fundamental concept for Tolkien (and the other Inklings) was that Evil
cannot create, only corrupt (the Boethian, as opposed to the Manichean,
concept of evil). In Letter 153 he explained that to a first approximation,
Treebeard was wrong ("Trolls are only counterfeits, made by the Enemy in the
Great Darkness, in mockery of Ents, as Orcs were of Elves." TT, p. 89) and
Frodo was right ("The Shadow that bred them can only mock, it cannot make:
not real new things of its own. I don't think it gave life to Orcs, it only
ruined them and twisted them ..." RK, p. 190). (Tolkien: "Treebeard is a
character in my story, not me; and though he has a great memory and some
earthy wisdom, he is not one of the Wise, and there is quite a lot he does
not know or understand." Letters, p. 190; "Suffering and experience (and
possibly the Ring itself) gave Frodo more insight ..." Letters, p. 191.)
("To the first approximation" [above] because in that same letter Tolkien
made some subtle distinctions between "creating" and "making", which cannot
be gone into here.) Tolkien stated explicitly in that letter (and several other places) that the Orcs are indeed "a race of rational incarnate creatures, though horribly corrupted". Also that "In the legends of the Elder Days it is suggested that the Diabolus subjugated and corrupted some of the earliest Elves, before they had ever heard of the 'gods', let alone of God." (Letters, p. 191). In fact, the Silm does state that Orcs were Avari (Dark Elves) captured by Morgoth (p. 50, 94), though strictly speaking, the idea is presented as the best guess of the Eldar, no more. Some have rejected the statements on those grounds, that the Elvish compilers of the Silm didn't actually know the truth but were merely speculating. But since Tolkien himself, speaking as author and sub-creator, more-or-less verified this idea, it's probably safe to accept it, as far as it goes.
It has been widely noted that this conception leaves several questions unresolved.