The idea made popular by Metchnikoff, that the body is injured by absorption of toxic substances formed in the intestine through the agency of putrefactive bacteria, rests upon evidence which can hardly be doubted. The beneficial effects resulting from taking a liberal milk diet, especially milk which has been soured by the bacillus acidophilus, are attested by a great many people. A very striking illustration of the manner in which a diet consisting largely of animal tissues modifies the bacterial flora of the intestine, and influences the physical and psychical condition of animals, is given by Herter and Kendall (5). These investigators restricted monkeys to a diet of eggs, and cats to a diet of meat for one or two weeks. They then shifted suddenly to a diet of milk and glucose. Previous studies had shown that eggs or meat encourage the growth of putrefactive organisms in the alimentary tract, whereas milk and glucose stimulates the growth of fermentative and lactic acid-forming organisms, just as does a diet consisting solely of carbohydrates. After one or two weeks on the milk and glucose diet, the animals were returned to the eggs or meat, respectively. These intervals were found sufficiently long to change entirely the character of the bacterial flora of the intestine.

As the proteolytic or putrefying type of bacteria began to predominate, which occurred promptly after restricting the monkeys to eggs, the animals became sleepy and rested their heads upon their hands in a bowed position. They were stupid and responded slowly to external stimuli, took their food very deliberately and manifested little interest in their surroundings. Not infrequently after a hearty meal the animals would spend much time trying to bite the woodwork of the cage. The urine voided was of small volume and highly colored, and amounted to approximately half that produced from the milk-carbohydrate diet. As the protein-digesting bacteria became established, in the intestine the amount of products of putrefaction which appeared in the urine increased markedly.

When the animals were changed to the milk-glucose diet, both the psychical and physical attitude underwent a great change. They no longer held their heads in their hands, but assumed an erect posture and were alert and bright, and showed a keen interest in their surroundings. Their appetites became sharp and the food was consumed with more avidity. The eyes, which were dull and lusterless while on the egg diet, became bright. They no longer attempted to chew the woodwork of the cage. The products of putrefaction of protein almost disappeared from the urine. The evidence from human experience seems sufficient to warrant the conclusion that similar changes in the diet of man induce similar modification in the bacterial flora of the intestinal tract. One may safely conclude that excessive consumption of meat, with but small and insignificant amounts of carbohydrates, tends to promote the generation of toxic substances which induce lethargy in both man and animals.