pulated with various new species of mastodons,
sabre-toothed tigers, camels, horses, deer, cats, wolves, hooved
creatures of strange shapes and some of them of giant size, all of
these being descended from the immigrant types; and side by side with
them there grew up large autochthonous [TR: original autochthonus]
ungulates, giant ground sloths well-nigh as large as elephants, and
armored creatures as bulky as an ox but structurally of the armadillo
or ant-eater type; and some of these latter not only held their own,
but actually in their turn wandered north over the isthmus and invaded
North America. A fauna as varied as that of Africa to-day, as abundant
in species and individuals, even more noteworthy, because of its huge
size or odd type, and because of the terrific prowess of the more
formidable flesh-eaters, was thus developed in South America, and
flourished for a period which human history would call very long
indeed, but which geologically was short.

Then, for no reason that we can assign, destruction fell on this
fauna. All the great and terrible creatures died out, the same fate
befalling the changed representatives of the old autochthonous fauna
and the descendants of the migrants that had come down from the north.
Ground sloth and glyptodon, sabre-tooth, horse and mastodon, and all
the associated animals of large size, vanished, and South America,
though still retaining its connection with North America, once again
became a land with a mammalian life small and weak compared to that
of North America and the Old World. Its fauna is now marked, for
instance, by the presence of medium-sized deer and cats, fox-like
wolves, and small camel-like creatures, as well as by the presence of
small armadillos, sloths, and ant-eaters. In other words, it includes
diminutive representatives of the giants of the preceding era, both of
the giants among the older forms of mammalia, and of the giants among
the new and intrusive kinds. The change was widespread and
extraordinary, and with our present means of information it is wholly
inexplicable. There was no ice age, and it is hard to imagine any
cause which would account for the extinction of so many species of
huge or moderate size, while smaller representatives, and here and
there medium-sized representatives, of many of them were left.

Now as to all of these phenomena in the evolution of species, there
are, if not homologies, at least certain analogies, in the history o