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136 to it. But a further question arises. Whence does the blood receive these new materials? Evidently from the food and from the oxygen we take into the blood by respiration. Food stuffs are then the source of the energy set free by the muscle.

A very little consideration shows us that animals live on food stuffs that are apparently very different from each other. An ox eats grass; a lion lives upon flesh; a man prefers a mixed diet, such as meat and potatoes. The diets of even the various races of mankind present remarkable differences. The native of Bengal lives largely on rice with a little fat; some Europeans, like the French and Italian peasantry, partake of a diet consisting almost wholly of vegetable substances; the Turkoman, in the steppes of Central Asia, consumes vast quantities of flesh; and the Esquimaux finds that a diet rich in fat best enables him to withstand the rigours of his severe and inhospitable clime. Men have found out by experience what suits them, and no doubt custom or habit has a great deal to do with the selection they make. Can we then compare their dietaries with the view of solving the