Relations between longevity and the structure of the digestive system.—The cæca in birds.—The large intestine of mammals.—Function of the large intestine.—The intestinal microbes and their agency in producing auto-intoxication and auto-infection in the organism.—Passage of microbes through the intestinal wall 59 IV MICROBES AS THE CAUSE OF SENILITY Relations between longevity and the intestinal flora.—Ruminants.—The horse.—Intestinal flora of birds.—Intestinal flora of cursorial birds.—Duration of life in cursorial birds.—Flying mammals.—Intestinal flora and longevity of bats.—Some exceptions to the rule.—Resistance of the lower vertebrates to certain intestinal microbes 73 V DURATION OF HUMAN LIFE Longevity of man.—Theory of Ebstein on the normal duration of human life.—Instances of human longevity.—Circumstances which may explain the long duration of human life 84 PART III INVESTIGATIONS ON NATURAL DEATH I NATURAL DEATH AMONGST 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 94 II NATURAL DEATH IN THE ANIMAL WORLD Different origins of death in animals.—Examples of natural death associated with violent acts.—Examples of natural death in animals without digestive organs.—Natural death in the two sexes.—Hypothesis as to the cause of natural death in animals 109 III NATURAL DEATH AMONGST HUMAN BEINGS Natural death in the aged.—Analogy of natural death and sleep.—Theories of sleep.—Ponogenes.—The instinct of sleep.—The instinct of natural death.—Replies to critics.—Agreeable sensation at the approach of death 119 PART IV SHOULD WE TRY TO PROLONG HUMAN LIFE? I THE BENEFIT TO HUMANITY Complaints of the shortness of our life.—Theory of “medical selection” as a cause of degeneration of the race.—Utility of prolonging human life 132 II SUGGESTIONS FOR THE PROLONGATION OF LIFE Ancient methods of prolonging human life.—Gerokomy.—The “immortality draught” of the Taoists.—Brown-Séquard’s method.—The spermine of Poehl.—Dr. Weber’s precepts.—Increased duration of life in historical times.—Hygienic maxims.—Decrease in cutaneous cancer 136 III DISEASES THAT SHORTEN LIFE Measures against infectious diseases as aiding in the prolongation of life.—Prevention of syphilis.—Attempts to prepare serums which could strengthen the higher elements of the organism 145 IV INTESTINAL PUTREFACTION SHORTENS LIFE