SOME SOUTH AMERICAN RULERS 143 



the ruler of the world. Just how long he has now 

 ruled we do not know, but some scientists think 

 that human beings have lived on this earth only 

 about fifty thousand years. Before this the mammals 

 were in power for three million years or more, and be- 

 fore them was the reign of the reptiles which lasted 

 fully seven million years. And then, before the age 

 of the reptiles, were other forms of life which can be 

 traced back for so many millions of years that the 

 mind cannot comprehend them. 



As we have already seen, the reason this life can 

 be traced so far back is because the fossil bones of 

 many animals which lived in those days are found 

 buried in rocks. And to-day other rocks are being 

 formed all over the world through the action of water 

 on mud and sand and shells, and the bones of crea- 

 tures familiar to us are being changed into fossils 

 within these rocks. Perhaps, millions of years 

 from now, men, women, and children will gaze with 

 astonishment upon these fossils, just as we now look 

 in wonder and in awe upon the skeletons of the 

 mighty animals which lived before man. 



