"Science is the great instrument of social change," wrote A. J. Balfour in 1908; "all the greater because its object is not change but knowledge, and its silent appropriation of this dominant function, amid the din of religious and political strife, is the most vital of all revolutions which have marked the development of modern civilization." The Birth Control movement has allied itself with science, and no small part of its present propaganda is to awaken the interest of scientists to the pivotal importance to civilization of this instrument. Only with the aid of science is it possible to perfect a practical method that may be universally taught. As Dean Inge recently admitted: "We should be ready to give up all our theories if science proved that we were on the wrong lines." One of the principal aims of the American Birth Control League has been to awaken the interest of scientific investigators and to point out the rich field for original research opened up by this problem. The correlation of reckless breeding with defective and delinquent strains, has not, strangely enough, been subjected to close scientific scrutiny, nor has the present biological unbalance been traced to its root. This is a crying necessity of our day, and it cannot be accomplished without the aid of science. Secondary only to the response of women themselves is the awakened interest of scientists, statisticians, and research workers in every field. If the clergy and the defenders of traditional morality have opposed the movement for Birth Control, the response of enlightened scientists and physicians has been one of the most encouraging aids in our battle. Recent developments in the realm of science,--in psychology, in physiology, in chemistry and physics--all tend to emphasize the immediate necessity for human control over the great forces of nature. The new ideas published by contemporary science are of the utmost fascination and illumination even to the layman. They perform the invaluable task of making us look at life in a new light, of searching close at hand for the solution to heretofore closed mysteries of life. In this brief chapter, I can touch these ideas only as they have proved valuable to me. Professor Soddy's "Science and Life" is one of the most inspiring of recent publications in this field; for this great authority shows us how closely bound up is science with the whole of Society, how science must help to solve the great and disastrous unbalance in human society.