Charles Darwin and the Golden Rule

In a footnote to his Descent of Man, Charles Darwin wrote:

“Nor is it probable that the primitive conscience would reproach a man for injuring his enemy: rather it would reproach him, if he had not revenged himself. To do good in return for evil, to love your enemy, is a height of morality to which it may be doubted whether the social instincts would, by themselves, have ever led us. It is necessary that these instincts, together with sympathy, should have been highly cultivated and extended by the aid of reason, instruction, and the love or fear of God, before any such golden rule would ever be thought of and obeyed.”1

How strange that the man who could make such a rational and obvious statement on the source of morality, could also be the prime mover of the theory which has led to such a widespread disbelief in God, and consequent increase in immorality.