Is there a natural way to substitute mascara?

Vaseline if course! It works so great, looks just like mascara you'll have all your friends fooled! Don't tell anyone though its our little secret ;)

Or you can make your own using the same ingredient used for a long time, even in ancient times, and that is guano (bat dung). However, you'd want to make sure that the droppings come from healthy bats that don't have rabies or other diseases. However, you can't get any more natural than this. That is likely a secret that you didn't want to know.