In the years following Darwin's return home from the journey of the Beagle, other naturalists were continuing to build collections of diverse species. For example, two other eminent collectors of Darwin's time also made contributions to the developing theory of evolution, Amazon explorers Henry Walter Bates and his schoolmate Alfred Russel Wallace.

From 1848 on, Bates and Wallace explored the Amazon Basin, then terra incognita for scientific collectors. Working together, they first noted that geographical factors had a great deal to do with where different species are found, a truism today that took their research to recognize. Bates in particular is known for his discovery of the principle of "mimicry," whereby a creature adopts the appearance of another species in the interest of his own self-preservation. The classic example is the butterfly with "eyes" on his wings, as if to appear a much larger, more dangerous animal. Wallace left the Amazon in 1852 to continue his collections and researches in Indonesia, while Bates stayed on until 1860.

Bates, Wallace, Thomas Huxley as well as Darwin and others sent literally thousands of samples back to museums and universities in Europe -- of birds and beetles and spiders and insects of all kinds, as well as curious mammals and reptiles.The presence of hundred of thousands of species made the old idea of creationism seem irrelevant. In most cases, these similar species had slight differences that made them better adapted to their specific geographic, climatic or other environmental situation: Did a God really have time to create dozens of closely related species that nonetheless differed from each other and could not interbreed, or was there another answer grounded in science, not faith?

Clearly there was an evolutionary force at work, on this most intelligent people agreed. What was missing was the key element -- not that it did work, but how it worked. There were two basic schools of thought on the questions. If species diversified, did they do so because they passed on developed characteristics from one generation to the next, as proposed by Lamarck early in the 19th century? Or was there another mechanism in nature that eventually led to the adaptation of species to their environment over the course of time?

Darwin's own travels and observations aboard the Beagle, especially during his Galápagos visit of 1835, resulted in the publication of his Voyage of the Beagle upon his return to England. Though his curiosity was certainly piqued by his travels, his later work kept him at his family's country estate, where he delved deeper into geological and biological observation. He wrote on coral reefs, barnacles, the fertilization of insects, insectivores and climbing plants, and bred pigeons as a hobby. These diverse researches proved invaluable when he eventually wrote on evolution itself, for his conclusions came out of observation and data, in the true scientific tradition, rather than speculation. Ironically, it was this very thoroughness that almost cost him the Big Prize -- history's mantle as founder of the theory of evolution. For while he pursued his garden researches, Alfred Russel Wallace was in the far more inspirational laboratory of the natural world.

Wallace had been in Indonesia since 1854, gathering a plethora of new species and speculating on the role of geography in the diversion of types. In the so-called Spice Islands, on the island of Ternate -- where the aromatic clove tree is endemic -- Wallace set up a base for his wide-ranging travels in 1858. Ironically, the Spice Islands also straddle the equator, just as do the Galápagos.

On Ternate, in the midst of a malarial fever, Wallace had a flash of insight. He put together the work on population dynamics by Malthus with his own extensive catalog of new species, and came up with the obvious answer: natural selection. Populations in nature are under stress from predation, disease and environmental change; variations occur naturally from one generation to another, due to the characteristics of heredity; therefore any "mutation" that serves to further the adaptability of a species can lead to the separation of a new species from the root stock of the old. Hence, the origin of species. "Natural selection" thus becomes the engine that drives the creation of species, and their demise -- the key to evolution.

Wallace wrote up his theory and sent it off to one of his correspondents in the scientific world, a companion beetler and author of Voyage of the Beagle. Darwin was stunned: here was the missing piece of the puzzle, proffered in innocence by a young collector on the far side of the globe. Darwin may already have identified the mechanism by of natural selection through his own researches, though he had not yet written on the subject. But in science, first out of the gate gets the prize, and Wallace unwittingly was threatening over 20 years of Darwin's work.

Darwin's supporters urged him to go public with his own theories on the matter, and in a generous gesture it was Darwin himself who presented Wallace's paper to the Royal Geographic Society later that same year, 1858. Darwin did so in association with a paper of his own on evolution, though natural selection was not mentioned in his writings at the time. But a year later, when On the Origin of Species appeared, natural selection was the key element to make the whole theory hang together, and modern science was born.

It was the bombshell Darwin had anticipated. His years of study and research had produced a closely reasoned, scientifically compelling argument for natural forces as the determinants of the creation and extinction of species. In contrast to Wallace's fevered inspiration, Darwin's methodology produced the kind of air-tight argumentation that could withstand the outraged antipathy of religious conservatives. In the scientific community Darwin's theories were embraced almost overnight; but even in the public mind, once the arguments became popularized and understood, the theory of evolution took its place among the verities of common sense.

Ironically, though Darwin continued to write on evolution -- his The Descent of Man outlined the evolution from earlier primates and apes to Homo sapiens -- one of the greatest popularizers of his theory became Alfred Russel Wallace himself. Wallace returned from Indonesia in 1861 to find his febrile inspiration, natural selection, the hot topic of parlor conversation as well as scientific argumentation. It turned out to be Wallace himself who coined the term "darwinism" -- in apparent innocence of the fact that without Wallace himself, there may never have been such a thing.