The key to an understanding of the importance of the libretto lies in the term Wagner used to describe the art-work of the future; he called this a "music-drama." The exclusive lover of music is tempted to look down on opera because its music is contaminated with drama; and for a similar reason, the exclusive lover of the drama is not attracted to opera because the drama is there more or less sacrificed to the music. But there are many opera-goers who best relish music and the drama when they are presented in conjunction. In a music-drama of the highest type, in Wagner's 'Tannhaüser,' for example, the music and the drama are Siamese twins; they were brought forth at a single birth. Each helps the other, and neither calls upon the other for any undue sacrifice. They can be enjoyed together better than they can be enjoyed apart, since each depends upon the other; and united they stand or fall. Mr. H. T. Finck was not overstating the case when he insisted that the ideal opera is one in which the book and the score are each of them of absorbing interest, "and yet make a doubly deep impression when heard together." The stories of 'Faust' and of 'Carmen' and of 'Lohengrin' are delightful in themselves, merely to read; and a musical expert can find pleasure in playing the music from them on the piano. "Yet how much more effective they are when we hear and see music and play together on the stage." And then the same writer goes on to point out that the best "libretto is one which tells its story to the eye," as in the case of 'Carmen,' for example. "No one with eyes to see can fail, for instance, to follow the career of 'Carmen,' from her flirtation with the young officer to the scene before the bullring where he stabs her." It was an acute French dramatic critic who once asserted that "the skeleton of every good play is a pantomime," and the assertion is more emphatically true when applied to the skeleton of a libretto. Indeed, as the words are rarely heard distinctly, and as they are often in a foreign language, there is double need of a story so clear and so straightforward that it can be caught by the eye alone from the actions and gestures and facial expressions of the performers without the aid of the actual words. But the inventing and the constructing of a plot of this seemingly simple effectiveness is a task of extraordinary difficulty--if we may judge by the infrequency of its achievement. And undoubtedly it is this difficulty which has led so many musicians to compose their scores to books only slightly altered from plays which had already an attested popularity in the theater. By so doing it has seemed to them that they were minimizing the risk of finding their music handicapped by an ineffective story. The danger in this case lies in the temptation to set to music any play which may chance to be successful without considering sufficiently whether it is really worthy of the composer's labor.