226 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY, 
occupies is to be the same year after year, the entire crop of 
the wood from which it comes must be made up of a series of age- 
classes equal in number to the years of the rotation ; and also 
that the age of these classes must run regularly up, without a 
break, from one year to the number of years in the rotation. 
To take a simple example:—Suppose it be desired to cut annually 
one acre of oak-coppice twenty years of age. To enable this to be 
done, there must be on the ground just before the cutting in any 
year, a series of twenty crops, each standing on an acre of ground, 
and running through all ages fronr one to twenty years. Under 
no other conditions can the desired yield be obtained ; for if the 
crops be older than this scale indicates, some of them, when 
their turn to be cut arrives, will be above twenty years of age ; 
and if they are younger, the converse will occur. Where such 
a series of age-classes exists, the classes standing on equal areas 
of similar productive power, and the ground being completely 
stocked, the forest is said to be normal; and if each annual 
felling-area be restocked as soon as the crop has been removed 
from it, the 20-acre wood will, in the absence of disturbing 
influences, yield in perpetuity an annual crop of one acre of 
oak-coppice twenty years of age. In other words, it will give 
a Sustained Yield of that amount. 
The advantages of organising woods worked for profit on the 
lines above indicated cannot be questioned. In the absence of 
such organisation, it is not possible for the proprietor to be sure 
whether he is taking from the property more or less than the 
amount to which he is entitled. On the one hand—and such 
instances are common—he may refrain from legitimate cutting 
through fear of taking too much ; and, on the other hand, acting 
in the most perfect good faith, and in complete ignorance that 
he is doing so, he may diminish the stock to a point at which 
the annual increment, or the amount of new growth put on each 
year is seriously lessened, while the producing power of the soil 
is at the same time greatly impaired. The latter is an even more 
common occurrence than the former; and this is attributable, in 
the majority of cases, to the absence of a working plan clearly 
indicating to the owner the fellings he is entitled to make. 
Instances are known in which proprietors hesitate to fell woods 
which are long past maturity, and have for many years past 
ceased to yield a fair increment, merely because they do not wish 
to expose themselves to remarks. But on any estate where the 
