Broca, being what today would be called a neurosurgeon, had taken an interest in the pathology of speech. He wanted to localize the difference between man and the other animals, which appeared to reside in speech. He discovered the speech center of the human brain, today called Broca's area after him. His interest was mainly in Biological anthropology, but a German philosopher specializing in psychology, Theodor Waitz, took up the theme of general and social anthropology in his six-volume work, entitled Die Anthropologie der Naturvölker, 1859–1864. The title was soon translated as "The Anthropology of Primitive Peoples". The last two volumes were published posthumously.