"Eugenics," wrote Francis Galton, who founded the science and coined the name, "is the study of agencies under social control that may improve or impair the racial qualities of future generations, either physically or mentally." The definition is universally accepted, but by its use of the word "study" it defines a pure science, and the present book is concerned rather with the application of such a science. Accepting Galton's definition, we shall for our purposes slightly extend it by saying that applied eugenics embraces all such measures, in use or prospect either individually or collectively, as may improve or impair the racial qualities of future generations of man, either physically or mentally, whether or not this was the avowed purpose. It is one of the newest of sciences. It was practically forced into existence by logical necessity. It is certainly here to stay, and it demands the right to speak, in many cases to cast the deciding vote, on some of the most important questions that confront society. The science of eugenics is the natural result of the spread and acceptance of organic evolution, following the publication of Darwin's work on _The Origin of Species by Means of Natural Selection_, in 1859. It took a generation for his ideas to win the day; but then they revolutionized the intellectual life of the civilized world. Man came to realize that the course of nature is regular; that the observed sequences of events can be described in formulas which are called natural laws; he learned that he could achieve great results in plant and animal breeding by working in harmony with these laws. Then the question logically arose, "Is not man himself subject to these same laws? Can he not use his knowledge of them to improve his own species, as he has been more or less consciously improving the plants and animals that were of most value to him, for many centuries?" The evolutionist answered both these questions affirmatively. However great may be the superiority of his mind, man is first of all an animal, subject to the natural laws that govern other animals. He can learn to comply with these laws; he can, therefore, take an active share in furthering the process of evolution toward a higher life.