12 TREES AND FLOWERS OF 



Into such a "stern and rock-bound" land came 

 the pioneers of the plant societies that followed the 

 retreating ice. There was no comeliness about them, 

 nor any stature; they were small, sober, patient 

 plants, whose descendants now dwell on the tundras 

 of the arctic regions. It is possible that they found 

 a little plant life when they came; lichens and moss- 

 es at least, and perhaps a few of their own kin, 

 relicts of the earlier world of plants that had fallen 

 before the invasion of the ice. At first they found 

 a foothold in sheltered crevices and on the sunny 

 sides of the hills. Very gradually they extended and 

 consolidated their holdings, seizing on the soil that 

 slowly weathered from the bare rock and weaving it 

 into their turf of roots, moving in wherever the 

 lichens and mosses prepared a place for them. The 

 rocky landscape lost much of its forbidding front, the 

 erosion of the morainal hills was checked, and life 

 became possible for other, less meager plants that 

 followed from the south as the climate continued to 

 modify. 



As the summers became warmer they also became 

 longer and drier. Less and less of the terrain was 

 occupied with permanent snow and ice, and life be- 

 came possible in constantly enlarging areas. The 

 longer growing seasons also made new forms of life 

 possible, and newcomers appeared in increasing num- 

 bers. Large bushes and even trees came in, and the 

 development of the forests began. The country be- 

 gan to take on a modern aspect. 



The drying-up process continued, and the hill- 

 sides at the lower altitudes began to be unable to sup- 

 port forests. The trees gave way before the incoming 

 sagebrush, and retreated to the park plateau, except 

 for certain drought-tolerating forms, like cedar and 

 limber pine. The earliest comers, forced by the com- 

 petition of the later arrivals still to keep to arctic 

 conditions, retired to the mountain-tops, or even dis- 



