CHAP. II THE STRUGGLE FOR EXISTENCE 15 



a number of the flowers. It is not so commonly knoAvn 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 l^y 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 Avhen 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 diff"erent behaviour of introduced wild plants or animals 

 into countries apparently quite as well suited to them as 

 those Avhich they naturally inhabit. Agassi z, in his work on 

 Lake Superior, states that the roadside weeds of the north- 

 eastern United States, to the number of 1 30 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 i:)lants in what appeared 

 to be the most favourable situations, but that, in hardly a 

 single case, has any one of them become naturalised.^ 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 A\-ould easily run 

 ^ Georjraphie Botanique, p. 798. 



