The origin of the third element, rhyme, is in dispute. In some instances it may have passed into Greek and Latin verses from Syrian hymns.[302] But on the other hand it had long been an occasional element in Greek and Latin rhetorical prose. Probably rhyme in Latin accentual verse had no specific origin. It gradually became the sharpening, defining element of such verse. Accentual Latin lent itself so naturally to rhyme, that had not rhyme become a fixed part of this verse, there indeed would have been a fact to explain. These, then, were the elements: accent, number of syllables, and rhyme. Most interesting is the development of verse-forms. Rhythmic Latin poetry came through the substitution of accent for quantity, and probably had many prototypes in the old jingles of Roman soldiers and provincials, which so far as known were accentual, rather than metrical. Christian accentual poetry retained those simple forms of iambic and trochaic verse which most readily submitted to the change from metre to accent, or perhaps one should say, had for centuries offered themselves as natural forms of accentual verse. Apparently the change from metre to accent within the old forms gradually took place between the sixth and the tenth centuries. During this period there was slight advance in the evolution of new verses; nor was the period creative in other respects, as we have seen. But thereafter, as the mediaeval centuries advanced from the basis of a mastered patristic and antique heritage, and began to create, there followed an admirable evolution of verse-forms: in some instances apparently issuing from the old metrico-accentual forms, and in others developing independently by virtue of the faculty of song meeting the need of singing. This factor wrought with power--the human need and cognate faculty of song, a need and faculty stimulated in the Middle Ages by religious sentiment and emotion. In the fusing of melody and words into an utterance of song--at last into a strophe--music worked potently, shaping the composition of the lines, moulding them to rhythm, insisting upon sonorousness in the words, promoting their assonance and at last compelling them to rhyme so as to meet the stress, or mark the ending, of the musical periods. Thus the exigencies of melody helped to evoke the finished verse, while the words reciprocating through their vocal capabilities and through the inspiration of their meaning, aided the evolution of the melodies. In fine, words and melody, each quickened by the other, and each moulding the other to itself, attained a perfected strophic unison; and mediaeval musician-poets achieved at last the finished verses of hymns or Sequences and student-songs.