138 



THE ORIGIN OF THE LAND & FRESHWATER MOLLUSCA 

 AT PRESENT LIVING IN THE BRITISH ISLES. 



(Presidential Address delivered at the Annual Meeting, October ist, 1904). 



By R. F. SCHARFF, Ph.D., M.R.I.A. 



When collecting land and freshwater shells you have no doubt 

 attempted to satisfy your curiosity as to the reason why some species 

 should be extremely restricted in range, whilst others may be found 

 distributed all over the country. The solution of this problem is not 

 easy, and in many cases we are as yet far from being able to point to 

 a satisfactory cause of such dissimilarity in range. In order to ascer- 

 tain the cause which produced these conditions, it is desirable, in the 

 first instance, to make a detailed study of the history of the species 

 and of the means of their dispersal. 



As regards the various modes by which the land and freshwater 

 mollusca have become dispersed over the British Islands, the most 

 natural and at the same time the usual one is either by slow progres- 

 sion on land for the terrestrial species, or by water for the aquatic 

 forms. But we are especially indebted to Mr. H. W. Kew^ for his care- 

 fully collected statistics as to other means of dispersal possessed by 

 land and freshwater mollusca. From tlie facts examined by him, he 

 thinks that " it may be safely concluded that the local distribution of 

 the smaller bivalves has been influenced in a marked degree by aquatic 

 insects" (p. 69); though in speaking of land-shells he acknowledges 

 (p. 119) "that we have little or no actual evidence of precise modes 

 of dispersal, even for short distances on land." 



The belief that terrestrial mollusca are able to cross the ocean by 

 occasional means of dispersal, is largely based on Dr. Wallace's 

 assumption that all animals now inhabiting truly "oceanic islands" 

 must have reached them either by crossing the ocean or that they 

 must be the descendants of ancestors which did so. More recent 

 researches, however, into the fauna and flora of several so-called 

 "oceanic islands" tend to shew that these were really parts of larger 

 land-masses, and that we are not justified in the supposition that the 

 animals and plants inhabiting them were obliged to cross the ocean 

 to reach them. The evidence, therefore, on which Dr. Wallace relied 

 to prove his theory is not so convincing as he thought. 



Some naturalists have spent much thought and labour in speculat- 

 ing on the possible modes of accidental or occasional dispersal, in 

 order to explain the presence ot land mollusca on what they consider 

 true "oceanic islands." Yet drift-timber, as Mr. Kew acknowledges, 

 has never been known to carry molluscs or their eggs (p. 138), and I 



I Kew, H. W., "The Dispersal of Shells," London, 1903. 



