492 POPULAR SCIENCE MONTHLY 



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 opti- 

 mistic substitute for the various forms of philosophy and religion 

 which many millions of people have found helpful in life, and cer- 

 tainly 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 Eussian, Professor Muhlmann. He has another theory in 

 regard to the fundamental nature of senility. He takes such in- 

 stances 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 medi- 

 cal 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 lec- 

 tures 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 

 vou that we are alreadv able to establish certain significant generaliza- 



