30 The Ma\e'Up and the Wor\ings of the Earth 



many may contain similar fossils, the rocks below the coal 

 beds may contain similar fossils in all three regions, and the 

 rocks above the coal beds may also contain similar fossils. 

 Yet all that we can say is that these various types of fossils 

 appeared in the same order in the three countries. We cannot 

 be sure that the corresponding layers were formed at ap- 

 proximately the same time. They may have been separated 

 from each other by millions of years. 



Fig. 3. A Trilobite 



Fossil remains of an animal related to the horse-shoe crab, 

 from Keokuk limestone. Photograph, courtesy of the Ameri- 

 can Museum of Natural History. 



The order in which different types of fossils appear in 

 the layers of rock is, however, of great importance in con- 

 sidering the question of evolution. The first significant set 

 of facts is this: in a series of fossils taken from successive 

 layers of rocks, the oldest or deepest correspond, in their 

 resemblances, to simpler plants and animals, while the more 

 recent, or those nearer the surface, correspond to more com- 

 plex plants and animals. Again, where we find sufficient 

 resemblances between the fossil forms and existing species 



