Occasionally, too, very minute land-snails would arrive alive on the
island after their long sea-voyage on bits of broken forest-trees--a
circumstance which I would perhaps hesitate to mention in mere human
society were it not that I have been credibly informed your own great
naturalist, Darwin, tried the experiment himself with one of the
biggest European land-molluscs, the great edible Roman snail, and found
that it still lived on in vigorous style after immersion in sea-water
for twenty days. Now, I myself observed that several of these bits of
broken trees, torn down by floods in heavy storm time from the banks of
Spanish or Portuguese rivers, reached my island in eight or ten days
after leaving the mainland, and sometimes contained eggs of small
land-snails. But as very long periods often passed without a single new
species being introduced into the group, any kind that once managed to
establish itself on any of the islands usually remained for ages
undisturbed by new arrivals, and so had plenty of opportunity to adapt
itself perfectly by natural selection to the new conditions. The
consequence was, that out of some seventy land-snails now known in the
islands, thirty-two had assumed distinct specific features before the
advent of man, while thirty-seven (many of which, I think, I never
noticed till the introduction of cultivated plants) are common to my
group with Europe or with the other Atlantic islands. Most of these, I
believe, came in with man and his disconcerting agriculture.

As to the pond and river snails, so far as I could observe, they mostly
reached us later, being conveyed in the egg on the feet of stray waders
or water-birds, which gradually peopled the island after the Glacial
epoch.

Birds and all other flying creatures are now very abundant in all the