Final answer:
Dubois argues that slavery allows the unfit to survive and prevents natural leadership among the enslaved.
Step-by-step explanation:
In his statement, Dubois is expressing his view that slavery is a system that allows those who are considered unfit or not capable of leading naturally to survive and thrive, while nullifying the potential for natural leadership among the enslaved individuals. He believes that slavery degrades the intellectual, moral, and natural abilities of those who are enslaved, while simultaneously benefiting those who are in power and perpetuating the institution.