In all defectives, in epileptics, idiots, the physical deformed, the insane, and the criminal, the prudence and self-restraint necessary to the limitation of families is either partially or entirely absent. To the poor in crowded localities, with limited room-space and insanitary surroundings, effective self-restraint is more difficult than in any other class of society. In all defectives the sexual instinct is as strong, if not stronger, than in the normal, and they have not that interest in life, and regard for the future that suggest restraint, nor have they the power to practise it though prudence were to guide them. The higher checks to population, as they exist among the better classes of people, do not obtain amongst the defectives taken as a class. Vice and misery are more active checks amongst the very poor, and abortion is practised to a very considerable extent, but the appalling fact remains, that the birth-rate of the unfit goes on undisturbed, while the introduction of higher checks amongst the normal classes has led to a marked decline, more marked than at first sight appears. The worst feature of the problem, however, is not so much the disproportion in the numbers born to the normal and the abnormal respectively, but the fact that the defectives propagate their kind. The defectives, whose existence and whose liberty constitute the greatest danger to the State, are the intermittent inhabitants of our lunatic asylums, prisons, and reformatories. There is one defect common to all these, and that is defective inhibition. All human activity is the result of two forces, motor impulses tending to action, and inhibition tending to inertia. The lower animals have strong motor impulses constantly exploding and expressing themselves in great activity, offensive, defensive, self-preservative, and procreative, being restrained only by the inhibitive forces of their conditions and environment. Children have strong motor impulses, which are at first little controlled. Inhibition is a late development and is largely a result of education. If the motor impulses remain strong, or become stronger in the presence of development with exercise, while inhibition remains weak, we have a criminal. Inhibition is the function performed by the highest and last-formed brain-cells. These brain cells may be undeveloped either from want of exercise, that is, education, or from hereditary weakness, or, having been developed may have undergone degeneration, under the influence of alcohol, or from hereditary or acquired disease. Motor impulses, as the springs of action, are common to all animals. In the lower animals inhibition is external, and never internal or subjective. In man it may be internal or external. It is internal or subjective in those whose higher brain centres are well developed and normal. Their auto-inhibition is such that all their motor impulses are controlled and directed in the best interests of society.