| Abstract/Notes | The distribution of land mollusca still remains one of the most perplexing of the problems to be solved by the geologist. Sir Charles Lyell seems to have been especially strack with their capricious distribution, particularly in Madeira. The facts are still substantially as stated in the tenth edition of his “Principles of Geology,” for Mr. Leacock, in showing me the results of many years collecting in Madeira, observed that his researches had not modified them in any important particulars. Nearly all the species are peculiar to the Madeira Archipelago, and the remarkable fact about their distribution is that, though there are 56 species in Madeira proper, and 42 in Porto Santo, only 12 are common to both islands, though in sight of each other. Still more unaccountable it seems, that of 19 species found on the Dezertas, three barren rocks which appear but little detached from the main island, only 12 are common to Madeira, and even each of these islets has species and varieties peculiar to itself. But two species of land-shells are in fact common to Madeira, the Dezertas and Porto Santo. Sir Charles Lyell infers the great antiquity of the Archipelago from this, contrasting it with the far more extensive group of the British Isles, numbering 200 islands, not one of which have yet developed peculiar species. The fact that such narrow seas have sufficed to keep the land-mollusca distinct conclusively proves, in his opinion, that they have no ready means of dispersal, and that their passage across even the narrowest sea must be of such extraordinary rarity that the possibility need hardly be taken into account. The presence of the same species throughout the whole of the British Islands is thus the strongest argument in favour of their having been joined together and to Europe at a very recent date. |
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