Consider the Gray Whale

Melville spends an entire chapter of Moby Dick on the
difficulty of drawing a whale.
It's really not that hard.

The Gray Whale, as any child can tell you, is a baleen whale.  They were once known as "devil fish" because when humans hunted them, they would fight back, which is sort of a double standard.  It is unknown what the Gray Whales called the humans.

Somewhere about fifty million years ago, some Pakistani Pakecetids decided to spend more time in the water.  They'd been hunting around fresh steams and floodplains, nabbing other smaller animals when they came to get a drink, and they collectively decided, "Let's just go for it."  This meant getting rid of their hooves, which weren't much use for swimming, but they probably figured if the whole aquatic thing didn't work out, they could always turn back.  They were only fooling themselves.

It only took another few million years for the nasal hole to move to the tip of the snout where it would later become a blow hole.  But the big deal was something you couldn't see.  The semicircular canals inside the ear had shrunk to vestigal organs.  This meant these proto-whales, which by this point were calling themselves Remingtonocetids, had passed the point of no return.  If they'd gone up on land, they'd just keep falling over.  No one can survive that way unless you're at a cocktail party where everyone else is doing the same thing.

Around the Miocene, the baleen whales appeared, one of which is the Gray Whale.  Remember?

Anyway, right along with the whales some other animals were doing some evolving too, specifically barnacles.  We don't know as much as barnacle evolution because of natural selection; it's all determined by who gets the opportunity to reproduce.  If you're at a party and tell a pretty girl, "I study whales," you have a much better chance of scoring than if you say, "I study barnacles."  Over time whale scientists reproduce more effectively than barnacle scientists, ensuring their survival.

In any case, it turns out somewhere along the way specific barnacles have evolved to live on the skin of Gray Whales and other baleens.  As soon as a baby whale is calved, a few little barnacle emissaries float off Mamma - in the larval stage barnacles are highly mobile - and latch onto baby.  Once they find a good spot, the barnacles create tube-shaped cavities in their shells that dig into the growing whale skin.  This sounds pretty gruesome to us, but to a Mamma barnacle, it's adorable.  By the time the whale is full grown, it can have 1,000 pounds of barnacles on its back.

Whale tourists think certain whales are friendly, because they'll come up for a "back rub."  In reality they're trying to relieve an itch that they have no other way to reach.  Everyone makes a big deal about how far the whales migrate - between 16,000 and 22,000 kilometers each year - but that's not nearly as impressive as it sounds.  They're whales.  What have they go to do but swim.  What impresses me is the barnacles.  A whale can live up to seventy-five years.  Think of that.  Seventy five years enduring a constant, maddening, unreachable itch.

Some people like to go around saying how wonderful creation is, and the beauty and wisdom of divine providence, etcetera, and how God has his eye on the sparrow and so forth.  God may have had his eye on the sparrow, but evidently he was looking the other way when that first Pakecetids decided to start spending more time in the water.  Someone should have warned them what lay in store for their descendants.

That's the thing about choices.  You're dipping your toes in the water, thinking how pleasant it looks deeper out, and how much opportunity and krill there might be; you don't find out about the barnacles until you've given up your toes and your semicircular canals and it's too late.