In the class of Mammals, with which we are here more particularly concerned, the males of almost all the species use their voices during the breeding-season much more than at any other time; and some are absolutely mute excepting at this season. Both sexes of other species, or the females alone, use their voices as a love-call. Considering these facts, and that the vocal organs of some quadrupeds are much more largely developed in the male than in the female, either permanently or temporarily during the breeding season; and considering that in most of the lower classes the sounds produced by the males, serve not only to call but to excite or allure the female, it is a surprising fact that we have not as yet any good evidence that these organs are used by male mammals to charm the females. The American _Mycetes caraya_ perhaps forms an exception, as does more probably one of those apes which come nearer to man, namely, the _Hylobates agilis_. This gibbon has an extremely loud but musical voice. Mr. Waterhouse states,[405] “It appeared to me that in ascending and descending the scale, the intervals were always exactly half-tones; and I am sure that the highest note was the exact octave to the lowest. The quality of the notes is very musical; and I do not doubt that a good violinist would be able to give a correct idea of the gibbon’s composition, excepting as regards its loudness.” Mr. Waterhouse then gives the notes. Professor Owen, who is likewise a musician, confirms the foregoing statement, and remarks that this gibbon “alone of brute mammals may be said to sing.” It appears to be much excited after its performance. Unfortunately its habits have never been closely observed in a state of nature; but from the analogy of almost all other animals, it is highly probable that it utters its musical notes especially during the season of courtship. The perception, if not the enjoyment, of musical cadences and of rhythm is probably common to all animals, and no doubt depends on the common physiological nature of their nervous systems. Even Crustaceans, which are not capable of producing any voluntary sound, possess certain auditory hairs, which have been seen to vibrate when the proper musical notes are struck.[406] It is well known that some dogs howl when hearing particular tones. Seals apparently appreciate music, and their fondness for it “was well known to the ancients, and is often taken advantage of by the hunters at the present day.”[407] With all those animals, namely insects, amphibians, and birds, the males of which during the season of courtship incessantly produce musical notes or mere rhythmical sounds, we must believe that the females are able to appreciate them, and are thus excited or charmed; otherwise the incessant efforts of the males and the complex structures often possessed exclusively by them would be useless.