plants, more than 100 species of which have spread widely over the
country, often displacing the native vegetation. On the other hand, of
the many hundreds of hardy plants which produce seed freely in our
gardens, very few ever run wild, and hardly any have become common. Even
attempts to naturalise suitable plants usually fail; for A. de Candolle
states that several botanists of Paris, Geneva, and especially of
Montpellier, have sown the seeds of many hundreds of species of hardy
exotic plants in what appeared to be the most favourable situations, but
that, in hardly a single case, has any one of them become
naturalised.[4] Even a plant like the potato--so widely cultivated, so
hardy, and so well adapted to spread by means of its many-eyed
tubers--has not established itself in a wild state in any part of
Europe. It would be thought that Australian plants would easily run
wild in New Zealand. But Sir Joseph Hooker informs us that the late Mr.
Bidwell habitually scattered Australian seeds during his extensive
travels in New Zealand, yet only two or three Australian plants appear
to have established themselves in that country, and these only in
cultivated or newly moved soil.

These few illustrations sufficiently show that all the plants of a
country are, as De Candolle says, at war with each other, each one
struggling to occupy ground at the expense of its neighbour. But,
besides this direct competition, there is one not less powerful arising
from the exposure of almost all plants to destruction by animals. The
buds are destroyed by birds, the leaves by caterpillars, the seeds by
weevils; some insects bore into the trunk, others burrow in the twigs
and leaves; slugs devour the young seedlings and the tender shoots,
wire-worms gnaw the roots. Herbivorous mammals devour many species
bodily, while some uproot and devour the buried tubers.