With the debate between Richard Owen and Thomas Huxley on the differences between the ape and human brains as its focus, this book explores some of the ways in which philosophical ideas and scientific practice influenced the discussion of evolution in the years before and after Darwin's publication of Origin of Species in 1859. It also shows how this episode can shed light on current philosophical notions of scientific practice and how they in turn influence our understanding of the history of science. The book advances the current historical discussion of the Owen-Huxley debate by making clear that Owen's anatomical claims had much more support than most historians and philosophers of science assume.