We assume that in general, a eugenically superior or desirable person has, to a greater degree than the average, the germinal basis for the following characteristics: to live past maturity, to reproduce adequately, to live happily and to make contributions to the productivity, happiness, and progress of society. It is desirable to discriminate as much as possible between the possession of the germinal basis and the observed achievement, since the latter consists of the former plus or minus environmental influence. But where the amount of modification is too obscure to be detected, it is advantageous to take the demonstrated achievement as a tentative measure of the germinal basis. The problem of eugenics is to make such legal, social and economic adjustments that (1) a larger proportion of superior persons will have children than at present, (2) that the average number of offspring of each superior person will be greater than at present, (3) that the most inferior persons will have no children, and finally that (4) other inferior persons will have fewer children than now. The science of eugenics is still young and much of its program must be tentative and subject to the test of actual experiment. It is more important that the student acquire the habit of looking at society from a biological as well as a sociological point of view, than that he put his faith in the efficacy of any particular mode of procedure. The essential points of our eugenics program were laid down by Professor Johnson in an article entitled "Human Evolution and its Control" in the _Popular Science Monthly_ for January, 1910. Considerable parts of the material in the present book have appeared in the _Journal of Heredity_. Helpful suggestions and criticism have been received from several friends, in particular Sewall Wright and O. E. Baker of the United States Department of Agriculture. PAUL POPENOE. WASHINGTON, _June, 1918._ TABLE OF CONTENTS PAGE PREFACE v INTRODUCTION BY EDWARD A. ROSS xi CHAPTER I. NATURE OR NURTURE? 1 II. MODIFICATION OF THE GERM-PLASM 25 III. DIFFERENCES AMONG MEN 75 IV. THE INHERITANCE OF MENTAL CAPACITIES 84 V. THE LAWS OF HEREDITY 99 VI. NATURAL SELECTION 116 VII. ORIGIN AND GROWTH OF THE EUGENICS MOVEMENT 147 VIII. DESIRABILITY OF RESTRICTIVE EUGENICS 167 IX. THE DYSGENIC CLASSES 176 X. METHODS OF RESTRICTION 184