In the fourth chapter I gave reasons to support my view that the large intestine in mammals had been developed because, by storing the products of digestion, it allowed them to run long distances without stopping, and so was an advantage in the struggle for existence. Moreover, the microbes which abound in the contents of the gut make it possible to use certain substances such as cellulose, that are difficult to digest. But these two advantages do not count in the case of the human race. Man does not secure his prey or escape from his enemies by the rapidity of his locomotion. The great development of his intellectual powers has given him advantages of another kind. Moreover, by the use of cooking and the cultivation of plants of high nutritive value, he is able to dispense with the digestion of cellulose. There is another side to the picture. Ignorant of death and of old age, mammals have acquired the advantages of a large intestine at the expense of longevity. I have already stated that birds live longer than mammals. Birds are practically devoid of a large intestine, and maintain a bacterial flora very much poorer than that found in mammals. There is one exception to this rule, an exception of great importance. Ostriches and their allies, the largest known birds, are characterised by absence of the power of flight and by rapidity of terrestrial locomotion by which they escape their enemies. These are the only birds in which the large intestine is well-developed. The duration of life is much less in their case than in that of smaller birds, such as parrots, ravens, and swans. According to M. Rivière, who has been engaged in ostrich farming in Algeria, these large birds do not live more than thirty-five years. The mode of life, and the shorter duration of life, the huge development of the large intestines and the rich bacterial flora found therein make the ostriches much more like mammals than birds. It is to be noticed that many birds in which the duration of life is long do not possess a cæcum, the portion of the alimentary canal that contains most bacteria. Examination of the intestinal contents of parrots shows that there exist in these birds very few microbes. A comparative study shows plainly that the existence of an abundant intestinal flora, useless for digestion, helps to shorten life by producing bacterial poisons which weaken the higher elements and strengthen the phagocytes. The human race has inherited from its ancestors an enormous large intestine and conditions favourable to the life of bacteria. It has to endure the disadvantages of this heritage. On the other hand, the brain of man is very highly developed, and with the increase of intellectual power has come a consciousness of old age and death. Our strong will to live is opposed to the infirmities of age and the shortness of life. Here lies the greatest disharmony of the constitution of man.