Although colors alone are usually unreliable as subgeneric characters, I am inclined to consider them as such in the case of these and some allied species, from their apparent connection with important organs. In fact the band, so constant in this large series of species, takes precedence of considerable variety of form, for the variations in outline, umbilicus, and peristome, though great in the extremes, are so gradually shaded and blended together in the whole series that no well-defined generic divisions can be founded on them, though useful for the minor grouping. The umbilicus especially is variable even in specimens of the same species, those from southern and arid regions being often nearly imperforate, and more conical than others.
Several Mexican species belong to the same series, such as H. Remondii Tryon, H. Griseola Pfeiff, and H. Berlandieriana Moric., the two last extending to Texas. Others, as H. Humboldtiana Val., scarcely differ from the typical Pomatia in form. I would, however, exclude the true Hygromias associated with these by Tryon. I would also exclude the plain or variegated species of Lower California, which approach nearer to Polymita. It must be observed that many of our species approach in form to others of allied groups, so that if we overlook characters of color and surface, we will be inclined to place in the same groups, Nos. 40 and 52, 24 and 32, 29 and 47, etc. Even in color Nos. 32 to 35 show an approach to the group of Lower California, but seem more closely allied to our series, having merely a geographical affinity to the former. Size is of little value, even as a specific character among the land shells, nearly all the species furnishing specimens twice as large as others of the same kind. The proportions of height to breadth are more reliable, but not constant.
The subgenus or division characterized by the band is scarcely distinguishable as a whole from the typical Helix, (type pomatia) of Europe, though the extremes vary greatly, simulating the three or more foreign genera to which various authors have attached them.[32]
Our species are distinct enough among themselves when the true specific characters here given are noted, though occasionally hybrids undoubtedly occur. Dr. W. Newcomb has raised many specimens in his garden in Oakland, combining the characters of Nos. 24, 25, 29, 31, and 43, in such manner that it is often impossible to determine which they belong to. Yet their natural locations are usually so widely separated that only occasionally can hybrids occur in a state of nature, and where several do inhabit one locality, as 24, 27, 28, 46 do at Santa Cruz, though nearly allied, intermediate forms are not found. Some of the so-called species are, however, scarcely more than hybrids or varieties, but the names are retained as indicating their differences, though almost every species is divisible into varieties as well marked or better. Thus the specimen described on p. 260 of this volume (from Mount Diablo) seems to be a hybrid between mormonum and ramentosa, and we also find specimens connecting the latter with 25, 26, and perhaps others.
Occasional links also occur, connecting many others of the banded species together.
A similar intermixing of species, where nearly allied, occurs among our marine shells wherever two or more encroach on each other’s limits; but the comparative rarity of the intermediate forms seems to indicate hybridity rather than specific identity of their allies.
It is probable that groups X and XI and XIII and XIV should be united, as the distinctive characters between them are not of first importance, and species of each are very closely similar otherwise. Parallel columns may be formed, as indicated on p. 260, in which close resemblances in form, number of whorls, etc., between species of the different groups may be shown, and this may be extended so as to show analogous parallels with those of other sub-families, or even families, but these resemblances do not indicate affinity, though very likely to mislead. A geographical arrangement of some groups is also indicated, though imperfectly, as there are no impassable limits between them. For special localities of many species, see vol. III, pp. 62, 115, 180, 259, and II, 91, 103.[33]
The Darwinian theory of development might be very beautifully illustrated by these banded snails, if we could find evidence that their various forms had all originated from a common stock (which might be the ramentosa, as that species now occupies a nearly central locality). But though fossil forms have been found differing considerably from their present representatives, there are others apparently as old, which show no such differences, and none of them show a tendency towards any common original type. The one referred to by Professor Whitney on p. 278, as found with the human skull of supposed pliocene (?) age, does not differ perceptibly from specimens of mormonum, now living near the locality. It retains even its band of color, which is soon lost in specimens imbedded near the surface, and this (if not preserved by its deep burial or incrustation) is strong evidence against a great antiquity of the skull. All other fossil Helicoids are considered postpliocene, at least so far as known in this State, though extinct species occur in Europe as far back as the Eocene.
The bandless species of the west coast slope, from lat. 33° to 49°, are added to the synopsis, to show their relations and analogies with the banded. The arrangement followed is essentially that of Tryon, except the addition of some he has omitted, or not yet published. The generic divisions are also reduced to groups, as the true generic characters are not yet settled. The lip is entirely wanting in the first family, but in the bandless Helicidæ of this coast, it becomes of great importance for grouping of species, (49 to 55) of which we have very few, while east of the Rocky Mountains there are more than fifty. Group III is also largely developed on the Atlantic slopes. The tendency now is to divide too much, which is as unnatural as to unite all under genus Helix, as many still do. It is probable that the divisions here called subfamilies, answer more nearly to the true genera than any others, though they require modification, and the selection of names applicable to them as genera, is a difficult task. To undertake to distinguish genera by the lingual teeth, mucous pores, or any other single character of the soft parts, is less practicable than to do it by the shells alone, and little if any more reliable. There may, however, be foreign shells closely resembling ours in form, which must still be separated on account of the animal.