THE PHYSICAL BASIS OF LIFE 89 



radio-activity could still exist. The idea is, and the 

 evidence is very strong indeed, that radium is the 

 direct descendant of uranium ; that, in fact, uranium 

 is slowly being converted into radium. Most prob- 

 ably, too, uranium is the descendant of something else, 

 or of a series of such elements that have long since 

 become extinct. The first appearance of radio-active 

 bodies is thus lost in obscurity not unlike that of the 

 first beginnings of life. But it must not be imagined 

 that, because the life of each radio-active substance 

 is so short, therefore, it is not^ to be found still in 

 such living bodies as may have originated contem- 

 poraneously with the remote ancestors of radium, 

 so that whatever the radio-active substance is, its 

 descendant would no doubt appear in some form and 

 even at the present day persist. 



