the beauty and harmony and enjoyment in nature is produced, and also
affords one of the most important elements in bringing about the origin
of species. We must, therefore, devote some time to the consideration of
its various aspects and of the many curious phenomena to which it gives
rise.

It is a matter of common observation that if weeds are allowed to grow
unchecked in a garden they will soon destroy a number of the flowers.
It is not so commonly known that if a garden is left to become
altogether wild, the weeds that first take possession of it, often
covering the whole surface of the ground with two or three different
kinds, will themselves be supplanted by others, so that in a few years
many of the original flowers and of the earliest weeds may alike have
disappeared. This is one of the very simplest cases of the struggle for
existence, resulting in the successive displacement of one set of
species by another; but the exact causes of this displacement are by no
means of such a simple nature. All the plants concerned may be perfectly
hardy, all may grow freely from seed, yet when left alone for a number
of years, each set is in turn driven out by a succeeding set, till at
the end of a considerable period--a century or a few centuries
perhaps--hardly one of the plants which first monopolised the ground
would be found there.

Another phenomenon of an analogous kind is presented by the different
behaviour of introduced wild plants or animals into countries apparently
quite as well suited to them as those which they naturally inhabit.
Agassiz, in his work on Lake Superior, states that the roadside weeds of
the northeastern United States, to the number of 130 species, are all
European, the native weeds having disappeared westwards; and in New
Zealand there are no less than 250 species of naturalised European