30 INTRODUCTORY EXPLANATIONS. 



character of the Zone ; and the upper and lower hmits of 

 all the other species may then be expressed by reference to 

 the Zones in which they are found. 



In selecting the species for this object, preference should 

 be given to those which are generally and abundantly dis- 

 tributed, sufficiently conspicuous not to be easily over- 

 looked, and the limits of which remain most miiform with 

 respect to each other. The commoner arborescent species 

 (Pinus, Betula, Alnus, Quercus, &c.) would best meet the re- 

 quired conditions, were it not for the interfering operations 

 of mankind ; whose own agency, rather than the hand of na- 

 ture, now assigns limits to the distribution of trees. In the 

 low country, which is subjected to the plough and scythe, 

 and mapped over with planted hedgerows, trees can scarcely 

 be pronounced really indigenous ; and on the mountains 

 and moors, they are often destroyed by fire, applied at short 

 intenals, for bm-ning off the natm-al vegetation, in order to 

 substitute a pastm-age better adapted for sheep-feeding. 

 Some of the Ericaceae, conspicuous by their size and 

 abundance, are so well fitted to flourish on the wastes of 

 nature, that they rapidly resume possession of the surface, 

 after each bm-ning ; and these accordingly will afford better 

 chai-acters for the indication of ascending Zones, above the 

 limit of cultivated gi'ound. 



Before proceeding to define om- Clunatic or Ascending 

 Zones, the tme meaning and use of such di^^sions of the 

 sm-face should be clearly understood. They are ai-bitraiy 

 and conventional sections, in so far- as the selected species 

 and fixed lines of demarcation are concerned ; and yet they 

 do correspond with and represent differences which actually 

 exist in natm-e ; inasmuch as the vegetation of each smgle 

 Zone differs considerably fi'om that of the other Zones 

 above or below. In truth, however, the natural changes of 

 vegetation being eveiywhere gradual, any line will in- 



