be allied in groups, from inheriting some inferiority in common;
and therefore as new and improved groups spread throughout the world, old
groups will disappear from the world; and the succession of forms in both
ways will everywhere tend to correspond.

There is one other remark connected with this subject worth making. I have
given my 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 {328} 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 occur 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