Metchnikoff made some laboratory animals ingest this microbe together with food, and ascertained that it reached the large intestine and decomposed in it the starch of farinaceous food into sugar, of which the acid products prevented the swarming of putrefying microbes. By this process it is possible to reduce to a minimum and even sometimes to eliminate the production of phenol and indol in rats subjected to a mixed diet and made at the same time to ingest cultures of the lactic bacillus and of the glycobacter. Metchnikoff applied these different diets to himself and to other individuals and obtained concordant results. However, he ascertained that it is not only the food diet which regulates the quantity of microbian poisons contained in the organism; that quantity sometimes varies very much in spite of an identical diet. He thought that a very important part of influence is due to pre-existing microbes which prevent or favour the development of microbes of putrefaction. All these questions, complicated by the richness and variety of the intestinal flora, still demanded a long series of laborious researches. At the end of the winter he felt tired, and we went to the seaside during the holidays. But the sharp sea air did not suit him; he had a beginning of cardiac asthma and nearly fainted during a walk. We therefore had to come away from the sea, and went inland, to Eu. At the beginning of our stay, Metchnikoff did not feel well, walking tired him, he suffered from cardiac intermittence; it was only gradually that his condition improved and he was able to write the preface to a Russian edition of his philosophical articles. This book was entitled _Forty Years' Search for a Rational Conception of Life_, and the articles record the evolution of his ideas and his search "not only for a rational understanding of life, but also for the solution of the problem of death, which is so full of contradictions." This collection of articles enables us at the same time to follow the gradual transition from the pessimism of his youth to the optimism of his maturity. His first writings[28] relate to the discords of human nature and the lack of a solid basis for morals. [28] _Education from an Anthropological Point of View_, _The Matrimonial Age_, _The Conception of Human Nature_, _The Struggle for Existence in a General Sense_. See Bibliography. But, already in 1883, he concluded an opening _Causerie_ at the Naturalists' Congress in Odessa, by the following words: "The theoretical study of natural history problems, in the widest sense of the word, alone can give a sound method for the comprehension of truth and lead to a definite conception of life--or at least to an approach to it." Another article, _The Curative Forces of the Organism_, sums up his phagocyte theory, and states the fact that the organism possesses special powers of struggle against enemy elements.