40 SCIENCE FROM AN EASY CHAIR 



its gradual development, according to Darwinian prin- 

 ciples, in the animal series, was shown twenty years ago 

 by Metchnikoff. His important work on "immunity" 

 and on infection and on protection against germ-caused 

 disease is thus seen to be one of the many flourishing and 

 valuable branches of knowledge which have originated 

 from Darwin's great conception and his example in 

 experiment and inquiry. 



Metchnikoff is now devoting all his attention to the 

 possibility of prolonging human life. The facts seem to 

 show that if we ate and drank only what is best for us, 

 and led lives regulated by reason and knowledge, we 

 should, nearly all, attain to 80 or even 100 years of age, 

 having healthy minds and healthy bodies. We should die 

 quietly and comfortably at the end, with much the same 

 feeling of contentment in well-earned final repose as 

 that which we now experience in going to sleep at the 

 end of a long and happy day of healthy exercise and 

 activity. Metchnikoff thinks that the causes of too early 

 death may be ascertained, and when ascertained avoided 

 or removed. In 1870, in a little book on Comparative 

 Longevity, I distinguished what we may call the "possible 

 life," or " potential longevity," of any given human being 

 from his or her " expectation " of life. Potential longevity 

 has been well called our " lease " of life. It is probably 

 not very different in different races of men or individuals 

 and is probably higher than King David thought, being 

 100 to 1 20 years, and not merely 70 years. We all, or 

 nearly all, fail to last out our " lease " owing to accidents, 

 violence, and avoidable, as well as unavoidable, disease; 

 so that 70 years is named as our tenure when the injury 

 done to us by unhealthy modes of life and by actual 

 disease are considered as inevitable. Metchnikoff pro- 

 poses to discover and to avoid those conditions which 

 "wear down" most of us and produce "senility" and 



