Forests are not the only environment where diversity peaks in the tropics. Grasslands contain more species nearest the equator and so do deserts. More species dwell in tropical lakes and rivers than in temperate fresh water. In the sea, diversity in fish, molluscs, and plankton peaks at the equator. Tropical seas contain more species both along coasts and in the open ocean. Nor is the diversity gradient a recent thing. Fossils, including those of trees and foraminifera, the planktonic creatures beloved by D’Arcy Thompson, show that the tropics have contained more species than the temperate zones for at least the past 250 million years.

The diversity gradient is pervasive and incontrovertible. But it is general and qualitative, and there are many complications and exceptions to the general pattern. In some groups—such as aphids, seabirds, and marine mammals—diversity peaks in the temperate regions. Points at the same latitude in different continents have different numbers of species. The steepness of the diversity gradient varies between groups and between the northern and southern hemispheres. And although diversity gradients have probably been constant throughout the earth’s history, the fossils suggest that they are steeper now than at any point in the past 65 million years. Nevertheless, such a widespread phenomenon cries out for a general explanation. Humboldt saw a clue as to what this might be in another pattern in diversity.

Humboldt could never see a mountain without needing to stand on its summit, and in the Andes this obsession led to an attempt on the extinct volcano Chimborazo, in Ecuador. The mountain is 20,700 feet high; at the time it was thought to be the world’s highest. Bonpland and Humboldt made it up past 19,000 feet, higher than any human had gone before, until a wall of ice and snow blocked their route to the summit. When news of the explorers’ feat reached Europe, it knocked Napoleon’s conquests off the front pages. Napoleon was not impressed. (Back in Paris, Humboldt was presented to the emperor: “I understand you collect plants, monsieur,” he said. “Yes, sire,” Humboldt replied. “So does my wife,” Napoleon responded.)

As he ascended Chimborazo, Humboldt decided that the bleeding gums and other unpleasant symptoms he was suffering must be due to

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