A critic has asked how the ears of man, and he ought to have added of other animals, could have been adapted by selection so as to distinguish musical notes. But this question shows some confusion on the subject; a noise is the sensation resulting from the co-existence of several aërial “simple vibrations” of various periods, each of which intermits so frequently that its separate existence can not be perceived. It is only in the want of continuity of such vibrations, and in their want of harmony _inter se_, that a noise differs from a musical note. Thus an ear to be capable of discriminating noises--and the high importance of this power to all animals is admitted by every one--must be sensitive to musical notes. We have evidence of this capacity even low down in the animal scale; thus crustaceans are provided with auditory hairs of different lengths, which have been seen to vibrate when the proper musical notes are struck. As stated in a previous chapter, similar observations have been made on the hairs of the antennæ of gnats. It has been positively asserted by good observers that spiders are attracted by music. It is also 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.” Therefore, as far as the mere perception of musical notes is concerned, there seems no special difficulty in the case of man or of any other animal. But if it be further asked why musical tones in a certain order and rhythm give man and other animals pleasure, we can no more give the reason than for the pleasantness of certain tastes and smells. That they do give pleasure of some kind to animals we may infer from their being produced during the season of courtship by many insects, spiders, fishes, amphibians, and birds; for, unless the females were able to appreciate such sounds and were excited or charmed by them, the persevering efforts of the males and the complex structures often possessed by them alone would be useless; and this it is impossible to believe. IX. DEVELOPMENT OF THE MORAL SENSE. [Descent of Man, page 97.] I fully subscribe to the judgment of those writers who maintain that, of all the differences between man and the lower animals, the moral sense or conscience is by far the most important. This sense, as Mackintosh remarks, “has a rightful supremacy over every other principle of human action”; it is summed up in that short but imperious word _ought_, so full of high significance. It is the most noble of all the attributes of man, leading him without a moment’s hesitation to risk his life for that of a fellow-creature; or, after due deliberation, impelled simply by the deep feeling of right or duty, to sacrifice it in some great cause. * * * * * [Page 111.]