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CHAPTER III
THE NUMERICAL DISTRIBUTION OF BRITISH PLANTS:
TEMPERATE FLORAS COMPARED
PROCEEDING from the more to the less familiar regions we
will begin with a few of the facts as to the flora of our own
country. Partly owing to its insular character, and also
because it has few lofty mountains or extensive forests, the
number of species of flowering plants is somewhat (but not
much) below that of most continental countries of equal area.
It contains about 1 800 species, as a rough mean between
the estimates of different botanists. 1
It may seem curious
that there should be any such difference of opinion, but one
of the facts that have always been adduced as showing that
species are not fixed and immutable entities is the frequent
occurrence of varieties, which are sometimes so peculiar and
so apparently constant that they are treated by some botanists
as distinct species, by others as sub-species, and by others
again as forms or varieties only. These modifications of a
species are usually confined to a more limited area than the
species itself, and are occasionally connected with each other
or with the parent species by intermediate forms. Again,
when these varieties are cultivated, and especially when a
large number of plants are raised from their seeds, they are
apt to revert partially or wholly to the parent form. Another
source of difference of opinion among botanists is, as to the
treatment of those plants, found usually near human habita-
tions, which are supposed to have been originally introduced,
either purposely or accidentally, from foreign countries.
1 In all the tables and comparisons of " Floras" in this work, unless where
ferns are specially noted, flowering plants only are intended, even when the term
" plants" is used.
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