the most disconcerting of any that had been revealed
since the days of Copernicus and Galileo. In its bald
essence, Smith's discovery was simply this: that the
fossils in the rocks, instead of being scattered haphazard,
are arranged in regular systems, so that any
given stratum of rock is labelled by its fossil population;
and that the order of succession of such groups of
fossils is always the same in any vertical series of strata
in which they occur. That is to say, if fossil A underlies
fossil B in any given region, it never overlies it in
any other series; though a kind of fossils found in one
set of strata may be quite omitted in another. Moreover,
a fossil once having disappeared never reappears
in any later stratum.

From these novel facts Smith drew the commonsense
inference that the earth had had successive populations
of creatures, each of which in its turn had become
extinct. He partially verified this inference by
comparing the fossil shells with existing species of similar
orders, and found that such as occur in older
strata of the rocks had no counterparts among living
species. But, on the whole, being eminently a practical
man, Smith troubled himself but little about the inferences
that might be drawn from his facts. He was
chiefly concerned in using the key he had discovered
as an aid to the construction of the first geological map
of England ever attempted, and he left to others the
untangling of any snarls of thought that might seem
to arise from his discovery of the succession of varying