The Epic Songs, properly speaking, are broadly divisible into three groups: the Cycle of Vladimir, or of Kieff; that of Novgorod; and that of Moscow, or the Imperial Cycle, the whole being preceded by the songs of the elder heroes. With regard to the first two, and the Kieff Cycle in particular, undoubtedly composed during the tenth, eleventh, and twelfth centuries, authorities on the origin of Russian literature differ considerably. One authority maintains that, although the Russian epics possess a family likeness to the heroic legends of other Aryan races, the Russians forgot them, and later on, appropriated them again from Ural-Altaic sources, adding a few historical and geographical names, and psychical characteristics. But this view as to the wholesale appropriation of Oriental myths has not been established, and the authorities who combat it demonstrate that the heroes are thoroughly Russian, and that the pictures of manners and customs which they present are extremely valuable for their accuracy. They would seem, on the whole, to be a characteristic mixture of natural phenomena (nature myths), personified as gods, who became in course of time legendary heroes. Thus, Prince Vladimir, "the Fair Red Sun," may be the Sun-god, but he is also a historical personage, whatever may be said as to many of the other characters in the epic lays of the Vladimir cycle. "Sadko, the rich Guest of Novgorod," also, in the song of that title, belonging to the Novgorod cycle, was a prominent citizen of Novgorod, who built a church in Novgorod, during the twelfth century, and is referred to in the Chronicles for a space of two hundred years. In fact, the Novgorod cycle contains less of the personified phenomena of nature than the cycle of the Elder Heroes, and the Kieff cycle, and more of the genuine historical element. A regular tonic versification forms one of the indispensable properties of these epic poems; irregularity of versification is a sign of decay, and a complete absence of measure, that is to say, the prose form, is the last stage of decay. The airs to which they are sung or chanted are very simple, consisting of but few tones, yet are extremely difficult to note down. The peasant bard modifies the one or two airs to which he chants his lays with astonishing skill, according to the testimony of Rybnikoff, who made the first large collection of the songs, in the Olonetz government (1859), and Hilferding, who made a still more surprising collection (1870), to the north and east of Olonetz.