Darwinists have stressed the advantages of the large intestine for convenience and the avoidance of the necessity of leaving frequent spoors by which animals might be tracked by their enemies. But many species are without it or have it only in rudimentary form and in man its removal by surgery results in no very serious impairment. We may add, too, that more recently psychoanalysts have described the anus and rectum and their functions as centers of various erotic activities, especially but by no means exclusively in children. Here the waste products of the digestive processes are dumped, awaiting removal, and it has long been known that their undue accumulation caused not only local troubles but general malaise, anxieties, and nervous and mental tensions. Metchnikoff and his pupils showed that very soon after birth noxious bacteria find their way to the large intestine and flourish thereafter in great profusion, especially in constipation, and that no cathartics can be relied on for permanent relief, salutary as medicine has always and everywhere found them for mitigation of many diverse ailments. It is the microbes that find their chief nidus here that are the principal cause of old age and if an antidote to their lethal action could be found Metchnikoff believes life could be very greatly prolonged. He attempts to show that among not only mammals but also birds, the species that have developed the large intestine are less long-lived than those in which it is rudimentary, so that in animals generally its relative size and individual longevity are inversely as each other. Most of the digestive processes are completed before food reaches this terminal part of the long alimentary canal and very little save water can be absorbed through its walls, so that rectal feeding contributes very little, indeed, to the total nutritive needs of the body. But it is here that death finds its chief armamentaria and establishes a receptacle, factory, or laboratory of poisons. Not only are there many microbes that here feed on food residues and occasionally pierce the intestinal walls themselves, but they produce putrefactive products that are still more lethal. The chief of these are phenol and indol, both very complex and due to the breaking down of albuminoids, the chief element in meat, peas, eggs, etc. Young people may for a long time show no trace of the deleterious effects due to the absorption of these toxins, but the slight wear and tear of the tissues they cause is cumulative. They produce in animals old-age effects in kidneys, arteries, liver, lungs, muscles, testes, ovaries, and even in the brain, for senility is due to the action of these bacterial invaders and not to time or to wearing out.