapted frog will escape 

 every attempt of his enemies to capture him. What actually 

 happens is a compromise between the more or less adequate 

 adaptations of the numerous individuals of the hundreds of 

 species of plants and animals to be found in a given area. 



Changes in Population 



Day by day changes in the temperature and in the mois- 

 ture bring with them changes in the proportions of the vari- 

 ous plants and animals. New species appear upon the scene, 

 those that emerge later than the others from the winter's 

 rest, those that push in from neighboring territory. The 

 migrating birds do not all arrive at the same time. And 

 each new arrival changes the environment for those already 

 present. The balance of life is both intricate and delicate. 

 Every change in detail brings with it many changes in other 

 details. The leaves of a plant are destroyed by caterpillars. 

 The plant dies down and becomes food for molds and bac- 

 teria. These make possible the rapid increase in the number 

 of various species of small animals, " worms," insects and 

 crustaceans. These in turn make the area favorable for 

 larger animals. At each point adaptation means fitness for 

 a particular situation, which for some species must be rather 

 restricted. Certain insects, for example, will feed only on 

 particular types of plants. Certain fungi will attack only 

 certain kinds of plants. 



These daily fluctuations in the actual make-up of the 

 life of a region set the limits for subsequent life. A given 

 area or a given body of water comes to contain a maximum 

 amount of life precisely because of the great variety of forms. 

 Every cranny comes to be occupied by some form. With the 

 coming of colder weather, we see again two kinds of changes 

 in the composition of the flora and fauna. There are changes 

 inherent in the life cycle of the plants and animals; other 

 changes are due directly to the changing temperature, mois- 

 ture and illumination. The production of seeds by many 



