COMMERCIAL EXPLOITABILITT. 47 



grow ou before they attain their highest degree of usefulness. It 

 often happens that these trees can be spared when the crop con- 

 taining them is exploited. In this manner we can follow up and 

 add to the results of National Exploitability, by keeping such trees 

 standing in an isolated state and not felling them except in accoru- 

 ance with the requirements of Qualitative Exploitability, that is to 

 say, only as each of these choice specimens of timber becomes indi- 

 vidually mature. 



Thus the three specific kinds of Exploitability treated of hitherto 

 all agree in having the public good for a common object. 



§ 4. Commercial Exploitability. 



Commercial Exploitability is based on the ratio of income to the 

 expenditure producing it. This is by no means a simple ratio. In any 

 given portion of a forest, the successive exploitations made therein are 

 naturally not annual, but periodical ; the consequence is that from 

 the very first appearance of the standing crop, the values, or sums of 

 money sunk in it, go on increasing, like any other kind of capital, 

 at compound interest. If then periodic exploitation, at intervals 

 of n years, yields receipts = R, the rate of interest i, which the origi- 

 nal value C ot the land with its stools or seedlings must be supposed 

 to be put out at is found from the equation 



C I (1+0-1 |=R. 



If we wish to find the rate of interest on capital invested, yielded 

 by a forest worked on a fixed rotation, we must employ this equa- 

 tion to determine it. Thus the comparison of income with capital 

 expenditure is always a question of figures. 



Actually a wood that is quite young has no convertible value. 

 Very soon it acquires such a value, at least as firewood, and thiff 

 value, towards the age of 20 or 30 years, reaches all of a sudden a 

 figure that is high compared to what it was only a few years pre- 

 viously. From this point onwards the value in question goes on 

 increasing, but this in strict accordance, Istly., with the natural laws 

 of tree growth, which always remain pretty uniform, and 2ndly., with 

 selling rate?, which as a rule rise slowly with increasing age. The 

 consequence is that the value of the crop on the land, in other 



