I, ASCENDING ZONES. 15 
It is to be observed also, that climatic or ascending zones of 
plants are designed to indicate the relative distribution of species 
under the joint influence of altitude and latitude, with other con- 
ditions of more local character, such as proximity to seas or moun- 
tains, the state of exposure or shelter, etc. Connexions may be 
traced between the distribution of plants and each of those con- 
ditions singly ; but everywhere the influence of any one condition 
is more or less disturbed and modified by the influence of other 
conditions. Their effect upon the species-flora, or upon the 
general vegetation, is mostly indirect or remote; that is, the flora 
varies with the climate, and the climate varies with altitude, lati- 
tude, and other conditions of place and surface. On a single 
isolated mountain floral stages are strongly marked in accordance 
with height ; some species disappearing, other species appearing, 
one above another, as we gradually ascend from base to summit. 
Yet on a single mountain the regularity of its ascending zones 
will be somewhat disturbed by local differences in the character of 
its surface, as even or rocky, dry or damp; and through dif- 
ferences of aspect on its acclivities, as facing to or from the sun. 
On an extended range of mountains the disturbing effect of local 
peculiarities will become much more obvious. And when we 
require to generalize our zones from the facts noted on several 
groups of mountains, dissimilar in extent, elevation, latitude, 
maritime proximity and other circumstances, it then becomes 
difficult to define them with any exactness. 
This difficulty is experienced within Britain. The absolute 
elevation at which the same species is found to grow, varies by 
several hundred feet on the different mountain ranges of our own 
island. And as this variation is by no means uniform among the 
different species, we find local changes in their relative elevation 
also, when the limit of some one species is constantly compared 
with the limit of some other species. Notwithstanding such local 
exceptions, however, the general rule may still be held true, that 
a species which ascends decidedly higher than another on one range 
of mountains, will also usually be found higher on other ranges ; 
