Final answer:
The person believed that race antagonism and the attitude of hate and desire separates whites from true heroism.
Step-by-step explanation:
The question is asking about what the person believed separates whites from true heroism. From the given information, it seems like the person believed that the separation comes from race antagonism and the attitude of hate and desire that white people have towards black people, which is representative of the whole white world's attitude towards black people.
This can be seen in the sentence: 'But that he despising her, hating her for not being white should yet desire her! It seemed to her that his attitude toward her-hate and yet desire, was the attitude in microcosm of the whole white world toward her own, toward that world to which those few possi-ble strains of black blood so tenuously and yet so tenaeciously linked her.'