Il. ASCENDING ZONES. 13 
apportion the whole height of the hills into successive stages; 
which are denominated Ascending Zones. If, for example, on one 
hill the Ulex ewrop@us should cease at 1000 feet of altitude, Erica 
cinerea at 2000 feet, Calluna vulgaris at 3000 feet, and Vaccinium 
Myrtillus at 4000 feet,—in such a case, their upper limits would 
divide the hill into four ascending zones of 1000 feet each. 
Again, on some more northerly hill, if the same shrubs should 
cease respectively at 500, 1500, 2500, and 3500 feet,—in this 
case, their upper limits would still form equivalent zones, rela- 
tively to each other, although their absolute altitudes differed by 
500 feet on the two hills. 
The low horizontal surface of the island equally admits of zonal 
division. A large number of the species which are seen in the 
southern counties of England, fail to reach the northern counties 
of Scotland ; disappearing or being left behind successively as a 
traveller proceeds from south to north. Conversely, various other 
species which flourish in the northern counties, will be found to 
run out and disappear as the traveller returns southward. But it 
is less easy to make or mark out horizontal zones on the low 
grounds, although in their general character they correspond with 
the vertical or montane zones, one single country being under 
view. Zones must be bounded by imaginary lines, while species 
seldom or never cease thus abruptly in hard lines. In the usual 
mode of cessation, especially on the horizoutal surface, a species 
first becomes less abundant and less continuous in its distri- 
bution ; its localities gradually becomimg wider and wider apart, 
and often less productive ; finally, a few outlying localities may be 
found, far separated from each other, in which the species almost 
seems to re-appear after a cessation. These extreme localities 
constitute the actual or true limits of the species, southerly or 
northerly, easterly or westerly, as the case may be. But if they 
were correctly marked on a map, such localities would be only 
minute dots wide apart, and not real lines. It is the conventional 
custom, however, to connect these dots on a map by tracing lines 
from one to another, and thus to make the terminal limits of 
plants to appear linear, instead of distantly dotted. Something of 
