lack of oxygen—making him the first person to diagnose the cause of altitude sickness. He also noticed that gaining altitude had the same effect on the vegetation as traveling north. As he climbed, so the tropical forest gave way to cypresses, oaks, and pines, much like the trees in European forests:

The extraordinary height to which not only individual mountains but even whole districts rise in tropical regions, and the consequent cold of such elevations, affords the inhabitant of the tropics a singular spectacle. For besides his own palms and bananas he is surrounded by those vegetable forms which would seem to belong solely to northern latitudes…. Thus nature has permitted the native of the torrid zone to behold all the vegetable forms of the earth without quitting his own clime.

The key to this changing vegetation, and therefore the explanation for tropical diversity, Humboldt suggested, was water. As you move away from the tropics, or gain altitude, freezing temperatures became steadily more common. In such an environment, plant life is suspended for much of the time: “Nature undergoes a periodic stagnation in the frigid zones: for fluidity is essential to life.” Humboldt believed that freezing was a stricture that relatively few forms of plant life could adapt to. Only those that could either withstand the cold or shed their leaves and wait for spring would survive. In the more permissive equatorial environment, life ran riot. “The nearer we approach the tropics, the greater the increase in the variations of structure, grace of form, and mixture of colours.”

A Harsh World?

The idea that the tropical climate is more conducive to life, and so leads to a greater variety of it, is a persistent theme of explanations for tropical diversity. But it is a more slippery notion than it first appears. The idea carries a powerful whiff of circularity. The tropics have lots of species because they are a benign environment. How do we know they are a benign environment? Because they have lots of species. We usually distinguish an environment as harsh or lush by its abundance of life, but this says nothing about what in that environment might cause that abundance or scarcity.

Anyway, who’s to say what’s harsh? True, environments that seem

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