In civilization, however, dancing is not only an incitement to love and a preliminary to courtship, but it is often a substitute for the normal gratification of the sexual instinct, procuring something of the pleasure and relief of gratified love. In occasional abnormal cases this may be consciously realized. Thus Sadger, who regards the joy of dancing as a manifestation of "muscular eroticism," gives the case of a married hysterical woman of 21, with genital anesthesia, but otherwise strongly developed skin eroticism, who was a passionate dancer: "I often felt as though I was giving myself to my partner in dancing," she said, "and was actually having coitus with him. I have the feeling that in me dancing takes the place of coitus."[42] Normally something of the same feeling is experienced by many young women, who will expend a prodigious amount of energy in dancing, thus procuring, not fatigue, but happiness and relief.[43] It is significant that, after sexual relations have begun, girls generally lose much of their ardor in dancing. Even our modern dances, it is worthy of note, are often of sexual origin; thus, the most typical of all, the waltz, was originally (as Schaller, quoted by Groos, states) the close of a complicated dance which "represented the romance of love, the seeking and the fleeing, the playful sulking and shunning, and finally the jubilation of the wedding."[44] Not only is movement itself a source of tumescence, but even the spectacle of movement tends to produce the same effect. The pleasure of witnessing movement, as represented by its stimulating effect on the muscular system,--for states of well-being are accompanied by an increase of power,--has been found susceptible of exact measurement by Féré. He has shown that to watch a colored disk when in motion produced stronger muscular contractions, as measured by the dynamometer, than to watch the same disk when motionless. Even in the absence of color a similar influence of movement was noted, and watching a modified metronome produced a greater increase of work with the ergograph than when working to the rhythm of the metronome without watching it.[45] This psychological fact has been independently discovered by advertisers, who seek to impress the value of their wares on the public by the device of announcing them by moving colored lights. The pleasure given by the ballet largely depends on the same fact. Not only is dancing an excitation, but the spectacle of dancing is itself exciting, and even among savages dances have a public which becomes almost as passionately excited as the dancers themselves.[46] It is in virtue of this effect of dancing and similar movements that we so frequently find, both among the lower animals and savage man, that to obtain tumescence in both sexes, it is sufficient for one sex alone, usually the male, to take the active part. This point attracted the attention of Kulischer many years ago, and he showed how the