Evidence of Progressive Change 35 



cession from more general to more special forms, and to 

 a real continuity of life from generation to generation, 

 throughout the ages. 



Evidence of Progressive Change 



If each new " creation " were unrelated to the life of 

 the preceding geological age or ages, such resemblances as 

 are found between fossils of different periods would be purely 

 chance or random resemblances. As has already been stated 

 (page 30), however, there is a constant increase in the re- 

 semblance to existing forms as we examine the fossils of later 

 and later periods. Moreover, a careful examination of the 

 fossils shows that the older forms are more " primitive," or 

 less specialized in their structure. For example, the earliest 

 fossils found in North America, in the Huronian formation, 

 under layers of rock miles in thickness, represent simple ma- 

 rine forms, one-celled animals and worms. These animals 

 were too soft to leave any record, but the protozoa formed 

 minute " shells " and the worms formed burrows or tubes of 

 sand, which remain after millions of years. In the next im- 

 portant layers are found representatives of all the principal 

 branches of animal life except backboned animals. The 

 most prominent form is the three-lobed " crab " or Trilobite, 

 which shows affinities to worms on the one hand and to the 

 king crab on the other. 



Later deposits show remains of land plants for the first 

 time. Corals and molluscs resembling the nautilus increase 

 in numbers, and the period is characterized by curious fish- 

 like animals with armored heads, the Ostracods (Fig. 4). 

 These animals show traits that link them to the king crab in 

 some respects, to the sea-squirts in other respects, and to the 

 jawless hagfish of recent times in still other respects. 



The first backboned animals to appear are the more 

 primitive fishes and sharks which were very abundant in the 

 period that produced the rocks underlying the coal measures. 

 Of the other four classes of backboned animals living today. 



