Emotion in animals specialised form of Occam's razor used in ethology, in which he stated: Charles Darwin initially planned to include a chapter on emotion in "The Descent of Man" but as his ideas progressed they expanded into a book, "The Expression of the Emotions in Man and Animals". Darwin proposed that emotions are adaptive and serve a communicative and motivational function, and he stated three principles that are useful in understanding emotional expression: First, "The Principle of Serviceable Habits" takes a Lamarckian stance by suggesting that emotional expressions that are useful will be passed on to the offspring. Second, "The Principle of
The Expression of the Emotions in Man and Animals the actor Henry Siddons in his "Practical Illustrations of Rhetorical Gesture and Action" (1807) and by the anatomist Robert Knox in his "Manual of Artistic Anatomy" (1852) – but abandoned them as unreliable, although the text of "The Expression" is littered with quotations from Shakespeare. Darwin opens the book with three chapters on "the general principles of expression", introducing the rather Lamarckist phrase "serviceable associated habits". With this phrase, Darwin seeks to describe the initially voluntary actions which come together to constitute the complex expressions of emotion. He then invokes a principle of "antithesis", through which opposite states of mind