14 INTRODUCTION. 
the linear character may often be really traced along the upper 
limits of species, as seen on the acclivities of mountains; although, 
even on the hill-sides, there is the like tendency to run out 
gradually in detached spots, rather than to cease suddenly along a 
continuous line. 
The mode in which it has been found most convenient to divide 
the surface of Britain into floral zones, horizontal and vertical 
taken together, is that of first tracing a line along the upper limit 
of grain crops, and then subdividing the two spaces above and 
below that line into three zones each. The two primary divisions 
have been designated the Agrarian Region, or region of culti- 
vation, and the Arctic Region, or region of (so-called) alpine plants. 
The three subordinate zones into which each of these two regions 
is subdivided, are similarly named by prefixing the words Infer 
Mid Super to the names of the regions. Tracing them upwards, 
—that is, from south to north, from lower grounds to higher 
grounds,—they may be said to stand thus 
I. Agrarian Region. II. Arctic Region. 
8. Super-agrarian Zone. 6. Super-arctic Zoue. 
2. Mid-agrarian Zone. 5. Mid-arctic Zone. 
1. Infer-agrarian Zone. 4. Infer-arctic Zone. 
The manner in which they succeed and overlap each other, is 
shown by a diagram at the lower corner of the map. These zones 
represent the facts of nature, but represent them by an artificial 
or conventional method. The zones are arbitrary sections, in so 
far as the selected species and fixed boundary lines are concerned ; 
and yet they do correspond with and represent differences which 
actually exist in nature, inasmuch as the flora (the species) and 
vegetation (general mass of plants) of each single zone differ con- 
siderably from those of the other zones above or below. In truth, 
however, the natural changes in flora and vegetation being every- 
where gradual, any line will unavoidably sever that which is 
nearly alike ; the vegetation being more similar on the contrary 
sides of any single dividing line, than it is on the two sides of the 
wide space between any two lines. 
