When Darwin's The Origin of Species was published in 1859, it was believed that he had put forward a theory that could account for the extraordinary variety of living things. He had observed that there were different variations within the same species. For instance, while wandering through England's animal fairs, he noticed that there were many different breeds of cow, and that stockbreeders selectively mated them and produced new breeds. Taking that as his starting point, he continued with the logic that "living things can naturally diversify within themselves," which means that over a long period of time all living things could have descended from a common ancestor.

However, this assumption of Darwin's about "the origin of species" was not actually able to explain their origin at all. Thanks to developments in genetic science, it is now understood that increases in variety within one species can never lead to the emergence of another new species. What Darwin believed to be "evolution," was actually "variation."