THE STEUGULE FOR EXISTENCE 145 



tliere are ''liglit forests." sueli as the oak forest, wliicli 

 l)crmit mucli low vegetation, as well as the shade forests, 

 such as beech forests, which permit very little. 



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

 ing is })ermaiient, 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 tlu- annual change of temperature is a ver}- im- 

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

 for it in one wav or another. In tracing the historv of 

 plants, however, liack into what are called ■" geological 

 times," we discover that there have been relatively per- 

 manent changes in temperature. Xow 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 iiifluence upon jilant life. 



104. 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 siiil malei'ial over very 

 extensive areas. Areas within rcadi "f nccasinnal Hoods, 

 also, may have the soil mndi changed in chai'acter by the 

 new deposits. Shifting ilunes are bilbiw-like masses of 

 sand, developed and kept in motion by strong prevailing 

 winds, and often encroach u])on other areas. Besiiles these 

 changes in the character of soil b\' natui'al 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. 



I(i5. Devastating animals. — The ravages of animals form 

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

 arrazing animals are wholesale destrovers of vegetation, and 

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

 leaf feeders among insects have frequently done a vast 



