Surveying insanity as a whole, one recognises as a fundamental fact that insane persons belong to a class who start life with a “deficient grade of organisation” of the nervous system called “hereditary predisposition.” Some authorities hold that this is the sine quâ non of insanity, but such a view, I consider, is not strictly accurate. It is certainly not borne out by statistics. All observers, using even indifferent discrimination, must have noticed cases in which no hereditary factor could be traced, but in which much self-abuse had occurred—either in the form of alcoholic, sexual, and other excesses—or where syphilis or other powerful toxæmic conditions had been contracted. Such conditions, I argue, may themselves break down the most hardy constitution and leave it a prey to secondary infections or intoxications, which may manifest themselves as insanity. Assuming that the insane, prior to their attack of mental disease, suffer from either some hereditary weakness, or some acquired constitutional degeneration, it is most probable that such defects act a dual part, weakening not only the nervous system, so that it is more susceptible to the actions of toxins and environments, but also weakening the natural defences of the body. The nervous system of these people is thus laid open to more severe and frequent attacks from poisonous substances, whether of bacterial, metabolic, or other sources, than is the nervous system of a more normally organised individual. In support of this statement I would mention that Dr. L. C. Bruce has pointed out that over 60 per cent. of maniacal patients were deficient in the normal protective agglutinin to certain strains of Staphylococcus aureus. This agglutinin is always present in healthy sera. Further, Dr. C. J. Shaw has ably demonstrated that the reason tubercular diseases account for so large a proportion of the deaths in asylums lies, not in any faulty hygienic precautions, but in the fact that the resistive power of the insane to tubercular infection is below par. Assuming that this hypothesis is true, we can more readily understand why the various insanities so frequently resist our efforts to cure them.