YELLOWSTONE NATIONAL PARK 13 



appeared altogether. Some of the more adaptable of 

 the newcomers followed on np the mountains and 

 took up their abode there. 



Even after all available room was taken up by the 

 plants the drama continued, and it still goes on. No 

 area where a plant can get a roothold is left bare, and 

 as soon as conditions become unfavorable for one 

 species, forcing it to give up the fight, its place is 

 taken over by another that is adapted to the changed 

 circumstances. If a lily pond dries up, the bottom is 

 soon occupied with cattails and rushes, which are 

 succeeded by sedges and grasses, and these by wil- 

 lows, and finally the forest comes in. If a dry, sage- 

 brush-covered hillside develops a gully where a trickle 

 of moisture persists, the sagebrush must yield to 

 plants that can make more advantageous use of the 

 water. First a few shrubs, then a thicket of aspen, 

 and finally fir or spruce or pine. The aspen always 

 pioneers, pushing up the slopes and out into the 

 open, carrying the new conditions to some extent in 

 its shade, and conquering new land for the trees that 

 follow. 



Thus the drama continues, year after year, cen- 

 tury after century, the plants sometimes fiercely com- 

 peting, sometimes accommodating each other, but al- 

 ways and unceasingly struggling against circumstance 

 and exploiting to the limit the opportunities that the 

 environment allows. What the last act will be is of 

 course outside human prophecy or even conjecture. 

 But in the meantime the student of nature, or the lay- 

 man with a philosophical turn of mind, will find much 

 fascinating food for thought in every meadow and on 

 every mountain-side. 



