196 



MONSTROSITIES OF LITTORINA RUDIS. 



influence of freshwater does not lead to the production of a peculiar 

 type of shell." I can hardly follow him to this latter length, 

 however, as an admixture of freshwater does materially alter the 

 thickness of most marine shells and after a certain point must 

 produce a general unfavourableness of the conditions, and must 

 tend to produce a form varying to some extent from the normal. 

 Varigny* has recorded that mollusca inhabiting the shore as L. 

 Rudis does are better able to endure the influx of freshwater than 

 those from the deep sea. The only thorough experiments made 

 with a view to testing the effect of transferring marine forms to 

 freshwater were made long ago by Beudant. No results have yet 

 been obtained by the breeding of marine forms in saltwater mixed 

 with fresh, owing probably to the difficulty of keeping the parents 

 alive for a sufficient time. Beudant experimented with (amongst 

 others) six species of marine gasteropoda by adding by degrees 

 freshwater to the salt, and I give the results abstracted from his 

 table : 



A curious fact appears from this table with regard to Littorina 

 Neritoides, a British species of ths same genus as the specimens we 

 are now considering. Actually more specimens of this species 

 which had been gradually accustomed to freshwater were alive at 

 the end of eight months than of those which had been kept in their 

 native saltwater. Thus it cannot be said that an admixture of 

 freshwater is injurious to the species during the first generation and 

 certainly not to the extent of making it become distorted. The 

 early death of these forms is also in my opinion against the theory 

 of a mingling of freshwater being the cause, as they should if 

 * C.B. Phys., vol. i., pp. 566-8. 



