CHAP. XIV.] 
ST. HELENA. 
293 
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 vulpince 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 water-cress, one or two 
species of Cyperus, and the Australian Isapis 'prolifer a. 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. 
