The oldest sedimentary rocks are devoid of fossil remains and so they are called the Azoic or Archaean.  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 Palaeozoic, 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 palaeozoic 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 to a class from which it may have taken its origin; in brief, there are no anachronisms in the record, which always corresponds with the record written by comparative anatomy, wherever the facts enable a comparison to be made.

But the extinct animals of the third and fourth ages are more interesting to us, because there are more of them and because they are more like the well-known organisms of our present era.  These two ages are called the Mesozoic or Secondary, and the Cenozoic or Tertiary.  The former is so named because it was a transitional age of animals that are intermediate in a general way between the primitive forms of the preceding age and those of the next period; the latter name means the “recent-animal” age, when evolution produced not only the larger groups of our present animal series, but also many of the smaller branches of the genealogical tree like orders and families to which the species of to-day belong.