Darwin’s most famous book, On the Origin of Species, triggered a paradigm shift that transformed biology into a modern science. His greatest scientific achievement, so nicely summed up by the evolutionary biologist Ernst Mayr, was freeing biology from “the paralyzing grip of essentialism.” Regarding emotion, however, Darwin made an inexplicable about-face thirteen years later by writing Expression, a book riddled with essentialism. In doing so, he abandoned his remarkable innovations and returned to essentialism’s paralyzing grip, at least where emotions are concerned.8
