INTRODUCTORY EXPLANATIONS. 29 



equal heights m all three lists ; but the Calluna is much 

 lower, and the Erica much higher, on Ben-na-muich-dhu. 

 , Such differences appeared explicable enough on the spot, 

 by attending to the physical conditions of the groimd ; but 

 these conditions will not be discussed here, because atten- 

 tion is at present desired to the simple facts, and not to the 

 causes or explanations of any want of strict uniformity 

 between them. 



Similar Usts to the preceding have been made by the 

 present writer, on many other hills in Scotland ; as also on 

 some of those in England and Wales. On compai-ing these 

 numerous Usts together, a strong general con-espondence 

 can be traced among them. Always, where the hills have 

 been left most nearly in a state of nature, the Calluna is 

 found growing to gi-eater elevations than the Erica Tetra- 

 lix; and the Erica as invariably rises higher than the 

 Myrica Gale. Moreover, in the great majority of instances, 

 the species which grow above the CaUuna on one hill, ai-e 

 the same which gi-ow above it on the others ; for example, 

 Luzula spicata and Gnaphalium supinum in the preceding 

 lists. The like con-espondence may be observed, on dif- 

 ferent hills, between the species which grow above the Erica, 

 and cease below the teraiination of the Calluna; as is the 

 case with Juniperus communis in the thi-ee preceding hsts. 

 Still the same fact prevails, if we compai-e the species whose 

 upper limit occurs between those of the Erica and Myi-ica. 

 As might be expected, the absolute altitudes attained by 

 the same species vaiy veiy much in different parts of Bri- 

 tain ; but the relative altitudes of the more generally dis- 

 tributed species, one being compared with another, remain 

 far more unifoiin. 



The Ascending or Climatic Zones of Plants ai-e foimded 

 upon this prevailing miiformity. Certain species are se- 

 lected, the presence or absence of which can be taken as a 



