Now, the means whereby barriers admit of being accidentally or occasionally surmounted are, of course, various; and they differ in the case of different organisms. Birds, bats, and insects, on account of their powers of flight, are particularly apt to be blown out great distances to sea, and hence of all animals are most likely to become the involuntary colonists of distant shores. Floating timber serves to convey seeds and eggs of small animals over great distances; and Darwin has shown that many kinds of seeds are able of themselves to float for more than a month in sea-water without losing their powers of germination. For instance, out of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an immersion of 137 days. As a result of all his experiments he concludes, that the seeds of at least ten per cent. of the species of plants of any country might be floated by sea-currents during 28 days, without losing their powers of germination; and this, at the average rate of flow of several Atlantic currents, would serve to transport the seeds to a distance of at least 900 miles. Again, he proved that even seeds which are quickly destroyed by contact with sea-water admit of being successfully transported during 30 days, if they be contained within the crop of a dead bird. He also proved that living birds are most active agents in the work of dissemination, and this not only by taking seeds into their crops (where, so long as they remain, the seeds are uninjured), but likewise by carrying seeds (and even young mollusks) attached to their feet and feathers. In the course of these experiments he found that a small cup-full of mud, which he gathered from the edges of three ponds in February, was so charged with seeds that when sown in the ground these few ounces of mud yielded no less than 537 plants, belonging to many different species. It is therefore evident what opportunities are thus afforded for the transportation of seeds on the feet and bills of wading-birds. Lastly, floating ice is well known to act as a carrier of any kind of life which may prove able to survive this mode of transit. Such being the nature of geographical barriers, and the means that organisms of various kinds may occasionally have of overcoming them, I will now give a few detailed illustrations of the argument from geographical distribution, as previously presented in its general form.