years) could not be included, and these are merely suggested 

 by the heavy line at zero time. 



The entire period encompassed by documented history is only 

 about six thousand years, or one hundred-thousandth of the 

 interval elapsing since the opening of the Paleozoic era (Cam- 

 brian period), when fossilized animals first begin to appear in 

 the sedimentary rocks. 



