THE FOREST AS A CONDITION. 55 



temperatures above the snow cover, droughty 

 winter storms, and frosts every month in the year 

 can be endured only by those plants which have a 

 rapid cycle of development, or are sheltered near 

 the ground by the snow cover; the wet soil on 

 the tundras, frozen for most portions of the year, 

 or the thin soil on the Alpine peaks, adds to the 

 difficulties for deep-rooting species in their contest 

 with the lower vegetation. Again, in the interior 

 of continents and other localities unfavorably situ- 

 ated with reference to the great sources of mois- 

 ture and moisture-bearing currents, deficiency of 

 water, namely scant rainfall or low relative humid- 

 ity, or both, and excess of evaporation, are inimi- 

 cal to tree growth. Occasionally soil conditions, 

 especially with reference to drainage, and climatic 

 conditions combined, may be more favorable to the 

 graminaceous vegetation, at least for a time, giving 

 rise to pampas, prairies, and savannas ; or else the 

 unfavorable conditions combine to such a degree 

 as to give rise to deserts. 



In addition, there are other inimical agencies in 

 the animal world, which prevent the progress of 

 forest growth and tend to preserve the prairie : 

 locusts, rodents, ruminants, buffalo, antelope, horses, 

 etc., impede the growth and spread of trees ; and 

 especially where compact soil and deficient mois- 

 ture conditions are leagued with these animals, the 

 change from prairie to forest is prevented, at least 

 for a time. 



