In his review of Darwin's Sacred Cause, by Adrian Desmond and James Moore, Terry Sullivan (Books, Socialist Review, February 2009) gives some praise to the book, but ends by saying that it is not totally convincing.
Desmond and Moore argue that Darwin's strong anti-slavery views fed into his evolutionary theory. Darwin rightly believed that all humans were the same species with a common ancestor, and he broadened this out to argue that all living creatures evolved from a common ancestor.
In that case, asks Sullivan, why did Darwin hardly mention humans in On the Origin of Species? I can't agree with Sullivan here. Darwin did not want to say much on humans at that time because he was terrified of losing his respectable social position by explicitly positing "ape-man" ancestors. He was also always reluctant to publish theories without having first accumulated evidence to back them up.
The fact that Darwin says nothing in his autobiography about his evolutionary theory being influenced by anti-slavery views is more of a problem for Desmond and Moore. Though it does not prove them wrong.
But I do think that Desmond and Moore are wrong to state that shared ancestry was "the unique feature of Darwin's peculiar brand of evolution". It was in fact the mechanism of natural selection that made Darwin's (and Alfred Russel Wallace's) brand of evolution unique.