The fundamental difference between the sonata and the polyphonic forms is that the sonata is written in the monophonic style. Polyphony is, indeed, occasionally employed, but the reigning style is that in which a melody, song-like in character and sung by a single part, is accompanied by other parts written in chord harmonies. The necessary repetition of the melodic ideas is made, not by the process of imitation, as in the fugue, but by what is called the cyclical method. In this a tune or a composition always returns to a restatement of the original theme from which it started. We have seen how this melodic style entered instrumental music in the days of Giovanni Gabrieli, and how Domenico Scarlatti transferred to the harpsichord the aria of the opera. From this time forward the monophonic style developed gradually from the initial impulse of the vocal solo. Composers who had not Bach's peculiar insight into polyphonic writing and profound genius for it naturally sought a form which would give their melodies coherence and intelligibility. The rapid development of the violin as a solo instrument was one of the influences in directing them along certain lines of construction. The violin naturally lent itself to a flowing, song-like style, yet it is easy to see how easily such a style would fall into monotony. The early violin composers, in their search after a form which would embrace coherency, variety, and contrast, did much toward assisting piano writers to reach the true method of composing. Arcangelo Corelli (1653-1713) was the most influential of these early violin composers. He endeavored to unite in his works the attractive and popular features of church music, song, and the dances of his time. The attractiveness of making compositions out of different kinds of dance movements in alternation had presented itself to composers at an early date. Morley, in his "Plain and Easy Introduction to Practical Music" (1597), says: "It is effective to alternate pavanes with galliards, because the former are a kind of staid music ordained for grave dancing, and the latter for a lighter and more stirring kind of dancing." This alternation of dances is what first suggested to composers the plan of following a slow movement with a lively one, or _vice versa_. As these different kinds of dances differed in rhythm,--the pavanes, for instance, being in common and the galliard in triple measure,--composers grasped the idea that changes in rhythm would heighten the contrast between movements. The one thing that did not seem to be settled at the outset--and that was due to the newness of harmonic as opposed to polyphonic style--was the matter of key contrasts. Sometimes these early writers put all their dance movements in one key, making what we now call a "suite," and sometimes they did not.