I just finished reading Robert Heinlein's "The Rolling Stones" and was a bit surprised to see "Tribbles" in the story. In "The Rolling Stones" they are called Flat Cats. The physical description is the same, the hypnotic purring is the same, even the massive reproducing is the same along with them threatening the ship's food supply! I've read where David Gerrold swears he didn't steal the idea from Heinlein but I have trouble believing that he came up with the same idea that just happened to have been thought of by someone else first. I think he probably read Heinlein's work and the Flat Cat idea stuck in his subconscious mind only to re-appear as tribbles later.