Although the intestinal wall in an intact state offers a substantial obstacle to the passage of bacteria, it is incontestable that some of these pass through it into the organs and the blood. Numerous experiments performed on different kinds of animals (horses, dogs, rabbits, etc.) show that some of the microbes taken with food traverse the wall of the alimentary canal and come to occupy the adjacent lymphatic glands, the lungs, the spleen and the liver, whilst they are occasionally found in the blood and lymph. Discussion has taken place as to whether the passage takes place when the wall of the gut is absolutely intact or only when it is injured to however small an extent. It would be extremely difficult to settle the question definitely, but it is easy to see that it has little practical bearing. It is known that the wall of the gut is damaged extremely easily, so that the bluntest sound can hardly be passed into the stomach without making a wound through which microbes can pass into the tissues and blood. In the ordinary course of life, the delicate wall of the gut must often undergo slight wounding, and the frequent presence of microbes in the mesenteric ganglia of healthy animals shows clearly what takes place.[56] It is indubitable, therefore, that the intestinal microbes or their poisons may reach the system generally and bring harm to it. I infer from the facts that the more a digestive tract is charged with microbes, the more it is a source of harm capable of shortening life. As the large intestine not only is the part of the digestive tube most richly charged with microbes, but is relatively more capacious in mammals than in any other vertebrates, it is a just inference that the duration of life of mammals has been notably shortened as the result of chronic poisoning from an abundant intestinal flora. IV MICROBES AS THE CAUSE OF SENILITY Relations between longevity and the intestinal flora—Ruminants—The Horse—Intestinal flora of birds—Intestinal flora of cursorial birds—Duration of life in cursorial birds—Flying mammals—Intestinal flora and longevity of bats—Some exceptions to the rule—Resistance of the lower vertebrates to certain intestinal microbes In the actual state of our knowledge it is impossible to make a final examination of my hypothesis, as there are many factors about which we are incompletely informed. Nevertheless, it is possible to confront the hypothesis with a large number of accurately established facts.