78 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
preserved. All trees may be treated under this system, but it 
is specially suited to the wants of the Norway spruce. 
The objection that the regeneration of a wood in this way is a 
process slow of accomplishment has not much force, and is altogether 
lost sight of after a wood has once been gone over, for then we 
have to deal with a series of age classes systematically arranged, 
the oldest of which is taken in hand first, and at a time when those 
which are younger are not ready for regeneration. 
When regeneration is accomplished in neither of the two ways 
already considered, but by taking certain irregularly distributed 
groups in hand as occasion demands, a third system presents itself, 
which is known as the shelter-wood group system. Under this 
system all groups of advance growth are specially attended to, 
and their occurrence regulates, to a considerable extent, the points 
where felling should be done. Groups of young seedlings which 
have found a footing in the open parts of the wood, even though 
embracing an area of only a few square feet, must not be over- 
looked, so long as they are not stunted in growth, and, owing to 
this cause, incapable of future development. Advance or seed 
fellings, properly so called, cannot be spoken of in-connection with 
these groups, for they have sprung into existence without any 
artificial aid being given. They must, however, be fostered by 
light fellimgs being made round about them, and these fellings 
become advance fellings for other groups which will subsequently 
spring up in their immediate neighbourhood. By this means each 
group is constantly being enlarged, like a crystal or a snowball, 
by the deposition of material on the outside, until ultimately, by 
group joining on to group, the regeneration of the whole wood is 
accomplished. 
Although groups of advance growth form special objects of care 
under this system, still there are certain circumstances when points 
of the wood must be attacked for other reasons than the fostering 
of young trees already established. This happens where patches 
of old trees occur which are in an unhealthy state, owing to such 
causes as insect or fungoid attack; or where, in a mixed wood, 
certain species are ready for felling sooner than others; or where 
extra old and heavy trees, the survivors of a former generation, 
have reached maturity, as well as where groups of trees have ceased 
to grow owing to the effects of situation, etc. 
In removing old trees, in order to provide sufficient light for the 
young groups, one must look not only to the vertical shade, but 
