Orcs

[Duncariel]

Tolkien spent most of his adult life writing and perfecting the Silmarillian. His son, Christopher, had it published posthumously.

These would be my reasons for believing that orcs were originally elves corrupted by Melkor. Orcs, in the beginning of Arda, did not exist, since they were not created directly by Illuvatar. It says that Melkor secretly captured the Elves that wandered too far from the others and took them to Utumno. (I think, don't quote me on that.) Since Aule "created" the Dwarves without the original permission of Illuvatar, and they did not have wills of their own, Melkor could not have created creatures of his own without the knowledge and permission of Illuvatar. Therefore, he had to find some way to bend already living being to his own warped uses.

I don't know how he accomplished this feat, nor do I want to, but it seems the only logical (and possible) way for him to bring these hideous beings into the world.