Final answer:
Three theories that supported Darwin's beliefs were Charles Lyell's Theory of Geology, Thomas Malthus' Theory of Population, and Alfred Russel Wallace's Theory of Evolution.
Step-by-step explanation:
- Charles Lyell's Theory of Geology: Charles Lyell's findings suggested an older Earth, which provided evidence for the long time needed for the evolution of life as Darwin proposed.
- Thomas Malthus' Theory of Population: Malthus' theory of population growth and limited resources helped Darwin understand the concept of a 'struggle for existence' and how it influences natural selection.
- Alfred Russel Wallace's Theory of Evolution: Wallace developed a similar theory of evolution by natural selection based on his own observations, which further supported Darwin's ideas.