It is one thing to note the preceding succession of steps as objective phenomena. It is quite another to explain the origin of the idea which transformed the _alleluia_ into the larger _iubilus_. This is the most obscure point in the musical development of the sequence, which, for lack of manuscript evidence cannot at present be clarified. At least three hypotheses have been offered. Arguing from the appearance of the trope, some have suggested that the _iubilus_ is a musical interpolation just as the trope is a textual interpolation. This is quite possible but perhaps too simple for an adequate solution. A much more tempting hypothesis has appealed to a variety of scholars,—that of the introduction of Greek melodies.[3] To these students it has seemed more than probable that the intercourse between western Europe and the Byzantine realms in the reign of Charlemagne constitutes a sufficient explanation for the appearance of fresh musical themes. Again, a possibility only has been suggested. So far manuscript evidence for the Greek melodies from which the Gregorian _alleluiae_ and their _iubili_ are derived, has not appeared. Blume, whose treatment of the subject forms the basis of this chapter, not only questions the hypothesis of Greek melodies but he offers a third suggestion and that tentatively; Gregory, he thinks, shortened the _alleluia_ brought over by the Greeks. When, later, a tendency was felt to elaborate the forms of worship, the longer melodies were once more revived in the sequence. This very interesting suggestion, if one day capable of proof, would harmonize the Byzantine and Gregorian influences which produced the initial extension of the final _a_ of the _alleluia_. For purposes of clearness a differentiation should be made between the musical and poetical development of the sequence as soon as the _sequentia cum prosa_ is reached. Manifestly it is impossible to do so, in any complete fashion, where words and music are so inextricably interwoven in a common development. It is better, however, to attempt the impossible and for the present, to ignore overlappings.[4]