THE PLANTS OF THE GRAMPIANS. 53 
aquilina always ceases long before Salix herbacea appears, 
as we ascend the hills of Aberdeenshire and Cumberland, we 
may reasonably infer that the same circumstance will be also 
found to hold in Caernarvon and Sutherland ; and that when 
we attain the height, be it what it may, at which Salix her- 
bacea grows in these two latter counties, we shall see no 
Pieris aquilina or other species which keep below its limits 
elsewhere. Though certain exceptions do occur, principally 
with respect to particularly local plants, the rule applies very 
extensively ; and it thus becomes comparatively easy to di- 
vide the flora of a country into ascending stages, and to refer 
each species to its proper stage or stages. 
But the absolute heights at which the same species will be 
found on different hills, and even on different parts of the 
same hill, are very variable; and, consequently, it becomes 
requisite to make numerous measurements in different places 
before we can ascertain either the mean height, or the range 
of height of any species that is at all widely distributed. If, 
for instance, the height of one species be ascertained in 
Sutherland, and that of another in Forfarshire, the results 
may be the same, and yet the two plants may be such as 
never appear at the same level, when growing upon one and 
the same hill. Alllists of species, designed to compare the 
absolute heights attained by them, must therefore relate to a 
small tract, that is, if intended to be more than general approxi- 
mations, But if sufficiently numerous measurements can be 
made at distant points, the averages of the whole will admit 
of comparison. 
In the Edinburgh New Philosophical Journal, some years 
ago, I gave lists of plants which had been ascertained to 
grow above 4000, 3000, 2000, and 1000 feet respectively, in 
the Scottish Highlands. Ina general way these lists indi- 
dicated the floras of so many successive stages of elevation ; 
but they still brought species into the same list or stage 
which really do never grow near together; much as in a 
British Flora, we promiscuously group all the species found 
im Britain, although many of them never associate under the 
