Darwin's most important observation on his famous voyage on the HMS Beagle was the amount of difference that existed between animals of the same species within the Galapagos Islands. Most famously, he detected divergence within finches on different islands, which suggested that they had changed, or evolved, over time from an ancestral finch. When he returned to England, he conducted intensive study of coral reefs, and began to think seriously about the implications of his observations in the Galapagos.
Over time, he developed a theory of evolution by natural selection, which, he argued, was what caused the changes he observed in species. Over time, these changes, based on favorable variations that enabled animals to better survive in their environments, would lead to the development of a distinct species. In short, his observations on the journey of the Beagle in the Galapagos and elsewhere convinced him that species were not unchanging, and this realization motivated him to seek an explanation as to what made change happen.