or
instance, if in Africa or S. America, we go from south to north{349}, or
from lowland to upland, or from a humid to a dryer part, we find wholly
different species of those genera or groups which characterise the
continent over which we are passing. In these subdivisions we may
clearly observe, as in the main divisions of the world, that
sub-barriers divide different groups of species, although the opposite
sides of such sub-barriers may possess nearly the same climate, and may
be in other respects nearly similar: thus it is on the opposite sides of
the Cordillera of Chile, and in a lesser degree on the opposite sides of
the Rocky mountains. Deserts, arms of the sea, and even rivers form the
barriers; mere preoccupied space seems sufficient in several cases: thus
Eastern and Western Australia, in the same latitude, with very similar
climate and soils, have scarcely a plant, and few animals or birds, in
common, although all belong to the peculiar genera characterising
Australia. It is in short impossible to explain the differences in the
inhabitants, either of the main divisions of the world, or of these
sub-divisions, by the differences in their physical conditions, and by
the adaptation of their inhabitants. Some other cause must intervene.

{349} _Origin_, Ed. i. p. 349, vi. p. 496.

We can see that the destruction of sub-barriers would cause (as before
remarked in the case of the main divisions) two sub-divisions to blend
into one; and we can only suppose that the original difference in the
species, on the opposite sides of sub-barriers, is due to the creation
or production of species in distinct areas, from which they have
wandered till arrested by such sub-barriers. Although thus far is pretty
clear, it may be asked, why, when species in the same main division of
the world were produced on opposite sides of a sub-barrier, both when
exposed to similar conditions and when exposed to widely different
influences (as on alpine and lowland tracts, as on arid and humid soils,
as in cold and hot climates), have they invariably been formed on a
similar type, and that type confined to this one division of the world?
Why when an ostrich{350} was produced in the southern parts of America,
was it formed on the American type, instead of on the African or on
Australian types? Why when hare-like and rabbit-like animals were formed
to live on the Savannahs of La Plata, were they produced on the peculiar
Rodent type of S. Am