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Show 2!)2 ISLAND LIFE. (PART II. proportion of Lhe St. Helena beetles live even'.in the yerfect st:te within the stems of plants or trunks of trees, while ~he. e?gs and larvre of a still larger number are likely to inhabit similar stations. Drift-wood might therefore be one of t~1e most important agencies by which these insects reached the Island. Let us now see how far the distribution of other groups support the conclusions derived from a consideration of the beet~es. The Hemiptera have been studied by Dr. F. Buchanan White, and though far less known than the beetles, indicate somew~at similar relations. Eight out of twenty-one genera are pec~lmr, n.nd the thirteen other genera are for the most part widely distributed, while one of the peculiar genera is of African type. The other orders of insects have not been collected or studied with sufficient care to make it worth while to refer to them in detail· but the land-shells have been carefully collected and minutely' described by Mr. Wollaston himself, a~d it is ~nteres~ing to see how far they agree with the msects m theu peculiarities and affinities. Land-shells of St. Helena.-The total number of species is only twenty-nine, of which seven are common in ~urope o~ the other Atlantic islands, and are no doubt recent mtroductwns. Two others, though described ~s distinct, are so closely allied to European forms, that Mr. Wollaston thinks they have probably even the 300 fathom line, one over 60 miles long ; and it is theref?re probable that a much larger island once occupie~ thi.s site. Now Asc~nswn is nearly equidistant between St. Helena and L1bena, and such an 1sland rnio-ht have served as an intermerliate station through which many of the irn~igrants to St. Helena passed. As the distances ~re hardly greater than in the case of the Azores, this removes whatever difficulty may have been felt of the possibility of any organisms reaching so remote an island. The present island of Ascension is probably only the summit of. a h_uge volcanic mass and any remnant of the original fauna and flora 1t m1ght have preserved may have been destroyed by great volcanic eruptions. .Mr. Darwin collected some masses of tufa which were found to be mamly organic containing besides remains of fresh-water infusoria, the siliceous tissue of plants ! In the light of the great extent of the submarine bank on which the island stands, Mr. Darwin's remark, that-" we may feel sure that at some former epoch, the climate anrl productions of Ascension wer~ very different from what they are now,"-has received a striking confirmation. (See Nat·umlist's Voyage Rouncl the World, p. 495.) CHAP. XIV.] ST. UELE~A. 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 surfa<.e 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 PG~.tula, 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 tl1e Bulimi (B. auris vulpince and B. darwinianus) are said to somewhat resemble Brazilian, New Zealand, and Solomo11 Island forms, while neither Bulimus nor Succinea occur at all in the Madeira group. Omitting the spe2ies 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 Jj''resh-wale?' Or,qctnisms.-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 p1·ol1feTa. TLe same absence of fresh-water shells characterises the Azores, where, however, there is one indigenous water-beetle. In the Sanrlwich Islands also re :::ent 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 ve:ry Temote oceani0 islauds, owing to causes of which ·we are nt present ignorant. |