ed them
carefully, that "we frequently find a genus represented in several
successive valleys by allied species, sometimes feeding on the same,
sometimes on different plants. 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.[17]

_The Variability of Insects_.

Among Insects there is a large amount of variation, though very few
entomologists devote themselves to its investigation. Our first examples
will be taken from the late Mr. T. Vernon Wollaston's book, _On the
Variation of Species_, and they must be considered as indications of
very widespread though little noticed phenomena. He speaks of the
curious little carabideous beetles of the genus Notiophilus as being
"extremely unstable both in their sculpture and hue;" of the common
Calathus mollis as having "the hind wings at one time ample, at another
rudimentary, and at a third nearly obsolete;" and of the same
irregularity as to the wings being characteristic of many Orthoptera and
of the Homopterous Fulgoridae. Mr. Westwood in his _Modern
Classification of Insects_ states that "the species of Gerris,
Hydrometra, and Velia are mostly found perfectly apterous, though
occasionally with full-sized wings."

It is, however, among the Lepido