Already some time previously, theoretical considerations on senile atrophies had directed his thoughts towards old age. His reflections now turned towards the psychological aspect of the problem; he analysed his personal sensations and realised that he, at the age of 53, felt an ardent desire to live. This imperious instinct for life, in spite of the inevitable evolution towards personal death and old age, brought his thoughts back to the disharmonies of human nature. But now, through all his gloomy reflections, he was borne up by the unshakable conviction that Science would succeed in correcting those disharmonies and he continued to work with untiring energy. He had prescribed for himself a hygienic diet, based on the idea that the cause of his own condition and senility in general was due to a chronic poisoning by intestinal microbes. This diet consisted in avoiding raw food in order not to introduce noxious microbes into the intestines, and in absorbing their useful enemies, the acid-forming microbes of sour milk. This diet was very favourable to his health. After he had finished his book on immunity he at last allowed himself to pass on to the new questions which preoccupied him, _i.e._ senility and death. He set forth a sketch of his ideas in 1901 in a paper which he read at Manchester (Wilde Lecture) on the "Flora of the Human Body." He reviewed this flora and pointed out the harmful effect of the microbes, especially those of the large intestine the toxins of which effect a chronic poisoning of the cells of our organism and thus provoke their gradual weakening. He then indicated the means of combating this evil, on the one hand by stimulating the vital activity of the cells exposed to enfeeblement, by means, for instance, of small doses of specific cytotoxins, and, on the other hand, by direct action on intestinal microbes. He concluded by saying that "the intestinal flora is the principal cause of the too short duration of our life, which flickers out before having reached its goal. Human conscience has succeeded in making this injustice obvious; Science must now set to work to correct it. It will succeed in doing so, and it is to be hoped that the opening century will witness the solution of this great problem." Metchnikoff considered that our chronic poisoning by intestinal microbes weakens our cellular elements; he supposed that the same cause might provoke senile phenomena, manifestly due to weakness of the tissues.