The science of eugenics consists of a foundation of biology and a superstructure of sociology. Galton, its founder, emphasized both parts in due proportion. Until recently, however, most sociologists have been either indifferent or hostile to eugenics, and the science has been left for the most part in the hands of biologists, who have naturally worked most on the foundations and neglected the superstructure. Although we are not disposed to minimize the importance of the biological part, we think it desirable that the means of applying the biological principles should be more carefully studied. The reader of this book will, consequently, find only a summary explanation of the mechanism of inheritance. Emphasis has rather been laid on the practical means by which society may encourage the reproduction of superior persons and discourage that of inferiors. – From the preface.