24 HISTORY OF THE EUROPEAN FAUNA. 



most cases, however, no particular reason can be 

 assigned why they should have been thus introduced, 

 and as a matter of fact there are always individual 

 differences of opinion as to the precise number of 

 such. Certain it is, that though the number of 

 supposed introductions from Europe to America 

 is very large, those which have been carried from 

 America to Europe is exceedingly small. In fact, 

 I remember only two instances of accidental animal 

 importations from America which have firmly 

 established themselves in Europe, viz., a small 

 fresh-water mollusc, Planorbis dilatatus^ and the 

 much-dreaded vine-pest, Phylloxera vastatrix. 



As a rule the animals die out very shortly after 

 their arrival on foreign soil. Many instances, 

 nevertheless, are on record, especially in the case 

 of molluscs, where snails thus transported have not 

 only survived but are apparently in a flourishing 

 condition and spreading. Helix aspersa, for example, 

 our large garden snail, has been naturalised in many 

 foreign countries by French and Portuguese sailors, 

 who had taken them on board their ships as food. 



It certainly cannot be denied that a number of 

 species among almost all groups of invertebrates have 

 been unintentionally conveyed by man from Europe 

 into foreign countries. It has been proposed by Dr. 

 von Ihering to apply the term "cenocosmic" to those 

 species which have become spread all over the world 

 through artificial means, and thus to distinguish 

 them from cosmopolitan ones which have attained a 



