Not only is there a variation in the rate of output of energy among various species of animals, but one finds also variations in the rate of output of energy among individuals of the same species. If our thesis that men and animals are mechanisms responding to environmental stimuli be correct, and further, if the speed of energy output be due to changes in the activating organs as a result of adaptive stimulation, then we should expect to find physical changes in the activating glands during the cycles of increased activation. What are the facts? We know that most animals have breeding seasons evolved as adaptations to the food supply and weather. Hence there is in most animals a mating season in advance of the season of maximum food supply so that the young may appear at the period when food is most abundant. In the springtime most birds and mammals mate, and in the springtime at least one of the great activating glands is enlarged--the thyroid in man and in animals shows seasonal enlargement. The effect of the increased activity is seen in the song, the courting, the fighting, in the quickened pulse, and in a slightly raised temperature. Even more activation than that connected with the season is seen in the physical state of mating, when the thyroid is known to enlarge materially, though this increased activity, as we shall show later, is probably no greater than the increased activity of other activating glands. In the mating season the kinetic activity is speeded up; in short, there exists a state--a fleeting state--of mild Graves' disease. In the early stages of Graves' disease, before the destructive phenomena are felt, the kinetic speed is high, and life is on a sensuous edge. Not only is there a seasonal rhythm to the rate of flow of energy, but there is a diurnal variation--the ebb is at night, and the full tide in the daytime. This observation is verified by the experiments which show that certain organs in the kinetic chain are histologically exhausted, the depleted cells being for the most part restored by sleep. We have seen that there are variations in speed in different species, and that in the same species speed varies with the season of the year and with the time of day. In addition there are variations also in the rate of discharge of energy in the various cycles of the life of the individual. The young are evolved at high speed for growth, so that as soon as possible they may attain to their own power of self-defense; they must adapt themselves to innumerable bacteria, to food, and to all the elements in their external environment. Against their gross enemies the young are measurably protected by their parents; but the parents--except to a limited extent in the case of man--are unable to assist in the protection of the young against infectious disease.