We can see why whole
groups of organisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands should
possess their own peculiar species of aerial mammals or bats. We can see
why, in islands, there should be some relation between the presence of
mammals, in a more or less modified condition, and the depth of the sea
between such islands 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 should likewise be
related, but less closely, to those of the nearest continent, or other
source whence immigrants might have been derived. We can see why, if there
exist very closely allied or representative species in two areas, however
distant from each other, some identical species will almost always there be
found.
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 apparent exceptions to the rule are so few that
they may fairly be attributed to our not having as yet discovered in an
intermediate deposit certain forms which are absent in it, but which occur
above and below: so in space, it certainly is the general rule that the
area inhabited by a single species, or by a group of species, is
continuous, and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by former migrations under different
circumstances, or through occasional means of transport, or by the species
having become extinct in the intermediate tracts.
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