We will here subjoin one important observation by way of elucidating the
connection of which we have spoken. The first general glance over the
vegetation of a vast extent of a continent shows us forms the most
dissimilar -- Graminae and Orchideae, Coniferae and oaks, in local
approximation to one another; while natural families and genera, instead of
being locally associated, are dispersed as if by chance. This dispersion
is, however, only apparent. The physical description of the globe teaches
us that vegetation every where presents numerically constant relations in
the development of its forms and types; that in the same climates, the
species which are wanting in one country are replaced in a neighboring one
by other species of the same family; and that this 'law of substitution',
which seems to depend upon some inherent mysteries of the organism,
considered with reference to its origin, maintains in contiguous regions a
numerical relation between the species of various great families and the
general mass of the phanerogamic plants constituting the two floras. We
thus revealed in the multiplicity of the distinct organizations by which
these regions are occupied; and we also discover in each zone, and
diversified according to the families of plants, a slow but continuous
action on the aerial ocean, depending upon the influence of light -- the
primary condition of all organic vitality -- on the solid and liquid surface
of our planet.
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