Locard in his monograph of this group, which he calls collectively the group of Helix Heripensis, tabulates 27 distinct named forms. The characteristics in which these forms differ have been reckoned as 17, and as several of these vary in degree of development, the number of modes may be increased to 109. For practical purposes however Coutagne considers that the various developments of 7 characteristics in their several combinations are enough to express the various forms, and he gives examples of this method of definition. As he observes, though names may be required to define the modes, no one need be alarmed at that, for the same names of modes will be applicable to a great range of distinct species, and the formulae expressing their combinations will replace the varietal names.
This particular example of polymorphism is but little limited by locality. Occasional colonies present some special physiognomy which may in a given place seem almost invariable, though in this very respect the colonies found elsewhere may be highly variable, but such limitations are exceptional for H. striata.
Some distinct and obvious susceptibilities to the influence of soil and climate are however noticeable. For example on siliceous ground the shells are thinner, while on calcareous soils they are thicker; similarly those from the Northern districts attain a larger size than those from further South. Moreover those subjected to curtailed development, whether from drought, heat or cold often show a shortening of the spire. In contrast with this case Coutagne describes the varieties of Helix caespitum, which he says are for the most part localised, quoting many illustrative cases.
Another remarkable case in which locality plays a curious part is provided by the two species Helix trochoides and pyramidata. In France generally they are distinct enough from each other, trochoides being smaller and having a characteristic keel. Coutagne says that after having collected these species from more than a score of localities he came upon a colony of trochoides on the island of Pomègues in which the shells were relatively enormous, most of them having only a slight keel, and a few none at all. On the other hand he received a consignment of pyramidata from four localities in Sicily, all small, and one of them exactly like the trochoides from Pomègues. Judging by the samples received from Sicily, trochoides is there not more variable than it is in Provence, while the Sicilian pyramidata is protean.
The relations of the two species Helix nemoralis and hortensis provide an illustration of another kind of manifestation of local peculiarity. H. hortensis and nemoralis as usually met with, are two very distinct forms. H. hortensis is smaller and duller, and its peristome is white. H. nemoralis is larger and more shiny, and its peristome is brown. In several anatomical points, moreover, especially in the shape of the dart, there are great differences. For a full account of these peculiarities of the two forms and a discussion of their inter-relations the reader is referred to the elaborate work of A. Lang[14] who has studied them extensively and has also succeeded in experimentally raising hybrids between them. These hybrids were in a slight degree fertile with both the parent species, but up to the time of publication no young had been reared from hybrids inter se.
Coutagne describes the result of collections made in 62 French localities. Some had exclusively hortensis, some exclusively nemoralis, and in some the two were found in association. He gives details of five of these collections from which I take the following summary of the more essential facts, omitting much that is almost equally significant.
Locality A, near Honfleur. Both forms present, each sharply and normally distinguished, without any intermediates. They are thus found in many places. Coutagne instances Müller's observations in Denmark, his own series from the Jura, etc.
Locality B. Vonges (Côte d'Or), 242 hortensis taken at random, showed 128 with light peristomes (either more or less pinkish or quite white) and 114 with dark brown peristomes; together with 26 nemoralis all with the usual brown peristomes.
Of the hortensis 50 were in ground-colour opalescens and 1 roseus; and in shape 5 were umbilicatus.
Locality C, about 3 kilometres from B. There were found 35 hortensis, of which 20 had light peristomes and 15 brown; together with 7 nemoralis.