If it be true that our precocious and unhappy old age is due to poisoning of the tissues (the greater part of the poison coming from the large intestine inhabited by numberless microbes), it is clear that agents which arrest intestinal putrefaction must at the same time postpone and ameliorate old age. This theoretical view is confirmed by the collection of facts regarding races which live chiefly on soured milk, and amongst which great ages are common. However, in a question so important, the theory must be tested by direct observations. For this purpose the numerous infirmaries for old people should be taken advantage of, and systematic investigations should be made on the relation of intestinal microbes to precocious old age, and on the influence of diets which prevent intestinal putrefaction in prolonging life and maintaining the forces of the body. It can only be in the future, near or remote, that we shall obtain exact information upon what is one of the chief problems of humanity. In the meantime, those who wish to preserve their intelligence as long as possible and to make their cycle of life as complete and as normal as is possible under present conditions, must depend on general sobriety and on habits conforming to the rules of rational hygiene. PART V PSYCHICAL RUDIMENTS IN MAN I RUDIMENTARY ORGANS IN MAN Reply to critics who deny the simian origin of man—Actual existence of rudimentary organs—Reductions in the structure of the organs of sense in man—Atrophy of Jacobson’s organ and of the Harderian gland in the human race Several critics of _The Nature of Man_ have protested against my theory of the simian origin of man. Some of these found my arguments unsatisfactory and unconvincing. Others have attacked generally my suggestion that some anthropoid had been suddenly transformed to a primitive human being. It is true that so long as we have little palæontological evidence as to the actual descent of man, we cannot discuss the subject without the aid of hypotheses. I think, however, that recent additions to knowledge confirm the theory of the descent of man in a way that ought to influence the most resolute opponents. I have in mind chiefly the arguments supplied by the embryology of anthropoid apes, and by the investigation of their blood. None the less, there are still many authors who maintain their opposition. One of my critics, Dr. Jousset,[150] enumerates certain differences in the structure of the skeleton in man and apes, and concludes that these radically separate man from apes.