As a digression here, it may be advisable to give full appreciation of the place of the Cymru in the 'well' of racial development in Britain.  By 350 e.v. the mainland of Britain was inhabited by the Keltic race of Britons from the Channel up to Antonine's Wall and a little beyond.  A reasonable boundary may be noted by drawing a line from the Isle of Arran, which lies off the West coast of Scotland, through Dumbarton, Dunblane, Perth, Forfar and Brechin to Stonehaven in the East.  To the North of this line lay Pictland.

The ancient name of Arran was Emain Ablach; that of Dumbarton (which means "fort of the Britons") was Kaer Glut; while Brechin was known by the longer form of Brecheinawc.  Perth to this day remains the untranslated Welsh word which means "bush" or "hedge."

Returning to the West, Bretrwn was the region around Troon and Coet Beit was the original name of the moor at Beith in Ayrshire.

It may be a revelation to many that the early names of these loci were in the Cymreig (Welsh) tongue, but it is no surprise to the student of history.  At this point in British history, apart from the wealthy and such as had scaled the then local government ladder by a-whoring with the occupying power and system imposed on the country by Imperial Rome, and who lived in the Roman style speaking Latin and subjecting their children to the Empire's standards in culture and education, the bulk of the population spoke 'Parent' British - a language now known as Primitive Welsh.

In 367 e.v. the mainland was attacked by the Teutonic Saxon warriors in the South East and by the Irish Scots in the North West.  The Scots were to settle the Northern Region in time, giving their name to it and totally absorbing the Pictish and Keltic cultures.

With the Roman administration and occupation virtually ended by c. 425, the unity of the mainland became eroded.  By the 540s there were three subdivisions of the Cymru (a word which meant "fellow countrymen."):

1. North Wales - from around the Bradford area up to
   Dumbarton.
2.Wales          - more or less according to its present
   boundaries.
3.West Wales  - from about Wells to Land's End.

In the 580s, Cumberland, then known as Rheged, was ruled by King Urbgen and Strathclyde by King Rhydderch.  Both these Cymreig monarchs valiantly but unsuccessfully attempted to rid the North of the Teutonic invaders.  The final and permanent wedge which was to separate the Northern Cymru from their Southern brethren was driven home by an Angle - King Aethelfrith of Northumbria when, in 616 e.v., he annihilated the Welsh army at Chester.

In 600 e.v. the tribe of the Gododin (later Middle Welsh spelling is "Gododdin"), which inhabited the region around Edinburgh, had marched against the Northumbrian king and had joined battle at Katraeth (Catterick in Yorkshire).  There the Gododin were completely overwhelmed, save for a handful of men which included the bard Aneurin.  He returned to the region of the Forth to write his epic tale of the encounter - "The Gododdin", a poem composed in Primitive Welsh and found in The Book of Aneurin.  This was called the earliest "Scottish" poem when published by Edinburgh University press c. 1971/2.

Geoffrey of Monmouth (12th cent.), in his "The History of the Kings of Britain" volume 2, recounts the story of King Ebraucus (founder of the city of Edinburgh), Maiden's Castle and the Dolorous Mountain.

In the Welsh romance "Peredur Son of Efrawg" which is linked with the Mabinogion, Peredur, who is the son of the same Ebraucus (the Latinised form of Efrawg), overcomes the serpent which lives in the "Dolorous Mound."  The Mound in Edinburgh, then, Arthur's Seat (or castle - Caer Arddu) and Maiden's Castle may be the few remaining links in legend with Cymreig traditions which at one time permeated the lowlands of Scotland and which is that region's birthright.

The Lion Rampant and the Red Dragon - co-guardians of the Northern lands, are quite content to be harnessed to the same chariot when viewed in this manner.  (In 1986 a political phenomenon made its appearance - a rudimentary alliance linking the Scottish National Party and Plaid Cymru.  Are Keltic and Gaelic group identities destined for a return to former greatness?  Only time will tell.

Certainly the leaders of both these parties, when appearing in tandem before the television cameras in the run-up to the general election in 1997, were virtually invincible during their interrogation by the interviewer!)

Lastly the similarity of many words in the Breton and Welsh languages never fails to amaze me, but, of course, many Cymru fled to Brittany during the above and later troubled times carrying much of the heritage beyond the British mainland shores.
 
This in answer to a few queries about the Keltic and Gaelic myths in Northern Britain:-

The Keltic myths are found in The Red Book of Hergest, The Black Book of Caermarthen and The White Book of Rhydderch with some traces in The Book of Taliesin.  The Gaelic myths are found in The Book of Invasions, The Book of The Dun Cow, The Book of Leinster, The Book of Ballymote, The Yellow Book of Lecan, The (later) Book of Lecan, and the Book of Lismore.

Manuscripts which corroborate the Irish legends and even supplement the Cuchullin Saga are house in the National Library of Scotland in Edinburgh.

So really, both Keltic and Gaelic mythology are valid in Scotland - the lowlands of the country being more concerned with the Keltic.  This of course is the ideal observation but is far from reality as Scotland is traditionally a Christian Calvinistic/Presbyterian country where very very few people remember any ancient legends or the personages therein.

If you ask me "By which myth do you live?" I would answer "The Keltic Myth of Math Ap Mathonwy."  If you had asked a Scot of over one hundred years ago, he may have replied "By the Old Testament and the Gospels of Christ."  But now, when church membership has plummeted to its lowest level - to such a degree that the Pope affirms that Britain is no longer a Christian country, most people would give NO answer to the question "By which myth do you live?"  So really, all the myths, both Keltic and Gaelic, have been forgotten and the people are left mythologically rootless.  Jung would have agreed with this.

Certainly there are some similarity between certain personages in the Keltic myths: Don, Beleu, Manawyddan, Llyr and the Gaelic Danu, Bile, Mannanan, Lir.  These similarities are more the exception than the rule.  The documented Irish Gaelic myths are far more prolific than the Keltic as can be seen by the various Irish books mentioned above.