Now the Arthurian legends ultimately derive from Celtic tales, which must be supposed to have travelled from Wales into France by way of Brittany--Little Britain, or Armorica--in the eleventh and twelfth centuries; for there are Welsh versions independent of the Breton forms, though closely akin. Students of early Celtic literature have not as yet agreed about the historical relations between Welsh and Irish stories--whether the Welsh imposed their mythology and heroic legends on the Irish, or _vice versa_; but the general similarity between them is undeniable, and easily explicable by a common Celtic source. Everything, then, points to the Celtic legends as the chief origin of the mediaeval fairy-lore; and the early Celtic literature, although its study, complicated by an unfamiliar language, has only recently been undertaken scientifically, has already revealed an extremely rich and complete store of romance that extends over a thousand years. From manuscripts which are attributed to the twelfth century (and even so contain matter rightly belonging to the ninth or tenth), we can trace the development of a creed concerning supernatural beings through the succeeding centuries, down to a time at which the written account is displaced by recorded oral tradition. A race of beings, who must originally have fallen from the Celtic Olympus, continue to appear, with characteristics that remain the same in essence, and under a designation that may be heard in Ireland today, through ten centuries of Irish tradition and literature.[76] These people are called in Irish mythology the _Tuatha De Danann_, described from at latest 1100 A.D. as _aes sidhe_, "the folk of the [fairy-] hillock;" the name for fairies in Ireland now is "the Sidhe."[77] Originally, it may be, the _aes sidhe_ were not identified with the _Tuatha De Danann_; and before the twelfth century the Sidhe were not associated with the Celtic belief in "a beautiful country beyond the sea," a happy land called by various names--Tir-nan-Og (the land of youth), Tir Tairngire (the land of promise)--which has now become "fairy-land." In the earliest heroic legends the _Tuatha De Danann_ assist or protect mortal champions, and fall in love with mortal men and maids; but with the spread of Christianity (as might be expected) they lost many of their previous characteristics.[78]