Ill VARIATION IN SNAILS 89 



the next generation of Limnaea was found to have resumed its 

 normal form. 



It has been noticed that a form of Helix caperata with a 

 flattened spire and wide umbilicus is restricted to tilled fields, 

 especially the borders of clover fields, while a form with a more 

 elevated spire and more compact whorls occurs exclusively in 

 open downs and uncultivated places. The Rev. S. S. Pearce 

 accounts ^ for this divergence by the explanation that the flatter 

 spire enables the shell of the fields to creep about more easily 

 under the leaves or matted weeds, seldom requiring to crawl up 

 a stalk or stem, while on the short turf of the downs and pas- 

 tures the smaller and more rounded shell enables the animal to 

 manoeuvre in and out of the blades of grass, and even to crawl 

 up them with considerable activity. The same writer endeav- 

 ours to explain the causes which regulate the distribution of H. 

 caperata var. ornata. He found that this variety (dark bands 

 on a white ground) occurred almost exclusively on downs which 

 were fed upon by sheep, associated with the common or mottled 

 form, while the latter form alone occurred in localities where 

 sheep were not accustomed to feed. Assuming then, as is prob- 

 ably the case, that sheep, in the course of their close pasturing, 

 devour many small snails, he believes that individuals of the 

 more conspicuous form ornata were more likely to be noticed, 

 and therefore avoided, by the sheep, than the mottled form, 

 which would more easily escape their observation. Hence the 

 var. ornata is due to the advantage which strikingly coloured 

 individuals obtained owing to their conspicuous habit, as com- 

 pared with the typical form, which would be less readily de- 

 tected. 



(5) Changes in Soil, Station, Character of Water, etc. — A 

 deficiency of lime in the composition of the soil of any particular 

 locality produces very marked effects upon the shells of the 

 MoUusca which inhabit it; they become small and very thin, 

 occasionally almost transparent. The well-known var. tenuis of 

 Helix aspersa occurs on downs in the Channel Islands where 

 calcareous material is scarce. For similar reasons, IT. arbusto- 

 rum develops a var. fusca, which is depressed, very thin, and 

 transparent, at Scilly, and also at Lunna L, E. Zetland. 



The common dog-whelk (^Purpura lapillus) of our own coasts 



1 Journ. of Conch, vi. p. 123. 



