220 NATURALISATIOX OF FOREST TREES. 



years, and much of that which was done was chiefly with quite 

 unsuitable species, while in all parts many species, likely to prove 

 suitable, have not yet been tried at all. 



In short, it must be said that, while we have ample practical 

 knowledge and experience to justify afforestation on a large scale 

 now, not only does much methodical experimentation remain 

 to be done but also w^e have so far gained only a somewhat vague 

 knowledge of the underlying principles which should both be based 

 on, and be the basis of, the practical work. 



It should be said that much useful work was done by Mr. D. E. 

 Hutchins in emphasising the importance of climatic suitability, 

 though possibly his special theory of the importance of the dif- 

 ference between a winter and summer rainfall climate, is not 

 entirely supported by practical experience. 



Before proceeding, it seems desirable to define some terms 

 which are often used loosely and interchangeably, but which should 

 have distinct meanings. 



The " Introduction " of an exotic does not necessarily imply 

 that the introduction is a desirable one or that the species becomes 

 naturalised. 



" Acclimatisation " implies that the species is not at first suited, 

 or is only partially ^uited, to its new surroundings, but becomes 

 adapted to them in one or more generations as the result of the 

 influence of environment. 



The " Natural Adaptability " of a species is not the same as 

 its capacity for acclimatisation, but denotes its inherent capacity 

 for thriving under conditions dissimilar to its native ones, without 

 undergoing any process of adaptation. 



" Naturalisation " takes place when the species is either already 

 suited to its new home, or becomes acclimatised to it, and, 

 though in the latter case it may sometimes be accompanied by 

 some deterioration, it necessarily implies that the species lives 

 to maturity and is capable, at least to some extent, of natural 

 reproduction. 



It may seem unnecessary to define a " Forest Tree " or a " Forest," 

 but when one is concerned with a country like this in which there 

 are almost all degrees of humidity, one must be ready to accept 

 very broad definitions of these terms. Broadly speaking, a forest 

 is any continuous arborescent growth which serves some useful 

 purpose and includes the growth of mimosa on the Modder River 

 as well as the Yellowwood forests of the coasts mountains. The 

 desirable exotic introductions may possibly include similar ex- 

 tremes of species and forest form. 



The extent to which acclimatisation of tree species can take 

 place is a doubtful question. From the analogy of other forms 

 of life there seems good reason to suppose that it is a possibility, 

 but it is probably usually a very slow process. At any rate 

 some cases of apparent acclimatisation are due to physical and 

 not physiological causes. For instance, when a tree appears 

 to become hardier to drought or frost as it passes out of the 

 seedling stage, it is usually because its crown rises above the 

 cold layer of air next to the ground, while its roots penetrate 



