46 NATIONAL EXPLOITABUilTT. 



But on the other hand the bulk of the produce is necessarily 

 always composed of a large proportion of firewood, even if the only 

 source thereof is the toppings. Besides this, the age at which the 

 mean rate of growth is highest, has already passed away, often 

 years upon years ago. The question therefore presents itself, " Does 

 it thence follow that we lose in quantity more than what we gain 

 by the enhanced usefulness of the large timber after a certain 

 definite age has been reached 1" 



To answer this question it will be enough to enquire in what 

 manner the quantity and the degree of usefulness of the produce 

 vary from the time that the individuals composing the crop in ques- 

 tion have become high forest trees, that is to say, have attained their 

 full length of bole. As long as the leaf-canopy is complete and the 

 trees are all vigorous and healthy, the annual sum of production 

 scarcely diminishes ; indeed we can expect no other result, and the 

 fact Is easy of proof. As long as the trees gain in diameter while 

 still remaining sound, their usefulness is continually Increasing. 

 This fact becomes obvious and striking if we compare the various 

 purposes for which large and moderate sized timber respectively are 

 employed. Indeed a comparison of receipts fo r equal periods of 

 time will show that the money returns go on steadily increasing 

 until the stage of maturity is reached, and thus proves that the 

 sum of utility has also greatly increased in the meantime.^ It hence 

 follows that the conditions of National Exploitability are not realiz- 

 ed except at the age of maturity. By felling forest crops when 

 they have reached that age, we obtain, as far as that is possible, 

 the most useful and considerable yield of material as well as the 

 largest money returns. 



Those forests of which the component species, the soil, or the 

 Regime in force do not permit of the production of any but firewood, 

 yield the highest sum of utility at the age which corresponds to that 

 of Quantitative Exploitability. But such cases are very rare or are 

 capable of being modified with advantage. On the other hand, 

 those forests which furnish us with the most valuable timber both 

 as regards quality and size, also contain numbers of trees that have 

 still, at the maturity of the collective crop, a great many years to 



(1) A high forest, mature say at 200 years and possessing at that age a 

 convertible value of J 670 per acre, is uot worth £ 500 per acre at 150 years, 

 and still less £ 335 at lOtt jears. (Author.) 



