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在1975的加拿大的动物学家发现，这些蝴蝶并不是什么亡灵，而现在我们比较熟悉的品种--夏天在美国和加拿大产卵、孵化的黑脉金斑蝶之后，从科学的角度而言，它们的故事依然动人。秋末，这些柔弱的几乎没有重量的蝴蝶拍打着翅膀，向南飞行约1500英里（或2400公里），到达墨西哥的几片冷杉林，并在此冬眠、交配。与候鸟不同，它们此前从未有过这样漫长的旅行，这一迁徙也因此成为北美最令人叹为观止的自然现象之一。

EDUARDO ZARZA GARCíA, a 55-year-old Nahuatl indian, remembers that when he was a boy, the arrival of millions of butterflies to the hills above his home in central Mexico was associated with the Day of the Dead on November 2nd. His grandparents would say that the palomas (doves), as they were called, were the spirits of his ancestors, paying an annual visit.

Even after Canadian zoologists discovered in 1975 that the butterflies were in fact the familiar monarchs that laid eggs and hatched during the summer in the United States and Canada, the science was no less alluring. In late autumn, the almost weightless butterflies flutter about 1,500 miles (2,400 km) south to reach a few clumps of fir trees in Mexico where they hibernate and mate. Unlike migrating birds, none has ever made the journey before, making it one of North America’s most extraordinary natural phenomena.

So it should be an inspiring symbol for North America, whose three leaders met on February 19th in Toluca, within 35 miles (56km) of the Piedra Herrada butterfly sanctuary, to celebrate the 20th anniversary of the North American Free Trade Agreement (NAFTA). But it isn’t. Data collected by the Mexican environment ministry and the World Wildlife Fund, a charity, show that over the last two decades, the size of the monarch population in Mexico has plummeted — at least as measured by the number of hectares they occupy. In a letter ahead of the summit, campaigners urged President Barack Obama and his counterparts to address the issue.

Traditionally, the finger of blame for the declining numbers has been pointed at the Mexicans. Illegal logging has affected much of the fir cover that the butterflies need to keep warm. However, the environmentalists who signed the letter grudgingly acknowledged that the Mexicans were becoming better custodians.

Not so Americans and Canadians, who they blamed for using modern farming methods that wipe out the milkweed on which the butterflies’ larvae feed on the insects’ staggered journey north, also hitting their numbers. The monarchs’ defenders say increasing use of genetically modified crops, which are resistant to herbicides, encourages farmers to spray a weedkiller, glyphosate, on their fields, destroying milkweed. They propose establishing a “milkweed corridor” along roadsides and on the edge of fields along their migratory route, especially in the United States and Canada.

The issue is likely to be ignored at the summit. But it should not have been. At a time when NAFTA is badly in need of a new impetus, the rescue of the monarch could have been a compelling metaphor. It is, after all, an example of seamless borders: the offspring (call them “dreamers”) is born in the United States and Canada; the oldies (call them sun-seekers) flock to Mexico. It also shows shared enterprise: The mating (call it assembly, or maquiladora) occurs in Mexico; the finished product delights everyone further north.

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