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The authors of this work, who are respectively experts in the study of genetics, of statistics and of psychology, have devoted ten years to the amassing of such a wealth of material—now presented in this book—as those interested in the subject had hardly dreamed of securing. The book opens with an account of investigations made in relation to fifty pairs of identical twins and the comparison of these with fifty pairs of like-sexed fraternal twins. The purpose of this was to arrive at a standard of comparison of the traits which may, with certainty or with probability, be looked for. In the second section of the book the findings thus obtained are compared with the most important part of the new material, namely, the results of investigation in relation to nineteen pairs of identical twins, i.e. twins of exactly the same heredity, who in every case had been separated at an early age, in most cases before they were a year old, and who were brought up apart from each other for periods varying from eleven to fifty years. Unfortunately the psychological side of the enquiry is confined to the sphere of achievements: the reader learns little of the way in which the subjects of the experiment had developed and next to nothing of their personalities, the method adopted being exclusively that of psychological tests. From motives of discretion and a desire to elude reporters, with their thirst for sensational news, the authors expressly state that they are not 'out to write stories'. Thus it comes about that far too little justice is done to the unique possibilities presented in this incomparable material. The final conclusion is disappointing but not surprising in view of the unpromising methods of investigation: what can be determined by heredity can also be determined by environment. It should be noted that the authors state that they are prepared to put at the disposal of those who are interested in the problem of heredity and environment the whole body of mat