Rheumatism, gout, and infectious diseases only play a secondary part in setting up arterial sclerosis. Edgren asserts, as the result of very careful calculation, that in nearly every fifth case he found it was impossible to account for the origin of arterial sclerosis. In the majority of cases the sufferers were elderly persons who, according to Edgren, “were afflicted with physiological sclerosis.”[301] I take it that this sclerosis of unknown origin was by no means physiological but was pathological like that set up by syphilis or alcoholism. The question then arises whence comes the poison in such cases? In syphilis there is a virus of a definite nature to deal with, which causes infection or poisoning, and brings about arterial sclerosis, general paralysis, and other serious ailments. Alcoholism is a poison arising from fermentation, excited by microscopical fungi related to true microbes. Instances of arterial sclerosis which are due neither to syphilis nor to alcohol poisoning nor to any other known cause, can only be accounted for as probably arising from poisoning set up by the mass of microbes congregated in the human intestines. Among these microbes there may be some that are harmless, and possibly even beneficial, but there are undoubtedly a great number the presence of which is extremely prejudicial to health and life. It is impossible to enter into the details of such an important question, and a brief mention must suffice. The human intestine contains an enormous quantity of bacteria, which, according to the recent investigations of Strassburger,[302] increase at the rate of 128,000,000,000,000 each day. These microbes, of which there are few in the digestive portion of the alimentary canal, are very numerous in the large intestines, _i.e._, in the lower part containing the waste material. The remains of undigested foods and the mucous secretions form a medium very favourable to the growth of microbes. This bacterial flora constitutes a third part of the human excreta. It is very varied, and contains an immense number of different species, among which are bacilli, cocci, and many kinds of other bacteria, about which little is known. The distribution of this bacterial flora shows that it contributes nothing to the well-being of man, being scanty in the digestive portions of the body, and abundant in other parts of the gut. This fact alone suffices to refute the theory of those who attribute great functional importance to the intestinal flora. This theory originated principally from the fact that certain animals perish when brought up under special conditions protecting them from the presence of microbes. Schottelius[303] was the first to try the experiment of rearing chickens in a cage specially constructed for this purpose. The chickens hatched out, and lived for a few weeks: then, there being no microbes within them and only sterilised food being given, instead of