32 INTRODUCTORY EXPLANATIONS. 



gi'adually ascend li'om base to summit. Yet on a single 

 mountain, as we have seen, local changes in the character 

 of its sm'face, and the difference of aspect on its declivi- 

 ties, will disturb the regularity of its ascending zones. On 

 an extended range of mountains the distm'bing effect of 

 local peculiarities will become much more obvious. And 

 when we h^ve to adapt om* zones to several gi'oups of 

 mountains, dissimilar in extent, elevation, latitude, maritime 

 proximity, and other circumstances, it then becomes dif- 

 ficult to define them with any exactness. This difficulty is 

 experienced in tracing the ascending zones of plants in 

 Britain. The absolute elevation, at which the same species 

 will grow, varies by many hundred feet on different moun- 

 tains. And as this variation is by no means unifonn with 

 different species, we find local changes in their relative ele- 

 vation also, the limit of one being compai'ed with the limit 

 of another. Notwithstanding such local exceptions, how- 

 ever, the general rule will be found true, that a species 

 which rises higher than another on one ratige of moun- 

 tains, will usualli/ be found higher on other ranges ; and 

 the commoner the species, the more exact is the rale found 

 to be. 



As before explained, it is upon the prevailing reg-ularity 

 of this fact or rule, that the chmatic zones of plants are 

 founded and defined; the presence or absence of some 

 common and conspicuous species being made the test of 

 the zone. The primary division which is here to be pro- 

 posed, as one best apphcable in Britain, is ostensibly 

 foimded upon an artificial character ; namely, the presence 

 or absence of cultivation. It is by this chai-acter that we 

 may distinguish the lower fi'om the upper zones of plants ; 

 giving to the former the common designation of Agrarian, 

 and calling the latter by the name of Arctic zones* Or, to 



