i. pp. 498, 559.

As every one would be surprised if two exactly similar but peculiar
varieties{414} of any species were raised by man by long continued
selection, in two different countries, or at two very different periods,
so we ought not to expect that an exactly similar form would be produced
from the modification of an old one in two distinct countries or at two
distinct periods. For in such places and times they would probably be
exposed to somewhat different climates and almost certainly to different
associates. Hence we can see why each species appears to have been
produced singly, in space and in time. I need hardly remark that,
according to this theory of descent, there is no necessity of
modification in a species, when it reaches a new and isolated country.
If it be able to survive and if slight variations better adapted to the
new conditions are not selected, it might retain (as far as we can see)
its old form for an indefinite time. As we see that some sub-varieties
produced under domestication are more variable than others, so in
nature, perhaps, some species and genera are more variable than others.
The same precise form, however, would probably be seldom preserved
through successive geological periods, or in widely and differently
conditioned countries{415}.

{414} _Origin_, Ed. i. p. 352, vi. p. 500.

{415} _Origin_, Ed. i. p. 313, vi. p. 454.

Finally, during the long periods of time and probably of oscillations of
level, necessary for the formation of a continent, we may conclude (as
above explained) that many forms would become extinct. These extinct
forms, and those surviving (whether or not modified and changed in
structure), will all be related in each continent in the same manner and
degree, as are the inhabitants of any two different sub-regions in that
same continent. I do not mean to say that, for instance, the present
Marsupials of Australia or Edentata and rodents of S. America have
descended from any one of the few fossils of the same orders which have
been discovered in these countries. It is possible that, in a very few
instances, this may be the case; but generally they must be considered
as merely codescendants of common stocks{416}. I believe in this, from
the improbability, considering the vast number of species, which (as
explained in the last chapter) must by our theory have existed, that
the _comparatively_ few fossils which have been found should chance to
be the im