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. JSTo 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 vv^here 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 vulpinm 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 speoies 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-iuater 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 Jsapis prolifei^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. 



