[Lalwendë]

I said there was something about the deterioration of Elf corpses in HoME X and there is, plus some more of interest.

...the body, deserted by the spirit was dissolved. This happened swiftly in Middle-earth. In Aman only was there no decay.

And the text further states that of course an Elf would only die due to great violence or grief; in the former case, the body would be a wreck and in the latter, the dead Elf would usually not be keen to enter that body (though Miriel eventually did, but then her body was not corrupted as she died of grief in Aman). Course, in Middle-earth, if those in the service of Morgoth and Sauron got to the corpse before it was utterly useless then what would stop such evil beings from making use of it?

It seems the Dark Lords made use of those Elves who ended up Houseless or Unbodied as some of the text hints at dark dealings of necromancy:

one of the hungry Houseless, if it is admitted to the friendship of the living, may seek to eject the fea from its body; and in the contest for mastery the body may be gravely injured, even if it not be wrested from its rightful inhabitant. Or the houseless may plead for shelter, and if it is admitted, then it will seek to enslave its host and use both his will and his body for its own purposes. It is said that Sauron did these things, and taught his followers how to achieve them.

and

the Unbodied, wandering in the world, are those who at the least have refused the door of life and remain in regret and self pity. Some are filled with bitterness, grievance, and envy. Some were enslaved by the Dark Lord and do his work still, though he himself is gone. They will not speak truth or wisdom. To call on them is folly. To attempt to master them and to make them servants of one's own will is wickedness. Such practices are of Morgoth; and the necromancers are of the host of Sauron, his servant.

To me, this all suggests a likely source for Orcs, and maybe for other creatures too. The refusal of the summons to Mandos was quite a serious infraction in terms of the nature of an Elf, and it was quite a dangerous thing to do, once Houseless as it made Elves very vulnerable to corruption - both as they had not got the protection of a body and because without one, they could not interact properly with other Elves. HoME tells us that some of the more benign Houseless 'haunted' places they had known and became rather like 'earth spirits', inhabiting trees and rocks etc, but it seems others, either through temptation or will, went down more sinister routes. Incidentally, this also gives us some justification for those in Middle-earth who did not entirely trust the Elves, as there must have been some who 'possessed' others.

So, maybe Orcs came about by Morgoth making use of the Houseless, giving them new forms either from fallen Elf corpses (in many forms of decomposition probably!) or who knows what Dr Frankenstein abominations...

And then having the bodies, what was to stop them reproducing in the normal way?