Final answer:
Thomas Jefferson justified the institution of slavery and racial hierarchy through his belief in polygenesis.
Step-by-step explanation:
Thomas Jefferson justified his belief in polygenesis as a means to justify the institution of slavery and the racial hierarchy that existed during his time. Polygenesis is the belief that different human races come from separate origins, suggesting that Black people were a separate and inferior race compared to the White race. Jefferson's belief in polygenesis allowed him to maintain the contradiction between his advocacy for equality and freedom in the Declaration of Independence and his ownership of slaves.