The cycle of greatest kinetic energy for physiologic ends is the period of reproduction. In the female especially there is a cycle of increased activity just prior to her development into the procreative state. During this time secondary sexual characters are developed-- the pelvis expands, the ovaries and the uterus grow rapidly, the mammary glands develop. Again in this period of increasing speed in the expenditure of energy we find the thyroid, the adrenals, and the hypophysis also in rapid growth. Without the normal development of the ovary, the thyroid, and the hypophysis, neither the male nor the female can develop the secondary sexual characters, nor do they develop sexual desire nor show seasonal cycles of activity, nor can they procreate. The secondary sexual characters--sexual desire, fertility--may be developed at will, for example, by feeding thyroid products from alien species to the individual deprived of the thyroid. At the close of the child-bearing period there is a permanent diminution of the speed of energy discharge, for energy is no longer needed as it was for the self-preservation of the offspring before adolescence, and for the propagation of the species during the procreative period. Unless other factors intervene, this reduction in speed is progressive until senescent death. The diminished size of the thyroid of the aged bears testimony to the part the activating organs bear in the general decline. We have now referred to variations in the rate of discharge of energy in different species; in individuals of the same species; in cycles in the same individual--such as the seasons of food supply, the periods of wakefulness and of sleep, the procreative period, and we have spoken of those variations caused artificially by thyroid feeding, thus far having confined our discussion to the conversion for adaptive purposes of latent into kinetic energy in muscular and in procreative action. We shall now consider the conversion of latent into kinetic energy in the production of heat,[*] and endeavor to answer the questions which arise at once: Is there one mechanism for the conversion of latent energy into heat and another mechanism for its conversion into muscular action? What is the adaptive advantage of fever in infection? [*] We use the terms "heat" and "muscular action" in the popular sense, though physicists use them to designate one and the same kind of energy. The Purpose and the Mechanism of Heat Production in Infections