346 



ANIMAL LIFE 



enemies sometimes perished, and their skeletons were like- 

 wise entombed in the sediment-forming rock and there 

 became fossilized. Therefore, each successive stratum or 

 layer of rock containing the remains of the life of former 

 ages serves as the leaf of a book giving a description of the 



animate world be- 

 fore the era of man. 

 By careful compu- 

 tation scientists 

 have determined 

 that life has proba- 

 bly existed on the 

 earth for a period 

 of from twenty mil- 

 lion to one hundred 

 million years. The 

 fossil remains show 

 that the inverte- 

 brates lived here more than a million years before there 

 were any vertebrates; the fish preceded the amphibians 

 by several million years, while as much more time elapsed 

 before the advent of reptiles, which dwelt here at least 

 two or three million years before the appearance of birds. 

 There is evidence indicating that man has been on the 

 earth from fifty thousand to a hundred thousand years, 

 and it is possible that he has been here even a million 

 years. 



" From lower to higher, 

 From simple to complete, 

 This is the pathway of eternal feet ; 

 From earth to lichen, 



FIG. 387. A piece of rock containing the fossil re- 

 mains of several animals. Photographed in Am- 

 erican Museum of Natural History. 



