A correction. Marx was NOT a Darwinist.
First he formed his ideas BEFORE Darwin.
Second, his word-view was not compatible on random mutations followed by free market like elimination of the weak. It was not based on racism either or irrational struggle for its own sake.
His theory of history was derived from Hegel, so he believed that historical process is guided by unifying reason and has final purpose.
That is why under Stalin, the late Marxists led by Lysenko attempted to defeat Darwin.
When American liberals in their fight against Protestant Fundamentalists try to bash them with dinosaurs, unwittingly they follow Hitler and not Marx.