At first sight this might appear to falsify the law of population enunciated by Malthus. Malthus maintained that population tended to increase beyond the means of subsistence; that three checks constantly operated to limit population--vice, misery, and moral restraint: vice, due largely to diseased conditions, misery, due to poverty and want, and moral restraint due to a dread of these. I shall show later that nothing has been said or written to add to or take away from the truth and force of these great principles, but, that the moral restraint of Malthus has been practised to an extent, and in a direction of which the great economist never dreamt. By moral restraint in the limitation of families Malthus meant only delayed marriage. In so far as men and women abstained from, or delayed their marriage, on the ground of inability to support a family, they fulfilled the law, and followed the advice of Malthus. Continence without the marriage bond was assumed; incontinence was classed with another check vice. Contrary to the expectations arising out of the famous progressions, wealth and production have increased and the birth-rate has decreased. It is the purpose of this work to show what are the causes that have led to this decline, that those causes are not equally operative through all classes of the people, and that the chief cause of the decline of the birth-rate is the desire on the part of both sexes to limit the number they have to support and educate. The considerations that lead up to, and, to some extent, justify this desire, will be discussed later. The fact remains that an increasingly large number of people have come to the conclusion that the burden and responsibility of family obligations limit their enjoyments in life, their ambition, and even their scope for usefulness, and have discovered, through the spread of physiological information, means by which marriage may be entered upon without necessarily incurring these responsibilities and limitations. It is the knowledge of these physiological laws and the practice of rules arising out of that knowledge, that account for the declining birth-rate of civilized nations. If it be true that the birth-rate is controlled by a voluntary effort on the part of married people to limit their families, and that that effort implies self restraint and self denial, it would not be too much to claim that those most capable of exercising self-control and with the strongest motives for such exercise, are those most responsible for the declining birth-rate, and that those with least self-control and the fewest motives for exercising the control they have, are most likely to have the normal number of children.