biological evidence of evolution provided by the results of this division
of zoölogical science. But all of the foregoing is fundamentally part of
this department of knowledge and it is absolutely essential for any one
who desires to understand what the fossils themselves demonstrate.

The oldest sedimentary rocks are devoid of fossil remains and so they are
called the Azoic or Archæan. They comprise about 30,000 feet of strata
which seem to have required at least 20,000,000 years for their formation.
This period is roughly two-fifths of the whole time necessary for the
formation of _all_ the sedimentary rocks, and this proportion holds true
even if the entire period of years should be taken as 100,000,000 instead
of 50,000,000 or less. The earth during this early age was slowly
organizing in chemical and physical respects so that living matter could
be and indeed was formed out of antecedent substances--but this process
does not concern us here. The important fact is that the second major
period, called the Palæozoic, or "age of ancient animals," saw the
evolution of the lowest members of the series,--the invertebrates,--and
the most primitive of the backboned animals, like fishes and amphibia. The
rocks of this long age include about 106,000 feet of strata, demanding
some 21,000,000 or 22,000,000 years for their deposition. Thus it is
proved that the invertebrate animals were succeeded in time by the higher
vertebrates, which is exactly what the evidences of the previous
categories have shown. When we remember that the lower animals are devoid
as a rule of skeletal structures that might be fossilized, and when we
recall the fact that the strata of the palæozoic provided the materials
out of which the upper layers were formed afterwards, we can understand
why the ancient members of the invertebrate groups are not known as well
as the later and higher forms like vertebrates. Yet all the fossils of
these relatively unfamiliar creatures clearly prove that no complex animal
appears upon a geological horizon until after some simple type belonging