BARBARA KLEIN: Charles Darwin read widely and sought ideas from other fields of study. He was influenced by Thomas Malthus’ work, "An Essay on the Principle of Population" written in seventeen ninety-eight. Malthus argued that populations are always limited by the food supply.

Darwin would later say that this work caused him to realize the struggle for limited resources was a fact of life. He said small changes took place in individual animals. Changes that helped them survive would continue. But those that did not would be destroyed. The result of this would be the formation of new species.

The British philosopher Herbert Spencer described this struggle as "survival of the fittest." But biologists use the term “natural selection” to describe the evolutionary process.

STEVE EMBER: Charles Darwin developed his idea slowly over more than twenty years. He was concerned that he would lose the support of the scientific community if he revealed it. He wrote to his friend, botanist Joseph Hooker, that speaking about evolution “was like confessing a murder.”

It was not until eighteen fifty-eight that Darwin was forced to release his theory to the public. Another naturalist, Alfred Russel Wallace, had independently written a paper that contained ideas similar to Darwin's concerning evolution. Wallace had reached these ideas from his studies on islands in the western Pacific Ocean.

With help from Darwin's friends, the two naturalists presented a joint scientific paper to the Linnean Society of London in July of eighteen fifty-eight. At first there was little reaction.