IV THE ARGUMENT FROM CHEMISTRY Having seen in the preceding chapters that man is adapted by nature of his constitution to live upon vegetable foods (meaning by this latter term not only vegetables, but fruits and nuts as well), we must next turn to a consideration of the question as to whether these foods would supply all the necessary elements for the nutrition of the human body. The bodily tissues being in a constant state of flux—worn-out particles of the body being continuously thrown off by means of the various eliminating organs, and fresh material constantly taking their place and being built into living tissue—it is obvious that the nature of this material supplied to the body should be of the best in quality; and that best adapted to maintain its structural integrity. If certain elements are lacking in the food material supplied, these elements will be lacking throughout every stage of the process of digestion, and the tissues ultimately become impoverished because of the lack of them. The chief reason why we eat meat (apart from mere custom), is that it contains a fairly large percentage of proteid—that material from which the muscles are largely built, and which physiologists have lately come to believe is one of the true sources of the bodily energy. Meat being a highly concentrated article of food, and, as before said, containing a large percentage of this proteid, it has always been considered necessary that more or less of it should be consumed in the course of the day in order to offset or replace the wastes necessitated by physical exercise and other causes. Professor Russell H. Chittenden, in speaking of the value of proteid in the human body says: