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."