Psychological observations allowed Metchnikoff to conclude that pessimism is much more frequent in youth than in maturity or in old age. He attributes this to the gradual development of the _vital instinct_ which is only completely manifested in middle age. Man then begins to appreciate life; made wiser by experience, he demands less and is therefore better balanced. Metchnikoff proffers examples in support of his theory. He analyses the psychic evolution of Goethe as reflected in his _Faust_ and describes that of "an intimate friend." These examples prove that natural psychological evolution already leads to a relative optimism. But, as long as senility is pathological and death premature, the apprehension that they inspire antagonises the normal evolution of optimism. A victory over those present evils will direct the normal course of life in the right way; one normal active period will succeed another; the accomplishment of individual and social functions corresponding with each period will become realisable; the death instinct will have time to develop, and Man, having been through his normal vital cycle, will sink, peacefully and without fear, into eternal sleep. CHAPTER XXVIII Researches on intestinal flora--Sour milk. The problem of our intestinal flora is so vast and so difficult that it demands years of research. Numerous facts had already been accumulated by Science on this subject, but it was still far from being elucidated. Certain scientists affirmed that microbes favour digestion by decomposing food residues in the intestine and are therefore not merely useful, but necessary to the organism. Others entertained a diametrically opposed opinion. The first thing, therefore, was to know which of the two opinions was founded on fact. Metchnikoff studied the case of the bat, in which the digestive tube is short and the large bowel not even differentiated. As he had supposed, _a priori_, in this animal, whose life duration is relatively long, the intestine contains few or no micro-organisms, which proves that digestion can be accomplished without their intermediary. Moreover, this was before long amply confirmed by the researches of MM. Cohendy, Wollman, and other scientists who succeeded in bringing up chickens and tadpoles in conditions of absolute sterility. Having acquired the conviction that microbes are not indispensable to digestion, Metchnikoff studied the part they play in the organism. It is universally admitted that the products of putrefaction are toxic, and he enquired whether the intestine sheltered putrefying microbes. This question had not yet been solved; certain bacteriologists thought that little or no putrefaction exists in a normal intestine. Metchnikoff ascertained through systematic researches that the intestinal flora includes several kinds of putrefying microbes which secrete highly toxic products.