Adrenalin alone, thyroid extract alone, brain activity alone, and muscular activity alone are capable of causing the body temperature to rise above the normal. The functional activity of no other gland of the body alone, and the secretion of no other gland alone, can cause a comparable rise in body temperature--that is, neither increased functional activity nor any active principle derived from the kidney, the liver, the stomach, the pancreas, the hypophysis, the parathyroids, the spleen, the intestines, the thymus, the lymphatic glands, or the bones can, _per se_, cause a rise in the general body temperature comparable to the rise that may be caused by the activity of the brain or the muscles, or by the injection of adrenalin or thyroid extract. Then, too, when the brain, the thyroid, the adrenals, the liver, or the muscles are eliminated, the power of the body to convert latent into kinetic energy is impaired or lost. I shall offer evidence tending to show that an excess of either internal or external environmental stimuli may modify one or more organs of the kinetic system, and that this modification may cause certain diseases. For example, alterations in the efficiency of the cerebral link may yield neurasthenia, mania, dementia; of the thyroid link, Graves' disease, myxedema; of the adrenal link, Addison's disease, cardiovascular disease. This introduction may serve to give the line of our argument. We shall now consider briefly certain salient facts which relate to the conversion of latent into kinetic energy as an adaptive reaction. The experimental data are so many that they will later be published in a monograph. The amount of latent energy which may be converted into kinetic energy for adaptive ends varies in different species, in individuals of the same species, in the same individual in different seasons; in the life cycle of growth, reproduction and decay; in the waking and sleeping hours; in disease and in activity. We shall here consider briefly the reasons for some of those variations and the mechanisms which make them possible. Biologic Consideration of the Adaptive Variation in Amounts of Energy Stored in Various Animals