reasons for believing that all our greater fossiliferous
formations were deposited during periods of subsidence; and that blank
intervals of vast duration occurred during the periods when the bed of
the sea was either stationary or rising, and likewise when sediment was
not thrown down quickly enough to embed and preserve organic remains.
During these long and blank intervals I suppose that the inhabitants
of each region underwent a considerable amount of modification and
extinction, and that there was much migration from other parts of the
world. As we have reason to believe that large areas are affected by the
same movement, it is probable that strictly contemporaneous formations
have often been accumulated over very wide spaces in the same quarter
of the world; but we are far from having any right to conclude that this
has invariably been the case, and that large areas have invariably been
affected by the same movements. When two formations have been deposited
in two regions during nearly, but not exactly the same period, we should
find in both, from the causes explained in the foregoing paragraphs, the
same general succession in the forms of life; but the species would not
exactly correspond; for there will have been a little more time in
the one region than in the other for modification, extinction, and
immigration.

I suspect that cases of this nature have occurred in Europe. Mr.
Prestwich, in his admirable Memoirs on the eocene deposits of England
and France, is able to draw a close general parallelism between the
successive stages in the two countries; but when he compares certain
stages in England with those in France, although he finds in both a
curious accordance in the numbers of the species belonging to the same
genera, yet the species themselves differ in a manner very difficult
to account for, considering the proximity of the two areas,--unless,
indeed, it be assumed that an isthmus separated two seas inhabited
by distinct, but contemporaneous, faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also,
shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; nevertheless he finds
a surprising amount of difference in the species. If the several
formations in these regions have not been deposited during the same
exact periods,--a formation in one region often corresponding with a
blank interval in the other