At the same time, it is true that Darwin rejected Lamarck's argument of an inherent tendency to climb the ladder of nature (similarly I accept Yoshie and Carrol's warning against understanding history as an inherent freeing of the individual from social being). And to have explained the emergence of capitalism on account of an inherent human (or European) tendency towards bourgeois society would have been as vacuous as saying that a man is fat because he has an inherent tendency towards obesity. But I do think Marx (wrongly) understood Darwin to have provided a non teleological and causal theory of progress in nature. Today we know that a branching tree is a better metaphor than Lamarck's ladder of nature and that even complexification (as say with greater DNA content of the genomes of later organisms) or a mean increase in fitness (in RA Fisher's terms) do not result from the evolutionary process.
But it seems clear to me that Marx understood capitalism to be progressive by some measure (complexification, productive power) over the modes of production that had preceded it. And it is also seems clear to me that he understood it for some time to be a gift that colonial powers were imposing on their subjects. For Marx capitalism was indeed higher on the evolutionary scale, however discredited such a metaphor may be today. I would love my claim to be disputed or at least nuanced.
Thanks. Rakesh