The series of facts that I have been discussing strengthens my conviction that the intestinal flora is an extremely important factor in the causation of senility. It must not be supposed, however, that all the known facts can be explained equally easily on this hypothesis. The harm done by microbes cannot always be measured by their abundance in the alimentary canal. In the first place, it must be remembered that some microbes are useful; moreover, microbes, even although their products are very dangerous, may exist in quantities in an organism, and yet do no harm if the organism has the power of resisting bacterial poisons. Thus, for instance, the bacillus of tetanus, which thrives in the alimentary canal, and which can endanger life if the wall of the gut is wounded, does not harm a crocodile or a tortoise, as these animals are extremely resistant to the poison of tetanus. Dr. Favorsky, by experiments at the Pasteur Institute, has shown that the poison of botulism can be absorbed with impunity by some birds, and by tortoises, although death follows if a very small quantity of it be introduced into the alimentary canal of a mammal. The bodies of man and of higher animals are possessed of a complex mechanism which resists the harmful action of bacteria and their poisons. The various parts of this mechanism may act differently, with the result that there is great variation in the power of resistance. Thus, however abundant microbes may be in the intestine, they may bring little harm to an organism that has a high power of destruction or neutralisation of the toxins, or when these harmful products are unable to pass through the intestinal wall. It is in this way that I explain some exceptions to the general rule, which are exceptions only in appearance. Such a case is that of the nocturnal birds of prey. Although the diurnal birds of prey (eagles, vultures, etc.) have very short cæca, in which the food is never found, owls have very large cæca, which may be as long as 10 cm. (Eagle-Owl, _Bubo maximus_). These long cæca, however, contain debris of the food only in the enlarged terminal portion, and the food masses contain a very small number of microbes. Notwithstanding a great difference in the length of the cæca between the owls and the eagles, these two groups of birds do not differ greatly in longevity. But the difference in the cæca does not imply a corresponding difference in the intestinal flora which appears to be very scanty in both cases. It is possible that the elephant is a more real exception to the rule. Here is a case of a mammal with an enormous large intestine and a capacious cæcum, and which none the less is capable of surviving for a century. I have had no opportunity of investigating the elephant from this point of view, and have no explanation to suggest.