d 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 areas completely sundered. Thus, I think, we
can understand the presence of many existing and tertiary representative
forms on the eastern and western shores of temperate North America;
and the still more striking case of many closely allied crustaceans
(as described in Dana's admirable work), of some fish and other marine
animals, in the Mediterranean and in the seas of Japan,--areas now
separated by a continent and by nearly a hemisphere of equatorial ocean.

These cases of relationship, without identity, of the inhabitants of
seas now disjoined, and likewise of the past and present inhabitants of
the temperate lands of North America and Europe, are inexplicable on the
theory of creation. We cannot say that they 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
the southern continents of the Old World, we see countries closely
corresponding in all their physical conditions, but with their
inhabitants utterly dissimilar.

But we must return to our more immediate subject, the Glacial period.
I am convinced that Forbes's view may be largely extended. In Europe we
have the plainest evidence of the cold period, from the western shores
of Britain to the Oural range, and southward to the Pyrenees. We may
infer, from the frozen mammals and nature of the mountain vegetation,
that Siberia was similarly affected. Along the Himalaya, at points 900
miles apart, glaciers have left the marks of their former low descent;
and in Sikkim, Dr. Hooker saw maize growing on gigantic ancient
moraines. South of the equator, we have some direct evidence of former
glacial action in New Zealand; and the same plants, found on widely
separated mountains in this isla