Evolution is the change of inherited characteristics within a population over time through natural selection, which may result in the formation of a new species.
Darwin ideas were documented in the book On the Origin of Species, which was published in 1859. The naturalist's ideas created controversy in Victorian society.
The theory of evolution through the process of natural selection was only gradually accepted because:
the theory challenged the idea that God made all animals and plants that live on Earth (creationism)
there was insufficient evidence when the theory was published to convince many scientists
the mechanism of inheritance and variation was not known until 50 years after the theory was published
Some scientists were reluctant to change their minds about the ideas of creationism, even when new evidence was discovered that contradicted their ideas.
Darwin's book, On the Origin of Species, was a world best seller and is still in print today. With every new edition of his book, more evidence was discovered to support Darwin's ideas. This led to the development of his theory over time. In a later edition, he introduced the famous term 'Survival of the fittest'.