CHAP. II 
THE STRUGGLE FOR EXISTENCE 
15 
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 north¬ 
eastern 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 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. 1 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 
1 Geogmphie Botanique, p. 798. 
