Looking for a science-fiction short story written before the 1970s, which involves starship travel in the 1950's style. Written from the point of view of a male researcher, who encounters a planet of humans who have stalled at a technologically primitive level despite high intelligence.
I remember a line from a kid:
Hey Mister, could I have a cracker for my Megatherium?
The humans see the Earthlings using wheeled carts, and think, ~"odd round legged little tables."
Over the course of their stay, the primitives go from using unwheeled sledges to inventing roller bearings, and the male researcher thinks they could advance quickly, but not so quickly as to be a threat to his galaxy spanning civilization.
Twenty years later, it didn't seem so funny.