[VadeRetro]

What Darwin did was to observe that living creatures have adapted to their environments.

Ongoing, all the time, yes.

He did not accurately explain how this happened in terms of a physical mechanism, because random mutations have not proven to give rise to sufficient sustainable changes.

Even most creationists accept something they call microevolution. I thus cannot even recognize what you think you're grandly proclaiming here. You are making an unsupported assertion which flies in the face of what most people on both sides of this debate know. I suggest you back it up.

In other words, by what mechanism does a species change from land dwelling to sea creatures?

Random variation and natural selection. When Darwin said that, the only evidence that land dwelling creatures become sea creatures was from comparative anatomy. That is, whales are mammals and sirenians are mammals and pinnipeds are mammals and sea otters are mammals, etc. Whales have almost a fish body plan, sirenians less so, seals less yet, and sea otters still look very terrestrial while spending much of their time in the water.

Darwin predicted evidence of actual transitions would be found. They were. Among the items in that preceding link may be found transitional sequences from land animals to whales. There's also a fossil legged sirenian. Virtually none of the items on that link were known to Darwin. They can all be regarded as successful predictions.

It isn't because one day a land creature happened to randomly give birth to a sea creature ...

It's useless to wave your wilfull ignorance of how a thing happens against evidence that it did and does. You don't know what Darwin said. You don't know what modern science is saying. You don't know what you're talking about.

[Williams]

You are quite mistaken. I have never doubted that evolution takes place. What you are ignorant of is that science has NOT found a mechanism for evolution. It happens, whole species change or branch off from the sea to land and vica versa. But the mechanism? Absolutely unknown. You think that all the sea otters that didn't go in the water had all their litters and just by random births the ones that went on and survived better became water adapted. Why? You think there is no why, it is just "mutations." Something goes wrong genetically, this otter has three heads and no eyes and dies, but this one's feet are webbed and it goes on to become a great success. Why would nature cause mutations that lead to ever more complex and survivable and intelligent forms of nature? You want to believe it's just random stuff happening in cells with no plan behind it. Life itself makes no sense from this perspective. Why should nature even produce life? This all leads to the scientist's view taht living things are just randomly constructed hunks of inanimate mechanical matter. And likewise their ability to observe and think about the World is just a randomly arrived at construct. Why? Why does nature care if species survive and adapt? If nature doesn't "care" then why does it happen?

There is an inate plan or design being carried out. That is as obvious as your observations of evolutionary change. Science does not understand the mechanism, much less the reason for these changes.