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 rial 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
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