In this thesis there has been an attempt made to trace the origin, growth and development, and modification of various tendencies in the Opera. The form only of the opera has been considered and no attempt has been made towards the harmonic analysis of the various operas. The main tendencies that have been discussed are, the differences in regard to; (1) the number of acts employed; (2) the number and kind of characters; (3) a comparison; (a) of the proportion of orchestral work, overtures, ballets and ritornelli; (b) of solo work and the various voices employed; (c) of recitative, spoken, accompanied and unaccompanied; (d) of ensemble work, duets, trios, quartets, quintets and sextets; and (e) separate from this last, a consideration of the proportion of chorus work, mixed, and that sung by the men and the women alone. A study has been made of thirty-four operas and in order to accurately consider these proportions, tables have been prepared, showing the percentage of the factors portrayed. These percentages have been obtained by actual count of the measures devoted to the solos, choruses, ensembles, etc. There are seven tables. Table I is a general table showing the number of characters, acts, percentage of orchestra, solo, recitative, ensemble and chorus. Table II shows the distribution and number of characters. Table III shows the percentage of orchestral work, and also that of the divisions of the orchestral work into overture, ballet and ritornelli. Table IV shows the percentage of solo work and its distribution to the various voices. Table V shows the percentage of recitative and its distribution to the various voices. Table VI shows the percentage of ensemble work and the various kinds. Table VII shows the percentage of the chorus work and its distribution as to mixed choruses and those sung by the men and women alone. "Rappresentatione de Anima" by Cavalieri is generally classed as an oratorio but many authorities consider it an opera, as he was one of Peri's contemporaries, and worked with the little band of Florentine nobles, and this was the first work which resulted from their meetings. However, whether it is an opera or an oratorio, the forms in those days were very vague, hence, the component factors are interesting as compared to the later operas, and inasmuch as no score could be obtained earlier than 1675, it will suffice as a type of that time. In studying the operas from Wagner on, the solos have been very difficult to distinguish from the recitative, and consequently any stretch of solo singing over twenty or twenty-five measures in length, with any sort of tune, has been considered as a solo. I. THE BEGINNINGS OF OPERA TO GLUCK. Before beginning the survey of modern operas and before tracing the development of the different tendencies, it will be interesting to consider briefly the historic source of the opera, and the part that music played in the various dramas, madrigals, ballets, church services, etc.