Charles Darwin's The Expression of the Emotions in Man and Animals (1872) is a very different kind of work from On the Origin of Species (1859). This "otherness" is most extreme in the character of the explanations that Darwin offers in the Expression. Far from promoting his theory of natural selection, the Expression barely mentions that theory, instead drawing on explanatory principles which recall less Darwinian than Lamarckian and structuralist biological theorizing. Over the years, historians have offered a range of solutions to the puzzle of why the Expression is so "non-Darwinian". Close examination shows that none of these meets the case. However, recent research on Darwin's lifelong engagement with the controversies in his day over the unity of the human races makes possible a promising new solution. For Darwin, emotional expression served the cause of defending human unity precisely to the extent that natural selection theory did not apply.