466 ANIMALS INTRODUCED UNAWARES 



the Canary Isles and such southern lands ; but it was first 

 noticed in England in a nursery near Bristol between 1812 

 and 1816, and in 1822 was described as breeding freely in 

 the open. Now it has obtained permanent footing in many 

 of the south-western counties of England and has been 

 found as far north as Cheshire. 



From Aberdeen comes a strange instance of a halfway 

 stage to naturalization. Three or four ponds at the Banner 

 Mill in Aberdeen were inhabited for many years by a 

 warm-country Freshwater Snail, Physa acuta, which is 

 common in pools in the West Indies and in southern France. 

 With it lived Goldfish and water-plants, as well as the 

 Common Pond Snail, Limncza peregra. There can be 

 little doubt that the foreigner, introduced with either the 

 Goldfish or the plants, was saved from extinction by the fact 

 that the ponds were filled by a flow of warm water dis- 

 charged from the mill. In these congenial habitations, the 

 visitors flourished and multiplied. If the Common Pond 

 Snail, accustomed to the icy pools of Aberdeenshire, could 

 bear with equanimity the artificial warmth of the Banner 

 Mill overflow, is it not possible that there might also be a 

 halfway stage by which the warm-temperate species might 

 step down from its own to our colder climate? Physa acuta 

 has been accidentally introduced and has been found to 

 thrive in the lily tanks of Kew, and in tanks and vessels con- 

 taining plants in the Royal Botanic Society's Garden in 

 Regent's Park, London. 



