Equality [of the races] is not given a priori. It is neither an ethical principle (though equal treatment may be) nor a statement about norms of social action. It just worked out that way. A hundred different and plausible scenarios for human history would have yielded other results (and moral dilemmas of enormous magnitude). They didn't happen.
The Contingency of History
The idea that historical and evolutionary outcomes are accidental results that could easily have turned out otherwise, rather than inevitable or predetermined.