NATURAL HISTORY OF THE FOREST. 143 



thus limited, soil conditions vary, and again dif- 

 ferentiate the distribution ; the frugal pines being 

 able to subsist on the deep, overdrained sands, the 

 shallow-rooted spruces on the thin soils of alpine 

 situations, the elms, swamp maples, tupelo, bald 

 cypress, being indifferent to excess of moisture at 

 their feet, the hickories, walnuts, and tulip trees 

 seeking the rich, loamy soils, and others again 

 being ubiquitous, adapted more or less readily to 

 any kind of soil. 



While, then, certain territory is assigned to the 

 different tree species, which through eras of evolu- 

 tion have adapted themselves to the climatic and soil 

 conditions, — and this is a very important eco- 

 nomic fact, since usefulness of species varies, — yet 

 the absence of a species from a given locality does 

 not necessarily predicate its inability to exist and 

 thrive in such a locality, since there are also me- 

 chanical barriers, like wide oceans and high moun- 

 tain ranges, or there may be absence of suitable 

 means of transportation for the seed, prevent- 

 ing its spread, and these difficulties man can 

 overcome. 



It is, therefore, not impossible to exchange and 

 distribute artificially the useful species, as has 

 been done in agriculture and horticulture. But 

 in the case of plant material for forest purposes 

 it is impracticable to give special protection to the 

 introduced species through the long term of its 

 growth to usefulness, as may be done in the case 



