YOUTH VERSUS AGE 



periods. None the less would they have failed of final 

 success had their perseverance and enthusiasm flagged. 

 Similar perseverance enabled Harvey at fifty to demon- 

 strate the circulation of the blood, and Jenner at forty- 

 seven to show the preventive power of vaccination over 

 small-pox. 



These illustrations from practical fields may be 

 readily paralleled from the records of theoretical workers. 

 Thus all of Aristotle's works that have come down to us 

 were composed after he was fifty. Copernicus finished 

 his revolutionary work on the solar system at fifty-seven. 

 Bacon published the Novum Organum at fifty-nine, 

 earning thereby the title of the "Father of Inductive 

 Philosophy. " Isaac Newton was forty-five when he 

 completed the Principia, the work which made his 

 contemporaries question whether he were a mere 

 mortal. Kant was fifty-six when he published the 

 first edition of the Critique of Pure Reason, and sixty- 

 two when he sent forth the modified edition to the 

 further bewilderment of legions of disciples and critics. 

 Lavoisier was forty-six when he revolutionized the ter- 

 minology of chemistry, thereby laying the foundations 

 of the modern science of that name. Dalton was forty- 

 one when his atomic theory gave a new insight into the 

 nature of matter. Darwin was almost fifty when his 

 Origin of Species appeared, to change the entire aspect 

 of nineteenth-century thought. 



Such illustrations seem to give ample proof that the 

 fifth and sixth decades may find men still in their 

 full flight of productive activity. Nor need we pause 



