West African slavery

Was West African slavery affected by racial attitudes? In what way? What significance should be attached to the observation that the Portuguese obtained most of their slaves through trade with African dealers? Conditions on the other side of the Atlantic are relevant too; before the importation of Africans to the Americas, the Spanish enslaved many Native Americans, especially in the Caribbean. How successful was this effort, and how did it foster the development of the transatlantic slave trade? What role did the geography and climate of the Americas play?