proportional weight to the following
considerations.

Although each formation may mark a very long lapse of years, each
perhaps 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 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 uppermost layers have been deposited, it would
be equally rash to suppose that it then became wholly 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.

With marine animals of all kinds, we may safely infer a large amount of
migration during 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 appeared 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 of 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 period, which forms only a part
of one whole geological period; and likewise to reflect on the great
changes of level, on the inordinately great change of climate, on the
prodigious 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 area during the whole of this period. It is not, for ins