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Vital organisms, whose relations in space are comprised under the head of
the geography of plants and animals, may be considered either according to
the difference and relative numbers of the types (their arrangement into
genera and species), or according to the number of individuals of each
species on a given area. In the mode of life of plants as in that of
animals, an important difference is noticed; they either exist in an
isolated state, or live in a social condition. Those species of plants
which I have termed 'social'* uniformly cover vast extents of land.
[footnote] *Humboldt, 'Aphorismi ex Physiologia Chemica Plantarum', in the
'Flora Fribergensis Subterranea', 1793, p. 178.
Among these we may reckon many of the marine Algae -- Cladoniae and mosses,
which extend over the desert steppes of Northern Asia -- grasses, and cacti
growing
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together like the pipes of an organ -- Avicennim and mangroves in the
tropics -- and forests of Coniferae and of birches in the plains of the
Baltic and in Siberia. This mode of geographical distribution determines,
together with the individual form of the vegetable world, the size and type
of leaves and flowers, in fact, the principal physiognomy of the district,*
its characteracter being but little, if at all, influenced by the
ever-moving forms of animal life, which, by their beauty and diversity, so
powerfully affect the feelings of man, whether by exciting the sensations of
admiration or horror.
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