I don't think it's safe to use a static variable here, because then the value will be shared between random number generators that should be separate, so calling `Normal` on one `Random` object will affect the results of a subsequent call to `Normal` on a *different* `Random` instance. We use RNGs for procedural generation of systems and so on, so it's very important that we can get a consistent sequence that isn't affected by what other code is doing.