Darwin did NOT argue this, anywhere, but certainly not in The Origin of Species. Humans and human evolution aren't even discussed in that book.
Try The Descent of Man. Darwin does make various points that assume the cultural superiority of Europeans. There are also some passages that are ambiguous as to whether you might interpret them as referring to biological or cultural causes. But there is no outright racism in Darwin, especially not by the standards of his time, in comparison to which Darwin was very liberal on race.
For instance Darwin is EASILY less racist than Abraham Lincoln, who DID explicitly believe that blacks were biologically inferior, even though he thought they still deserved completely equal political and moral treatment. I'm not putting down Lincoln, btw. I'm a huge fan of Lincoln. As to the issue of race I evaluate him in historical context and reject the ahistorical, leftist gotcha tactics.
Yes, you're right. I should have said Descent of Man.