uch 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;
{295} 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 instance, probable that sediment was deposited
during the whole of the glacial period near the mouth of the Mississippi,
within that limit of depth at which marine animals can flourish; for we
know what vast geographical changes occurred in other parts of America
during this