It appears unquestionable that marine genera from high northern latitudes are provided with shells of uniform colour, or whitish with a pale brown epidermis; spots, bands, or stripes seldom occur. The arctic forms of Buccinum, Trophon, Chrysodomus, Margarita, Crenella, Leda, Yoldia, Astarte illustrate this fact. In the more temperate seas of Europe, colours tend on the whole to increase, although there are certain genera (e.g. Pecten) which are not more brightly coloured in Mediterranean than in Icelandic waters.
Land Mollusca inhabiting the mainland of a continent not unfrequently become smaller when they have spread to adjacent islands where perhaps the rainfall is less abundant or the soil and food-supply less nicely adjusted to their wants. Orthalicus undatus is decidedly larger on the mainland of S. America than on the adjacent islands of Trinidad and Grenada. Specimens of Bulimulus exilis from Barbados are invariably broader and more obese than those from S. Thomas, while those from the volcanic island of S. Lucia, where lime is deficient, are small and very slender. Streptaxis deformis, as occurring at Trinidad, is only half the size of specimens from Georgetown, Demerara.[189]
Certain localities appear, for some unexplained reason, to be particularly favourable to the production of albino varieties. The neighbourhood of Lewes, in Sussex, has produced no fewer than fourteen of these forms of land Mollusca and five of fresh-water.[190]
Our common Helix aspersa, as found near Bristol, is said to be ‘dark coloured’; about Western-super-mare ‘brown, with black markings’; near Bath ‘very pale and much mottled’; at Cheddar ‘very solid and large.’[191] Sometimes the same kind of variation is exhibited by different species in the same locality. Thus specimens of H. aspersa, H. nemoralis, and H. hortensis, taken from the same bank at Torquay, presented a straw-coloured tinge of ground colour, with red-brown bands or markings. Trochiform H. nemoralis and H. arbustorum, sinistral H. hortensis and H. aspersa, sinistral H. aspersa and H. virgata, and similarly banded forms of H. caperata and H. virgata, have been taken together.[192]
The immediate neighbourhood of the sea appears frequently to have the effect of dwarfing land Mollusca. Thus the var. conoidea of Helix aspersa, which is small, conical, with a compressed mouth, occurs ‘on sandhills and cliffs at the seaside.’ The varieties conica and nana of Helix hispida are found ‘near the sea.’ Helix virgata is exceedingly small in similar localities, and tends to become unicoloured. H. caperata var. Gigaxii, a small depressed form, occurs at ‘Sandwich and Falmouth.’[193] Sometimes, however, the exact opposite is the case, for H. nemoralis var. major, which is ‘much larger’ than the type, occurs on ‘sandhills and downs’ and is ‘remarkably large in the I. of Arran, Co. Galway.’ The dwarf form of Limnaea peregra known as maritima appears to be confined to the neighbourhood of the sea.
Dwarfing of the shell seems frequently to be the result of an elevated locality, not perhaps so much as the direct consequence of purer air and less barometric pressure, as of changes in the character of the food supply and in the humidity of the air. Several species of Helix have a variety minor which is characteristic of an Alpine habitat. Helix arbustorum var. alpestris, which is scarcely two-thirds the size of the type, occurs on the Swiss Alps in the region of perpetual snow. Sometimes a very slight elevation is sufficient to produce the dwarfed form. At Tenby the type form of Helix pisana is scattered in countless numbers over the sandhills just above high-water mark. At the extreme western end of these sandhills rises abruptly to a height of over 100 feet the promontory known as Giltar Head, the vegetation of which is entirely distinct from that of the burrows below. There is a colony of H. pisana at the end of Giltar, all of which are devoid of the characteristic markings of the typical form, and most are dwarfed and stunted in growth.
Occasionally the same variety will be found to be produced by surroundings of very different nature. Thus the var. alpestris, of H. arbustorum mentioned above, besides being characteristic of high Alpine localities, also occurs abundantly in low marshes at Hoddesdon on the River Lea. Helix pulchella var. costata, according to Jeffreys, is found in dry and sandy places, often under loose stones and bricks on walls, while other authorities have noticed it in wet and dry localities quite indifferently.
Sometimes the production of a variety may be traced to the intrusion of some other organism. According to Brot, nine-tenths of the Limnaea peregra inhabiting a certain pond near Geneva, were, during one season, afflicted with a malformation of the base of the columella. This deformity coincided with the appearance, in the same waters, of extraordinary numbers of Hydra viridis. The next season, when the Hydra disappeared, 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[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.