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 120 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 proposes to discover and to avoid those conditions which “wear down” most of us and produce “senility” and “death” before we have really run out our lease of life.