alone have passed the barrier, and may be considered as wanderers from
the south, such as the puma, opossum, kinkajou, and peccari. South
America is characterized by possessing many peculiar gnawers, a family
of monkeys, the llama, peccari, tapir, opossums, and, especially,
several genera of Edentata, the order which includes the sloths,
ant-eaters, and armadilloes. North America, on the other hand, is
characterized (putting on one side a few wandering species) by numerous
peculiar gnawers, and by four genera (the ox, sheep, goat, and
antelope) of hollow-horned ruminants, of which great division South
America is not known to possess a single species. Formerly, but within
the period when most of the now existing shells were living, North
America possessed, besides hollow-horned ruminants, the elephant,
mastodon, horse, and three genera of Edentata, namely, the Megatherium,
Megalonyx, and Mylodon. Within nearly this same period (as proved by
the shells at Bahia Blanca) South America possessed, as we have just
seen, a mastodon, horse, hollow-horned ruminant, and the same three
genera (as well as several others) of the Edentata. Hence it is
evident that North and South America, in having within a late
geological period these several genera in common, were much more
closely related in the character of their terrestrial inhabitants than
they now are. The more I reflect on this case, the more interesting it
appears: I know of no other instance where we can almost mark the
period and manner of the splitting up of one great region into two
well-characterized zoological provinces. The geologist, who is fully
impressed with the vast oscillations of level which have affected the
earth's crust within late periods, will not fear to speculate on the
recent elevation of the Mexican platform, or, more probably, on the
recent submergence of land in the West Indian Archipelago, as the cause
of the present zoological separation of North and South America. The
South American character of the West Indian mammals [6] seems to
indicate that this archipelago was formerly united to the southern
continent, and that it has subsequently been an area of subsidence.
When America, and especially North America, possessed its elephants,
mastodons, horse, and hollow-horned ruminants, it was much more closely
related in its zoological characters to the temperate parts of Europe
and Asia than it now is. As the remains of these genera are found on
both sides of Behring's Straits [7] and on the plains of Siberia, we
are led to look to the north-western side of North America as the
former point of communication between the Old and so-called New World.
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