NATURAL HISTORY OF THE FOREST. 141 



necessarily differ, inasmuch as the former is 

 merely the result of a natural evolutionary strug- 

 gle among the different forms of vegetation, in 

 which the "most fit" survivors may not be the 

 economically desirable, while the forester substi- 

 tutes arfificial selection for natural selection, and 

 makes sure of the protected survival of the most 

 useful. Within limits, at least, he has it in his 

 power to influence the seemingly lawless mixture 

 of species which the virgin forest offers into a 

 form more suitable for his purposes. The limits 

 are set by the adaptability of the species to climate 

 and soil, and by the skill of the forester in recog- 

 nizing and utilizing the laws under which the 

 natural forest develops. 



Climatic factors, temperature and moisture con- 

 ditions, determine, in the first place, the field of 

 natural distribution of the various species. Differ- 

 ent species are adapted to live within different 

 ranges of temperature and of relative humidity, 

 or the combination of both ; hence, different types 

 of forest occupy the different regions through 

 which we pass from the tropics, with their palms 

 and broad-leaved evergreen trees, through the de- 

 ciduous-leaved forest of the middle latitudes, com- 

 posed of oaks, hickories, chestnut, and tulip tree, 

 to the northern latitudes, where birch, maple, 

 beech, with pine, and hemlock, and finally, only 

 aspen and spruce, can brave the wintry blasts. 

 And beyond the last outposts of these, tousled and 



