Fixed.
Both Chris and Orion seem to have hit the magic combo - worked exactly like I wanted. Tried Chris's, spotted Orion's (which looks almost identical) solutions, and it's all good.
Thanks guys!
Any deity worthy of a graven image can cobble up a working universe complete with fake fossils in under a week - hey, if you're not omnipotent, there's no real point in being a god. But to start with a big ball of elementary particles and end up with the duckbill platypus without constant twiddling requires a degree of subtlety and the ability to Think Things Through: exactly the qualities I'm looking for when I'm shopping for a Supreme Being.