At the same time there are certain species which exhibit a curiously opposite tendency, and which seem capable of flourishing in almost any part of the world, and under the most varied surroundings. 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, Mauritius, 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 Montevideo; _Helix similaris_, whose fatherland is Eastern Asia, has been transported to Mauritius, Bourbon, West Africa, West Indies, Brazil, and Australia; _Ennea bicolor_ (Eastern Asia) to India, Bourbon, Mauritius, West Indies; _Stenogyra decollata_ (Mediterranean basin) to South Carolina; _S. Goodallii_ (West Indies) to British pineries; _Helix Hortensis_ to New Jersey. Seven common English species (_Limax gagates_, _Hyalinia cellaria_, _H. alliaria_, _Helix aspersa_, _H. pulchella_, _Pupa umbilicata_) have become naturalised in St. Helena,[355] and as many as nineteen in Australia.[356] 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 Mollusca 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 little effect in checking dispersal. There is no appreciable difference between the land Mollusca north and south of the Ganges, or north and south of the Amazon. Living snails, or their ova, are no doubt transported from one bank to another on floating débris of various kinds. The barrier offered by the sea is obvious, and at first sight appears insurmountable; but the facts with regard to oceanic groups of islands like the Azores and Canaries (see p. 297) show that even a stretch of salt water many hundred miles in breadth may be ineffectual in preventing the dispersal of Mollusca.