THE STRUGGLE FOR EXISTENCE 145 



there are ^Miglit forests," such as the oak forest, which 

 permit much low vegetation, as well as the shade forests, 

 such as beech forests, which permit very little. 



In the forest regions of the tropics, however, the shad- 

 ing is 2)ermanent, since there is no annual fall of leaves. 

 In such conditions the climbing habit has been extensively 

 cultivated. 



103. Change in temperature. — In regions outside of the 

 tropics the annual change of temperature is a very im- 

 portant factor in the life of plants, and they have provided 

 for it in one way or another. In tracing the history of 

 plants, however, back into what are called " geological 

 times, '^ we discover that there have been relatively i)er- 

 manent changes in temperature. Now and then glacial 

 conditions prevailed, during which regions before temperate 

 or even tropical were subjected to arctic conditions. It is 

 very evident that such permanent changes of temperature 

 must have had an immense influence upon plant life. 



101. Change in soil composition. — One of the most ex- 

 tensive agencies in changing the compositions of soils in 

 certain regions has been the movement of glaciers of conti- 

 nental extent, which have deposited soil material over very 

 extensive areas. Areas within reach of occasional floods, 

 also, may have the soil much changed in character by the 

 new deposits. Shifting dunes are billow-like masses of 

 sand, developed and kept in motion by strong prevailing 

 winds, and often encroach upon other areas. Besides these 

 changes in the character of soil by natural agencies, the 

 various operations of man have been influential. Clearing, 

 draining, fertilizing, all change the character of the soil, 

 both in its chemical composition and its physical properties. 



105. Devastating animals. — The ravages of animals form 

 an important factor in the life of nuiny plants. For example, 

 grazing animals are wholesale destroyers of vegetation, and 

 may seriously affect the plant life of an area. The various 

 leaf feeders among insects have frequently done a vast 



