by phagocytes to poisoned parts of the body, the poisoning being due to the fermentation in the large intestine. Now it has been observed by some of the German investigators of these matters that the presence of lactic acid interferes with this fermentative process as it goes on in the intestine. Lactic acid, as its name implies, is the characteristic acid which occurs in milk when it becomes sour. An Italian friend of Professor Metchnikoff tried drinking some sour milk with the idea of stopping the fermentation in the intestine, and so putting an end to the deleterious change, and he believes in the short time that he tried it that it did him good—quite, you see, in the way of a patent medicine. Professor Metchnikoff, on this basis, has recommended, in his book on the 'Nature of Man,' the regular drinking of sour milk, in the hope apparently that that will postpone senility, and will leave us our powers in maturity long beyond that period when we at present reach the fullness of our vigor, and advance the period of time when the changes of the years put us out of court. He regards this as an optimistic substitute for the various forms of philosophy and religion which many millions of people have found helpful in life, and certainly it is the cheapest substitute which has ever been seriously proposed.
There is another writer who, though having a German name, is in reality a Russian, Professor Mühlmann. He has another theory in regard to the fundamental nature of senility. He takes such instances as that which I spoke of, of respiration in connection with the production of warmth in the child's body and in the body of the adult, and finds that the diminution of the surface in proportion to the bulk of the body is characteristic of the old, and he concludes that we become old because we do not have proportionately surface enough left. His view implies, apparently, that if we could keep ourselves more or less of the stature of pygmies we should be healthier and better off. I confess these theories, and many others which I might enumerate to you, seem to me to be somewhat fantastic—odd rather than valuable. Yet they all spring from this one common feeling, which is, I believe, a sinister influence upon the thought of the day, in regard to the problem of age—they spring from the medical conception that age is a kind of disease, and that the problem is to explain the condition as it exists in man. Now that is precisely what I wish to protest against. What I hope to accomplish in these lectures is to build up gradually in your minds some acquaintance with the fundamental and essential changes, which are characteristic of age and in regard to which we have been learning something during the last few years—-I might almost say only within recent years—and by means of this exposition to give you a broader view and a juster interpretation of the problem. I hope, before I finish, to convince you that we are already able to establish certain significant generaliza-
