202 BOTANICAL INFORMATION. 
found true, that a species which rises higher than another on 
one range of mountains, will usually be found higher on other 
ranges ; and the commoner the species, the more exact is the 
rule found to be. 
It is upon the prevailing regularity of this fact or rule, that 
the climatic zones of plants are founded and defined ; the pre- 
sence or absence of some common and conspicuous species ' 
being made the test of the zone. The primary division which 
I have proposed, as one best applicable in Britain, is osten- 
sibly founded upon an artificial character; namely, the pre- 
sence or absence of cultivation. It is by this character that I 
would distinguish the lower from 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 
prevent confusion with subordinate divisions, it may be well 
to say, in the first instance, Agrarian Region, and Arctic 
Region. 
In the spontaneous vegetation of Britain, we can find no 
character equally obvious and general with that afforded by 
the cultivation of grain. The interests of mankind are so 
intimately connected with the production of corn, that we 
shall every where find cultivated fields as far up the valleys 
and acclivities of the mountains as their climate will allow. 
No doubt we may see many spots where the nature of the 
soil or surface, rather than the climate, forbids success in 
cultivation. Buta correct observer can scarcely be misled in 
such instances, since he will usually find cultivation suffi- 
ciently near these spots, to show that it has not been prevent- 
ed by inferiority of climate. Moreover, nature will afford us à 
second test of the Agrarian region, by the presence of a very 
common and conspicuous fern, the Pteris aquilina. This fern 
is distributed throughout the region, and from one extremity - E 
of our island to the other. I have observed itin many places, - 
and always found its upper limit running nearly uniform with 
the limit of corn cultivation; so that the two characters M 
connexion form a very satisfactory test of the region. The — 
plough is soon fatal to the Pteris, nor can it long resist the | - 
