From 40
to 60 per cent of the fossils belong to existing species. On
the Loup Fork of the river Platte, in the Upper Missouri region,
are strata which are also believed to be referable to the Pliocene
period, and probably to its upper division. They are from 300 to
400 feet thick, and contain land-shells, with the bones of numerous
Mammals, such as Camels, Rhinoceroses, Mastodons, Elephants, the
Horse, Stag, &c.
As regards the _life_ of the Pliocene period, there are only
two classes of organisms to which our attention need be
directed--namely, the Shell-fish and the Mammals. So far as the
former are concerned, we have to note in the first place that
the introduction of new species of animals upon the globe went
on rapidly during this period. In the Older Pliocene deposits,
the number of shells of existing species is only from 40 to 60
per cent; but in the Newer Pliocene the proportion of living
forms rises to as much as from 80 to 95 per cent. Whilst the
Molluscs thus become rapidly modernised, the Mammals still all
belong to extinct species, though modern generic types gradually
supersede the more antiquated forms of the Miocene. In the second
place, there is good evidence to show that the Pliocene period
was one in which the climate of the northern hemisphere underwent
a gradual refrigeration. In the Miocene period, there is evidence
to show that Europe possessed a climate very similar to that
now enjoyed by the Southern United States, and certainly very
much warmer than it is at present.
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