_Theory of Oceanic Islands._--In 1859 the _Origin of Species_ was published, and in the thirteenth chapter of this celebrated work Mr. Darwin put forth his views on oceanic islands or such as are situated far away from any continent and are surrounded by deep oceans. It had been up to this time believed that in most cases these islands were fragments of ancient continents; as an example of which we may refer to the Azores, Madeira, and the other Atlantic islands, which were thought to support the notion of an Atlantic or western extension of the European continent. In order to ascertain what was the condition of these islands when first discovered, Mr. Darwin searched through all the oldest voyages, and found that in none of them was a single native mammal known to exist, while in almost all of them frogs and toads were also absent. All the Atlantic isles from the Azores to St. Helena; Mauritius, Bourbon, and the other isles of the Indian Ocean; and the Pacific islands, east of the Fijis, as far as the Galapagos and Juan Fernandez are thus deficient. They all of them, however, possess birds, and most of them bats; and whenever small mammalia, such as goats, pigs, rabbits, and mice have been introduced they have run wild and often increased enormously, proving that the only reason why such animals were not originally found there was the impossibility of them crossing the sea; while such as could fly over--birds, bats, and insects--existed in greater or less abundance. If, on the other hand, they had once formed part of the continent, it is impossible to believe that some of the smaller mammalia, as well as frogs, would not have continued to exist in the islands to the present day.