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24 THE WORLD OF LIFE CHAP.
termed the British type ; that is, they are found in suitable
places over the whole of Great Britain, and in most districts
are so plentiful that they may be termed common plants
such are the Alder, Birch, and Hazel among trees and shrubs ;
the Honeysuckle, Ivy, Heather or Ling, Daisy, Chickweed,
Nettle, and a host of others. Another group is abundant
in England, but absent from the Highlands or from Scotland
generally, such as the Dwarf Gorse and Yellow Dead-Nettie.
Several arctic or alpine plants are peculiar to the Highlands,
a considerable number of species are found only in our
eastern counties, while as many or more are characteristic of
the west
More curious perhaps than all these are the cases of
plants found only in one small area, or two or three isolated
patches ; and of others which are limited to a single station,
sometimes of a few acres or even a few yards in extent.
Such are the Cotoneaster, found only on Great Orme's Head
in N. Wales ; the Yellow Whitlow-Grass, on Worms Head
in S. Wales ; the pretty white-flowered Potentilla rupestres,
on a single mountain-top in Montgomeryshire ; the small
liliaceous plant, Simethus bicolor, in a single grove of pine
trees near Bournemouth, now probably exterminated by the
builder, and another plant of the same family, Lloydia
serotina, limited to a few spots in the Snowdon range ; the
beautiful alpine Gentiana verna, in upper Teesdale, Yorkshire,
and others confined to single mountains in the Highlands.
Between the extremes of widespread abundance and the
greatest rarity, every intermediate condition is found ; and
this is so far as we know, a characteristic of every part of
the world. This, again, affords a striking proof
of that
struggle for existence which has already been referred to,
acting, as Darwin was the first to point out, first to limit the
in two more or
range of a species, often so that it exists only
less isolated areas, then to diminish the number of individuals
in these areas, and finally to reduce them to a single group
which ultimately succumbs to an increased stress of competition
or of adverse climatal changes, when a species which
may have once been flourishing and widespread altogether
ceases to exist. The rarity of a species may thus be
considered as an indication of approaching extinction.