Hence we see that the phenomena of the emotions may within certain limits be increased, diminished, or abolished by increasing, diminishing, or totally excluding the secretion of the thyroid gland. Graves' disease may be increased by giving thyroid extract and by fear. It may be diminished by removing a part of the gland, or by interrupting the blood and nerve supply, or by complete rest. In addition, at some stage of Graves' disease there is an increase in the size and in the number of the secreting cells. These facts regarding the normal and the pathologic supply of thyroid secretion point to this gland as one of the sources of the energizing substance or substances, by means of which the motor phenomena of animals are executed and their emotions expressed. Anger is similar to fear in origin and, like fear, is an integration and stimulation of the motor mechanism and its accessories. Animals which have no natural weapons for attack experience neither fear nor anger, while the animals which have weapons for attack express anger principally by energizing the muscles used in attack. Although, as has already been stated, the efficiency of the hands of man has largely supplanted the use of the teeth, he still shows his teeth in anger and so gives support to the theory that this emotion is of remote ancestral origin and proves the great persistence of phylogenetic association. On this conception we can understand why it is that a patient consumed by worry--which to me signifies interrupted stimulation, a state of alternation between hope and fear--suffers so many bodily impairments and diseases even. This hypothesis explains the slow dying of animals in captivity. It explains the grave digestive and metabolic disturbances which appear under any nerve strain, especially under the strain of fear, and the great benefits of confidence and hope; it explains the nervousness, loss of weight, indigestion--in short, the comprehensive physical changes that are wrought by fear and by sexual love and hate. On this hypothesis we can understand the physical influence of one individual over the body and personality of another; and of the infinite factors in environment that, through phylogenetic association, play a role in the functions of many of our organs. It is because under the uncompromising law of survival of the fittest we were evolved as motor beings that we do not possess any organs or faculties which have not served our progenitors in accomplishing their survival in the relentless struggle of organic forms with one another. We are now, as we were then, essentially motor beings, and the only way in which we can meet the dangers in our environment is by a motor response. Such a motor response implies the integration of our entire being for action, this integration involving the activity of certain glands, such as the adrenals (Cannon), the thyroid, the liver, etc., which throw into the blood-stream substances which help to form energy, but which,