Without a doubt, my all-time favorite animal is the duck-billed platypus. It appeals to my nonconformist instincts because it breaks so many rules of biology. Consider: The platypus has a flat, rubbery bill, no teeth, and webbed feet, like a duck. Yet it has a furry body and beaverlike tail, and nurses its young like a mammal.

But wait-it walks with a lizard gait and lays leathery eggs like a reptile! And the male can use venomous hind-leg spurs to strike like a snake.

The strange animal stymied scientists for years, and in fact the first platypuses shipped back to England in 1800 were judged frauds. Europeans were still reeling from an expensive and popular fad item: imported "genuine mermaids," which turned out to consist of monkeys' heads stitched to the bodies of fish from the China Sea. They were not about to fall for a bizarre concoction of duck's bill, webbed feet, and beaver's body.

The platypus holds a certain charm precisely because it does break all the rules. ...