chains, deserts, etc., are not as impassable, or likely to have
endured so long as the oceans separating continents, the differences are
very inferior in degree to those characteristic of distinct continents.

Turning to the sea, we find the same law. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of
the eastern and western shores of South and Central America; yet these
great faunas are separated only by the narrow, but impassable, isthmus
of Panama. Westward of the shores of America, a wide space of open ocean
extends, with not an island as a halting-place for emigrants; here we
have a barrier of another kind, and as soon as this is passed we meet
in the eastern islands of the Pacific, with another and totally
distinct fauna. So that here three marine faunas range far northward
and southward, in parallel lines not far from each other, under
corresponding climates; but from being separated from each other
by impassable barriers, either of land or open sea, they are wholly
distinct. On the other hand, proceeding still further westward from the
eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places,
until after travelling over a hemisphere we come to the shores of
Africa; and over this vast space we meet with no well-defined and
distinct marine faunas. Although hardly one shell, crab or fish is
common to the above-named three approximate faunas of Eastern and
Western America and the eastern Pacific islands, yet many fish range
from the Pacific into the Indian Ocean, and many shells are common to
the eastern islands of the Pacific and the eastern shores of Africa, on
almost exactly opposite meridians of longitude.

A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or sea, though the
species themselves are distinct at different points and stations. It is
a law of the widest generality, and every continent offers innumerable
instances. Nevertheless the naturalist in travelling, for instance,
from north to south never fails to be struck by the manner in which
successive groups of beings, specifically distinct, yet clearly related,
replace each other. He hears from closely allied, yet distinct kinds of
birds, notes nearly similar, and sees their nests similarly constructed,
but not quite alike, with eggs coloured in nearl