The idea of evolution and that earth's species descended from common ancestors had already been around for some time when Darwin published On the Origin of Species in 1859. What was new about his work was that it explained evolution, using a theory called natural selection. This claimed that species change in small ways, gradually, over long periods of time; the individuals best suited to their environment survive and live to produce offspring with the same traits that they have. Darwin's theory challenged the idea that God created plants and animals in their finished and perfect state and radically changed scientific and religious thinking for all time.