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.
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