pencils could be found anywhere in the box. Yet most small-ranged species are still found near the equator. Colwell agrees that the mid-domain effect is not the sole explanation for the latitudinal gradient in diversity, but argues that it was never meant to be. It is, he says, one more factor that must be taken into account. It is also a reminder that, before we propose an explanation for a biological trend, it helps to work out exactly what needs to be explained and gives a glimpse of what biology would look like if there were nothing biological to explain. Models such as the mid-domain effect and neutral ecology, which treat species or individuals in the same way that theories from physics treat particles, looking for the emergent properties in their behavior without worrying about the biological causes, are proving fruitful in this regard. Often the results they produce are very different from what we would have assumed—a world without ecology would not be a world without patterns.

The Importance of History

Evelyn Hutchinson described nature as an evolutionary play acted out on an ecological stage. All the ideas about tropical diversity that we have encountered so far—the school of thought that Humboldt founded—are ecological. They deal with the stage and offer reasons why the tropics might be able to hold more actors. They ignore history—what has been going on in the drama up until the point we began watching. The other main strand of ideas used to explain the latitudinal gradient in diversity looks instead to history, suggesting that the play’s script has been different in different parts of the earth. To put it another way, past events might control diversity more than current conditions. Another great scientific traveler, Alfred Russel Wallace, founded this intellectual lineage.

Born in Wales in 1823, Wallace came from a family impoverished by his father’s succession of business failures. Alfred had little formal schooling and left home at 13. Soon after, his elder brother William began to train him to be a surveyor. While learning his trade, Alfred became interested in geology and natural history.

When their father died in 1843, William could no longer afford to



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