Well, that's only true in the biological cycle but what are the physical proof that whales and dolphins ever lived on land before, seeing we also have reptiles living both on land and water? The biological evolution is highly questionable and lack proofs because because if a specie developed new features and change environment , we should'nt have other species living and surviving in the harbitat others migrated from. Example, if Africans are living in Africa because they are blacks, then only blacks should live in Africa. But if both white and black people can live in Africa, there's no justification that we are here because we are black.