98 The Scottish Auitura/ist. 



sents in miniature and in some degree the botanical or zoolo- 

 gical phenomena of the whole terrestrial mass. Each of these 

 summits is a facsimile on a very small scale of the north or south 

 pole, and the base of each is, i?i comparisoji to the apex, a tropical 

 region. Near the arctic (or antarctic) circle a hill of moderate 

 height will present these features, but as we go southwards an 

 ever-increasing altitude is necessary. Thus we find that plants 

 or animals which at the arctic circle are found at the sea-level, 

 do not, in middle Europe, descend below one or more thousand 

 feet above sea-level ; in the Alps or Pyrenees several additional 

 thousands of feet are necessary ; and on hills south of the lati- 

 tude of Europe a still greater altitude. So with species that 

 inhabit the sea-level of northern (but not arctic) Europe ; they, 

 too, when existing further south, keep above a certain altitude, 

 always increasing as the tropics are approached. Some species 

 are very exorbitant in this respect, but others are better able to 

 accommodate themselves to altered conditions, and hence have 

 many advantages in the fight for existence. 



From all this we may gather the intimate relations existing 

 between latitude and altitude in the distribution of species. 



From the zones of latitude or altitude inhabited by them, 

 species may be classed in groups. Some are confined to the 

 region within or near the arctic circle, and hence may be called 

 arctic species ; others are confined to the great mountain chains 

 (including the Alps, Pyrenees, and other ranges north, south, 

 east, or west of them), and are termed alpine species ; others 

 occur in both these regions, but not on the intervening lower 

 ground, and to them the title of arctic-alpine species may be 

 given. 



To one or other of these groups all the species about to be 

 treated of belong. 



Of the 2000 species of Lepidoptera known to inhabit the 

 British islands, not more than about fifteen can be considered 

 as mountain species — meaning thereby species confined to the 

 higher mountain ranges, and not usually descending below an 

 altitude of about 1500 feet above sea-level. The majority of 

 the species alluded to do not descend as low as 1500 feet, but 

 some, at least in the far north of Scotland, descend much 

 lower. Many other species of Lepidoptera may be frequently 

 found at high altitudes, but as they descend to the low grounds, 

 ihey cannot be considered as mountain species proper; whilst 



