People have heard much of the flora of the digestive tract, until they have come to think with anxiety of the almost infinite number and multitudinous variety of the minute plant life that finds a habitat in the human intestine. Most people think that all of these are, in tendency at least, {96} harmful, and are only kept from being positively dangerous by the overwhelming vital activity of the mucous membrane and the secretions which keep them from exerting their malign activity. Very few appreciate the fact that the intestinal flora, far from being a disturbing factor, are often an aid to digestion, and that the equilibrium established among them favors many biological and chemical processes which help in the preparation of food and in the breaking up of waste products that might be dangerous if reabsorbed during their stay in the intestinal tract. Microbes we have always with us and always will have, and men have lived to round old age, not only in spite of them, but very probably partially because of them. They are part of that beneficent mystery of nature of which as yet, in spite of scientific progress, we know comparatively little. _Opposing Favorable Suggestion_,--A recent striking change of sentiment with regard to one form of food material furnishes a good example of how little we know about the real effect of bacterial life within the digestive tract. There was a time, not so long since, when sour milk was supposed to be especially harmful, or at least only likely to do good to those of particularly strong digestive vitality. Metchnikoff's work on the influence of sour milk on the digestive tract, however, has brought a complete reversal of opinion in this matter. Now most physicians are convinced that the bacillus of sour milk, acts in the intestinal tract to inhibit the reproduction and growth of other, and possibly more disturbing, bacterial agents. Sour milk is looked upon as one of the things that, by neutralizing certain unfortunate bacterial processes in the digestive tract, lead to longevity. There seems no doubt at all, that those who consume a great deal of it, live longer lives than the average, and many old men have taken to its use with a consequent amelioration of digestive annoyances.