s, are in so mysterious a manner
linked together by affinity, and are likewise linked to the extinct
beings which formerly inhabited the same continent. Bearing in mind
that the mutual relations of organism to organism are of the highest
importance, we can see why two areas having nearly the same physical
conditions should often be inhabited by very different forms of
life; for according to the length of time which has elapsed since
new inhabitants entered one region; according to the nature of the
communication which allowed certain forms and not others to enter,
either in greater or lesser numbers; according or not, as those which
entered happened to come in more or less direct competition with each
other and with the aborigines; and according as the immigrants were
capable of varying more or less rapidly, there would ensue in different
regions, independently of their physical conditions, infinitely
diversified conditions of life,--there would be an almost endless amount
of organic action and reaction,--and we should find, as we do find,
some groups of beings greatly, and some only slightly modified,--some
developed in great force, some existing in scanty numbers--in the
different great geographical provinces of the world.

On these same principles, we can understand, as I have endeavoured to
show, why oceanic islands should have few inhabitants, but of these a
great number should be endemic or peculiar; and why, in relation to the
means of migration, one group of beings, even within the same class,
should have all its species endemic, and another group should have all
its species common to other quarters of the world. We can see why whole
groups of organisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands possess
their own peculiar species of aerial mammals or bats. We can see why
there should be some relation between the presence of mammals, in a more
or less modified condition, and the depth of the sea between an island
and the mainland. We can clearly see why all the inhabitants of an
archipelago, though specifically distinct on the several islets, should
be closely related to each other, and likewise be related, but less
closely, to those of the nearest continent or other source whence
immigrants were probably derived. We can see why in two areas, however
distant from each other, there should be a correlation, in the presence
of identical species, o