THE STRUGGLE FOR EXISTENCE 145 
there are "light 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 permanent, 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 per- 
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. 
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 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 many 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 
