To dance is to trip with measured steps, and, whilst primarily referring to human movement, the word is secondarily applied to rapid rhythmic movements even of inanimate objects. Rhythm is what distinguishes dancing from ordinary movement of progression or from simple gesture or mere antics. Dancing on the part of man or animal implies a sense of rhythm. Though not common amongst animals, it is exhibited by many birds, by spiders, and by some crustaceans! Rhythm is an essential feature of the sequence of sounds which we call "music." The singing of birds is related to their perception of and pleasure in rhythm, and it is not, therefore, surprising that they should also dance. It is, however, curious that the birds which "dance" are not the "singing birds," and that there are many birds which neither sing nor dance. The dancing of birds is usually part of the "display" of the males for the purpose of attracting the females at the breeding season. It is well known in some African cranes, as well as in rails and other similar birds, and may be witnessed at the Zoological Gardens in London. Other birds "strut" rather than dance, whilst displaying their plumage, as, for instance, the turkey and pheasant tribe and the bustards. Parrots and cockatoos will often make a rhythmical up-and-down movement of the neck in time to music, but usually the "dance" is the accompaniment of definite emotion. The male spider of some species courts the female by making dancing movements and posing itself in a very curious way, so as to display a spot of bright colour on the head to her observation. The same kind of movement and action has been observed in marine shrimp-like creatures. Some spiders are excited and made to dance by the vibrating note of a tuning-fork set going near them. I once had the chance to observe a male octopus in the aquarium at Naples, who was displaying himself to the female, changing colour rapidly from one shade to another, and rolling his long sucker-bearing arms in the form of spirals. Probably one should not consider this as a "dance," since no rhythmic interruption or succession of movements was observable.
It is established that in mankind, as well as many animals, when in a state of emotion, movement and gesture, as well as the vocal utterance, take on a rhythmic character, that is to say, become a dance and a song. The emotion is not necessarily that of amorous passion; in mankind it is frequently of a warlike or religious character, and is worked up by the sympathy, imitativeness, and desire for unison in expression which is common in troops or large gatherings of animals of social habits. Man presents a more advanced development in variety, sensitiveness, and abandonment to social or combined action and expression than do other animals, and this is equally true of the more civilized and of the more barbarous races. Apparently in obedience to the same tendencies as those which convert simple forms of movement into a rhythmic dance, the speech of man, under conditions of emotion, assumes a rhythmic form, so that dancing bears the same relation to the ordinary movements of locomotion and gesture which verse does to ordinary speech, or, again, which song bears to mere exclamations and cries, indicative of feeling. Dancing is the universal and most primitive expression of that sense of rhythm which is a widely distributed attribute of the nervous system in animals generally. In primitive men it is a simple but often very violent demonstration of strong emotion, such as social joy, religious exaltation, martial ardour, or amatory passion. The voice and the facial muscles, as well as those of the limbs and body, are affected, and the dancers derive an intense pleasure from the excitement, which so far from exhausting them leads them on to more and more violent rhythmic or undulatory action. In its purest form this ecstatic condition is seen in the spinning dervishes. It was developed into the mad and dangerous festivals of the worshippers of Bacchus and other deities in ancient Greece. It has been seen in mediaeval Europe as the dancing mania and tarantism. The liability to this and similar forms of "mania" lurks beneath the surface among populations which are nevertheless staid and phlegmatic in their usual behaviour. The Romans in ancient times recognized its unhealthy character, and though fond of ceremonial dances and theatrical shows, and even of the performances of dancing girls from Greece and the East, disapproved of dancing on the part of a Roman citizen. Cicero says, "As a rule no one, who is not drunk, dances--unless he is, temporarily, out of his mind."