The work outlines a systematic ethnic psychology that affirms the fundamental unity of human mental life while tracing how individual minds combine to form group or ethnic minds. It surveys cultural and natural histories of collective mentality: definitions of the ethnic mind, modes and rates of progressive, regressive, and pathological variation, and physiological, hereditary, social, and geographic influences on mental traits. The author draws on comparative evidence and theoretical reflection to explain the origins, transmission, and modification of group characteristics, and to show how collective mental patterns shape social relations and historical development.