NATURAL HISTORY OF THE FOREST. 151 



This decimation is in exact mathematical rela- 

 tion, except for accidents, with the development of 

 the dominant, especially in height growth. At the 

 age of eighty, of the several thousand trees which 

 started in the race, only a portion — not more than 

 400 to 500 — are left. Then the diminution pro- 

 ceeds at a slower rate, until finally only 200 to 300 

 occupy the ground, or as many as can conveniently 

 fill the air space in the upper story, the number 

 varying according to soil and climatic conditions 

 and species. 



The time has arrived when the height growth is 

 practically finished. The branches cannot lengthen 

 any more to occupy the air space. After this a nu- 

 merical change can take place only as a result of 

 casualties, caused by fungi, insects, fires, or wind- 

 storms ; these of course may also from the start in- 

 terfere in the regular progress of adjustment which 

 takes place under the effect of physiological laws. 



In reality the conditions of soil, climate, and 

 species in combination are so various that this pro- 

 cess of evolution does not appear so simple, yet the 

 seemingly lawless, yet actually law-directed, appear- 

 ance of a forest growth explains itself by these 

 few observations of the results of action and reac- 

 tion of its surroundings and of the single compo- 

 nents. 



The factor of light is not only the most impor- 

 tant one in bringing about the evolution of the 

 natural forest, but practically almost the only one 



