Is Morality Human Instinct?

The argument is that what C.S. Lewis calls the Moral Law, or an innate moral agreement all humans share, is simply a developed instinct for species survival. Lewis admits that this may be the case, but there is also the instinct for self-preservation which often runs against species survival. Why is it that we know the “right” choice is to help the other person in need and not self-preservation? Lewis argues that it is because there is a third, mysterious inner heart that decides which instinct is more moral.