Western Europe and North America, a similar parallelism in the forms of
life has been observed by several authors: so it is, according to
Lyell, with the several European and North American tertiary deposits.
Even if the few fossil species which are common to the Old and New
Worlds be kept wholly out of view, the general parallelism in the
successive forms of life, in the stages of the widely separated
pal ozoic and tertiary periods, would still be manifest, and the
several formations could be easily correlated.
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. So, again, several
highly competent observers believe that the existing productions of the
United States are more closely related to those which lived in Europe
during certain later tertiary stages, than to those which now live
here; and if this be so, it is evident that fossiliferous beds
deposited at the present day on the shores of North America would
hereafter be liable to be classed with somewhat older European beds.
Nevertheless, looking to a remotely future epoch, there can, I think,
be little doubt that all the more modern _marine_ formations, namely,
244