In the wild state of animal life in which only the fittest survive in the struggle for existence, every point of advantage has its value. An animal engaged in battle or in a desperate effort to escape will be able to give a better account of itself if it have some means of accelerating the discharge of energy-- some influence like that of pouring oil upon the kindling fire. There is evidence, though perhaps it is not conclusive, that such an influence is exerted by the thyroid gland. In myxedema, a condition characterized by a lack of thyroid secretion, there is dulness of the reflexes and of the intellect, a lowered muscular power, and generally a sluggish discharge of energy. In Graves' disease there is an excessive production of thyroid secretion. In this disease the reflexes are increased, the discharge of energy is greatly facilitated, and metabolism is at a maximum. The same phenomena occur also after the administration of thyroid extract in large doses to normal subjects. In the course of sexual activities there is an increased action of the thyroid, which is indicated by an increase in its size and vascularity. That in fear and in injury the thyroid, in cases of Graves' disease, is probably stimulated to increased activity is indicated by the increased activity of the thyroid circulation, by an increase in the size of the gland, by the histologic appearance of activity in the nuclei of the cells, and by an increase of the toxic symptoms. Finally, Asher has stated that electric stimulation of the nerve supply of the thyroid causes an increased secretion. The origin of many cases of Graves' disease is closely associated with some of the causes of the discharge of nervous energy, depressive influences especially, such as nervous shocks, worry and nervous strain, disappointment in love, business reverses, illness and death of relatives and friends. The association of thyroid activity with procreation is well known, hence the coincidence of a strain of overwork or of fear with the sexual development of maturing girls is obviously favorable to the incidence of Graves' disease. The presence of a colloid goiter is a suitable soil for the development of Graves' disease, and I fully recognize also the evidence that infection or auto-intoxication may be contributing factors and must be assigned their role.