12 INTRODUCTION. 
on the other hill. Those really seen would seldom or never be 
found exactly in the same series or order of succession on both. 
Real differences would be traced, through some of the species 
themselves not invariably keeping the same upper and lower limits 
relatively to each other. Apparent differences would arise, through 
the Botanist failing to observe the highest or lowest points for 
some of the species. But the absolute heights by measurement, 
at which the same single species appeared or disappeared on the 
two hills respectively, would usually be found to differ more than 
the relative heights of the several species, one compared with 
another. 
Again, the Reader will please to suppose that our botanical 
observer has repeated the described process on numerous hills 
within Britain; making out so many lists of the species, as they 
were first observed in ascending or descending the hills. The 
series of names in his various lists will present an approximate 
similarity, although not an actual sameness. The closest approxi- 
mation towards sameness will be found among the very common 
species which occur almost everywhere; the rarer species in 
general being less uniform in their relative position. For 
instance, it will be found almost invariably that Evica cinerea 
ascends higher than Ulex europaus; that Calluna vulgaris ascends 
higher than the Erica; that Vaccinium Myrtillus ascends higher 
than the Culluna. Conversely, Saxifraga stellaris will almost 
always be found to descend Jower than Surifraga nivalis ; Eimpe- 
trum nigrum will be seen lower down than Azalea procumbens ; 
and Luzula spicata will appear at a much lower elevation than the 
rarer Luzula arcuata is ever found at in Britain. ‘ 
This tendency of plants to cease at different altitudes, to have 
upper and lower limits, and for several also usually to keep an 
approximate similarity in the distance apart of those limits, one 
species relatively to another, affords the facility for forming 
ascending zones of plants. Common and conspicuous species are 
selected, which usually cease at some considerable distance from 
each other, a distance tolerably uniform in measurement. The 
intervening spaces between the limits of these species are taken to 
