ies and groups are usually well defined
and strictly limited in range. Again, their relations with other lands are
often direct and simple, and even when more complex are far easier to
comprehend than those of continents; and they exhibit besides certain
influences on the forms of life and certain peculiarities in their
distribution which continents do not present, and whose study offers many
points of interest.

In islands we have the facts of distribution presented to us, sometimes in
their simplest forms, in other cases becoming gradually more and more
complex; and we are therefore able to proceed step by step in the solution
of the problems they present. But as in studying these problems we have
necessarily to take into account the relations of the insular and
continental faunas, we also get some knowledge of the latter, and acquire
besides so much command over the general principles which underlie all
problems of distribution, that it is not too much to say that when we have
mastered the difficulties presented by the peculiarities of island life we
shall find it comparatively easy to deal with the more complex and less
clearly defined problems of continental distribution.

_Classification of Islands with Reference to Distribution._--Islands have
had two distinct modes of origin--they have either been separated from
continents of which they are but detached fragments, or they have
originated in the ocean and have never formed part of a continent or any
large mass of land. This difference of origin is fundamental, and leads to
a most important difference in their animal inhabitants; and we may
therefore first distinguish the two classes--oceanic and continental
islands.

Mr. Darwin appears to have been the first writer who called attention to
the number and importance, both from a geological and biological point of
view, of oceanic islands. He showed that with very few exceptions all the
remoter islands of the great oceans were of volcanic or coralline
formation, and that none of them contained indigenous mammalia or amphibia.
He also showed the connection of these two phenomena, and maintained that
none of the islands so characterised had ever formed {243} part of a
continent. This was quite opposed to the opinions of the scientific men of
the day, who almost all held the idea of continental extensions, and of
oceanic islands being their fragments, and it was long before Mr. Darwin's
views obtained general accepta