[GLDNGUN]

The environment in which the original fish (and other creatures) became amphibious and then land dwellers no longer exists. Now there is a great deal of competition from existing amphibians and land dwellers, so there's little opportunity or pressure for sea creatures to become land creatures.

So let me see if I have this straight. The female fish eggs and the male fish sperm just somehow "know" that "there's little opportunity or pressure for sea creatures to become land creatures" so they don't even bother producing fish with little legs. Hmmm. Talk about "fishy". ;-)

[NonZeroSum]

So let me see if I have this straight.

You don't.

The female fish eggs and the male fish sperm just somehow "know" that "there's little opportunity or pressure for sea creatures to become land creatures" so they don't even bother producing fish with little legs.

No. What happens is that any mutation that has a tendency to try to come up on the land has to compete with the creatures already there, and will be unsuccessful, so it won't reproduce. In the past, this would have been a successful mutation. Presently, it would be a failed one.