PDF - Wallace Online

ix THE DEBT OF SCIENCE TO DARWIN 469of special creations, had no scientific existence. It is toDarwin that we owe the establishment of the distinction ofoceanic from continental islands, while he first showed us thevarious modes by which the former class of islands have beenstocked with life.By a laborious research in all the accountsof old voyages, he ascertained that none of the islands of thegreat oceans very remote from land possessed either landmammalia or amphibia when first visited ;and on examinationit is found that all these islands are either of volcanicorigin or consist of coral reefs, and are therefore presumablyof comparatively recent independent origin, not portions ofsubmerged continents, as they were formerly supposed to be.Yet these same islands are fairly stocked with plants, insects,land-shells, birds, and often with reptiles, more particularlylizards, usually of peculiar species, and it thus becomesimportant to ascertain how these organisms originally reachedthe islands, and the comparative powers different groups ofplants and animals possess of traversing a wide extent of ocean.With this view he made numerous observations and someingenious experiments. He endeavoured to ascertain howlong different kinds of seeds will resist the action of saltwater without losingtheir vitality, and the result showedthat a large number of seeds will float a month withoutinjury, while some few survived an immersion of one hundredand thirty-seven days. Now, as ocean currents flow on theaverage thirty-three miles a day, seeds might easily be carried1000 miles, and in very exceptional cases even 3000 miles,and still grow. Again, it is known that drift-timber is oftencarried enormous distances, and some of the inhabitants ofthe remote coral-islands of the Pacific obtain wood by thismeans, as well as stones fastened among the roots. Now,Darwin examined torn -up trees, and found that stones areoften inclosed by the roots growing round them so as to leaveclosed cavities containing earth behind; and from a smallportion of earth thus completely inclosed, he raised threedicotyledonous plants. Again, the seeds that have passedthrough the bodies of birds germinate freely, and thus birdsmay carry plants from island to island. Earth often adheresto the feet of aquatic and wading birds, and these migrate toenormous distances and visit the remotest islands, and from