Fig. I. Genealogical tree of animal life. 

 Figures at left give estimated time in millions of years from beginning of 

 each epoch to present, according to Barrell's estimates, based on measure- 

 ment of rate of disintegration of radioactive ores since they were crystallized 

 at different geological ages. (After Gregory, Our Face from Fisli to Man. G. 

 P. Putnam's Sons.) 



f54l 



