STUDIES ON THE DURATION OF LIFE 199 



BACTERIA AND DURATION OF LIFE IN DROSOPHILA 



But clocks may be stopped in other ways than by 

 ninning down. It will be worth while to consider with 

 some care a considerable mass of most interesting, and 

 in some respects even startling, experimental data, re- 

 garding various ways in which longevity may be influenced 

 by external agents. Since we have just been considering 

 Drosophila it may be well to consider the experimental 

 evidence regarding that form first. It is an obviously 

 well-known fact that bacteria are responsible in all higher 

 organisms for much organ breakdown and consequent 

 death. An infection of some particular organ or organ 

 system occurs, and the disturbance of the balance of the 

 whole so brought about finally results in death. But is 

 it not possible that we overrate the importance of bacter- 

 ial invasion in determining, in general and in the broad- 

 est sense, the average duration of life? May it not be 

 that when an organ system breaks down under stress 

 of bacterial toxins, it is in part at least, perhaps 

 primarily, because for internal organic reasons the resis- 

 tance of that organ system to bacterial invasion has nor- 

 mally and naturally reached such a low point that its 

 defenses are no longer adequate? All higher animals 

 live constantly in an environment far from sterile. Our 

 mouths and throats harbor pneumonia germs much of 

 the time, but we do not all or always have pneumonia. 

 Again it may fairly be estimated that of all persons who 

 attain the age of 35, probably at least 95 per cent, have 

 at some time or other been infected with the tubercle 

 bacillus, yet fewer than one in ten break down with 

 active tuberculosis. 



What plainly is needed in order to arrive at a just 

 estimate of the relative influence of bacteria and their 



