Maybe throw them up in the air and see which side they land on for true natural selection?
Perfect!!

I rarely put my babies on their tummies. I never did "tummy time" where I consciously put them there and I never left them on their tummies if they weren't liking it. I usually laid them down on their backs or else I was holding them (often upright in a sling or lying on their tummy on my chest). They both developed motor skills on the early side of normal.