[footnote] *On the physiognomy of plants, see Humboldt, 'Anischten der
Natur', bd. ii., s. 1-125.
Agricultural nations increase artificially the predominance of social
plants, and thus augment, in many parts of the temperate and northern zones,
the natural aspect of uniformity; and while their labors tend to the
extirpation of some wild plants, they likewise lead to the cultivation of
others, which follow the colonist in his most distant migration. The
luxuriant zone of the tropics offers the strongest resistance to these
changes in the natural distribution of vegetable forms.
Observers who in short periods of time have passed over vast tracts of land,
and ascended lofty mountains, in which climates were ranged, as it were in
strata one above another, must have been early impressed by the regularity
with which vegetable forms are distributed. The results yielded by their
observations furnished the rough materials for a science, to which no name
had as yet been given. The same zones of regions of vegetation which, in
the sixteenth century, Cardinal Bembo, when a youth,*described on the
declivity of Aetna, were observed on Mount Ararat by Tournefort.
[footnote] *Aetna Dialogus.' 'Opuscula', Basil., 1556, p. 53, 54.
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