ermany between others which agree perfectly in
their organic remains with our Lias and Magnesian Limestone. Until
lately the fossils of the Coal-measures were separated from those of the
antecedent Silurian group by a very abrupt and decided line of
demarcation; but recent discoveries have brought to light in Devonshire,
Belgium, the Eifel, and Westphalia, the remains of a fauna of an
intervening period. This connecting link is furnished by the fossil
shells, fish, and corals of the Devonian or Old Red Sandstone group, and
some species of this newly intercalated fauna are found to be common to
it and the subjacent Silurian rocks, while other species belong to it in
common with the Coal-measures. We have also in like manner had some
success of late years in diminishing the hiatus which still separates
the Cretaceous and Eocene periods in Europe. Still we must expect, for
reasons before stated, that some such chasms will forever continue to
occur in some parts of our sedimentary series.

_Consistency of the theory of gradual change with the existence of great
breaks in the series._--To return to the general argument pursued in
this chapter, it is assumed, for reasons above explained, that a slow
change of species is in simultaneous operation everywhere throughout the
habitable surface of sea and land; whereas the fossilization of plants
and animals is confined to those areas where new strata are produced.
These areas, as we have seen, are always shifting their position; so
that the fossilizing process, by means of which the commemoration of the
particular state of the organic world, at any given time, is affected,
may be said to move about, visiting and revisiting different tracts in
succession.

To make still more clear the supposed working of this machinery, I shall
compare it to a somewhat analogous case that might be imagined to occur
in the history of human affairs. Let the mortality of the population of
a large country represent the successive extinction of species, and the
births of new individuals the introduction of new species. While these
fluctuations are gradually taking place everywhere, suppose
commissioners to be appointed to visit each province of the country in
succession, taking an exact account of the number, names, and individual
peculiarities of all the inhabitants, and leaving in each district a
register containing a record of this information. If, after the
completion of one census, another is imme