These facts are only a special case of the general rule that small doses of poisons increase the activity of the elements that are killed by large doses. In order to increase the activity of the heart, medical men give successfully small doses of cardiac poisons such as digitalis. As a commercial process, the activity of yeasts is increased by submitting them to weak doses of substances (fluoride of sodium) which, given in larger quantities, would kill them. My general conclusion from these facts is that it is logical to lay down the principle that the higher elements of our body could be strengthened by subjecting them to the action of small doses of the appropriate cytotoxic serums. There is, however, much difficulty in putting this into practice. It is quite easy to obtain human blood to inject into animals with the object of preparing a serum which can increase the number of red corpuscles. On the other hand, it is extremely difficult to get human bodies sufficiently fresh to use them for a practical purpose. According to law, _post mortem_ examinations can be made only after an interval of time in course of which the tissues have changed; besides, the organs obtained in this way are frequently affected by injuries or diseases militating against their use. Even in Paris, with its three million inhabitants, it is extremely rare that there is a good opportunity for the preparation of human cytotoxic serums. In two or three years, during which Dr. Weinberg has collected the organs from human bodies fairly fresh, he has been unable to obtain sufficiently active serums. The best results have been obtained from new-born infants which have been killed by some accident in the process of child-birth, as in them the organs are in a normal state. However, owing to the advance in the practice of obstetrics, such accidents, already infrequent, are becoming extremely rare. In such conditions we may have to wait long before getting a positive result, unless the future will find some method of obtaining the necessary materials for this difficult and interesting purpose. As it is so difficult to prepare a remedy which can strengthen the weakened higher elements of the body, it may be easier to find a means of preventing the weakening which interferes so much with our desire to live long. As the products of microbes are the most active agents in deteriorating our tissues, we must look towards them for the solution of the problem. IV INTESTINAL PUTREFACTION SHORTENS LIFE Uselessness of the large intestine in man—Case of a woman whose large intestine was inactive for six months—Another case where the greater part of the large intestine was completely shut off—Attempts to disinfect the contents of the large intestine—Prolonged mastication as a means of preventing intestinal putrefaction