18 INTRODUCTION. 
ground, the presence of Erica cinerea might be taken for a test 
instead. 
The Agrarian Reyion cannot be subdivided into its three zones 
rigidly by the presence or absence of single species. As corn 
cultivation of some kind extends throsigh the whole length of 
Britain, even into Shetland where crops of oats and barley are 
still grown, the whole low surface of the country has to be 
apportioned between the three Agrarian zones, along the sea coast, 
and thence upwards to the line or spots where cultivation ceases 
on the acclivities of the hills, and where the Pieris aquilina also 
usually disappears. If we had to deal with a truly level or only 
slightly undulated surface, the region might conveniently be 
divided into three zones simply in accordance with latitude ; 
namely, South—Middle—North. But the three zones must also 
be considered to overlap each other in succession on the lower 
acclivities of the mountains, and in their intervening valleys; 
increasing elevation above the level of the sea, in any more 
southerly province, corresponding in a general manner with 
increasing latitude in a northerly direction. Our indications of 
the zones must thus keep in view both elevation and latitude, and 
be adapted to both conditions. On comparing a flora of one of 
the south-eastern provinces of England with a flora of the High- 
land valleys, or of the plains near the north coast of Scotland, the 
differences would be found decidedly marked, and this both in the 
species and in their comparative numbers or frequency; and an 
intermediate tract between these two extremes of the agrarian 
region, would be sufficiently well characterised by its own dis- 
tinctive features taken in the aggregate. But the difficulty is 
always renewed when we attempt to divide natural gradations by 
abrupt lines, or to lay down distinctions which rest upon single 
characteristics to be applied under varying circumstances. Keeping 
this practical difficulty im recollection, the three Agrarian zoncs 
may now be shortly described, rather than very rigidly defined. 
The Super-ayrarian zone may be said to comprise three different 
kinds or portions of the surface of Britain. First, all the coast- 
line and low plains or moors in the north and north-west of 
