SCHARFF : ORIGIN OF LAND AND FRESHWATER MOLLUSCA. 1 39 



know of no authenticated record of a living terrestrial mollusk having 

 been carried across the sea to an island and established itself there 

 by any means except by man. We must also remember that un- 

 successful attempts at colonization by man, often in most suitable 

 localities, clearly indicate to us how very small must be the chance of 

 the ultimate establishment of a new colony on an island as the result 

 of the transportal of a few individuals by accidental means under the 

 forces of nature. 



On the other hand, we do possess positive proof that terrestrial 

 species, particularly suited to withstand the deleterious action of sea- 

 water, have been stranded on islands probably for many centuries, 

 and yet have not become established there. I need only refer to 

 Cyclostoma elegans^ which has been picked up on the Irish coast, and 

 specimens of which must be constantly washed up on the shore 

 owing to the facility with which it floats on the surface. It has not, 

 however, succeeded in securing a permanent abode in Ireland. 



Though I do not believe that the moUuscan fauna of the British 

 Isles owes its presence there, in any appreciable degree, to occasional 

 means of introduction from the continent by birds, drift-timber, or 

 hurricanes, local distribution is no doubt somewhat influenced by 

 these causes. I fully agree, however, with Mr. Kew in his opinion 

 " that both snails and slugs, during the last three centuries at least, 

 have been more dispersed by human agency than by all other causes 

 together" (p. 178). Nevertheless, among the species of land and 

 freshwater mollusca inhabiting the British Islands, there are only few 

 which can be reasonably regarded as human importations. Mr. 

 Adams,^ for instance, in his excellent little manual, expresses the 

 opinion that four terrestrial species were thus introduced into our 

 fauna, viz.. Helix villosa, H. terrestris, Stenogyra goodalli, and Clau- 

 silia parinda. Similarly Helix pofiiatia, H. aspersa, H. cantiana, 

 and the freshwater species Physa acuta, Planorbis dilatatus, Dreis- 

 sensia polyinorpha\ and Sphcerium pallidum are looked upon by some 

 authorities as recent introductions. Now these, after all, are only 

 about a dozen out of nearly one hundred and forty species known to 

 inhabit the British Isles, so that there seems to be no reasonable 

 doubt that the remaining one hundred and twenty-eight species are 

 mostly indigenous to these islands. If these reached us from the con- 

 tinent they must either have come by land or by accidental dispersal 

 in remote times. It is impossible for us, in the present state of our 

 knowledge, to arrive at a clear estimate of what proportion of these 

 one hundred and twenty-eight species reached our islands by migration 



r I Throughout this address I have adhered to the old nomenclature, as I understood that 



many members were not sufficiently familiar with the revised one, and had not yet adopted it. 



2 Adams, L. E., "The Collector's Manual of British Land and Freshwater Shells," second 

 edition, Leeds, i8q6. 



