In every such case the valleys that are
nearest to each other furnish the most nearly allied forms; _and a full
set of the varieties of each species presents a minute gradation of
forms between the more divergent types found in the more widely
separated localities_."
In most land-shells there is a considerable amount of variation in
colour, markings, size, form, and texture or striation of the surface,
even in specimens collected in the same locality. Thus, a French author
has enumerated no less than 198 varieties of the common wood-snail
(Helix nemoralis), while of the equally common garden-snail (Helix
hortensis) ninety varieties have been described. Fresh-water shells are
also subject to great variation, so that there is much uncertainty as
to the number of species; and variations are especially frequent in the
Planorbidae, which exhibit many eccentric deviations from the usual form
of the species--deviations which must often affect the form of the
living animal. In Mr. Ingersoll's Report on the Recent Mollusca of
Colorado many of these extraordinary variations are referred to, and it
is stated that a shell (Helisonia trivolvis) abundant in some small
ponds and lakes, had scarcely two specimens alike, and many of them
closely resembled other and altogether distinct species.
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