n geological structure they agree generally
with the more recent islands; like them they possess mammalia and amphibia,
usually in considerable abundance, as well as all other classes of animals;
but these are highly peculiar, almost all being distinct species, and many
forming distinct and peculiar genera or families. They are also well
characterised by the fragmentary nature of their fauna, many of the most
characteristic continental orders or families being quite unrepresented,
while some of their animals are allied, not to such forms as inhabit the
adjacent continent, but to others found only in remote parts of the world.
This very remarkable set of characters marks off the islands which exhibit
them as a distinct class, which often present the greatest anomalies and
most difficult problems to the student of distribution.

_Oceanic Islands._--The total absence of warm-blooded terrestrial animals
in an island otherwise well suited to maintain them, is held to prove that
such island is no mere fragment of any existing or submerged continent, but
one that has been actually produced in mid-ocean. It is true that if a
continental island were to be completely submerged for a single day and
then again elevated, its higher terrestrial animals would be all destroyed,
and if it were situated at a considerable distance from land it would be
reduced to the same zoological condition as an oceanic island. But such a
complete submergence and re-elevation appears never to have taken place,
for there is no single island on the globe which has the physical and
geological features of a continental, combined with the zoological features
of an oceanic island. It is true that some of the coral-islands may be
formed upon submerged lands {245} of a continental character, but we have
no proof of this; and even if it were so, the existing islands are to all
intents and purposes oceanic.

We will now pass on to a consideration of some of the more interesting
examples of these three classes, beginning with oceanic islands.

All the animals which now inhabit such oceanic islands must either
themselves have reached them by crossing the ocean, or be the descendants
of ancestors who did so. Let us then see what are, in fact, the animal and
vegetable inhabitants of these islands, and how far their presence can be
accounted for. We will begin with the Azores, or Western Islands, because
they have been thoroughly well explored by naturalists, and in