This is a classic.
For what Mencius identifies as the religious progressive tradition, nothing can just happen to be true. Discovering something new about the world would not have been sufficient reason for Darwin to have formed the theory of evolution by Natural Selection. For it to have been worthwhile there must have been some ulterior political motive.
It's always a bit of a shock to find that one's allies are right only for bad reasons. One of the most startling books I've read is Correlli Barnett's The Lost Victory, in which he showed quite convincingly that the Western leaders who argued against Soviet communism on the basis of the efficiency of the market system did not in fact believe their own (correct) arguments.
In the same way, it seems the ruling class of the 20th century accepted and promoted scientific knowledge not because it was true, but because it was politically convenient. We were just lucky it happened to be true too.
Darwin, whatever Desmond and Moore said, had no such attitude. It is very telling that the modern centre-left projects its priorities onto him.