isolated, within a much more recent period, on the
several mountain ranges and on the arctic lands of Europe and North
America. Hence, it has come, that when we compare the now living
productions of the temperate regions of the New and Old Worlds, we find
very few identical species (though Asa Gray has lately shown that more
plants are identical than was formerly supposed), but we find in every
great class many forms, which some naturalists rank as geographical
races, and others as distinct species; and a host of closely allied or
representative forms which are ranked by all naturalists as specifically
distinct.

As on the land, so in the waters of the sea, a slow southern migration
of a marine fauna, which, during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar
Circle, will account, on the theory of modification, for many closely
allied forms now living in marine areas completely sundered. Thus, I
think, we can understand the presence of some closely allied, still
existing and extinct tertiary forms, on the eastern and western shores
of temperate North America; and the still more striking fact of many
closely allied crustaceans (as described in Dana's admirable work), some
fish and other marine animals, inhabiting the Mediterranean and the seas
of Japan--these two areas being now completely separated by the breadth
of a whole continent and by wide spaces of ocean.

These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America,
the Mediterranean and Japan, and the temperate lands of North America
and Europe, are inexplicable on the theory of creation. We cannot
maintain that such species have been created alike, in correspondence
with the nearly similar physical conditions of the areas; for if we
compare, for instance, certain parts of South America with parts of
South Africa or Australia, we see countries closely similar in all their
physical conditions, with their inhabitants utterly dissimilar.

ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.

But we must return to our more immediate subject. I am convinced that
Forbes's view may be largely extended. In Europe we meet with the
plainest evidence of the Glacial period, from the western shores of
Britain to the Ural range, and southward to the Pyrenees. We may infer
from the frozen mammals and nature of the mountain vegetation, th