When the sediments were accumulating age after age, it naturally came
about that remains of the plants and animals living at the time were
buried, and these formed the fossils by the aid of which it is possible
to read the story of the past. By careful piecing together of evidence
the geologist is able to determine the order in which the different
sedimentary rocks were laid down, and thus to say, for instance, that
the Devonian period was the time of the origin of Amphibians. In other
cases the geologist utilises the fossils in his attempt to work out the
order of the strata when these have been much disarranged. For the
simpler fossil forms of any type must be older than those that are more
complex. There is no vicious circle here, for the general succession of
strata is clear, and it is quite certain that there were fishes before
there were amphibians, and amphibians before there were reptiles, and
reptiles before there were birds and mammals. In certain cases, e.g. of
fossil horses and elephants, the actual historical succession has been
clearly worked out.
If the successive strata contained good samples of all the plants and
animals living at the time when the beds were formed, then it would be
easy to read the record of the rocks, but many animals were too soft to
become satisfactory fossils, many were eaten or dissolved away, many
were destroyed by heat and pressure, so that the rock record is like a
library very much damaged by fire and looting and decay.
§ 2