Evidence of evolution

I don’t remember for sure, but I think it was in a book by Dawkins that I ran across an argument for the evolution of marine mammals from land mammals: to wit: marine mammals propel themselves through water by wagging their tails up-and-down instead of side-by-side like fish.

Even if this indicated common ancestry, I don’t see how it indicates the direction of descent. Why not say that land mammals were descended from marine mammals?

But that question aside, dogs wag their tails side-by-side, and I also observed, on a recent walk, that ducks also wag their tails side-by-side. So do crocodilians.

So, by evolutionary logic, land mammals and marine mammals belong to one evolutionary tree while dogs, ducks, and alligators belong to a different evolutionary tree.

In addition, if marine mammals are descended from land animals since they both wag their tails up-and-down, then, by parity of reasoning, ducks, as aquatic animals, are descended from dogs or protodogs since they both wag their tails side-by-side.

Now all we need to cinch the argument is a fossil dog with a duckbill and feathers.

Where would we be without the theory of evolution to help us sort things out?