Obesity may be hereditary; that is to say, the father or the mother may transmit to their children a peculiar formation, having a tendency to make fat. From certain physiological conditions, we may recognize at an early age a natural tendency on the part of some persons to become corpulent. In the young of both sexes, where this predisposition exists, the face is broad and short, the eyes round, and the nose short and thick; the hands and feet are small, and there is a general roundness of limb. When possessed of such an organization, obesity may be warded off by a rational system of diet, to be indicated in the following pages. But the immediate and producing cause of corpulence is to be sought and discovered in the character of the food. The present system is founded upon this principle. Medical authors assert that food has a most important bearing in the production of corpulence. They forbid the use of meat, and recommend watery vegetables, such as spinage, sorrel, salad, fruit, &c., and for beverage water; and at the same time they direct the patient to eat as little as possible. These instructions, like too many others, are given because they are asked, and that in every disease, curable or incurable, the physician is bound to offer some advice. Medical men themselves put no faith in them, since they pronounce obesity to be incurable. Having devoted a great deal of attention to this enquiry, I have arrived at the conclusion that it is not to be wondered at that obesity should be incurable, because the very means which have been recommended to overcome it, are exactly those best fitted to induce and maintain it. I lay it down as an axiom, in opposition to the received opinion of centuries, that a very substantial diet, such as meat, does not develop fat, and that nothing is more capable of producing the latter than aqueous vegetables and water. It is a principle which at first sight may appear inadmissible. Nevertheless, the consideration of a few physiological and chemical facts, within the comprehension of everybody, will suffice to prove its correctness.