What a load of bullshit. Most of the great civilizations were monoracial bar some slight admixtures inevitable with including other nations into the empire. The Chinese were mono-racial, the Egyptians, the Mayans, the Romans and Greeks, as well as the British and French. White people not only kept their whiteness after conquering colonies, they actually acted as population donors, spreading their seed far and wide, which is why there are now blond blue-eyed people from Canada to New Zealand. Victorian Britain was incomparable to modern times, when you can barely find a British child in a British school.