[Snowdog]

Good topic A_Brandybuck!
I'll answer your questions in your starting post according to my thoughts as theyhave to do with Middle Earth and not bother addressing things brought up in subsequent posts (nazism, christianity, etc.).

Originally Posted by A_Brandybuck

What exactly could be the reason for the swifter waning of the Dúnedain of Gondor?
Does the reason only lie in the mingeling with other folk without númenórian ancestry?

I would have to say that the proximity to Mordor, the battles, and keeping the watch on incursions into the lands were a primary cause. The men were evermore consumed with fighting and honing of their skills that they took less interest in marriage and and family. They married late and less children were born. This caused many of the pure Dúnedain lines to dwindle. Also, intermingling with the lesser peoples, and later with the Northmen led to their waning. There however was times and individuals who came along that seemed to embody the recessed genes of Numenor (Faramir for example). But I think less breeding with other Dunedain clans and more breeding with other was the primary cause.

Originally Posted by A_Brandybuck

Was there no or less mingeling in Eriador? In Eriador there has also lived other non-númenórian folk, which could have mingeled with the Dúnedain.

There was definitely much less intermingling in Eriador. I think the only place that it occured was in Rhuadur after the division of Arnor in TA 861. Surely many of the Dúnedan intermingled with the hillmen of the Ettenmoors, and later those who hadn't were either slain or driven out and came back to Arthedain.

Originally Posted by A_Brandybuck

Or does the reason lie in the different behavior of both lines? In later times it seems to me, that the Rangers of Eriador, have kept more true to their ideals than the Dúnedain of the South, although or just hence there was no more great realm in the North. The Dúnedain of the South acted more in things of power and glory, while the Dúnedain of the North acted more unselfishly. In example, by protecting the borders of the Shire. Maybe the presence of Elves or the contact to them let them treat their ideals more noble, than the southern Dúnedain, who had not really much contact to the Elves.

Hmm.... I agree to some degree and disagree to some degree with this.
I don't think the Dúnedain of the north were less selfish, but initially more so. For while they had no real external enemies after the Last Alliance until the coming of the Witch King to Carn Dûm a thousand years later, they seemed to have slowly deteriorated into factions and feud among themselves which came to a head when King Eärendur divided Arnor between his sons. I have theories on how this came about but won't go into it here (at least not yet). After the division, they continued to fight and some erupted into all out war, first between Cardolan and Rhuadur over possession of Amon Sûl and its Palantiri ( though I'm sure Arthedain was not complacent during this time), and later between Rhuadur and Arthedain, with Rhuadur falling under the control of Carn Dûm. With the coming of the Witch King to the north, the northern Dúnedain now had a formidable external enemy, but their strength had been eroded by their in-fighting and sickness when the final blow came and Fornost was sacked. Now the elves of Lindon and Rivendell did help fight the Witch King in the years leading up to the fall of Arthedain, but it was the great armada that sailed from Gondor under Prince Eärnur that finally threw the tide of the war in the north in their favor, and the Witch King was drivin out of the north. The greatness of this armada really shows how much greater in might that Gondor hadcompared to Arnor. Despite centuries of war in the south and the plague, plus the fact Gondor had just battled the Wainriders only 30 years before when King Ondoher and his sons were slain, King Eärnil was able to muster such a force that he could spare to sail in aid of Arnor.

And afterward, Aranarth, eldest son of King Arvedui, and his brothers and the menfolk who remained of the Dunedain of the north took up the ways of the Rangers, and they rode as shadows, opposing evil whenever and wherever they could. I think in the fall of Arnor, they realized thenorthern Dúnedain had in part caused their own downfall, and learned from it all, and were evermore vigilent in their opposition of evil. As Formendacil said, the northern Dúnedain was not morally superior, they just declined differently. I think the enviroment was a considerable part of the difference(military might, proximity to the Eldar, etc.).