92 DOCTRINE OF EVOLUTION 



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 forma- 

 tion. 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 evi- 

 dences of the previous categories have shown. When 

 we remember that the lower animals are devoid as a rule 

 of skeletal structures that might be fossihzed, and when 

 we recall the fact that the strata of the palaeozoic pro- 

 vided 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 



