ot half that number, inhabiting any given locality. There may
be no want of space in the supposed tract: it may be a large mountain,
or an extensive moor, or a great river plain, containing room enough for
individuals of every species in our island; yet the spot will be
occupied by a few to the exclusion of many, and these few are enabled,
throughout long periods, to maintain their ground successfully against
every intruder, notwithstanding the facilities which species enjoy, by
virtue of their power of diffusion, of invading adjacent territories.

The principal causes which enable a certain assemblage of plants thus to
maintain their ground against all others depend, as is well known, on
the relations between the physiological nature of each species, and the
climate, exposure, soil, and other physical conditions of the locality.
Some plants live only on rocks, others in meadows, a third class in
marshes. Of the latter, some delight in a fresh-water morass,--others in
salt marshes, where their roots may copiously absorb saline particles.
Some prefer an alpine region in a warm latitude, where, during the heat
of summer, they are constantly irrigated by the cool waters of melting
snows. To others loose sand, so fatal to the generality of species,
affords the most proper station. The _Carex arenaria_ and the _Elymus
arenarius_ acquire their full vigor on a sandy dune, obtaining an
ascendancy over the very plants which in a stiff clay would immediately
stifle them.

Where the soil of a district is of so peculiar a nature that it is
extremely favorable to certain species, and agrees ill with every other,
the former get exclusive possession of the ground, and, as in the case
of heaths, live in societies. In like manner the bog moss (_Sphagnum_)
is fully developed in peaty swamps, and becomes, like the heath, in the
language of botanists, a social plant. Such monopolies, however, are not
common, for they are checked by various causes. Not only are many
species endowed with equal powers to obtain and keep possession of
similar stations, but each plant, for reasons not fully explained by the
physiologist, has the property of rendering the soil where it has grown
less fitted for the support of other individuals of its own species, or
even other species of the same family. Yet the same spot, so far from
being impoverished, is improved, for plants of _another_ family. Oaks,
for example, render the soil more fertile for the fir trib