THE BIOLOGICAL BASIS OF FORESTRY. 215 



ticular area. In the course of the tree's subsequent development 

 its physiological peculiarities influence the practice of silviculture. 

 Consider, for instance, the tree's demand for light. Space does 

 not allow of a detailed treatment, but briefly it may be indicated 

 that since species vary in this respect, since too the demand for 

 light varies during the lifetime of the individual, so must practice 

 conform to the tree's requirements. Distance of planting, the 

 age at which thinning takes place, the degree of thinning at 

 various ages, the number of trees in the final crop, the number 

 of trees left per unit area if natural regeneration is to follow — all 

 depend on a knowledge of the relation of the tree to light 

 intensities. Not only do trees' light-demands differ but these 

 latter vary with variation of the habitat. It is sometimes due 

 to misunderstanding in this respect that we condemn the 

 unsuccessful introduction of continental methods. The principles 

 remain the same, but varying conditions necessitate modifi- 

 cations in practice. " What we can learn from Germany and 

 other countries with highly developed forestry is not so much 

 their methods and systems but the principles upon which they 

 are based, for these principles are determined by the universal 

 laws of nature and human society." 



Our study of fundamentals is by no means complete, if we 

 confine our attention to the individual tree. Of late years the 

 study of plant communities has progressed so far that we are in 

 a position to apply the knowledge gained to the practice of 

 silviculture. Just as there is a definite relation between any 

 plant and its habitat, so there is an intimate relation between a 

 community of plants and environment. The plant community 

 of direct interest to us is the forest, which behaves like an 

 organism varying from day to day and from century to century, 

 passing through various life stages from infancy to old age and, 

 like an organism, possessing functions and structure responding 

 to and recording in the latter alterations of the habitat. The 

 functions of the forest organism of chief interest here are its 

 capacity to invade other plant communities whose habitats are 

 being gradually modified by the addition of humus ; and its power, 

 after gaining a foothold, of ejecting ultimately the previous 

 tenant. How then can we make use of this knowledge? 

 According to some, millions of acres of our Scottish Highlands 

 are afforestable, but if we reflect that for centuries seeds of forest 

 trees have been falling on some of these desolate areas without 



