in scanty numbers--and this we do find in the several 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 that of
these, a large proportion should be endemic or peculiar; and why, in
relation to the means of migration, one group of beings should have all
its species peculiar, and another group, even within the same class,
should have all its species the same with those in an adjoining quarter
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 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. Both
in time and space species and groups of specie
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