Chapter IV: 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

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