434 Proceedings of the Royal Trhli Academy. 



in the great majority of cases, it is easy to distinguish between a 

 true native and one which has only strayed to Ireland accidentally, 

 and the latter only form an extremely small and insignificant per- 

 centage of the Irish fanna and flora. Mr. Murray (60, p. 16) was 

 of opinion that only a slight intermixture occurs in the flora of an 

 island from occasional dispersal, and he was disposed to reckon the 

 proportion of such colonization at not more than two per cent, in the 

 most favonrable circumstances. In the fanna he thought it mnst he 

 much less. • 



As regards Ireland, I believe the animals derived from accidental 

 or occasional means of dispersal amount to five per cent, of the whole 

 fauna at the most. Land and freshwater MoUnsea are generally 

 looked upon as particulariy liable to accidental dispersal. 



If they had been to any great extent carried to Ireland by 

 occasional means of dispersal instead of gradually spreading to that 

 country on a former continuous land-surface, spectilations based on their 

 range would be futile. Dr. Blandford (8, p. 43), who speaks with an 

 authoritative voice on the subject of distribution of mollusca, says : 

 "The prevalent idea that land- mollusca, or their eggs, are trans- 

 ported by floating logs, appears to me extremely improbable in a great 

 number of forms, because, so far as is known, very few hibernate in 

 wood, or lay their eggs there ; and as the wood is carried to the sea 

 during floods caused by heavy rains, which would certainly make 

 every snail leave its hiding place, the notion that some would remain 

 ensconced in the clefts appears to me quite opposed to the habits of 

 the animal." 



Darwin (21 a, p. 353) tells ns how Baron Aucapitaine immersed a 

 number of Cyelostoma eUgam for a fortnight in the sea, by way of 

 experiment, and that almost all survived the treatment. He naturally 

 concluded that the operculum, with which these shelb are furnished 

 was of distinct advantage in enabling them to float across arms of the 

 sea to an island. Supposing Ireland had been stocked in this manner 

 with shells, we should expect operculate species, or even such which 

 provide themselves with a membranous diaphragm during winter, to 

 be abundant. But this is not the case. Xeither Cychstoma eleyans 

 nor Helix pomatia, the two species which were experimented on, 

 inhabit Ireland, though both occur in England and in France. More- 

 over, as a matter of fact, the shells of the former species have again 

 and again been washed ashore within recent years on the Irish coast ; 

 but though this mnst have been going on for centuries, yet Cyclostoma 

 elegant has not established itself in this country. The only Irish 



