In his studies on the Galapagos Islands, Charles Robert Darwin observed patterns in animals and plants that suggested to him that species changed over time to produce new species. Darwin collected several species of finches; they were all similar, but each had developed beaks and bills specialized to catch food in a different way. Some species had heavy bills for cracking open tough seeds, while others had slender bills for catching insects; still another species used twigs to probe for insects in tree cavities. All the species resembled one species of South American finch. In fact, all the plants and animals of the Galapagos Islands were similar to those of the nearby coast of South America. Darwin felt that the simplest explanation for this similarity was that a few species of plants and animals from South America must have migrated to the Galapagos Islands, then changed as they adapted to the environment on their new home, eventually giving rise to many new species. These observations led to part of his theory on evolution—that species change over time in response to environmental challenges.

The work of Darwin and Wallace generated controversy on at least two fronts. First, their theory directly countered Christian teaching on how species do not change and all organisms being special creations by God. Second, by presenting arguments for common descent, Darwin and Wallace showed that humans were related to other animals— apes in particular. This insulted those who felt that humans were unique and not part of the animal kingdom. However, it should be noted that as the work was read among scientists, it gained general acceptance in the late decades of the nineteenth century.