TAYLOR: VARIATION OF LIMN^A PEREGRA. 285 



ever not generally accepted, and in this paper I propose to regard 

 the American forms as specifically different from our shell. 



Some years ago Limnaa peregra was recorded under 

 several new specific names from Tasmania, but the correctness 

 of the view which regards the Tasmanian shells as forms of 

 Z. peregra has recently been seriously called in question. But 

 leaving these countries out of account altogether, the undoubted 

 range of the species is exceedingly great, its ability to adapt 

 itself to such widely different conditions as are afforded by 

 turbulent streams or the most tranquil pools, the margin of 

 the water, or immersion in its most profound depths, brackish 

 water or streams and pools loaded with impurities, altitudes 

 up to 18,000 ft. — assuming Reeve to be correct in referring 

 Limiicea hookeri to our species, I however regard L. hooker I as 

 much more closely related to the American L. catascopium, 

 which in the present paper I have considered to be a distinct 

 species — and temperatures varying from near loo'' Fahr., to 

 the icy-cold waters of the elevated Pyrenean lakes, would lead 

 us naturally to expect the great modifications in size, form, 

 substance and sculpture, which actually do occur, and which are 

 but a reflex of the widely different environments to which the 

 animals are subjected. 



In this country the life-term of this animal would seem to 

 be about one year, the adults dying off usually during the early 

 summer, but on the other hand some European conchologists 

 regard the continental individuals as requiring 2 or 3 years to 

 arrive at full growth, and figures have been actually published 

 showing the annual stages in the progress towards maturity. 



The causes promoting the evolution of any particular type 

 of variation are not definitely established, but it is hoped that 

 precise and accurate observations may bring to lightsome of the 

 factors governing or influencing varietal differentiation. 



Brot has shown how in all probability some of the lowest 

 organisms may affect the forms of the shells of mollusks, 

 instancing how nine-tenths of the Limncea peregra found during 



