292 Plant Life and Evolution 



to forest, and vigorous growths of the same trees 

 pines, oaks, maples, and walnuts that once covered 

 the whole region with an unbroken forest, are again 

 taking possession of the soil which for a hundred 

 years or more was devoted to agriculture. 



Effects of Clearing the Forest. The clearing of 

 the country has also affected the vegetation in an- 

 other way. By the removal of the forest cover, 

 especially in the mountainous regions, there has 

 often resulted a disastrous denudation of the soil, 

 due to the washing of heavy rains, and to landslides. 

 The washing away of the fertile surface soil makes 

 it impossible for many plants, which formerly oc- 

 cupied these places, to grow, and the bare slopes can 

 only support species which are fitted to live in an 

 impoverished soil. The drying up of the lowlands, 

 due also to the interference with the water supply, 

 the result from the clearing off of the forest, must 

 necessarily affect very strongly the vegetation of the 

 region. There is little evidence that the total amount 

 of rain in a forested region is materially diminished 

 by clearing the land; but the effect upon the flow 

 of springs and streams is very marked. On a for- 

 ested mountain side the shade of the trees checks 

 the evaporation from the soil, and the undergrowth 

 and spongy masses of decaying leaves and twigs 

 allow the water to percolate slowly through, reach- 

 ing the sources of the springs and streams gradu- 

 ally and keeping the flow steady. When the land 

 is cleared, the water runs off quickly, making the 



