I read it back in junior high, and google isn't helping. EDIT: Someone pointed out how ambiguous that is. Sorry. I would have read it about a decade ago, but I think it's from the seventies. From what I remember, a group of astronauts lands on a planet that has a bunch of animals on it. What they observe is that some of the animals have two limbs, some four, some six, some eight. What they eventually found strange was that animals seeming to be of the same level of evolutionary development would have such a disparate number of limbs. Looking at Earth, for example, what we might think of as "upper level" animals have four limbs. Then they notice other things that hint that the animals all may be from different planets. They conclude that they're in some kind of zoo or collection. I don't remember if they get off the planet or anything.
Curiously I was led to this answer by another question/answer on this site, though the question description there makes no mention of evolution (or legs).
'Catch Em All Alive!' is a predictable story of two zoological hunters on a new world who discover they have become the latest exihibits.
I’ve seen some fairly strange creatures since I was assigned to the zoological department, but this one took the grand prize.
It was about the size of a giraffe, moving on long, wobbly legs and with a tiny head up at the end of a preposterous neck. Only it had six legs and a bunch of writing snakelike tentacles as well, and its eyes, great violet globes, stood out nakedly on the ends of two thick stalks.
“Hold it, fellows,” I said. I peered at the animal in Davison’s hands and glanced up. “This is a curious beast,” I said. “It’s got eight legs.”
“You becoming a zoologist?” Holdreth asked, amused.
“No—but I am getting puzzled. Why should this one have eight legs, some of the others here six, and some of the others only four?”
They looked at me blankly, with the scorn of professionals.
“I mean, there ought to be some sort of logic to evolution here, shouldn’t there? On Earth we’ve developed a four-legged pattern of animal life; on Venus, they usually run to six legs. But have you ever seen an evolutionary hodgepodge like this place before?”
“Forget the giraffes. They tried to warn us, but it’s too late. They’re intelligent beings, but they’re prisoners just like us. I’m talking about the ones who run this place. The super-aliens who make us sabotage our own ship and not even know we’re doing it, who stand someplace up there and gape at us. The ones who dredged together this motley assortment of beasts from all over the galaxy. Now we’ve been collected too. This whole damned place is just a zoo—a zoo for aliens so far ahead of us we don’t dare dream what they’re like.”
I looked up at the shimmering blue-green sky, where invisible bars seemed to restrain us, and sank down dismally on the porch of our new home. I was resigned. There wasn’t any sense in struggling against them.
I could see the neat little placard now: Earthmen. Native Habitat, Sol III.