“Evil cannot create anything new, they can only corrupt and ruin what good forces have invented or made.” -- J.R.R. Tolkien

Not entirely true, but mostly true.

EDIT: In reality, this isn't a direct quote but a paraphrase of two things J.R.R. wrote in his books:

"No, they eat and drink, Sam. 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 the orcs, it only ruined them and twisted them; and if they are to live at all, they have to live like other living creatures." -- The Return of The King

For the Orcs had life and multiplied after the manner of the Children of Ilúvatar; and naught that had life of its own, nor the semblance of life, could ever Melkor make since his rebellion in the Ainulindalë before the Beginning: so say the wise. -- The Silmarillion