X ARTIFICIAL TRANSPORT OF SPECIES 2/9 



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 flourish- 

 ing in almost any part of the world, and under the most varied 

 sm-roundings. Our own common garden snail {Helix aspersa) is 

 a striking instance of this adaptability to new conditions. 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, 

 Mam-itius, Loyalty Islands, and Australia. The great Achatina 

 fulica of East Africa has been established first in Mauritius, and 

 from thence has been carried to the Seychelles and Calcutta. 

 Helix lactea, a common Mediterranean species, has been carried 

 to Teneriffe and Monte Video ; Helix similaris, wdiose 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. Goodallii 

 (West Indies) to British pineries ; Helix liortensis to New Jersey. 

 Seven common English species {Lirtiax gagates, Hjalinia cellaria, 

 H alliaria, Helix aspersa, H. pulchella, Fupa umhilicata) 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 packing. 

 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 

 ]\Iollusca brings to bear upon the general problems of distribution 

 will be confused by any intermixture of fauna naturally distinct. 



Land Mollusca : Barriers 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. ^ e_ a. Smith, P.Z.S. 1892, p. 259. 



» C. T. Miissoii, Proc. Linn. Soc. iV. S. Wales (2), v. p. 883. 



