increasing in weight, they became thin and showed signs of starvation. Schottelius supplied them with food from which bacteria were no longer excluded, upon which the chickens rallied, and soon became completely restored to health. Madame Metchnikoff[304] tried a similar experiment with tadpoles, which, when kept in vessels and fed upon bread containing the usual microbes, developed normally, but which, when reared under conditions entirely free from the presence of microbes, lived on for some months, but in a degenerate condition, their development being arrested. On the other hand, Nuttall and Thierfelder[305] succeeded in keeping alive for several days new-born guinea-pigs, the alimentary canals of which were free from microbes, and which were fed only on absolutely sterilised milk and vegetable matter. Notwithstanding this complete absence of microbes the guinea-pigs developed well. As the two sets of experiments were conducted under conditions arranged so carefully that the chance of error was excluded, it is important to try to reconcile the apparently contradictory results. There is one point common to these three experiments, _i.e._, that they were all executed upon newly born creatures. Now it is well known that at birth the digestive juices are often very imperfectly secreted. In the case of the guinea-pigs, these juices sufficed in quantity for the digestion of the diet provided, whereas in the cases of the chickens and the tadpoles, the digestive juices were incapable of fulfilling their function satisfactorily, and the introduction of microbes endowed with considerable digestive capacity into the intestines compensated for the functional inefficiency of the gastric juices. In addition to the guinea-pigs experimented upon by Nuttall and Thierfelder, there may be mentioned a whole series of lower creatures such as the larvæ of mites and other insects which are able to digest such indigestible material as wax and wool in spite of the total absence of microbes within their intestinal tubes. These experiments are corroborated by the established physiological fact that the gastric and pancreatic juices of mammals easily digest the most varied kinds of foods, even if treated so antiseptically as to ensure the total exclusion of microbes from the intestines.