"First, do no harm" is not a bad principle

The Deposition, Raphael's sixteenth-century depiction of the Apostles carrying Jesus' lifeless body from the cross, is the rare painting the eye can't release—the kind that connoisseurs travel halfway around the world to see and which you typically have to elbow through a crowd merely to glimpse. But at Rome's Borghese Gallery and Museum this summer, at the height of the tourist season, I had the masterpiece all to myself.

The Borghese is not a place you can stroll into on a whim: You have to have a reservation (you can make one online). This requirement enables the museum to strictly control the number of visitors, which both protects the art and makes it possible to enjoy the collection the way the cardinal who amassed it did—without having to battle for position. With travelers running roughshod over some of the world's most fragile places, we could learn a lot from the Borghese.

In Antarctica, for instance, the number of tourists has climbed 90 percent in the past six years, the penguin population has declined, and a cruise ship recently ran aground, spilling 200 gallons of diesel into the sea. The United States and several other nations are pressing for a limit on visitors to the continent, but at the moment these are little more than good intentions—and we all know where those lead.

The new rail line linking Beijing and Lhasa has been hailed as one of the world's great engineering feats, but it may not be so great for Tibet: It's projected to pump up tourist traffic by 15 to 20 percent a year, and Lhasa's Potala Palace has already raised its daily visitor limit from 1,500 to 2,300 to accommodate the influx. But at least it has a limit. Uncontrolled tourism just won Machu Picchu a spot on the World Monuments Fund's 2008 Watch List of the 100 Most Endangered Sites, and in June, alarm bells were sounded for Ecuador's Galápagos Islands when UNESCO put them on its World Heritage in Danger List. A ship that can carry up to 500 passengers began shuttling tourists to the islands last year, a major hotel boom is under way, and there has even been talk of a casino. The very creatures that led Darwin to his epiphany are now in danger of disappearing under beach towels and poker chips. "The Galápagos is a fragile place that should be exclusively for small-scale, nature-based tourism," says David Blanton, president of the nonprofit International Galápagos Tour Operators Association, which promotes the protection of the islands. "We hope that restrictions will be put in place before the islands are overrun."

Perhaps the most powerful example of tourism's mixed blessing can be seen in Cambodia, where the once-sleepy town of Siem Reap became a tourist mecca almost overnight: A million visitors are expected to descend this year on the nearby temples at Angkor Wat—a 400 percent increase over 2001. Revenues from the boom have paid for the restoration and preservation of the temples, and hotel construction has provided residents with jobs. But the town's water supply is quickly being drained to keep hotel pools filled and new golf courses green. "The quality of life for the locals is worsening," says Bonnie Burnham, president of the World Monuments Fund. "At some point, they may not have any water."

For 20 years, we've observed with some measure of pride that Condé Nast Traveler readers view travel as a right, not a privilege. Now, as the world's iconic destinations begin to literally crumble underfoot, it's time to accept that this right comes with a responsibility: First, do no harm. Call it the traveler's Hippocratic oath. But we can't do it alone. The governments that control these world treasures must stop using them as national mints and start setting limits on the number of people permitted to visit. Unless preservation becomes at least as important as economic development, we'll all end up the poorer. The Borghese has it right. Let's hope that life starts imitating art before it's too late.