that the islands are subject to fierce and frequent storms blowing from every point of the compass,--and we have all the facts requisite to enable us to understand how this remote archipelago has become stocked with animal life without ever probably being much nearer to Europe than it is now. For the islands are all volcanic, the only stratified rock that occurs being believed to be of Miocene date. _Madeira and the Canaries._--Coming next to Madeira, we find the number of genera of land birds has increased to twenty-eight, of which seventeen are identical with those of the Azores. Some of the commonest European birds--swallows, larks, sparrows, linnets, goldfinches, ravens, and partridges, are among the additions. A gold-crested warbler, _Regulus Maderensis_, and a pigeon, _Columba Trocaz_, are peculiar to Madeira. In the Canaries we find that the birds have again very much increased, there being more than fifty genera of land birds; but the additions are wholly European in character, and almost all common European species. We find a few more peculiar species (five), while some others, including the wild canary, are common to all the Atlantic Islands or to the Canaries and Madeira. Here, too, the only indigenous mammalia are two European species of bats. _Land Shells._--The land shells of Madeira offer us an instructive contrast to the birds of the Atlantic Islands. About fifty-six species have been found in Madeira, and forty-two in the small adjacent island of Porto Santo, but only twelve are common to both, and all or almost all are distinct from their nearest allies in Europe and North Africa. Great numbers of fossil shells are also found in deposits of the Newer Pliocene period; and {209}although these comprise many fresh species, the two faunas and that of the continent still remain almost as distinct from each other as before. It has been already stated (p. 31) that the means by which land mollusca have been carried across arms of the sea are unknown, although several modes may be suggested; but it is evidently a rare event, requiring some concurrence of favourable conditions not always present. The diversity and specialization of the forms of these animals is, therefore, easily explained by the fact, that, once introduced they have been left to multiply under the influence of a variety of local conditions, which inevitably lead, in the course of ages, to the formation of new varieties and new species.