PART II

LONGEVITY IN THE ANIMAL KINGDOM

I

THEORIES OF LONGEVITY

Relation between longevity and size.—Longevity and the period of growth.—Longevity and the doubling in weight after birth.—Longevity and rate of reproduction.—Probable relations between longevity and the nature of the food 39

II

LONGEVITY IN THE ANIMAL KINGDOM

Longevity in the lower animals.—Instances of long life in sea-anemones and other vertebrates.—Duration of life of insects.—Duration of life of “cold-blooded” vertebrates.—Duration of life of birds.—Duration of life of mammals.—Inequality of the duration of life in males and females.—Relations between longevity and fertility of the organism 47

III

THE DIGESTIVE SYSTEM AND SENILITY

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