he
temperature of the water may constitute a barrier as effectual as the
presence of land; and also that, in the case of all shallow-water
faunas, a tract of deep ocean constitutes almost as complete a barrier
as it does to terrestrial faunas.

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
organ