16 INTRODUCTION. 
and the commoner the species, the more constant is this rule 
found to be. 
The primary division into Agrarian and Arctic Regions, adopted 
as the one best applicable in Britain, is ostensibly founded upon 
an artificial character; namely, the presence or absence of culti- 
vation. In the spontaneous flora or vegetation of Britain we can 
find no character equally obvious and general as is that afforded 
by the cultivation of grain. The interests of mankind are so 
closely 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. We may doubtless see 
many spots where the nature of the soil or surface, rather than 
the climate by itself, forbids success in cultivation. But a correct 
observer can scarcely be misled in such instances, since he will 
usually find cultivation sufficiently near to those spots, to show 
that it has not been prevented by inferiority of climate. More- 
over, nature will afford us a second test of the Agrarian region, in 
the presence of a very common and conspicuous fern, the Pteris 
aquilina. This fern is abundantly distributed through the lower 
region, and from one extremity of our island to the other; its 
upper limit usually running almost uniform with the climatic 
limit of corn cultivation ; so that the two characters in connexion 
afford a satisfactory test of the region. 
The height to which cultivation ascends among the mountains 
varies much with aspect and position. Among the Grampian 
mountains of Scotland, about the latitude of 57, the general line 
of corn cultivation runs from 1000 to 1200 feet; rising in some 
favorable situations in Aberdeenshire even to 1500 or 1600 feet. 
So likewise with the Pteris; which is seldom seen above 1200 
feet on the exposed moors of the Highlands, unless in sheltered 
depressions of the surface, or on acclivities which front to the sun; 
while in the woods of Lochnagar, in Aberdeenshire, it was ob- 
served in two spots respectively estimated, and perhaps somewhat 
over-estimated, to be 1760 and 1900 fect of elevation. In the 
North of England, we are told by Mr. J. G. Baker, oats have 
been grown as a field crop at 1650 feet, and that in Weardale he 
