The waning of the elves and Melkor's influence on the history of Arda

Question

I'm currently struggling my way through Morgoth's ring, and am reading the commentary on the Athrabeth. However, one quote in the commentary is confusing me, and I was hoping someone understood it better than me. In note 7 (p. 382) it is said re: the waning of the elves:

They eventually became housed, if it can be called that, not in actual visible and tangible hroar, but only in the memory of the fea of its bodily form, and its desire for it; and therefore not dependent for mere existence upon the material of Arda.(20) But they appear to have held, and indeed still to hold, that this desire for the hroa shows that their later (and present) condition is not natural to them, and they remain in estel that Eru will heal it. 'Not natural', whether it is due wholly, as they earlier thought, to the weakening of the hroa (derived from the debility introduced by Melkor into the substance of Arda upon which it must feed), or partly to the inevitable working of a dominant fea upon a material hroa through many ages. (In the latter case 'natural' can refer only to an ideal state, in which unmarred matter could for ever endure the indwelling of a perfectly adapted fea. It cannot refer to the actual design of Eru, since the Themes of the Children were introduced after the arising of the discords of Melkor. The 'waning' of the Elvish hroar must therefore be part of the History of Arda as envisaged by Eru, and the mode in which the Elves were to make way for the Dominion of Men.

What is meant by the last sentences, where it's stated that the elves waning must be part of Eru's plan, since the theme of the children was introduced after Melkor's initial discord? Wouldn't it mean that it was not part of Eru's plan, due to the discord? I suppose the question is why can the natural state described not be the actual design of Eru, but also part of “the history of Arda envisaged by Eru”? Additionally, I thought it had been established that the waning was due to the fact that Melkor so thoroughly diffused his power in to Arda, and as such all matter was corrupted in some way, which is mentioned both in the quote I included, and I believe in Laws and Customs.

Anyone able to help me out, or point me towards a good summary of the Athrabeth that explains this?

Answer 1

I think the key to understanding this is a passage early in The Music of the Ainur:

Then Iluvatar spoke, and he said 'Mighty are the Ainur, and mightiest among them is Melkor; but that he may know, and all the Ainur, that I am Iluvatar, those things that ye have sung, I will show them forth, that ye may see what ye have done. And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined.'

What Melkor did in corrupting Arda, while evil, would still be turned to "things more wonderful" than before.

So, in essence, both are completely true: Melkor did mar Arda. It is changed in terrible ways from what it might have been. Yet Eru Iluvatar took Melkor's destructive meddling and turned it into something greater.

It's impossible to say for sure what comes from the "original" plan and what from Melkor's actions. But Tolkien does provide hints:

Then again Iluvatar arose, and the Ainur perceived that his countenance was stern; and he lifted up his right hand, and behold! a third theme grew amid the confusion, and it was unlike the others. For it seemed at first soft and sweet, a mere rippling of gentle sounds in delicate melodies; but it could not be quenched, and it took to itself power and profundity. And it seemed at last that there were two musics progressing at one time before the seat of Iluvatar, and they were utterly at variance. The one was deep and wide and beautiful, but slow and blended with an immeasurable sorrow, from which its beauty chiefly came. The other had now achieved a unity of its own; but it was loud, and vain, and endlessly repeated; and it had little harmony, but rather a clamorous unison as of many trumpets braying upon a few notes. And it essayed to drown the other music by the violence of its voice, but it seemed that its most triumphant notes were taken by the other and woven into its own solemn pattern.