According to communists, what is the key to improving human society?

Reynolds's political map of the united states shows free states, slave states, and areas open to be free or slave after the repeal of the missouri compromise. reynolds’s political map of the united states showing comparative area of the free and slave states following the repeal of the missouri compromise. southern states are gray. northern states and california are red. territories to the west are green. kansas is not filled. by reynolds [public domain], via wikimedia commons how is the compromise of 1850 reflected in the map?