“Nature,” PBS’ 25-years-old-this-year series about life in the natural world, kicked off two weeks ago with a quite unnatural story, got back on track with a visit to the penguins of the Antarctic last Sunday, and this week barrels headlong into the holiday season with a stop in Yellowstone, America’s first national park.

The boys and girls at WNET in New York do keep getting it just right year after year, though delivering these sizzling slices of life in the natural world must become more and more difficult.

To backtrack, go to pbs.org or thirteen.org, click on “Nature” and search “Chimpanzees: An Unnatural History” and “Penguins of the Antarctic.”

Or just jump in this Sunday when “Nature” examines the toughest time of the year for wildlife in Yellowstone. The struggles of resident bears, otters, American bison and other park creatures may evoke empathy but the landscapes will take your breath away. This one’s also available in high-definition, for those who care about such things, and if you miss it this weekend, expect an encore on Christmas Eve.

Coming up throughout winter and spring of 2007 will be the following:

“Best of Nature – 25 Years”: Pretty straightforward, this one, which delivers 90 minutes of greatest moments; host is Lynn Sherr.

“Incredible Hulks”: We’re talking here about rhinos, of which there once were millions on the planet. Rarer today, these beasts are being studied, protected, bred in captivity and, with luck, transferred to more hospitable habitats.

“Andes: The Dragon’s Back”: This title refers to the spiny South American mountain range where environments vary dramatically enough to host such diverse birds as penguins and flamingos.

“Raptor Force”: Another high-def offering, this one pulls out the stops for sheer drama. Filmmakers mount tiny cameras on the bodies of birds with beaks and talons to help viewers understand why they’re called masters of the sky.

“Dogs That Changed the World”: This high-def two-part program shows how DNA analysis and other research has pieced together the puzzle of canine evolution. The upshot? What your fifth grade natural science teacher told you is true. All dogs, from Chihuahuas to St. Bernards, descended from the wolf.