About orcs...

I don´t know if you have discussed that before, but i want to know the different theories you may have about orcs´ genesis. I cannot remember what exact references may exist in the books, but i believed there are some whispered rumours saying that orcs could be elfs corrupted by Morgoth and that uruk-hai could be men corrupted by Saruman. Thouhg i didn´t know what sort of corruption was it refering, i always assume that as their true origin. However, yesterday when i was reading an excellent essay about Tom Bombadil´s identity (link provided by Hernalt in other thread), i found these lines: "There are, finally, two accounts given by Tolkien of the origins of the Orcs, both of which cannot be true." Anybody knows what accounts is that refering and why aren´t they possible?