ived at its commencement and close, but I cannot assign
due proportional weight to the following considerations.

Although each formation may mark a very long lapse of years, each
probably is short compared with the period requisite to change one
species into another. I am aware that two palaeontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or thrice
as long as the average duration of specific forms. But insuperable
difficulties, as it seems to me, prevent us from coming to any just
conclusion on this head. When we see a species first appearing in the
middle of any formation, it would be rash in the extreme to infer
that it had not elsewhere previously existed. So again, when we find
a species disappearing before the last layers have been deposited, it
would be equally rash to suppose that it then became extinct. We forget
how small the area of Europe is compared with the rest of the world;
nor have the several stages of the same formation throughout Europe been
correlated with perfect accuracy.

We may safely infer that with marine animals of all kinds there has been
a large amount of migration due to climatal and other changes; and when
we see a species first appearing in any formation, the probability is
that it only then first immigrated into that area. It is well known, for
instance, that several species appear somewhat earlier in the palaeozoic
beds of North America than in those of Europe; time having apparently
been required for their migration from the American to the European
seas. In examining the latest deposits, in various quarters of the
world, it has everywhere been noted, that some few still existing
species are common in the deposit, but have become extinct in the
immediately surrounding sea; or, conversely, that some are now abundant
in the neighbouring sea, but are rare or absent in this particular
deposit. It is an excellent lesson to reflect on the ascertained amount
of migration of the inhabitants of Europe during the glacial epoch,
which forms only a part of one whole geological period; and likewise to
reflect on the changes of level, on the extreme change of climate,
and on the great lapse of time, all included within this same glacial
period. Yet it may be doubted whether, in any quarter of the
world, sedimentary deposits, INCLUDING FOSSIL REMAINS, have gone on
accumulating within the same ar