Progress as a word describing a process. Im aware, however, that one mans progress can be another mans excrement sandwich. Motives and objectives define progress. Progress can be used as a political tool (the Progressive Party or progressive political policies contrasted to anothers ill-conceived or evil intentions) suggesting a superiority of political motives and goals. Im sorry, but I just cant get very enthused over schools of thought, preferring ideas instead.
Darwin was mystified by the extension of his insight to things other than biological species
Im not sure mystified, or simply horrified. In any event, I take it that he treated the interested attentions of Marx as though encountering a skunk under the front porch. Marx was thrilled to make a connection between Darwins science and Marxian political and economic theories. Darwin couldnt see it, and refused to affirm the relationship. Avoided it like the plague, as a matter of fact.
Improvement is the meaning as used by sociologists such as Marx and by historians.