To our eyes today, both these elements seem a bit naive. Most artist have now long since given up on the idea that there is any kind mainstream art, much less any linear development of it. It seems the our definitions of art have become too broad for it to have a mainstream. And, if ever there WAS a time when art had much of an impact on society's major problems, I think we're safe in saying it has now passed. WHEN did it pass? That's a little harder to say. Possibly around the end of the 1960s when art historians started talking about the POST Modern age. But the transition wasn't like passing through a door; it was more accurately like entering a fog.
Although there are those who might cite others, Paul Cezanne is often credited with having been the FIRST modern artist. Although the Impressionists fulfilled the first criteria of Modernism to some degree, they were not so much interested in linear DEVELOPMENT amongst the Impressionists, but in REBELLION against what the generation before had wrought. Cezanne, on the other hand, tried to take Impressionism, and as he put it, "...make something solid of it." Which IS linear development, and on top of that, Cezanne was very much taken with the idea that art could have a profound civilizing effect upon mankind. Perhaps in his day, it could, and did. Whatever the case, whatever Modernism means, Cezanne certainly fills the bill as the first of them.