New Guinea and some small adjoining islets only by a
narrow and shallow strait; whereas New Guinea and its adjoining islets
are cut off from the other East Indian islands by deep water. These
latter islands, I may remark, which fall into the great Asiatic group,
are separated from each other and the continent only by shallow water;
and where this is the case we may suppose, from geological oscillations
of level, that generally there has been recent union. South America,
including the southern part of Mexico, is cut off from North America by
the West Indies, and the great table-land of Mexico, except by a mere
fringe of tropical forests along the coast: it is owing, perhaps, to
this fringe that N. America possesses some S. American forms. Madagascar
is entirely isolated. Africa is also to a great extent isolated,
although it approaches, by many promontories and by lines of shallower
sea, to Europe and Asia: southern Africa, which is the most distinct in
its mammiferous inhabitants, is separated from the northern portion by
the Great Sahara Desert and the table-land of Abyssinia. That the
distribution of organisms is related to barriers, stopping their
progress, we clearly see by comparing the distribution of marine and
terrestrial productions. The marine animals being different on the two
sides of land tenanted by the same terrestrial animals, thus the shells
are wholly different on the opposite sides of the temperate parts of
South America{346}, as they are (?) in the Red Sea and the
Mediterranean. We can at once perceive that the destruction of a barrier
would permit two geographical groups of organisms to fuse and blend into
one. But the original cause of groups being different on opposite sides
of a barrier can only be understood on the hypothesis of each organism
having been created or produced on one spot or area, and afterwards
migrating as widely as its means of transport and subsistence permitted
it.

{345} On the general importance of barriers, see _Origin_, Ed. i.
p. 347, vi. p. 494.

{346} _Origin_, Ed. i. p. 348, vi. p. 495.

_Relation of range in genera and species._

It is generally{347} found, that where a genus or group ranges over
nearly the entire world, many of the species composing the group have
wide ranges: on the other hand, where a group is restricted to any one
country, the species composing it generally have restricted ranges in
that country{348}. Thus among mammifers the f