Certain it is that in any group the constituent minds are controlled at a given time by some one idea common to all. This is, in one sense, a perversion of the intellect. The dominant idea assumes a magnitude out of proportion to its actual value; and by this disproportion—that is, by the undue attention it receives, others, often of equal or greater value to the group, are neglected. These dominant ideas form the national ideals, after which the individual lines are consciously patterned, and by the practical application thus given, add to the cohesion of the group through the unification of its members. Acting under natural laws, common to organic forms as well as to societies, these ideas are the chief agents in social selection, and thus control almost absolutely the traits and destinies of nations, as has been traced in a masterly manner by Vacher de Lapouge. Such ideas are easily recognised in a community. A slight acquaintance with history and literature teaches us that the early Romans were exclusively possessed by the military ideal, the lust of conquest; that the ideal of the Israelites has always been the thirst for commercial gain; and that art was the ruling aim in the palmy days of Greece. But the finest example that occurs to me of many different peoples being dominated by a fixed idea is seen in the votaries of the Mohammedan religion. They are bound together by one sacred language, in which one book, the Koran, lays down all law, civil, criminal, and ecclesiastical, and the expressed dicta of which set them in sharpest opposition to all who do not accept it. The religious idea, thus stimulated out of all proportion to others, has developed in them a fanatical force which at one time almost enabled them to conquer the known world, and which has since resulted in the inevitable decay of their greatest states, their literature and arts. II. PSYCHOPATHIC CONDITIONS IN THE EMOTIONAL LIFE.—Apart from the perversions of intelligence which cloud the reasoning faculties of nations, they are subject to widespread and persistent disturbances of their emotional lives, which frequently react disastrously on the common weal. Following the division adopted by some competent alienists in individual cases, I may with propriety classify these into two divisions, as they represent, on the one hand, excessive, misdirected, and morbid activity, or, on the other, unhealthy depression and apathy. 1. CONDITIONS OF HYPERSTHENIA.—It is a popular error in scientific circles that diseases of the nervous system increase with civilisation. The opposite is true. The lowest stages of culture are far more pathological than the higher, in this, as well as in most respects. True that certain neuroses belong to cultured peoples; but morbid emotional states are especially prevalent in lower conditions.