THE GEOLOGICAL BACKGROUND OF EVOLUTION 



393 



history and their sequence is shown. This time scale includes both the 

 known parts of the history and the gaps; the latter probably account 

 for at least as much time as the periods of which there are records. The 

 complete time scale of modern geology is nearly as complicated and 



Fig. 25.2. Types of fossils. The symbols given after each refer to the table of modes of 

 fossilization on p. 396. A, skeleton of phytosaur, I:A2a. B, fossil fish, I:A2a. C, fossil cycad 

 leaves, I:B3. D, crinoid or sea lily, I:A2a. E, part of stem of giant rush, Calamites, I:A3. 

 F, trilobite, Illaenus americanus, I:B2 + 3. G, cycad cones, I:A3. (Courtesy Ward's Natural 

 Science Establishment, Inc., except C, American Museum of Natural History.) 



contains nearly as many names and events as, for example, a moderately 

 condensed treatise on the history of Europe. For our purposes only the 

 most important periods and events are needed. A simplified time scale 

 giving this information is presented herewith. Knowledge of the names of 

 the periods and their sequence is essential to an understanding of the 

 chapters that follow. 



