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-characterised
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.
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