In the United States the average length of life is about forty-two years; but a large and growing school of modern scientists (comparative anatomists) declare that the natural age of the human family cannot be much less than from one hundred, to one hundred and twenty-five years. Any death that comes at least before one hundred years, is not a natural death but accidental or violent. From the point of view of science, death through disease is just as accidental and violent as the extinguishment of life in a railway wreck or by drowning in the sea; and the fact that the average life of man is to-day only about one-third of that which nature designed for him is due to the operation of autointoxication more than to any other cause. Natural death in man is therefore more a possibility than an actual occurrence. Nevertheless, instances have been recorded of the actual appearance of the instinct in aged people, where the wished-for death came not because life was burdensome, not because of poverty, disease, or loneliness, but seemed to arrive as naturally as sleep to a younger person, or the wish for more extended life which all of us possess. Metchnikoff states that instances of veritable cases showing an instinct of death are extremely rare, yet this instinct really does seem to lie deep in the constitution of man. And if the cycle of human life followed an ideal course, he concluded men and women after living a healthy and useful life extending over at least a century, with their usefulness and satisfaction in life at maximum during the latter portion of that period, would then give themselves up calmly and gracefully to the arms of death, as to the arms of a friend laying them down to earned and wished-for rest. Old age would have no terrors, and death no victory. It has been, perhaps, Metchnikoff’s crowning discovery, that the immediate cause of old age is not merely the accumulation of poisonous wastes, but is due to a destruction of the tiny cells which make up the tissues by certain cells of the body, which he describes as macrophages. These are of an especial kind, which wander through the body and devote their energies to the destruction of waste particles and organic débris—particles of material which are not used in the building up of tissue, just as particles of brick and wood might be left on the ground after the erection of a house. These macrophages enact the part of scavengers, very much like the turkey buzzards, which in southern cities eat up the refuse from the back alleys. Just so long as these wandering cells confine themselves to this useful and necessary work, all goes well; but when the vigor of the body cells has been lowered by the accumulation of tissue poisons, these scavenger cells turn traitor to the cause of the body and attack the very cells which they formerly guarded. They have been photographed in the very act of devouring nerve cells in the brains of old people.