364 STRUGGLE BETWEEN PLANT-COMMUNITIES SECT, xvn 



CHAPTER XCVII. CHANGE OF VEGETATION WITHOUT 

 CHANGE OF CLIMATE OR OF SOIL 



NUMEROUS facts have proved that there are many species which are 

 still migrating and have not attained the distribution that soil, climate, 

 their means of travelling, and other relations, would permit. Such species 

 are able to emerge triumphant from struggles in many communities, 

 without requiring the aid of any change in the inanimate surroundings. 

 Senecio vernalis in northern Germany has spread towards the west as 

 a pestilent weed, within a period scarcely more than twice a man's life. 1 

 We have already mentioned 2 the hordes of European plants that have 

 entered Argentina and have destroyed the indigenous vegetation here 

 and there. On the other hand, American plants introduced into Europe 

 have locally suppressed native species : among North American immi- 

 grants are Elodea canadensis in fresh water of Central Europe, Opuntia and 

 Agave americana in Mediterranean countries, and a number of weeds, 

 including Oenothera biennis and Erigeron canadensis. In the same way 

 several hundred foreign species have reached New Zealand, where some 

 of them defeat the native vegetation. 3 It is essential that climate and soil 

 shall suit immigrant plants ; otherwise they fail to gain an entrance, 

 even when protected by man, as is proved by unsuccessful attempts to 

 introduce certain trees. One species that is perhaps still moving westward 

 is Picea excelsa. Travelling from the east, it has entered the northern 

 part of the Scandinavian peninsula and advanced towards the south, but 

 has not yet reached the south of Sweden nor Denmark. In several places 

 in Norway it has swept through passes and vanquished the Scots pine, 

 but has not been able to establish itself everywhere, so that there are 

 remarkable gaps in its distribution. That the common spruce can defeat 

 the Scots pine largely depends upon its hardier nature and upon its faculty 

 of enduring shade. 4 



In every region, usually without any change in the physical environ- 

 ment, the vegetation indubitably undergoes slow changes, which may be 

 appreciable only after the lapse of ages and are the result of struggles among 

 species. This conclusion is forced upon us when we see a tract of soil that 

 has been laid bare successfully occupied by a long series of different kinds 

 of vegetation. In this connexion we may mention Hult's 5 description of 

 the district of Blekinge in the south of Sweden : here most of the ' forma- 

 tions of vegetation are merely transitional stages towards a few final 

 communities, whose distribution within the area is ultimately determined 

 by soil'. Nevertheless we must assume that the struggles in question 

 are rare in very ancient countries, whose vegetation is not appreciably 

 disturbed by man or animals, and which has been exposed for long ages 

 past to immigration from adjoining countries : in this case a certain 

 condition of equilibrium must have been attained. Yet nearly all changes 

 in vegetation that we see taking place for instance, in forests in various 

 parts of the earth seem to be due to recent physical changes, and par- 

 ticularly to such as were caused by the destruction of forest through 



1 Ascherson, 1863. * See p. 287. 3 See Cheeseman, 1882. 



' For details consult G. Andersson, 1896. * Hult, 1885. 



