Whenever I see this picture I think of this:
| Creature | Swims | Walks | Flies |
| -------- | ----- | ----- | ----- |
| Dog | X | X | |
| Fish | X | | |
| Bird | | X | X |
| Duck | X | X | X |
Notice how for each combination of swimming, walking and flying, there is a living creature that can do just that, with the exception of swimming and flying.
Sure there are flying fish, but they glide rather than fly. Did evolution really forget to cover this niche?
huh, is it restrictive to say that you need some number of legs to be able to walk?
It seems the non-restrictive definition would just be propelling yourself against the ground somehow. At least in this context, where the others are against the air or against the water.