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[194] 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 pastures 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 endeavours 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 probably 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 compared with the typical form, which would be less readily detected. (_b_) _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 Mollusca 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, _H. arbustorum_ develops a var. _fusca_, which is depressed, very thin, and transparent, at Scilly, and also at Lunna I., E. Zetland. [Illustration: FIG. 35.--19 specimens of _Purpura lapillus_ L., Great Britain, illustrating variation.