Victorian Englishmen were not known for feeling comfortable displaying their emotions. Charles Darwin, exceptional in so many other ways, was like his countrymen in this regard, and considered the display of emotions in adult humans to be vestigial, something left over from our evolutionary past. That didn't stop him from publishing, in 1872, what remains the most comprehensive text on the nature of emotions.

In "The Expression of the Emotions in Man and Animals," Darwin argued that like any other trait, emotions and emotional expressions had evolved by natural selection. Ever observant, Darwin had noticed how his dog would go happily before him as he left his house, anticipating being taken for a long walk. But often Darwin would instead take a path that led to his hothouse, containing his experimental plants.

"The instantaneous and complete change of expression which came over him as soon as my body swerved in the least towards the path (and I sometimes tried this as an experiment) was laughable," wrote Darwin. "His look of dejection was known to every member of the family, and was called his hot-house face. This consisted in the head drooping much, the whole body sinking a little and remaining motionless; the ears and tail falling suddenly down, but the tail was by no means wagged."