her Atlantic
islands, and are no doubt recent introductions. Two others, though
described as distinct, are so closely allied to European forms, that Mr.
Wollaston thinks they have probably been introduced and have become
slightly modified by new conditions of life; so that there remain exactly
twenty species which may be considered truly indigenous. No less than
thirteen of these, however, appear to be extinct, being now only found on
the surface of the ground or in the surface soil in places where the native
forests have been destroyed and the land not cultivated. These twenty
peculiar species belong to the following genera: Hyalina (3 sp.), Patula (4
sp.), Bulimus (7 sp.), Subulina (3 sp.), Succinea (3 sp.); of which, one
species of Hyalina, three of Patula, all the Bulimi, and two of Subulina
are extinct. The three Hyalinas are allied to European species, but all the
rest appear to be highly peculiar, and to have no near allies with the
species of any other country. Two of the Bulimi (_B. auris vulpinae_ and
_B. darwinianus_) are said to somewhat resemble Brazilian, New Zealand, and
Solomon Island forms, while neither Bulimus nor Succinea occur at all in
the Madeira group.

Omitting the species that have probably been introduced by human agency, we
have here indications of a somewhat recent immigration of European types
which may perhaps be referred to the glacial period; and a much more
ancient immigration from unknown lands, which must certainly date back to
Miocene, if not to Eocene, times.

_Absence of Fresh-water Organisms._--A singular phenomenon is the total
absence of indigenous aquatic forms of life in St. Helena. Not a single
water-beetle or fresh-water shell has been discovered; neither do there
seem to be any water-plants in the streams, except the common {305}
water-cress, one or two species of Cyperus, and the Australian _Isapis
prolifera_. The same absence of fresh-water shells characterises the
Azores, where, however, there is one indigenous water-beetle. In the
Sandwich Islands also recent observations refer to the absence of
water-beetles, though here there are a few fresh-water shells. It would
appear therefore that the wide distribution of the same generic and
specific forms which so generally characterises fresh-water organisms, and
which has been so well illustrated by Mr. Darwin, has its limits in the
_very remote_ oceanic islands, owing to causes of which we are at present
ignorant.

The ot