A man can trust his luck, but a society can't; and cultural change, like random mutation, may make things chancier. So they have gone very slowly. At any one point in their history a hasty observer would say that all technological progress and diffusion had ceased. Yet it never has. Compare the torrent and the glacier. Both get where they are going.
Chapter 8 Another Way into Orgoreyn (p. 98) - Hainish Cycle - The Left Hand of Darkness (1969)