'
Looking at the effect of climate on vegetation, we find that as we
proceed from the north towards the south, the number and luxuriance of
plants increase in a remarkable degree, and the same result is
observable in altitude as in latitude. 'Step by step,' writes Mr
Henfrey, 'as the land rises in any mountain region, the vegetation
assumes, more and more, a polar character; and in the mountains of the
tropics, a succession of stages has been distinguished, corresponding
in the general peculiarities of the plants which clothe them, to
tracts extending horizontally, in succession, on the sea-level, from
the base of these mountains to the frozen regions within the arctic
and antarctic circles. Increase of elevation is accompanied by an
alteration of climate, bringing with it a set of conditions analogous
to those prevailing at certain distances further from the sun.
Ascending the Peak of Teneriffe, a series of regions are traversed,
one above another, displaying with the approach to the summit a
continually closer approximation in character to the polar regions,
till the traveller who left the palm, the cactus, and the thousand
varied forms of tropical vegetation at the foot, finds himself at last
among the stunted shrubs and scaly lichens, the borderers who hold the
outposts on the limits of the eternal snow.'
It might be expected that places on the same parallel of latitude
would be equal in temperature; but on tracing out the distribution of
heat over the globe, and laying it down in what are called
_isothermal_ lines on a map, most striking deviations are found to
exist, and the contour of the lines is anything but regular.
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