ch 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 {409} 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, of varieties, of
doubtful species, and of distinct but representative species.

As the late Edward Forbes often insisted, there is a striking parallelism
in the laws of life throughout time and space: the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see
this in many facts. The endurance of each species and group of species is
continuous in time; for the exceptions to the rule are so few, that they
may {