I'd say it's rather Euro-centric to assume caste and race are the same. Caste is more like institutional feudalism than it is like race—the people of different castes are not genetically dissimilar. As for the Arabs and slavery, some had prejudices that were much like those of the Greeks—Africans might be assumed to be inferior, but that seems to have been more a case of using skin as a tribal marker. Eric Williams' observation applies there, too: racism was born from slavery for Arabs as well as Europeans.