The story had reached this stage of development when the northward-wandering tribes carried it to Norway, and in due course to Iceland, where it underwent much less modification than it did among those who remained, or who finally settled down, in central Europe. What changes it underwent during the wanderings of the tribes, by what influences and by what steps a legend originally heathen and tribal was modified by Christianity and feudalism, till after some six hundred years it emerges to view in something approaching its present form--of all this we have no real knowledge, and no subject of literary criticism has been more fruitful of conjecture. We may assume that it was handed down by oral tradition until, with the development of chivalry, with its natural affinity for romance and poetry, there came in the 12th and 13th centuries a great revival of interest in the old heroic literature. Its cultivation became a passion with the nobility, who followed it on two main lines, leading to the production (or revival) of epic poetry of two classes:--(1) the Court Epic, which took for its subject the romance of knight-errantry, and (2) the National Epic, which took the old popular heroic tradition, and gave it permanence in a metrical form peculiar to itself. The _Nibelungenlied_ is essentially, in its subject and spirit, a national epic; but, as it was remodelled by courtier-poets, their treatment of it made it approximate in some respects very closely to the court epic, especially in what we may call the veneer of chivalrous refinement laid over the more elemental characters of the original story. Hence it bears throughout, both in characters and incidents, evidences of the influence of feudalism and chivalry, on the one hand, and of Christianity on the other.