At the very end of a book I can manage to work for longer stretches, but mostly, making stuff up for three hours, that's enough. I can't do any more. At the end of the day I might tinker with my morning's work and maybe write some again. But I think three hours is fine.
Although I insist that God has always had the power to intervene directly in nature to create new forms, I am willing to be per-suaded that He chose not to do so and instead employed secondary natural causes like random mutation and natural selection.