Famous today for his theories of evolution and natural selection, naturalist Charles Darwin also explored the relationship between the emotions expressed by both humans and animals. The third most influential in his list of works of evolutionary theory, The Expression of the Emotions in Man and Animals proposes the animal origins of human expressions such as the lifting of the eyebrows in a moment of surprise. Of particular delight is the use of images throughout (including a once cutting-edge form of photo printing called a Heliotype) to show the reader various dramatic expressions.