of species; and consequent
breaks, small or large as the case may be, in the paleontological
series. Other and more special geological changes must produce other and
more local blanks in the succession. By some inland elevation the
natural drainage of a continent is modified; and instead of the sediment
previously brought down to the sea by it, a great river brings down
sediment unfavourable to various plants and animals living in its delta:
whereupon these disappear from the locality, perhaps to re-appear in a
changed form after a long epoch. Upheavals or subsidences of shores or
sea-bottoms, involving deviations of marine currents, remove the
habitats of many species to which such currents are salutary or
injurious; and further, this redistribution of currents alters the
places of sedimentary deposits, and thus stops the burying of organic
remains in some localities, while commencing it in others. Had we space,
many more such causes of blanks in our paleontological records might be
added. But it is needless here to enumerate them. They are admirably
explained and illustrated in Sir Charles Lyell's _Principles of
Geology_.

Now, if these minor changes of the Earth's surface produce minor breaks
in the series of fossilized remains; must not great changes produce
great breaks? If a local upheaval or subsidence causes throughout its
small area the absence of some links in the chain of fossil forms; does
it not follow that an upheaval or subsidence extending over a large part
of the Earth's surface, must cause the absence of a great number of such
links throughout a very wide area?

When during a long epoch a continent, slowly sinking, gives place to a
far-spreading ocean some miles in depth, at the bottom of which no
deposits from rivers or abraded shores can be thrown down; and when,
after some enormous period, this ocean-bottom is gradually elevated and
becomes the site for new strata; it is clear that the fossils contained
in these new strata are likely to have but little in common with the
fossils of the strata below them. Take, in illustration, the case of the
North Atlantic. We have already named the fact that between this country
and the United States, the ocean-bottom is being covered with a deposit
of chalk--a deposit which has been forming, probably, ever since there
occurred that great depression of the Earth's crust from which the
Atlantic resulted in remote geologic times. This chalk consists of the
minute