It is well known that endurance, both physical and mental, is one of the most variable of the human faculties, and it is usually considered that exercise or training is the chief cause of the differences so frequently seen. The Maine guide will row a boat or paddle a canoe for the entire day without undue fatigue, while the novice, though he may have the necessary strength, lacks the endurance to continue the task longer than a few hours. As expressed by Professor Fisher, “Some persons are tired by climbing a flight of stairs, whereas the Swiss guides, throughout the summer season, day after day spend the entire time in climbing the Matterhorn and other peaks; some persons are ‘winded’ by running a block for a street car, whereas a Chinese coolie will run for hours on end; in mental work, some persons are unable to apply themselves more than an hour at a time, whereas others, like Humboldt, can work almost continuously through eighteen hours of the day.” Again, Fisher states that “among some 75 tests of different persons holding their arms horizontal, many were found whose arms actually dropped against their will inside of 10 minutes, whereas several were able to hold them up over 1 hour, and one man held them 3 hours and 20 minutes, or a round 200 minutes, and then dropped them voluntarily. Similarly with deep knee-bending, some persons were found physically unable to rise again from the stooping posture after accomplishing less than 500 bendings, whereas several succeeded in stooping 1000 times, and in one case, 2400.” Here, we have inherent differences in endurance not associated with training or exercise, and the question may well be asked, What is the cause of these radical variations in the ability to repeat a simple muscular exertion? Hitherto, little attention has been paid to the possible influence of diet upon this faculty. It has always been assumed that endurance, like physical strength, is augmented by a rich proteid diet, but it has never been considered that diet by itself was a factor of any great moment as compared with training or persistent exercise. It is true that claims have been advanced from time to time concerning the beneficial effects on endurance of a vegetable diet, and vegetarians have frequently presented glowing reports of the great increase in endurance they have experienced, but little attention has been given to such statements, and the matter has remained more or less in obscurity.