In addition to the effects of musical sound already mentioned, it may be added that, as Epstein, of Berne, has shown,[106] the other senses are stimulated under the influence of sound, and notably there is an increase in acuteness of vision which may be experimentally demonstrated. It is probable that this effect of music in heightening the impressions received by the other senses is of considerable significance from our present point of view. Why are musical tones in a certain order and rhythm pleasurable? asked Darwin in _The Descent of Man_, and he concluded that the question was insoluble. We see that, in reality, whatever the ultimate answer may be, the immediate reason is quite simple. Pleasure is a condition of slight and diffused stimulation, in which the heart and breathing are faintly excited, the neuro-muscular system receives additional tone, the viscera gently stirred, the skin activity increased; and certain combinations of musical notes and intervals act as a physiological stimulus in producing these effects.[107] Among animals of all kinds, from insects upward, this physiological action appears to exist, for among nearly all of them certain sounds are agreeable and attractive, and other sounds indifferent and disagreeable. It appears that insects of quite different genera show much appreciation of the song of the Cicada.[108] Birds show intense interest in the singing of good performers even of other species. Experiments among a variety of animals in the Zoölogical Gardens with performances on various instruments showed that with the exception of seals none were indifferent, and all felt a discord as offensive. Many animals showed marked likes and dislikes; thus, a tiger, who was obviously soothed by the violin, was infuriated by the piccolo; the violin and the flute were preferred by most animals.[109] Most persons have probably had occasion to observe the susceptibility of dogs to music. It may here suffice to give one personal observation. A dog (of mixed breed, partly collie), very well known to me, on hearing a nocturne of Chopin, whined and howled, especially at the more pathetic passages, once or twice catching and drawing out the actual note played; he panted, walked about anxiously, and now and then placed his head on the player's lap. When the player proceeded to a more cheerful piece by Grieg, the dog at once became indifferent, sat down, yawned, and scratched himself; but as soon as the player returned once more to the nocturne the dog at once repeated his accompaniment.