18 
these is marked by the line of the limit above which grain cannot be 
cultivated : or when this test is not easy of application, by the line of 
cessation of the common brake, (Pteris aquilina,) which is generally dif- 
fused throughout the whole extent of the island ; but only grows where 
cultivation is practicable. The lower of these divisions possesses a mean 
annual temperature, ranging from 52 to about 42 degrees. It comprises a 
much larger extent of surface than the upper, but its conditions are con- 
siderably modified by the operations of agriculture and the influence of 
man. It is therefore designated the region of cultivation or Agrarian 
region. The upper division possesses a range of mean annual temperature 
from 42 to about 32 degrees, and remains nearly in its natural state. It 
includes that portion of the surface of the Scotch Highlands which 
rises above 450 or 500 yards in elevation above the level of the sea : and 
of the English and Welsh mountains above about 600 yards. Its charac- 
teristic flora bears a strong analogy to that of the north of Europe, and it 
is therefore distinguished as the Arctic region. 
In order to supply a means of registering the vertical distribution of 
species, the two climatic regions are each subdivided into three zones. 
Each of these zones has a range of about three degrees of mean annual 
temperature, but the extent of surface which they severally embrace is 
necessarily very unequal. Each is characterised by its peculiar flora, and 
as an aid to their recognition in practice we may regard each as bounded 
by the line of limit of some conspicuous species. The author of the 
“ Cybele” furnishes the following table. 
II.— ARCTIC REGION. 
Name. 
Characteristic Species. 
6. Superarctic zone 
5. Midarctic zone 
4. Inferarctic zone 
. Salix herbacea without Calluna 
. Calluna vulgaris without Erica 
. Erica Tetralix without Pteris. 
Name. 
I.— AGRARIAN REGION. 
Characteristic Species. 
3. Superagrarian zone . . . Pteris aquilina without Rhamnus 
2. Midagrarian zone . . . Rhamnus catharticus without Clematis 
1. Inferagrarian zone . . . Clematis, Rubia, Cyperus longus. 
The area of Yorkshire is embraced within the second, third, and fourth 
