<P> 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 Antithesis suggests that some expressions exist merely because they oppose an expression that is useful . Third, The Principle of the Direct Action of the Excited Nervous System on the Body suggests that emotional expression occurs when nervous energy has passed a threshold and needs to be released . </P> <P> Darwin saw emotional expression as an outward communication of an inner state, and the form of that expression often carries beyond its original adaptive use . For example, Darwin remarks that humans often present their canine teeth when sneering in rage, and he suggests that this means that a human ancestor probably utilized their teeth in aggressive action . A domestic dog's simple tail wag may be used in subtly different ways to convey many meanings as illustrated in Darwin's The Expression of the Emotions in Man and Animals published in 1872 . </P> <Ul> <Li> Examples of tail position indicating different emotions in dogs </Li> <Li> <P> "Small dog watching a cat on a table" </P> </Li> <Li> <P> "Dog approaching another dog with hostile intentions" </P> </Li> <Li> <P> "Dog in a humble and affectionate frame of mind" </P> </Li> <Li> <P> "Half - bred shepherd dog" </P> </Li> <Li> <P> "Dog caressing his master" </P> </Li> </Ul> <Li> Examples of tail position indicating different emotions in dogs </Li>

Emotions are present in humans and other animals