Why did slavery spread in the Spanish colonies?

Slavery spread throughout the Spanish colonies because the natives that were forced to work for the Spanish under the Encomienda system were under harsh conditions, which decimated much of its population. So the Spanish didn’t have enough natives to work on the sugar plantations, mines, etc., so they went to depend on the African slaves who they saw as “dispensable”, and there were a large amount of slaves being traded. The Spanish also treated the slaves with harsh work conditions, and if they died or got ill, they easily replaced them.

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(d). gave voting rights to free african americans and white women.