l relation between
certain areas of the world, and the production of certain organic forms;
but we now see that such an assumption would have to be complicated by
the admission that such a relation, though holding good for long
revolutions of years, is not truly persistent.

{395} <Note by the author.> All this requires much verification.

{396} This point seems to be less insisted on in the _Origin_.

{397} _Origin_, Ed. i. p. 356, vi. p. 504.

I will only add one more observation to this section. Geologists
finding in the most remote period with which we are acquainted, namely
in the Silurian period, that the shells and other marine
productions{398} in North and South America, in Europe, Southern Africa,
and Western Asia, are much more similar than they now are at these
distant points, appear to have imagined that in these ancient times the
laws of geographical distribution were quite different than what they
now are: but we have only to suppose that great continents were extended
east and west, and thus did not divide the inhabitants of the temperate
and tropical seas, as the continents now do; and it would then become
probable that the inhabitants of the seas would be much more similar
than they now are. In the immense space of ocean extending from the east
coast of Africa to the eastern islands of the Pacific, which space is
connected either by lines of tropical coast or by islands not very
distant from each other, we know (Cuming) that many shells, perhaps even
as many as 200, are common to the Zanzibar coast, the Philippines, and
the eastern islands of the Low or Dangerous Archipelago in the Pacific.
This space equals that from the Arctic to the Antarctic pole! Pass over
the space of quite open ocean, from the Dangerous Archipelago to the
west coast of S. America, and every shell is different: pass over the
narrow space of S. America, to its eastern shores, and again every shell
is different! Many fish, I may add, are also common to the Pacific and
Indian Oceans.

{398} <Note by the author.> D'Orbigny shows that this is not so.

_Summary on the distribution of living and extinct organic beings._

Let us sum up the several facts now given with respect to the past and
present geographical distribution of organic beings. In a previous
chapter it was shown that species are not exterminated by universal
catastrophes, and that they are slowly produced: we have also seen that
each species is