These observations, however, relate to the marine inhabitants of
distant parts of the world: we have not sufficient data to judge
whether the productions of the land and of fresh water change at
distant points in the same parallel manner. We may doubt whether they
have thus changed: if the Megatherium, Mylodon, Macrauchenia, and
Toxodon had been brought to Europe from La Plata, without any
information in regard to their geological position, no one would have
suspected that they had coexisted with still living sea-shells; but as
these anomalous monsters coexisted with the Mastodon and Horse, it
might at least have been inferred that they had lived during one of
the latter tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same thousandth or hundred-thousandth year,
or even that it has a very strict geological sense; for if all the
marine animals which live at the present day in Europe, and all those
that lived in Europe during the pleistocene period (an enormously
remote period as measured by years, including the whole glacial
epoch), were to be compared with those now living in South America or
in Australia, the most skilful naturalist would hardly be able to say
whether the existing or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere.
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