NATURALISATION OF FOREST TREES. 221 



below the surface layer of soil which is most liable to drying- 

 out and to freezing. On the other hand, the tendency of de- 

 ciduous trees to become evergreen when transferred to a warmer 

 climate and vice versa, appear to be cases of true acclimatisation. 



The basis for the distinction between natural adaptability and 

 capacity for acclimatisation is the fact that the native environ- 

 ment of a species is not necessarily that which suits it best 

 and that it may have been excluded by the competition of other 

 species or by physical barriers from other environments which 

 suit it equally well, if not better. 



A species is found in a certain habitat not so much because 

 the latter is the one most suited to it as because it is more suited 

 to the habitat than are the other species which have had a chance 

 of establishing themselves in it. To take an instance from the 

 introduction of an exotic, Rohinia makes better growth and reaches 

 a greater height in England than in its home in the Appalachians. 



At any rate, different species show a great difference in their 

 degree of adaptability when transferred to new habitats. While 

 some Eastern American trees thrive well in England, others, 

 associated with them in America, thrive badly in England. 



Though adaptability to a whole new set of conditions may 

 be out of the question, yet adaptability to a certain factor is quite 

 possible and may be of practical consequence, for instance, the 

 greater hardiness to frost of one out of two species which come 

 from the same habitat and are both introduced to a colder but 

 otherwise similar one. 



Perhaps we may expect to find the greatest capacity for ac- 

 climatisation in species whose wide range includes several variations 

 in locality, or in species which show a great tendency to botanical 

 variation and appear to be in an uncertain stage of evolution, 

 such as many of the Mexican pines. On the other hand, we may 

 perhaps expect the greatest natural adaptability in species with 

 narrow ranges to which they have been confined by some physical 

 cause. 



Acclimatisation and natural adaptability can usually only be 

 ascertained by prolonged experiments and meanwhile it is safest 

 to place no reliance on them. The possibility of them, however, 

 must always be borne in mind when drawing conclusions from 

 experiments. 



The introduction of an exotic which does not reach maturity or 

 become naturalised may yet, under some circumstances, serve a 

 useful forestal purpose and give financially profitable results. Its 

 cultivation under such conditions cannot be denied the name of 

 forestry and may be by no means undesirable, at least until a 

 suitable species which serves the same purpose is discovered. 

 However the idea of permanancy is so intimately wrapped up 

 with that of forestry that naturalisation must always be our aim, 

 and it is only by working in accordance with the laws of nature 

 that truly successful and permanent results are to be obtained. 



One sometimes hears it said that Nature evidently did not mean 

 a treeless country to be afforested, and that for man to attempt 

 to do it will prove a useless fight against her. At first sight there 



