DISPERSAL BY MAN. l8l 



anything ever experienced in England/ Similarly 

 among fresh-water groups the notoriously rapid 

 multiplication and spread of the introduced zebra- 

 mussel {Dreissena polytnorphd) in England and certain 

 parts of the continent of Europe may be instanced ; a 

 statement by Sir C. Lyell that a Livincea introduced 

 unintentionally into Madeira by the Portuguese ran all 

 over the island in thirty years has already been referred 

 to. It will be borne in mind of course that molluscs, no 

 matter how quickly they increase, can never spread 

 with much rapidity of their own accord ; obviously, 

 migration at a " snail\s pace " can hardly be rapid, and 

 those kinds which have quickly occupied large areas 

 must certainly have been largely helped by external 

 causes. 



Man's influence upon a fauna, it need hardly be 

 remarked, is not always by way of addition, for he 

 often drives away or exterminates many aboriginal 

 forms. The introduction of new and dominant kinds 

 must in itself be highly prejudicial, while the disastrous 

 results attending the destruction of forests and the 

 breaking up of land for cultivation are but too well 

 known. St. Helena furnishes a striking illustration. 

 When discovered in 1501, the little island is said to 

 have been clothed with a luxuriant forest vegetation, but 

 the woods have been almost wholly destroyed by man 

 and the animals introduced by him; and notwithstanding 

 the establishment of a large number of foreign plants, 

 its general aspect is now described as barren and 

 ' John Carson, " Garden," xiii. (1878), p. 273. 



