CHAPTER IV 



THE COMING OF VERTEBRATES AND THE RISE OF 



FISHES 



IT was a long time after life began before 

 any back-boned animals made their appearance 

 time enough for the formation of vast beds of 

 sand and mud that subsequently hardened into 

 layers of rock from 3 to 5 miles in thick- 

 ness. And when vertebrate animals did appear 

 they were small, and for the most part -very 

 unlike any now living sham vertebrates as 

 we might call them, or forerunners of verte- 

 brates as they have been styled by some 

 cautious naturalists. The very earliest indica- 

 tions of these animals that have yet been dis- 

 covered were found in the Lower Silurian 

 rocks of Colorado, and consist of bony plates 

 and traces of a back-bone of some fishlike 

 creature believed to be related to our lamprey. 

 These fossils are certainly insignificant enough 



85 



