This is the new edition of a textbook dealing with the nature, origins, development, and causes of human variety, and with the biological organization of past and present human populations. The familiar structure of the previous editions has been slightly modified, and the book is divided into four sections. These cover some of the general principles of evolutionary theory; the history of Primates; the fossil evidence for human evolution; aspects of Primate behaviour; the principles of genetics as applied to population studies; systematic descriptions of human variation; the nature of human growth and factors which determine it; varieties of human physique and other constitutional traits; and the ecology of human populations, considered especially in terms of adaptation processes. Two new authors, Professor D. Pilbeam and Professor P.T. Baker, both in the United States, have contributed the parts on human evolution and human adaptability respectively to this substantially revised and rewritten edition.