5}

The land-shells are not abundant--about twenty in all, most of them
peculiar species, but not otherwise remarkable. The observation of Captain
Collnet, quoted by Mr. Darwin in his _Journal_, that drift-wood, bamboos,
canes, and the nuts of a palm, are often washed on the south-eastern shores
of the islands, furnishes an excellent clue to the manner in which many of
the insects and land-shells may have reached the Galapagos. Whirlwinds also
have been known to carry quantities of leaves and other vegetable _debris_
to great heights in the air, and these might be then carried away by strong
upper currents and dropped at great distances, and with them small insects
and mollusca, or their eggs. We must also remember that volcanic islands
are subject to subsidence as well as elevation; and it is quite possible
that during the long period the Galapagos have existed some islands may
have intervened between them and the coast, and have served as
stepping-stones by which the passage to them of various organisms would be
greatly facilitated. Sunken banks, the relics of such islands, are known to
exist in many parts of the ocean, and countless others, no doubt, remain
undiscovered.

_The Keeling Islands as Illustrating the Manner in which Oceanic Islands
are Peopled._--That such causes as have been here adduced are those by
which oceanic islands have been peopled, is further shown by the condition
of equally remote islands which we know are of comparatively recent origin.
Such are the Keeling or Cocos Islands in the Indian Ocean, situated about
the same distance from Sumatra as the Galapagos from South America, but
mere coral reefs, supporting abundance of cocoa-nut palms as their chief
vegetation. These islands were visited by Mr. {286} Darwin, and their
natural history carefully examined. The only mammals are rats, brought by a
wrecked vessel and said by Mr. Waterhouse to be common English rats, "but
smaller and more brightly coloured;" so that we have here an illustration
of how soon a difference of race is established under a constant and
uniform difference of conditions. There are no true land-birds, but there
are snipes and rails, both apparently common Malayan species. Reptiles are
represented by one small lizard, but no account of this is given in the
_Zoology of the Voyage of the Beagle_, and we may therefore conclude that
it was an introduced species. Of insects, careful collecting only produced
thirteen species belon