INSECTS AND DISEASES. 141 



tree becomes feeble and diseased, and, finally, dies of 

 starvation. It is a case of arbor-consumption. It is then 

 constitutional and chronic ; and not only so, but conta- 

 gious so far that it affects those in its immediate vicin- 

 ity. Ifc is a well-known fact in agriculture, that by suo- 

 oessive crops of one variety of grain the soil is exhausted 

 of the essential elements for the production of that vari- 

 ety, and that the crops must be changed, or the yield will 

 gradually dwindle down to nothing, or the exhaustive 

 process must be counteracted by fresh supplies of the 

 aliment consumed. Hence arises the practice of rotating 

 crops, and also of using concentrated fertilizers. By the 

 first, we use a different feeder to consume a different ali- 

 ment ; by the latter, we use the same feeder, but intro- 

 duce a new supply of the same aliment. But, as all 

 plants consume a portion of the same aliment, the rota- 

 tion practice cannot be pursued indefinitely without los- 

 ing the highest benefit ; but the soil must be enriched 

 occasionally by fresh supplies of the ingredients extracted. 

 So it is in arboriculture ; for while it is true that the 

 leaves and rotten wood do much to supply nourishment 

 for the forest growth, yet it is equally true that they are 

 not equivalent to the special aliment consumed. If the 

 whole forest were leveled at once, and allowed to rot where 

 it fell, it would, probably, be an equivalent, and the soil 

 would then be prepared to produce and sustain another 

 crop of the same variety. Bnt not so, if the forest has 

 been cut down, and carried away. Do this, in any in- 

 stance, and see if the next growth will be the same as that 

 removed. Not at all ; but a totally different variety. If 

 the first was pine, the next will likely be oak, and vice versa. 

 We have seen beautiful illustrations of this in several 

 instances. We will mention one. Some twenty years 

 ago a forest of white oak, of very heavy growth, was 

 removed for the timber. Very soon after, the whole 

 tract was covered with a thick growth of young white 

 pine, while not a single young tree of the former growth 



