as the scientific discovery of America, and it became the foundation of Humboldt’s life’s work. The accounts of his voyage inspired a generation of nineteenth-century naturalists. He was a hero to Darwin, who described him as “the greatest scientific traveler who ever lived” and joined the crew of the Beagle to follow in Humboldt’s footsteps. He recommended Humboldt’s writings to his friends and once wrote: “I shall never forget that my whole course of life is due to having read and re-read as a youth his Personal Narrative [of the American expedition].” One twentieth-century biographer wrote that Humboldt “combined meteorology, geography, geology, botany and zoology and, single-handed, created the science of ecology.”
Back in Paris, Humboldt spent his inheritance—within a few years he was nearly as poor as Bonpland—on publishing his observations and ideas from the voyage. The core of his thinking about tropical diversity is contained in an 1807 essay, Ideas for a Physiognomy of Plants. Like Willdenow, he equated warmer climes with more species:
The verdant carpet which a luxuriant Flora spreads over the surface of the earth is not woven equally in all parts; for while it is most rich and full where, under an ever-cloudless sky, the sun attains its greatest height, it is thin and scanty near the torpid poles, where the quickly-recurring frosts too speedily blight the opening bud or destroy the ripening fruit…. Those who are capable of surveying nature with a comprehensive glance, and abstract their attention from local phenomena, cannot fail to observe that organic development and abundance of vitality gradually increase from the poles to the equator, in proportion to the increase of animating heat…. It is beneath the glowing rays of the tropical sun that the noblest forms of vegetation are developed.
Discoveries made since Humboldt marveled at the Orinoco have only reinforced our view of tropical life. There are not just more plant species in the tropics, there are more woodpeckers, ants, and monkeys—and birds, insects, and mammals in general. The same goes for parasites: Most human diseases are tropical diseases, and the tropics harbor more pathogens of all kinds. Even cultural diversity follows a similar pattern. In sub-Saharan Africa, the areas that contain the most species also contain the most linguistic groups.