Earlier quoted context omitted.
I don't know what Darwin's Descent of Man says, but considering the immediate reaction, I'd say it's not worth defending.
It probably wasn't. Better to write and defending texts that can actually change the public position until the Descent of Man becomes not quite as unpalatable, and therefore eventually worth defending. I'm prefer effectiveness over martyrdom.
Also, the strength of the reaction against a non-conformist text is not really strongly correlated with the degree to which it's non-conformist. Dogmatics police small deviations precisely because they don't want an incremental strategy to work.