In some cases centenarians have been much addicted to the drinking of coffee. The reader will recall Voltaire’s reply when his doctor described the grave harm that comes from abuse of coffee which acts as a real poison. “Well,” said Voltaire, “I have been poisoning myself for nearly 80 years.” There are centenarians who have lived longer than Voltaire, and have drunk still more coffee. Elisabeth Durieux, a native of Savoy, reached the age of 114. Her principal food was coffee, of which she took daily as many as forty small cups. She was jovial and a boon table companion, and used black coffee in quantities that would have surprised an Arab. Her coffee-pot was always on the fire, like the tea-pot in an English cottage (Lejoncourt, p. 84; Chemin, p. 147). It has been noticed that many centenarians do not smoke, but this like all other traits is not universal. M. Ross, who gained a prize for longevity in 1896 at the age of 102, was an inveterate smoker. In 1897, a widow named Lazennec, died at La Carrière, in Kérinou, Finistère, at the age of 104. She lived in a hovel on charity, and she had smoked a pipe ever since she was quite young. It is plain that any factor to which long duration of life has been attributed disappears when many cases are examined. Naturally a sound constitution and a simple and sober life are favourable to longevity, but apart from these, there is something unknown which tends to long life. The celebrated physiologist of Bonn, Pflüger,[70] came to the conclusion that the chief condition of longevity is something “intrinsic in the constitution,” something which cannot be defined exactly, and which must be set down to inheritance. In the present state of knowledge, we cannot denote the chief cause of human longevity, but the proper course will be to seek it out as we would seek out that of animal longevity. As human longevity is often local in its character, and is exhibited by married people who have nothing in common except their mode of life, we may enquire into the intestinal flora and the mechanism by which the organism resists its harmful effect as factors which influence the duration of life. It is reasonable to suppose that in persons living in the same district or under the same roof, the intestinal flora may be similar. The problem can be settled only by a series of laborious researches which have yet to be made. At present I can do no more than bring together a large number of facts regarding the duration of life in man and in animals, with the hope of suggesting the lines for future investigation. PART III INVESTIGATIONS ON NATURAL DEATH I NATURAL DEATH AMONG PLANTS Theory of the immortality of unicellular organisms—Examples of very old trees—Examples of short-lived plants—Prolongation of the life of some plants—Theory of the natural death of plants by exhaustion—Death of plants from auto-intoxication