NATURALISATION OF FOREST TREES. 223 



The same considerations obviously apply equally to the even 

 less well founded objection that it is contrary to Nature to attempt 

 to introduce exotic trees into indigenous forests. 



Even the better founded statement of some authorities that it 

 should never be attempted to extend the range of a species within 

 its forest region on the grounds that it would have been extended 

 naturally, if it were possible, is not necessarily always true, be- 

 cause it neglects the facts that climates are changing and species 

 are migrating (for instance, Norway Spruce) and that their migra- 

 tions lag behind the changes in climate ; it neglects also the 

 influences of the competition of the other species and of the control 

 of man. A notable case of the successful extension of the range 

 of a species within its forest region is that of the introduction of 

 Silver Fir into Scandinavia. 



When practical German foresters are departing from their ultra- 

 conservative attitude on the introduction of exotics and are being 

 persuaded that the latter is possible and will improve their already 

 excellent forests, we need hardly hesitate as to the soundness of 

 the same policy with regard to our indigenous forests. B. E. 

 Fernow may be quoted on this point. He says : — 



" While it may still be safest to rely upon the native flora, yet if exotics- 

 climatically adapted, promise more rapid growth, larger production, sylvi- 

 cultural quantities or quality of wood superior to the native, as for in- 

 stance the Norway Spruce, it is proper policy to supplant the inferior native." 



There certainly seems room for improvement in the production 

 of our native forests, and there are a large number of exotics with 

 rapid growth and valuable products, which are climatically suited 

 to the indigenous forest country of South Africa. 



The Principles of Naturalisation. 



To come, at length, to the principles of naturalisation, there is 

 if w^e neglect acclimatisation and natural adaptability, in reality 

 only one principle ; that is — fit the local habitat with species whose 

 native home has the same factors of habitat or their equivalent. 

 In other words, it is merely the consideration of what are the 

 essential factors of locality and to what extent and in which direc- 

 tion a certain difference in any one of the factors will operate. 



There are two kinds of factors of locality — i.e., those of climate, 

 and those of soil ; and of these the former, namely. Temperature, 

 Moisture, and Light, are usually by far the most important. 



Temperature 



The mean annual temperature is of very little value for eco- 

 logical purposes, and should be entirely disregarded except in 

 connection with other data. As Professor Mayr points out, the 

 annual temperature at the upper limit of tree growth on mountains 

 at the equator is 50° F., or the same as that of the parts of Europe 

 where oak, tobacco, and the grape find their optimum. In other 

 words, the total amount of heat received during the year is by 

 itself of no consequence to plant distribution. 



What are, however, of vital importance are the amount and 

 degree of heat during the growing season, and these are measured 



