These observations, however, relate to the marine inhabitants of the world:
we have not sufficient data to judge whether the productions of the land
and of fresh water at distant points change 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 co-existed with sea-shells all still living; but as
these anomalous monsters co-existed with the Mastodon and Horse, it might
at least have been inferred that they had lived during one of the later
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 year, or even to the same century, or even
that it has a very strict geological sense; for if all the marine animals
now living in Europe, and all those that lived in Europe during the
pleistocene period (a very remote period as measured by years, including
the whole glacial epoch) were compared with those now existing in South
America or in Australia, the most skilful naturalist would hardly be able
to say whether the present or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere. So, again,
several highly competent observers maintain that the existing productions
of the United States are more closely related to those which lived in
Europe during certain late tertiary stages, than to the present inhabitants
of Europe; and if this be so, it is evident that fossiliferous beds now
deposited on the shores of North America would hereafter be liable to be
classed with somewhat older European beds.
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