X ARTIFICIAL TRANSPORT OF SPECIES 279 



permanent root. Mr. H. W. Kew ^ has collected a good many 

 of these attempts to acclimatise species, the general success of 

 which seems to depend almost entirely on a restoration of the 

 old conditions of life. 



At the same time there are certain species which exhibit a 

 curiously opposite tendency, and which seem capable of flour- 

 ishing in almost any part of the world, and under the most 

 varied surroundings. Our own common garden snail (^Helix 

 asper^ci) is a striking instance of this adaptability to new condi- 

 tions. It has been established, by art or by accident, in Nova 

 Scotia, Maine, South Carolina, New Orleans, California, Mexico 

 city, Cuba, Hayti, Cayenne, Brazil, Valparaiso, Cape Town, the 

 Azores, St. Helena, Mauritius, Loyalty Islands, and Australia. 

 The great Achathia fidica of East Africa has been established 

 first in Mauritius, and from thence has been carried to the 

 Seychelles and Calcutta. Helix laetea^ a common Mediterranean 

 species, has been carried to Teneriffe and Montevideo ; Helix 

 similaris, whose fatherland is Eastern Asia, has been transported 

 to Mauritius, Bourbon, West Africa, West Indies, Brazil, and 

 Australia ; Ennea hicolor (Eastern Asia) to India, Bourbon, 

 Mauritius, West Indies ; Stenogyra decollata (Mediterranean 

 basin) to South Carolina ; S. Groodallii (West Indies) to British 

 pineries ; Helix Hortensis to New Jersey. Seven common Eng- 

 lish species (^Limax gagates^ Hyalinia cellaria^ H. alliaria^ Helix 

 asjjersa^ H. jjulchella^ Pupa umbilicatct) have become naturalised 

 in St. Helena,^ and as many as nineteen in Australia.'^ 



Cases of artificial transport of this kind are readily detected ; 

 they follow the lines of trade. The snails themselves or their 

 ova have been accidentally enclosed with plants or mould, or 

 have adhered to packing-cases, or to hay and grass used in pack- 

 ing. Thus they constitute no disturbance to the general rule of 

 the persistent localisation of species and genera, and there is little 

 fear that the evidence which the geographical distribution of the 

 Mollusca brings to bear upon the general problems of distribution 

 will be confused by any intermixture of fauna naturally distinct. 



Land McUusca : Bai^riers to Dispersal. — The chief natural 

 barriers to dispersal are extremes of temperature, the sea, 

 mountain ranges, and deserts. Rivers, however large, seem of 



1 The Dispersal of Shells, pp. 182-195. 2 e. A. Smith, P.Z.S. 1892, p. 259. 



3 C. T. Musson, Proc. Linn. Soc. X. S. Wales (2), v. p. 883. 



